摘要:

Synthesis, polymorphic characterization and structural comparisons of the non-linear optically active and inactive forms of polymorphs of 34nitroanilino)c y cloalk-2-en-bones Kin-Shan Huang,?" Doyle Britton? the late Margaret C. Etter' and Stephen R. Byrnxb 'Department of Chemistry, University of Minnesota, Minneapolis, MN 55455, USA bDepartment of Medicinal Chemistry and Pharmacognosy, Purdue University, IN 47907, USA Three 3-(nitroanilino)cycloalk-2-en-l-oneshave been synthesized and their non-linear optical (NLO) properties investigated. Two of these compounds have been found to exist in two polymorphic forms (a and 8)that exhibit different second-order NLO properties. These polymorphic forms were prepared and characterized by second-harmonic generation measurements as well as the more conventional methods of X-ray powder diffraction and infrared spectroscopy.Of these polymorphs, the a- and p-forms of 3-(4-nitroanilino)cyclohex-2-en-1-one(4NACHD) have been further characterized by X-ray single-crystal diffraction and the crystal structures obtained have been compared with each other to rationalize why these two crystalline forms exhibit different second-order NLO properties. The a-form of 4NACHD crystallizes in a centrosymmetric space group (P2,lc) with a =6.863(7), b =12.767( 3), c =13.410(4)A,p= 104.59 (5)", 2=4, D,=1.356 g cm-3 and R =0.046, yhereas the p-form crystallizes in a non- centrosymmetric space group (P2,2,2,) with a=7.228(3), b= 12.064(3),c= 13.104(3)A, 2=4, D,= 1.350 g cm-3 and R=0.076. Except for the slight difference in bond distances, both the a- and p-forms have the same orientation of the carbonyl group and hydrogen-bonding interactions. The carbonyl group is anti to the N-H group in both the two forms that result in the lambda (A) conformation.The whole molecule of 4NACHD is more twisted in the p-form than in the a-form. Based on structural comparisons of the polymorphs of 4NACHD and other compounds, the more twisted conformation in the /%formmay bias molecules to pack into a non-centrosymmetric structure. Preliminary results suggest that 3-(nitroanilino)cycloalk-2-en-l-onecompounds may have a higher chance of forming non-centrosymmetric crystal structures than normal achiral organic molecules. Materials possessing non-linear optical (NLO) properties have received considerable attention in recent years because of their potential applications to laser devices and to optical communi- cation, information processing and computing.' Second-har- monic generation (SHG), the second-order effect of NLO properties, of crystalline materials depends both on the magni- tude of the molecular hyperpolarizability (B) (microscopic 4NACHDnonlinearity) and on the orientation of the molecules in the crystal lattice.2 Organic NLO materials, in general, are thought to possess many potential advantages over existing inorganic materials such as large p-values, fast response time, high resistance to optical damage and the possibilities of designing molecules highly suitable for SHG.3 Many organic compounds such as p-nitroaniline that have inherently large 8-values, however, do not exhibit SHG in the solid state, since they pack into centrosymmetric crystal structures for which the macroscopic polarizability, x( 2), is necessarily zero.4 Most of the presently known organic molecules with non-centrosym- metric crystal structures, including urea, rn-nitroaniline and p-nitro-o-toluidine, were found by chance.Hydrogen-bond directed cocrystallization has been demon- strated to be a useful way to form cocrystals (or molecular complexes) for studying the molecular recognition and hydro- gen-bonding properties of a class of related host molec~les.~-~ In our continuing research on designing organic NLO mate- rials using hydrogen-bonding interactions,8 we unexpectedly obtained a class of compounds, 3-(nitroanilino)cyclohex-2-en-1-ones, from an attempt to cocrystallize the host nitroanilines with the guest cycloalkane-1,3-diones, as shown in Scheme 1.This class of compounds has been found to exist often in ?Current address: Parke-Davis/Werner Lambert, 180 Tabor Road, Morris Plains, NJ, USA. $Address for correspondence: Professor Stephen R. Byrn, School of Pharmacy and Pharmaceutical Sciences, Purdue University, West Lafayette, IN 47907-1333, USA. 3NACHD 4NACPD Scheme 1 General synthetic method for 3-(nitroanilino)cyclohex-2-en-1-ones. 4NACHD =3-(4-nitroanilino)cyclohex-2-en-l-one;3NACHD= 3-(3-nitroanilino)cyclohex-2-en-1-one; 4NACPD =3-( 4-nitroani1ino)- cyclopent-2-en-1-one. more than one polymorphic form.Interestingly, these poly- morphic forms usually exhibit different second-order NLO properties. For instance, the a-form of 3-(4-nitroanilino)-cyclohex-2-en- 1-one, referred to as 4NACHD hereafter in this study, is SHG inactive, whereas its corresponding 8-form is SHG active. In this paper, besides the synthesis and poly- morphic characterization of the different polymorphs of three 3-(nitroanilino)cyclohex-2-en-l-onesby X-ray powder diffrac- tion, SHG measurement, infrared (IR) spectroscopy, hot-stage Ir,;croscopy and differential scanning calorimetry (DSC) analy-sis, we have determined the crystal structures of the a- and 8-forms of 4NACHD using X-ray single-crystal diffraction to rationalize why these two polymorphs exhibit different second- order NLO properties.In addition, the chance of forming non- centrosymmetric crystal structures for this class of compounds is preliminarily investigated and compared with that of the normal achiral organic molecules. Experimental p-Nitroaniline, rn-nitroaniline, cyclohexane-1,3-dione and cyclopentane- 1,3-dione were purchased from Aldrich Chemical J. Muter. Chern., 1996, 6(2), 123-129 123 Table 1 Summary of the melting points of nitroanilines, cycloalkane-1,3-diones and the resulting products nitroaniline m.p./”C cycloalkane-1,3-dione m.p./”C reaction temp.“/”C product m.p./”C p-nitroaniline 149-151 cyclohexane- 1,3-dione 103-105 80 4NACHD 187- 189 rn-nitroaniline 112-1 14 cyclohexane-1,3-dione 103-105 55 3NACHD 172-174 p-nitroaniline 149-151 cyclopentane-1,3-dione 151-153 110 4NACPD 239-242 “ The lowest temperatures required to start the reactions in the solid state.Table 2 NMR data of 4NACHD, 3NACHD and 4NACPD 4NACHD 3NACHD 4NACPDb 8.45 (1 H, s), 8.24 (2 H, dd, J 2.0, 7.0), 7.43 (2 H, dd, J 2.0, 7.0), 5.76 (1 H, s), 2.63 (2 H, t, J 6.2), 2.27 (2 H, t, J 5.6) and 2.00 (2 H, quintet, J 5.6, 6.2) 9.16 (1 H, s), 7.86-7.94 (2 H, m), 7.60 (2 H, dd, J 1.1, 1.6), 5.48 (1 H, s), 2.53 (2 H, t, J 5.8), 2.20 (2 H, t, J 6.2) and 1.92 (2 H, dd, J 5.8, 6.2) 10.12 (1 H, s) 8.17 (2 H, ddd, J 2.0, 3.1, 9.2), 7.36 (2 H, ddd, J 2.0, 3.1, 9.2), 5.73 (1 H, s), 3.39 (1 H, s), 2.77 (2 H, m) and 2.26 (2 H, m) 197.6, 160.3, 147.1, 142.5, 126.9, 126.2, 121.2, 113.2, 103.2, 37.3, 29.5 and 22.2 196.4, 160.8, 148.3, 140.7, 130.7, 128.2, 117.9, 116.0, 99.5, 36.5, 28.6 and 21.3 206.5, 205.1, 170.0, 146.8, 141.5, 125.4, 118.6, 104.7, 32.7, 30.7 and 29.1 a J Values are given in Hz.Results and Discussion section). Company, Milwaukee, WI. Cyclopentane- 1,3-dione was puri- fied by recrystallization from acetone, whereas p-nitroaniline, rn-nitroaniline and cyclohexane- 1,3-dione were all purified by recrystallization from toluene. All solvents used in this study are spectroscopic grade that were purchased from Fischer Scientific Company, Pittsburgh, PA, and were used without further purification. Melting points were determined using a Fisher-Johns appar- atus and are uncorrected. Solution ‘H and 13C nuclear mag- netic resonance (NMR) spectra were recorded on a Varian VXR-200 and VXR-300 spectrometer, respectively.Chemical shifts are reported in parts per million (6) with respect to tetramethylsilane (TMS). UV-VIS spectra were recorded using a Hewlett Packard 8452A diode array spectrophotometer. Elemental analyses were performed at the microanalysis labor- atory of Purdue University. 4NACHD was synthesized by dissolving a 1 : 1 molar ratio of p-nitroaniline and cyclohexane- 1,3-dione in acetonitrile at 50°C for 5 min, and the resulting solution was allowed to evaporate slowly at room temperature. The a-form (orange plates) and the 1-form (yellow rods) of 4NACHD were pre- pared by recrystallization from acetonitrile and a 3: 1 meth- anol-water mixture, respectively.343-nitroanilino)cyclohex-2-en-l-one, referred to as 3NACHD, was synthesized from rn-nitroaniline and cyclohex- ane-1,3-dione using the same method as described above for 4NACHD. This compound also exists in two polymorphic forms (a and 1).The a-form was prepared by recrystallization from acetone as yellow prisms, whereas the p-form was pre- pared by recrystallization from methanol as yellow fine needles. 344-nitroanilino)cyclopent-2-en-l-one,referred to as 4NACPD, was synthesized from p-nitroaniline and cyclopen- tane-1,3-dione using the same method as described above for 4NACHD. Preliminary tests showed that only one poly-morphic form was observed for this compound.All the three compounds synthesized were purified by recrys- tallization and characterized by melting point measurements, solution ‘H and 13C NMR spectroscopy and elemental analy- ses. Melting points, NMR characterizations and elemental analyses of these compounds are given in Tables 1, 2 and 3, respectively. All the polymorphic forms obtained were dried in a silica gel desiccator under a vacuum at room temperature overnight before polymorphic characterization. A Zeiss optical micro- scope equipped with a Mettler FP 52 temperature controller was used for the microscopic investigation. The crystals were 124 J. Mater. Chern., 1996, 6(2), 123-129 In C2H,]DMS0, 4NACPD may exist in both the ketone and enol tautomeric forms (see illustration below in Table 3 Elemental analyses of 4NACHD, 3NACHD and 4NACPD compound analysis 4NACHD Found C, 61.7; H, 5.1; N, 11.8 Calc.for CI2Hl2N2O3: C, 62.06; H, 5.21; N, 12.06 3NACHD Found: C, 61.7; H, 5.0; N, 11.7 Calc. for C12H12N,03: C, 62.06; H, 5.21; N, 12.06 4NACPD Found C, 60.2; H, 4.5; N, 12.5 Calc. for Cl,Hl,N20,: C, 60.55; H, 4.62; N, 12.84 heated at a rate of 10 “C min-’ and their melting points and physical changes were examined. X-Ray powder diffraction patterns were recorded by a Siemens D-500 Diffractomete!- Kristalloflex with Ni-filtered Cu-Ka radiation (A= 1.5418 A) over the interval 4-40”/28. IR spectra of the samples were recorded from wavenumber 4000 to 450 cm-’ using a Perkin- Elmer 1600 FTIR spectrophotometer with both the Nujol mull and KBr pellet methods. Measurements of powder SHG efficiencies were performed on a modified apparatus of Kurtz powder techniqueg with the fundamental wavelength ( 1064 nm) of a Q-switched Nd:YAG laser.DSC analyses were carried out using a Perkin-Elmer DSC-4 instrument calibrated with an indium standard, and 3-5mg samples were heated at a rate of 20 “C min-’ from 50-210°C under a constant N2 purge with the sample sealed in crimped aluminium pans. Single crystals of the a-and 1-forms of 4NACHD were obtained as orange plates from acetonitrile and yellow rods from a 3 : 1 methanol-water mixture, respectively. The crystals used for the diffraction studies were 0.45 x 0.35 x 0.20 mm (the a-form) and 0.60 x 0.30 x 0.20 mm (the 1-form).Crystallographic data for both the two polymorphic forms are given in Table 4.T Preliminary examination and data collection were performed with an Enraf Nonius CAD-4 single-crystal X-ray diffractometer with Mo-Ka radiation (A=0.71069 A). The centrosymmetric space group of the a-form, P2,/c, was determined based on the systematic absences of h01:1# 2n and OkO : k # 2n and the successful solution and refinement of the structure. Similarly, the non-centrosymmetric space group of the 1-form, P2,2121, was determined based on the systematic absences of hOO:h#2n, OM):k#2n and 001:l#2n, and was confirmed by the positive powder SHG test. Data collection on the diffractometer was performed at 24°C using the w26 tsupplementary data available from the Cambridge Crystallographic Data Centre: see ‘Information for Authors’, J.Mater. Chern., 1995, Issue 1. Table 4 Crystallographic data for the a- and p-forms of 4NACHD a-form p-form chemical formula C12H12N203 lZH 1ZNZ03 formula mass 232.24 232.24 sp$ce group a/+ bl+ 6.863( 7) 12.767( 3) P21lc p2121217.228( 3) 12.064( 3) Z 13.410(4) 104.59( 5) 1137(2) 4 13.1O4( 3) 90 1143(1) 4 DJg ~rn-~ p(Mo-Ka)/cm -scan type scan rate/degrees min- (in w) 20m,,/degrees range of h, k, 1 no. observed reflections" F(cKw 1.356 488 0.93 cu-20 5.0-20.0 49.9 *8; 0-15; 1157 f15 1.350 488 0.92 w20 1.5-10.0 59.9 f8; 0-14; 953 0-16 R; RWb 0.046; 0.059 0.076; 0.093 goodness-of-fit 1.55 2.15 max.shiftlesd' APmax /e 4-APmin/e A-3 0.03 0.14 -0.17 0.01 0.26 -0.25 " I >3.0o(I) for the a-form and I >3.Oo(I) for the b-form. R = p,=[(C,,(IF,ICIIF,I -~Fc~~/C~Fo~~-IF,I)2/X~F,2)]"2. 'esd=esti-mated standard deviation. scan technique to a maximum 28 of 49.9" for the a-form and 59.9" for the p-form. All data were corrected for Lorentz and polarization effects. An empirical absorption correction using the program DIFABS" was applied to the a-form only which resulted in transmission factors ranging from 0.81 to 1.13. The crystal structures were solved by direct rnethods.l1*l2 The final cycle of full-matrix least-square refinements was based on 1157 observed reflections [I >3.00a(I)] and 157 variable parameters for the a-form and 953 observed reflections [I >2.00a(I)] and 153 variable parameters for the p-form.Scattering factors for neutral atoms and AY and Af" were taken from ref. 13. All calculations were performed using a TEXSAN crystallographic software package.14 Results and Discussion Three 3-(nitroanilino) cycloalk-2-en- 1 -ones, 4NACHD, 3NACHD and 4NACPD, were synthesized from cocrystalliza- tions of nitroanilines with cycloalkane-1,3-diones (Scheme 1). This type of reaction is believed to proceed by a standard enamine formation in which nucleophilic addition to the carbonyl group of cycloalkane- 1,3-diones by the amino group of nitroanilines is followed by dehydration. Since the amino groups of nitroanilines, particularly p-nitroaniline, are poor nucleophiles compared with other amines, the presence of acid catalysts and prolonged reflux are usually required to accelerate the reaction between nitroanilines and cycloalkane- 1,3-diones in s~lution.'~ Interestingly, all three compounds can also be directly prepared in the solid state by heating equimolar amounts of nitroanilines and cycloalkane- 1,3-diones at elevated temperatures.The reaction temperatures for the formation of 4NACHD, 3NACHD and 4NACPD are 80"C, 55°C and 110 "C, respectively, which are all below the melting points of their starting materials (Table 1). Nevertheless, these reactions may not be regarded as the 'true' solid-state reactions16 since nitroaniline-cycloalkane-1,3-dione mixtures in all the three cases melted first and then the condensation took place.rn-Nitroaniline seems to be more reactive than p-nitroaniline with cyclohexane-1,3-dione since the solid-state reaction occurred at a lower temperature for 3NACHD (55°C) than for 4NACHD (80 "C). This is understandable because the amino group of rn-nitroaniline is considered to be a stronger nucleo- phile than that of p-nitroaniline. In addition, the melting points of 4NACHD (187"C), 3NACHD (172°C) and 4NACPD (259°C) are all much higher than those of the individual nitroanilines (149 "C for p-nitroaniline and 112 "C for rn-nitroaniline), indicating that this class of compounds would have better thermal stability than nitroaniline compounds. To understand the solvatochromic properties of 3-(nitroanili- no)cycloalk-2-en-l-ones,the UV-VIS spectra of 4NACHD and 3NACHD as well as p-nitroaniline and rn-nitroaniline in different solvents were investigated, and the results are listed in Table 5.The wavelength of maximum absorption (Lax)of 4NACHD has a red shift relative to p-nitroaniline, particularly in nonpolar solvents like toluene. In contrast, the A,,, of 3NACHD has a blue shift with respect to rn-nitroaniline. p-Nitroaniline has much larger maximum molar absorption coefficient than rn-nitroaniline, whereas 4NACHD and 3NACHD have similar E,,, (Table 5). The solvatochromic properties of 3-(nitroanilino)cycloalk-2-en- l-ones and nitro- anilines are significantly affected by the polarity of solvents. For instance, all the Laxof these compounds have a red shift in weakly polar solvents such as chloroform compared with strongly polar solvents such as DMF and DMSO.Interestingly, the red-shift effect is smaller for 3-(nitroani1ino)- cycloalk-2-en-l-ones than for nitroanilines. The AA (ADMSO-A~~~~~,,~)for 4NACHD and 3NACHD are 24 and 10 nm, respectively, and the corresponding AA for p-nitroaniline and rn-nitroaniline are 46 and 34 nm, respectively. Like nitro- aniline compounds, 4NACHD and 3NACHD do not absorb radiation in the visible region, which is an important property in considering the applications for SHG in the visible ~pectrum.'~ Both the a-and P-polymorphic forms of 4NACHD and 3NACHD were obtained by the preparation procedures described in the Experimental section and identified with data from X-ray powder diffraction, IR spectroscopy and powder SHG measurement.The X-ray powder diffraction profiles of these polymorphic forms are shown in Figs. 1 and 2. For 4NACHD, the sharp X-ray diffraction peaks of the a-form occurred at 28=9.85, 13.74, 15.50, 19.50, 21.73, 22.71, 25.25, 27.01 and 30.30", and those of the p-form were observed at 28= 10.14, 13.66, 14.88, 15.33, 16.36, 20.54, 23.90, 24.81, 25.88, 26.85 and 37.61'. At a glance these two crystalline forms do not exhibit noticeably different X-ray powder diffraction patterns. However closer examination shows that the a-and b-forms of 4NACHD can be distinguished from each other by their different peak positions. In contrast, the X-ray powder patterns of the a-and p-forms of 3NACHD can be easily distinguished from each other.The sharp X-ray diffraction peaks of the a-form of 3NACHD occurred at 28 =8.95, 12.06, 13.74, 14.62, 16.41, 17.81, 21.99, 22.57, 26.64, 35.59 and 36.90", and those of the p-form were observed at 28=9.96, 10.58, 15.92, 19.26, 25.80, 26.49 and 26.93". Different polymorphic forms usually give different solid- state IR spectra because IR absorptions are very sensitive to Table5 Summary of UV-VIS data for 4NACHD, 3NACHD, p-nitroaniline and rn-nitroaniline in different solvents La,/nm lo^ dm3 mol-' cm-') compound toluene chloroform DMF DMSO 4NACHD 3 64 365 382 388 (18.2) (17.8) (19.5) (20.5) 3NACHD 302 304 3 10 3 12 (18.8) (22.9) (23.7) (24.9) p-nitroaniline 344 348 382 390 (29.9) (14.7) (14.5) (13.3) rn-nitroaniline 364 366 390 398 (1.7) (1.4) (0.9) (0.8) J.Muter. Chern., 1996,6(2), 123-129 125 10 15 20 25 30 35 28ldegree.s Fig. 1 X-Ray powder diffraction of the a-form (a) and the P-form (b) of 4NACHD 5 10 15 20 25 30 35 4 28 /degrees Fig.2 X-Ray powder diffraction of the a-form (a) and the P-form (b) of 3NACHD the solid-state environments, particularly hydrogen-bonding environments. The solid-state IR spectra of these polymorphic forms as well as 4NACPD are shown in Fig. 3. Interestingly, both the a-and /?-forms of 4NACHD exhibit the same IR spectrum, suggesting that these two polymorphs may have similar molecular conformations or hydrogen-bonding inter- actions in the solid state.In contrast, the IR absorption spectra of the a-and /3-forms of 3NACHD show recognizable differ- ences in the positions and intensities of the absorption bands between the regions of 3600-3000 cm-l and 1800-2000 cm-I (Fig. 3). The major absorption bands observed in these two regions are 3277, 3203, 3131, 1602, 1579, 1533, 1357 and 1251 cm-l for the a-form and 3305, 3217, 3139, 1614, 1579, 1549, 1531, 1480 and 1268 cm-' for the p-form. In addition to polymorphic characterization, solid-state IR spectroscopy could be useful for studying the solid-state struc- tures of 3-(nitroanilino)cycloalk-2-en-l-ones.These com-pounds, in principle, can have three tautomeric forms, a$-unsaturated ketone (I), /?-anilino-enolic (11) and /?-anilino ketone (111) structures, shown below.4NACHD, 3NACHD and 4NACPD all exhibit N-H stretching bands in the region 3300-3000 cm-' (Fig. 3).18 In addition, all the three com-pounds exhibit a broad and strong C=O stretching band near 1540 cm-I which is often observed in a number of open-chain compounds with the R-CO-C=C-N<stru~ture.'~ The pres- ence of both the N-H and C=O stretching bands suggest that 4NACHD, 3NACHD and 4NACPD all exist in the a$-unsaturated ketone form (I). The preference of the form I for the a-and p-forms of 4NACHD is also confirmed by X-ray crystal structure determination. 126 J. Muter. Chem., 1996, 6(2), 123-129 w -1700 1500 1300 wavenurnbedcm-' Fig. 3 Solid-state IR spectra of 4NACHD, 3NACHD and 4NACPD.(a) 4NACHD (its a-and p-forms exhibit the same IR spectrum); (b) the a-form of 3NACHD; (c) the P-form of 3NACHD; (d)4NACPD. I I1 .I11 Since the second-order NLO property is sensitive to the orientation of molecules in the solid-state medium,2 SHG measurements are particularly useful for characterizing poly- morphic forms when one of them has a centrosymmetric crystal structure and the other has a non-centrosymmetric crystal structure. The results of SHG measurements are given in Table 6. The two polymorphic forms of 4NACHD are differen- tiated by their different SHG properties: the a-form is SHG inactive and the /?-form is SHG active. Similarly, the a-and p-forms of 3NACHD can also be distinguished from each other because of their different SHG activities (Table 6).Hot-stage microscopy and DSC were used for investigating the phase transformation of the polymorphic forms of 4NACHD and 3NACHD. For 4NACHD, the a-form endo-thermally transforms to the p-form at 132°C and then melts at 187"C, whereas the /?-form directly melts at 187°C. The transformation energy between the a-form and /?-form was approximately 5 kJ mol-' based on two determinations. For 3NACHD, both the a-and /?-forms melt at 172°C. However, Table6 Summary of powder SHG values of the polymorphic forms of 4NACHD, 3NACHD and 4NACPD (values relative to urea) compound polymorph SHG (1064 nm) 4NACHD 0 4NACHD 2 3NACHD 0 3NACHD 1.5 4NACPD 0 01 03 iI II Fig.4 Chemical structure of 4NACHD and atomic numbering used in this study Table 7 Selected bond distances (A),bond angles (degrees) and torsion angles (degrees) in the a-and p-forms of 4NACHD a-form p-fom ~ C( 1)-N( 1 1 c(4)-N (2) C(7)-N( 1) C(9)-O(3) C( 7)-C( 8 )C(7)-C( 12) C( 8 )-C(9) C( 10)-C( 11) C(8W(9 1-0(31 C( 10)-C( 11)-C( 12) C(7)-C(8)-C(9)-0(3) C(7)-C(8)-C(9bC( 10) C(9)-C( 8)-C(7)-C( 12) C( 8)-C( 9)-C( 10)-C( 11) C( 1)-N( 1)-C(7) C( 7)-C( 8)-C( 9) 1.401 (3) 1.459( 4) 1.369( 3) 1.217(3) 1.343(4) 1.506( 4) 1.436(4) 1.498 (4) 130.4(2) 12 1.9 (3) 121.4( 3) 11 1.1 (2) -5.5(4) -176.9(3) 2.6(4) -27.3 (3) 1.445(8) 1.456(8) 1.314(7) 1.231 (7) 1.357( 8) 1.554(9) 1.455( 8) 1.48( 1) 128.6( 6) 122.5( 6) 120.1 (7) 114.0( 7) 174.4( 6) -6.1 (9) -4.8(9) 20(1) the p-form was obtained when the melt of the a-form solidified.DSC analysis showed that there is only one fusion peak at 172°C for both forms, the transformation between the a-form and /?-form may occur at 172 "C or the transformation energy is very small. All the thermal phase transformation results were tested and confirmed by SHG measurements and X-ray powder diffraction. The atomic numbering of the a-and p-forms of 4NACHD is depicted in Fig. 4. Selected bond distances, bond angles and torsion angles of the a-and p-forms are given in Table 7. The crystal packing of the two polymorphic forms are illustrated by stereoviews in Fig. 5. Except for the C( 1)-N( 1) and C(7)-N( 1) bond lengths, the a-and /?-forms of 4NACHD generally have a good agreement in their bond distances, bond angles and torsion angles (Table 7).The molecule of 4NACHD is composed of a nitroani- lino moiety and a cyclic a$-unsaturated ketone moiety. The carbonyl group is anti to the N-H group in both the two forms that result in the lambda (A) conformation. In both the two polymorphic forms, the nitro group and the phenyl ring of the nitroanilino moiety are quite planar and are coplanar with respect to each other. The cyclic ketone ring in the other moiety is in a puckered conformation where )he C( 11) is out of the plan? of the remaining atoms, 0.642A for the a-form and 0.554A for the p-form. The puckered conformation is common for cyclohexyl rings that contain three adjacent sp2 carbon atoms and three adjacent sp3 carbon atoms.20 To reduce the hydrogen-hydrogen repulsion between C( 6) and C( 8), the nitroanilino plane and the a$-unsaturated ketone plane are twisted from each other, particularly in the p-form.The dihedral angle between the two least-squares planes is calculated as 25.1" for the a-form and 35.7" for the p-form. Fig. 5 Stereoviews of the crystal packing for the a-form (a) and the p-form (b) of 4NACHD Since the nitro and N-H groups are para-related to each other in the nitroanilino moiety an! since the bond distpce of C(7)-N( 1) in the a-form ($369 A) and p-form (1.314 A) is noticeably shorter than 1.47 A of the normal C-N (~p~-N),~l three resonance forms are proposed for 4NACHD (shown below). Since the bond +stance of C(7)-N(1) in !he two polymorphic forms (1.369 A for the a-form and 1.314 A for th,e p-form) is considerably shorter than that of C( 1)-N( 1) (1.401 A for the a-form and 1.445 A for the /?-form), the lone pair electrons in the N-H group would be more easily delocalized into the cyclic a$-unsaturated ketone group than into the nitroanilino group. As a result, the resonance form A should be more favourable than the resonance form B.The preference for form A is $so supported by the shortet C(8)-C(9) bond length (1.436 A for the a-form and 1.455 A for the /?-form) compared with 1.48A for the normal sp2-sp2 CIC bonds2' and by the longeor C( 1)-N( 1) bond length (1.401 A for tbe a-form and 1.445 A for the /I-form) compared with 1.371 A for the corresponding C-N bond observed in p-nitroaniline.22 As illustrated in Fig.6, the amino proton in the a-and /?-forms participates in hydrogen bonding as a proton donor. The carbonyl oxygen, rather than the nitro group, is the sole proton acceptor in the two structures that accepts the amino proton to form an intermolecular N-H...O hydrogen bond. As a consequence, the 4NACHD molecules aggregate to form an infinite hydrogen-bonded chain along the two-fold screw axis in which the nitroanilino group is positioned alternatively in the chain. The N-H...O hydrogen bond observed in thes? two structures is very stroFg since the N.e.0 distance, 2.801 A for the a-form and 2.813 A for the /?-form (Table 8), is much shorter than van der Waals distances (3.14 A for N.e-0 inter- action~)~~and shorter than most intTa- and inter-molecular N...O hydrogen bond distances (2.85 A).24 Because the a-and p-forms are all involved in the same hydrogen-bonding inter- actions, they exhibit the same IR absorption spectrum (Fig.3). Although the a-and p-forms exhibit the same orientation of the carbonyl group and hydrogen-bonding interactions, they 0 0 H IH IH A B J. Muter. Chem., 1996, 6(2), 123-129 127 H H 03 H Fig.6 The hydrogen-bonding interactions in the a-form and the p-form of 4NACHD. Hydrogen bonds are indicated by dashed lines. pack into quite different crystal structures, P2,/c (monoclinic) for the a-form and P2,2,2, (orthorhombic) for the p-form.Like nitroaniline 3-(nitroanilino)cycloalk-2-en-1-ones such as 4NACHD might have large intrinsic molecu- lar hyperpolarizability; thus this class of compounds could be good candidates for NLO materials. Since the a- and /I-forms of 4NACHD crystallize in the centrosymmetric (P2,lc) and non-centrosymmetric (P212121) space groups, respectively, the a-form is SHG inactive and the p-form is SHG active. It is well known that intermolecular interactions are important in the crystal packing of molecules; however, how these inter- actions affect and control the crystal packing is still not clearly known. As discussed earlier, except for the differences in the bond lengths of C( 1)-N( 1) and C(7)-N( 1) and in the dihedral angle between the nitroanilino and a$-unsaturated ketone planes (25.1" for the a-form and 35.7" for the p-form), both the a-and p-forms of 4NACHD have the same orientation of the carbonyl group and hydrogen-bonding interactions.In the previous study of the relationship between the NLO properties and polymorphism of 1,3-bis (m-nitrophenyl) urea and 1,3-bis (o-anisoy1)urea (shown below), we found that polymorphs with the more twisted conformations (the larger dihedral angles between the two phenyl rings) crystallized in a non-centrosym- metric space group whereas those with the less twisted confor- mation crystallized in the centrosymmetric space group.26 A reasonable rationale for this phenomenon is that the more twisted conformation would result in a smaller net dipole moment for the molecule, a similar effect to the vanishing dipole moment proposed by Zyss et Interestingly, the same trend is also observed in the polymorphs of 4NACHD in which the more twisted conformer crystallized in a non- centrosymmetric space group (p-form), whereas the less twisted conformer crystallized in a centrosymmetric space group (a- form).I ,3-bis(m-nitropheny1)urea 1,3-bis(o-anisoyI)urea As with other organic and pharmaceutical c~rnpounds,~~~~~ two of the three compounds synthesized, 4NACHD and 3NACHD, have been found to exist in two polymorphic forms (a and p). Of the four polymorphs, two of them do not exhibit SHG and the other two do exhibit SHG (Table 6). Since SHG is exhibited in the non-centrosymmetric media only, these two polymorphs with positive SHG should have non-centrosym- metric crystal structures. Thus, the chance of having a non- centrosymmetric crystal structure is 40% for the five samples which is significantly higher than 11% for the normal achiral organic compounds.30 Although the number of samples studied here may not be large enough, these preliminary results suggest that 3-(nitroanilino)cycloalk-2-en-l-onecompounds may have a higher chance of forming non-centrosymmetric crystal struc- tures than normal achiral organic compounds. If this is true, the chemical structure and molecular conformation of this class of compounds could be important to their high incidence of forming non-centrosymmetric crystal structures.Curtin and Paul have demonstrated that the crystal structures of metu-substituted compounds such as m-nitroaniline are more likely to be non-centrosymmetric than the other organic com-pound~,~~probably because the non-centrosymmetric mol- ecules could induce non-centrosymmetricity in developing crystal nucleation sites.In addition, the lambda (A) confor-mation observed in the a- and j3-forms of 4NACHD may be important to encourage the molecules to pack into a non- centrosymmetric crystal structure since Watanabe and co-workers have reported that a number of the lambda (A) molecules such as N,N'-bis(4-nitrophenyl)methanediamine, shown below, often tend to crystallize in a non-centrosym- metric space OzNn'N-N /DNo2 IIHH N, N'-bis (44trophenyl) mtthanediamine Conclusions Three 3-(nitroanilino)cycloalk-2-en-l-ones, including 4NACHD, 3NACHD and 4NACPD, were synthesized from the reactions of nitroanilines and cycloalkane-1,3-diones. 4NACHD and 3NACHD have been found to exist in two polymorphic forms (a and p) that exhibit different second- order NLO properties.The a-polymorph of 4NACHD trans- forms to the p-polymorph at 132 "C, whereas the a-polymorph of 3NACHD melts at 171 "C and then transforms into the p-Table 8 Hydrogen-bond parameters in the a-and /?-forms of 4NACHD D-H acceptor He -.A/A D..-A/A D-H..-A/" symm. oper. 1.81(3) 2.801 (3) 170.3 -x, 1/2+y, 112-2 1.96(5) 2.813(7) 168.5 2-x, -1/2+y, 112-2 128 J. Muter. Chem., 1996, 6(2), 123-129 polymorph.To understand why these polymorphic forms exhibit different SHG properties, the crystal structures of the a-and p-forms of 4NACHD were determined by X-ray single- crystal diffraction. Except for the slight difference in bond distances, both the a-and p-forms have the same orientation of the carbonyl group and hydrogen-bonding interactions. The carbonyl group is anti to the N-H group in both the two forms that result in the lambda (A) conformation. The whole molecule of 4NACHD is more twisted in the p-form than in the a-form. Based on the crystal structural comparisons of the polymorphs of 4NACHD and other compounds, the different NLO properties of these two polymorphs may be related to the twisted conformation between the nitroanilino and a,p-unsaturated ketone planes. The preliminary results show that 3-(nitroanilino)cycloalk-2-en-1-onecompounds may have a higher chance of forming non-centrosymmetric crystal struc- tures than the normal achiral organic molecules.We gratefully acknowledge financial support for this research from the Office of Naval Research and the Byrn/Zografi joint project for study of the effect of water on the molecular mobility of pharmaceutical solids. References 1 P. N. Prasad and D. J. William, Introduction to Nonlinear Optical Effects in Molecules and Polymers, Wiley, New York, 1991. 2 D. J. Williams, Angew. Chem., Int. Ed. Engl., 1984,23,690. 3 R. Rytel, G. F. Lipscomb, M. Stiller, J. Thackara and A. J. Ticknor, in Nonlinear Optical Effects in Organic Polymers, ed.J. Messier, F. Kajzar, P. Prasad and D. Ulrich, Kluwer Academic Publishers, Dordrecht, 1988, pp. 277-289. 4 J. Zyss and J. L. Oudar, Phys. Rev. A, 1982,26,2016. 5 M. C. Etter and P. W. Baures, J.Am. Chem. SOC., 1988,110,639. 6 M. C. Etter, Z. Urbanczky-Lipkowska, M. Zia-Ebrahimi and T. W. Panunto, J.Am. Chem. SOC., 1990,112,8415. 7 M. C. Etter and S. M. Reutzel, J.Am. Chem. SOC., 1991,113,2586. 8 M. C. Etter, K. S. Huang, G. M. Frankenbach and D. A. Adsmond, in Materials for Nonlinear Optics: Chemical Prespectives, ed. S. R. Marder, J. E. Sohn and G. D. Stuky, ACS Symp. Ser. 455, American Chemical Society, Washington DC, 1991, pp. 446-456. 9 S. Kurtz and T. T. Perry, J.Appl. Phys., 1968,39,3798. 10 N.Walker and D. Stuart, Acta Crystallogr., Sect. A, 1983,39,158. 11 C. J. Gilmore, J. Appl. Crystallogr., 1984, 17,42. 12 P. T. Beurskens, DIRDIF, an automatic procedure for phase extension and refinement of difference structure factors, technique report 1984/1, Crystallography Laboratory, Toernooiveld, The Netherlands, 1984. 13 R. Steward, E. R. Davidson and W. T. Simpson, International Tables for X-ray Crystallography, Kynoch Press, Birmingham, 1974, VO~.IV, pp. 202-207. 14 Molecular Structure Corporation, TEXSAN, TEXRAY structure analysis package, MSC, 3200A Research Forest Drive, The Woodlands, TX 77381, USA, 1985. 15 K. Ramalingam, M. Balasubramanian and V. Baliah, Indian J. Chem., 1972, 10, 62. 16 I. C. Paul and D. Y. Curtin, Acc.Chem. Res., 1973,7,223. 17 J. F. Nicoud and R. J. Twieg, in Nonlinear Optical Properties of Organic Molecules and Crystals, ed. D. S. Chemla and J. Zyss, Academic Press, Orlando, FL, 1987, vol. I, pp. 249-253. 18 L. J. Bellamy, in The Infrared Spectra of Complex Molecules, Chapman and Hall, London, 1975, pp. 277-291. 19 N. H. Cromwell, F. A. Miller, A. R. Johnson, R. L. Frank and D. J. Wallace, J. Am. Chem. SOC., 1949,71, 3337. 20 A. Katrusiak, Acta Crystallogr., Sect. B, 1990,46,246. 21 F. H. Allen, 0.Kennard, D. G. Watson, L. Brammer, A. G. Orpen and R. J. Taylor, J. Chem. SOC., Perkin Trans. 2, 1987, S1. 22 K. N. Trueblood, E. Goldish and J. Donohue, Acta Crystallogr., 1961,14,1009. 23 A. Bondi, J.Phys. Chem., 1964,68,441. 24 R. J. Taylor and 0.Kennard, Acc. Chem. Res., 1984,17,320. 25 J. L. Oudar and D. S. Chemla, J. Chem. Phys., 1977,66,2664. 26 K. S. Huang, D. Britton, M. C. Etter and S. R. Byrn, J. Muter. Chem., 1995,379. 27 J. Zyss, D. S. Chemla and J. F. Nicoud, J. Chem. Phys., 1981, 74,4800. 28 S. R. Byrn, D. Y. Curtin and I. C. Paul, J. Am. Chem. SOC., 1972, 94,890. 29 J. Haleblian and W. McCrone, J.Pharm. Sci., 1969,58,911. 30 M. C. Etter and K. S. Huang, Chem. Muter., 1992,4,824. 31 D. Y. Curtin and I. C. Paul, Chem. Rev., 1981,81,524. 32 T. Watanabe, H. Yamamoto, T. Hosomi and S. Miyata, in Organic Molecules for Nonlinear Optics and Photonics, ed. J. Messier, F. Kajzar and P. Prasad, Kluwer Academic Publishers, Dordrecht, 1991, pp. 151-159. Paper 5/020401; Received 31st March, 1995 J. Muter. Chem., 1996, 6(2), 123-129 129

ISSN:0959-9428    

DOI:10.1039/JM9960600123

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Z-type Langmuir-Blodgett film structures: surface plasmon resonance, second harmonic generation and fibre optic devices Geoffrey J. Ashwell,"" Gary Jefferies," Christopher D. George," Rakesh Ranjan," Robert B. Chartersband Ralph P. Tatamb "Centrefor Molecular Electronics, Cranfield University, Cranfield, UK MK43 OALT boptical Sensors Group, School of Mechanical Engineering, Cranfield University, Cranfield, UK MK43 OALT Second harmonic generation (SHG)from Langmuir-Blodgett (LB) films of the iodide (I) and octadecylsulfate (11)salts of E-N-octadecyl-4-[ 2-(4-dibutylaminophenyl)ethenyl]quinolinium and from films of a related pyridinium dye (111)increases with the number of layers. The LB film structures are non-centrosymmetric (Z-type) and have high second-order susceptibilities, x(')zzz,of 120 pm V-' for dye 1(20 layers), 80 pm V-' for dye I1 (100 layers) and 30 pm V-' for dye I11 (160 layers) at Am= 1.064 pm.The values are resonantly enhanced but for dye I11 the absorbance is very weak at the harmonic wavelength. The real and imaginary parts of the dielectric permittivity of dye I, derived from the surface plasmon resonance (SPR), are 3.14 f0.06 and 0.66 f0.03, respectively, at 532 nm and, with the exception of the monolayer, the thickness is 3.0 f0.3 nm layer-'. Waveguiding overlays of dye I, evanescently coupled to side-polished optical fibres, have resulted in useful in-line wavelength-selective elements. The LB technique allows the necessary control of film thickness for such devices. Langmuir-Blodgett (LB) films have been extensively studied for second harmonic generation (SHG) in thr~ugh-planel-~ and guided-wave configuration^.'-^ Pitt and Walpita' first established that LB multilayers of fatty acids could be utilised as planar waveguides and numerous methods of guiding in both passiveg*10 and active overlay^^-^ have appeared.For SHG it is necessary for the structure to be non-centrosym- metric. This can be realised by interleaving the layers with compatible spacers'-4 and, as predicted by theory, the second- harmonic intensity increases quadratically with the number of active layers if long-range order is maintained. The interleaved films include bilayer structures with interlocking geometries ('molecular zips'),' those stabilised by interlayer hydrogen bonding' and polymeric LB films.3 A four-fold improvement in the SHG is feasible if each layer is active, but there have been few examples of homomolecular films showing quadratic enhancement to greater than ten layers.''-'6 The first reported example, 2-docosylamino-5-nitropyridine(DCANP), has a Y-type structure in which the molecular layers pack head-to- head and tail-to-tail." Such arrangements are usually centro- symmetric but, in this case, the layers adopt a non-centrosym- metric herringbone arrangement with the charge-transfer axis parallel to the plane of the substrate.Another example is the quinolinium zwitterion, Z-P-(N-hexadecyl-4-quinolinium)-a-cyano-4-styryldicyanomethanide (C16H33-Q3CNQ), which adopts a Z-type structure with a head-to-tail packing arrange- ment displaying a quadratic SHG dependence to 200 layers.'2 The structure is stabilised by the negative charge on the terminal dicyanomethanide group which would repel in a Y-type film.The use of unconventional two-legged cationic dyes with hydrophobic chains at opposite ends of a hydrophilic chromophore has recently provided additional examples for SHG.13-16Such materials invariably form Z-type structures and quadratic SHG enhancement has been observed to more than 100 layers in three instance^.'^-'^ In this work we report that the Z-type deposition and associated non-linear optical properties of some unconven- tional cationic dyes may be improved by the addition of a t Both groups are affiliated to the Centre for Photonics and Optical Engineering.third alkyl chain and the use of an amphiphilic anion (Fig. 1). The SHG from films of one of these dyes has been shown to increase quadratically with the number of LB layers to thick- nesses of cu. 0.6 pm;this is the thickest Z-type structure to date, although thicker non-centrosymmetric films have been obtained by interleaving the layers with inactive spacer^.^ We also report the application of such multi-legged dyes in wave- guiding overlays, evanescently coupled to optical fibres, for use as in-line channel dropping filters. Our current work has demonstrated the value of the LB technique in controlling the thickness to satisfy the phase-matching condition for coupling optical power from the fibre to the LB film.Q X-L==/ 'C4H9 Fig. 1 Molecular structures of (a) dye I (X-=iodide) and dye I1 (X-=octadecylsulfate), and (b) dye I11 J. Muter. Chem., 1996,6(2), 131-136 131 Experimental The dyes were synthesised using the procedure reported in ref. 17, recrystallised from methanol and purified by column chromatography before use. Dye I was spread from dilute chloroform solution (0.1mg ml-') onto the pure water sub- phase of one compartment of an alternate-layer LB trough (Nima Technology, model 622), left for 5 min at 20 & 2 "C,and then compressed at 0.5 cm2 s-l (0.1% s-' of area). The films were transferred at 30mN m-' by passing a glass slide (for SHG) or a silver-coated slide (for SPR) via the second compart- ment of the trough, under the fixed surface barrier separating the compartments, to deposit on the upstroke at a rate of 5 mm min-'.Films were also transferred to annularly side- polished optical fibres, at 5 mm min-' for the first ten passes and then subsequently at a rate of 25 mm min-', by lowering and raising the fibre through the floating layer compressed to 30 mN m-'. The device transmission was measured in situ as deposition proceeded using the method previously described." Dyes I1 and I11 were obtained by the metathesis of their iodide salts and sodium octadecylsulfate at the air-water interface of the LB trough. The materials were spread in a 1:1 mole ratio from dilute chloroform-methanol solution onto the subphase, causing the water soluble ions, Na+ and I-, to dissolve and leaving octadecylsulfate salts of the dyes at the surface.The Langmuir films were compressed at 0.5 cm2 s-' and then transferred on the upstroke to a glass slide at 40 mN m-' (dye 11) and 35 mN m-l (dye 111) in the manner described for the iodide salt above. Results and Discussion Is0therms The pressure-area isotherm of dye I is featureless and collapses at 32 mN m-' [Fig. 2(a)]. Grazing incidence synchrotron X-ray diffraction studies on tbe floating monolayer have provided an in-plane area of 46.6 A2 molecule-' at 30 mN mo-' and 20.7 "C,compared with the corresponding value of 62 A2 from the isotherm. The discrepancy is not unusual and is readily explained.The value from X-ray diffraction only arises from the ordered domains whereas the isotherms aver!ge over the entire film including the voids. Furthermore, 46.6 A2 is in close agreement with the cross-sectional van der Waals area of the dibutylamino group and thus, the molecules probably adopt a vertical alignment at higher pressures with the hydrophobic dibutylamino group adjacent to the water subphase. This unusual arrangement has been reported for two-legged dyes with hydrophobic end-groups and has been confirmed by X- ray synchrotron diffraction studies at the air-water interfa~e.'~ The isotherms of the two octadecylsulfate salts show similar trends with the onset of plateau-like regions at 23 mN m-' (dye 11) and 19mN m-' (dye 111) and with corresponding areas of 92 and 100 A2 molecule-', respectively [Fig.2(b)]. The areas are consistent with those obtained from the van der Waals dimensions of the chromophores, whereas at collapse the areas reduce to 35 and 40A2 molecule-' and match the cross-sections Ff the dibutylamino group and the two octadecyl chains (ca. 40 A2). Thus, the plateau regions may be attributed to structural rearrangements which involve a change from horizontal to vertical alignment of the chromophore. The octadecylsulfate counter-ion improves the stability of the film and for the quinolinium dye the collapse pressure increases to above 50 mN m-', compared with 32 mN m-l for the iodide. Spectra The spectra of films of the quinolinium dye are similar but the broad absorption maximum of the charge-transfer band is shifted from 530 nm for the iodide to 515 nm for the octadecyl- sulfate salt (Fig.3). In both cases the films show negligible 132 J. Muter. Chern., 1996, 6(2), 131-136 t -10 0 '"1'"' 0 25 50 75 100 125 150 175 200 area/A* molecule-' Fig. 2 Pressure-area isotherms at 2042 "C for (a) dye I; (b) dye I1 (-) and dye I11 (---) 0.4 1 wavelengthhm Fig.3 UV-VIS spectrum of a 100 layer LB film of dye I1 (Lax=515 nm, A,,, =0.0037 layer-'). The spectrum of dye I is very similar but the broad maximum is shifted to 530 nm with A,,, =0.007 layer-'. absorbance above ca. 750nm but absorb strongly at the harmonic wavelength. In contrast, the weaker pyridinium acceptor causes the charge-transfer maximum to be shifted to 425 nm (Fig.4). In this case, the absorbance at the harmonic wavelength is very weak (6 x lop4 layer-') compared with 7 x lop3for dye I and 3.5 x lop3for dye 11. Thus, the pyridin- ium dye is an ideal candidate for SHG because there is still a degree of resonant enhancement, although the absorbance is meagre. wavelengthhm Fig. 4 UV-VIS spectrum of a 160 layer LB film of dye I11 (~,,,= 425 nm, A,,, = 0.0040 layer -') Surface plasmon resonance SPR studies were carried out using an attenuated total reflec- tion geometry in the Kretschmann configuration.'* Silver was vacuum-deposited onto clean glass substrates to a thickness of approximately 46nm and the substrate index matched to one face of a 60" BK7 crown glass prism.Reflectivity data were collected as a function of incidence angle using a p-polarized frequency-doubled Nd:YAG laser beam (A = 532 nm) and subsequently corrected for reflections at the entrance and exit faces of the prism prior to analysis by using the Fresnel reflection f~rmulae.'~ The real (E,) and imaginary (E~)compo-nents of the relative permittivity and film thickness (1) obtained for freshly deposited silver were used in the subsequent analysis of the glasslAg structures coated, in turn, with one to seven LB layers of the iodide salt (dye I). The experimental data for the silver and the first five LB layers and the theoretical fit to the experimental data for the three layer film are illustrated in Fig. 5 and 6.The plots show characteristic broadening and decreasing minima as the thickness increases beyond the bilayer, whereupon absorption contributes significantly to increased losses within the LB overlay. Analysis of the SPR data gave real and imaginary compo- nents of the dielectric permittivity of 3.14 f0.06 and 0.66 f0.03, respectively, the imaginary part being in close agreement with the value calculated from the absorbance at 532nm. The derived thickness is 2.47 nm for the first layer and 3.0 f0.3 nm 1.0 0.8 .-0 .-> CI0 0.6 a L -0 3 0.4 E 8 c 0.2 0.0 angle of incidenceldegrees Fig. 5 Normalized reflectivity data at 532 nm for a glass slide overlaid with a 46 nm thick silver film (far left) and progressively, from left to right, overlaid with one to five layers of dye I.For clarity the theoretical fits for all films and the experimental data for six and seven LB layers have been omitted from the figure. angle of incidenceldegrees Fig. 6 Experimental (+) and theoretical (-) reflectivity data for the glasslAgltrilayer structure of dye I at 532 nm. The theoretical fit corresponds to E, = 3.08, E~ =0.64 and I = 8.23 nm. layer-' for all subsequent layers. This suggests that the mono- layer adopts a more tilted arrangement on silver, whereas the larger value for the bulk film is consistent with the mean thickness of 3.0 nm layer-' from ellipsometry for 20 LB layers on glass. For comparison, ellipsometry has provided a thickness of 3.5 nm layer-' for I11 and a refractive index of ca.1.5 at 670 nm. The thicker film is consistent with a different molecular tilt of 30+ 1" for I11 compared with 41 f5" for I (see below). Second harmonic generation SHG measurements were performed in transmission using a Nd:YAG laser (A= 1.064 pm) with the beam at 45" to the film, as described in ref. 4. The SHG from films comprising from one to twenty layers of the iodide salt have been reported previously and show quadratic SHG enhancement with increasing thi~kness.'~ The octadecylsulfate salt, reported in this work, shows similar behaviour to 20 layers but for greater thicknesses the SHG deviates from the quadratic dependence, L(N) =LJ(l)N2 (1) where N is the number of LB layers.The second-harmonic intensity from a freshly deposited 100 layer film is 5 x lo3times the monolayer signal, but has been found to increase with time to an upper limit of ca. 7 x lo3 (Fig. 7). The film absorbs at LVV 160 + + + t + n 2 100 t t + 60 t+ *O *++;++++ number of layers Fig. 7 Variation of the square root of the second-harmonic intensity with the number of LB layers of dye I1 when first deposited (+) and four weeks later after reaching a constant value (0) J. Muter. Chem., 1996, 6(2), 131-136 133 the harmonic wavelength and the expected dependence approximates to: j=N 12w(N)=IZW(1) 1 N*TN-j j= 1 where T, the mean layer transmittance, is 0.992 at 532nm. Eqn. (2) predicts 12w(N)/12w(1)M 6.9 x lo3 for N =100 and this corresponds to the upper limit observed for the 100 layer film. Allowing for absorbtion losses, the SHG dependence is consist- ent with theory and, therefore, the alignment must be non- centrosymmetric (Z-type) and reasonably ordered throughout the 100 layers.The SHG polarization dependence, 12J p-+p)/12Js-+p), is similar for the iodide and octadecylsulfate salts and, using the method of Kajikawa et ~l.,~'the data correspond to chromo- phore tilt angles of 4 =41 f5" from the substrate normal. The validity of this approach is not clear as intermolecular charge transfer as well as intramolecular processes can give rise to strong SHG.21 Nonetheless, the product of molecular length and COS~closely matches the thickness obtained from SPR and ellipsometry.Using the data obtained from these tech- niques the second-order susceptibility, x(2)zzz,of dye I is 120 pm V-'. Unfortunately, the ellipsometry studies on films of dye I1 have been unsuccessful to date. However, as the chromophore tilt angle is the same for both dyes it may be assumed that the thickness of the octadecylsulfate salt is similar and for 1= 3.0 nm layer -'the susceptibility, corrected for absorbance by the 100 layer film, is 80 pm V-' (dye 11).Although these values are resonantly enhanced, the films may be applicable to frequency doubling at more suitable wavelengths where they are transparent. Importantly, we have demonstrated that the presence of the hydrophobic dibutylamino end-groups causes the structures to be non-centrosymmetric (Z-type).To overcome the problem of absorbance at 532nm (AJ our recent work has concerned a novel pyridinium analogue (dye 111). The SHG has been investigated to 160 layers and the intensity varies by no more than &4% along a 30 mm length of the film. The normalised intensity, 120(N)/N2,is higher for the LB monolayer and this may be attributed to a slightly different tilt angle for molecules adjacent to the glass substrate than in the bulk film. However, quadratic SHG enhancement has been realised from 4 to 160 layers and this is clearly indicative of an ordered Z-type arrangement (Fig. 8). The chromophore tilt angle from the SHG polarization dependence and the second-order susceptibility are 4 =30 f1"and x(2)zzz= 30 pm V-l.The film absorbance is 6 x lop4layer-' at 532 nm and thus, the transparency/efficiency trade-off is very favour- able. Moreover, the Z-type structures of dye I11 are stable and 100 80 + tt t + t + 20 '0 20 40 60 80 100 120 140 160 number of layers Fig. 8 Variation of the square root of the second-harmonic intensity with the number of LB layers of dye I11 134 J. Muter. Chem., 1996, 6(2), 131-136 in the year since the 160 layer film was first fabricated the SHG has shown no sign of deterioration. Fibre optic studies We have studied the possibility of evanescent coupling to the LB film using side-polished single mode optical fibres to realise in-line channel dropping filters.The fibre cladding was removed using an annular polishing process'' to access the propagating optical field and an LB film of dye I was deposited onto the polished fibre over an interaction length of 9.6k0.3 mm (Fig. 9). By assuming weak coupling the device may be viewed as an asymmetric directional coupler; the strong differential waveguide dispersion between the fibre-guided mode and the LB-guided mode results in a bandstop or channel dropping spectral response centred on the synchronous wavelength (A,) at which phase matching between the two guided modes occurs. This simple description yields accurate predictions for A, as a function of the LB film thickness," but the true device geometry of cylindrical fibre and planar overlay must be taken into account to determine the form of the spectral res~onse.~~.~~ The LB film was deposited onto the optical fibre during 188 passes through the Langmuir film (upstroke and downstroke) and the normalised spectral responses from this device are shown in Fig.10 (see also Table 1). It is difficult to assign polarization states to the resonances at 744 and 820 nm since the optical nature of the films is not well enough established. Indeed, with an assumed tilt angle of ca. 41",taken from the SHG polarization dependence of films deposited onto micro- scope slides, it may well be that the normal modes of the film are hybrid.24 In terms of device performance this would manifest itself as TE/TM (s/p) polarization cross-coupling.In Interaction Fibre Region Fibre Core \ t-l "Trig\ I I Langmuir-Blodgett Film Fig. 9 Schematic representation of the structure used for the in-line fibre optic device 700 740 780 820 860 900 wavelengthhn Fig. 10 Spectral responses of the fibre optic device of dye I for 188 passes (94 each on the upstroke and downstroke) through the floating monolayer. Each trace corresponds to a different polarization state. Table 1 Spectral characteristics of a polished fibre device overlaid with an LB film of the iodide salt (dye I). The LB film was deposited during 188 passes through the floating monolayer (94 each on the downstroke and upstroke) .synchronous modulation wavelength/nm depth/dB 3 dB linewidthlnm 744 11.7 29 820 14.7 42 this respect, the use of a highly birefringent polarization- maintaining fibre would be beneficial allowing an accurate TE-like ( LPOly) and TM-like (LP,,,) polarization state to be established in the device before dep~sition.'~ This would allow the magnitude of such cross-coupling effects to be ascertained.Wavelength scans in the range 700 to 950nm revealed no polarization-dependent responses other than those shown in Fig. 10 and thus, it is likely that the normal modes are predominantly TE and TM. The effects of absorption upon the bandstop responses are evident in the lower-wavelength trace with an increased insertion loss of 0.69k0.05 dB at 700 nm compared to 0.20 & 0.05 dB at il>880 nm.By compari- son with previously reported data for o-tricosenoic acid" the occurrence of the resonance at 820 nm from 188 passes through the floating layer arises from deposition on the upstroke only, i.e. with 94 deposited layers. If this is the case, then consider- ation of the SHG data suggests that the LB film structure is also likely to be Z-type and, as such, the device should exhibit an electro-optic response. Consequently, we are currently investigating the addition of a suitable electrode structure to the device. By assuming an all-electronic contribution to the second- order non-linearity of dye I the high susceptibility ( x(z)zzz= 120pm V-') implies that the molecule should also exhibit a strong electro-optic coefficient (r33).However, since the second- harmonic wavelength lies within an electronic charge-transfer resonance (Table 2) the data are unsuitable for modelling bandstop responses at longer wavelengths and, in view of the significant tilt angle, it is difficult to model the device response with any degree of accuracy.Nonetheless, with reference to its absorption spectrum it is clear that by operating above 800 nm one would expect, by the Kramers-Kronig relations,26 a sig- nificant material dispersion with negligible optical absorption. Broberg et aLZ7 have already shown that this can augment the differential waveguide dispersion between the fibre and the LB film guided modes, resulting in linewidth narrowing and poss- ibly even a degree of resonance enhancement of the susceptibility. Fibre optic overlays could be assumed to be a possible alternative to prism coupling but the geometrical structure of the cylindrical fibre and planar overlay must be taken into account since the lack of lateral confinement in the latter results in losses in this dimension." A theoretical description of the experimentally observed spectral responses, although possible, is complicated even for optically isotropic overlays and, as such, precludes the use of this method as a reliable Table 2 Spectra, non-linear optical properties, chromophore tilt angle and thickness dye nlnaxl nm AmI1 layer- p'ZZZI pm V-' #/degrees l/nm layer-' I 530 0.007 120 41 3.0 I1 515 0.0037" 80 41 3.0b I11 425 0.0040" 30 30 3.5 "Absorbance derived from thick LB films comprising 100 layers of dye I1 and 160 layers of dye 111.Thickness assumed in the calculation of the susceptibility. thin film characterisation tool. In certain cases, however, it could be used to augment existing methods such as SPR. Conclusion The use of chromophores with hydrophobic end-groups has been shown to facilitate Z-type deposition to thicknesses suitable for waveguiding and the non-centrosymmetric align- ment has resulted in high second-order non-linearities, albeit resonantly enhanced, for dyes I and 11. The SHG from films of dye I11 has been shown to increase quadratically with the number of LB layers to thicknesses of 0.56 pm, this being the thickest homomolecular Z-type structure obtained to date.In this case the use of a pyridinium acceptor in place of quinolin- ium has resulted in negligible absorbance at the harmonic wavelength. The LB technique has also been shown to provide precise thickness control for the formation of in-line fibre optic channel dropping filters and our current work on such overlays involves the development of suitable electrode structures to demonstrate intensity modulation. We are grateful to Dr. I. R. Gentle and Professor C. H. L. Kennard (The University of Queensland) for the X-ray synchrotron diffraction data and acknowledge the EPSRC (UK) for support of the non-linear optics programme at Cranfield. The EPSRC are also acknowledged for providing an assistantship (to G.J.) and studentships (to R. B. C. and R. R.) and British Gas and EPSRC for a Total Technology Studentship (to C. D. G.). References 1 G. J. Ashwell, E. J. C. Dawnay, A. P. Kuczynski and P. J. Martin, SPIE Int. SOC. Opt. Eng., 1991, 1361, 589. 2 M. Era, K. Nakamura, T. Tsutsui, S. Saito, H. Niino, K. Takehara, K. Isomura and H. Taniguchi, Jpn. J. Appl. Phys., 1990,29, L2261. 3 P. Hodge, Z. Ali-Adib, D. West and T. A. King, Macromolecules, 1993,26,1789. 4 G. J. Ashwell, P. D. Jackson, D. Lochun, P. A. Thompson, W. A. Crossland, G. S. Bahra, C. R. Brown and C. Jasper, Proc. R. SOC. London A, 1994,445,385. 5 M. Kupfer, M. Florsheimer, M. Bosshard and P. Gunter, Electron. Lett., 1993, 29, 2033. 6 I. Fujiwara, N. Asai and V.Howarth, Thin Solid Films, 1992, 221,285. 7 T. L. Penner, H. R. Motschrann, N. J. Armstrong, M. C. Ezenyilimba and D. J. Williams, Nature, 1994,367,49. 8 C. W. Pitt and L. M. Walpita, Electron. Lett., 1976, 12,479. 9 W. L. Barnes and J. R. Sambles, Thin Solid Films, 1986, 143,237. 10 R. B. Charters, A. P. Kuczynski, S. E. Staines, R. P. Tatanband G. J. Ashwell, Electron. Lett., 1994, 30, 594; R. B. Charters, S. E. Staines and R. P. Tatam, Optics Lett., 1994,19,2036. 11 G. Decher, B. Tieke, C. Bosshard and P. Gunter, J. Chem. SOC., Chem. Commun., 1988,933; Ferroelectrics, 1989,91, 193. 12 G. J. Ashwell, G. Jefferies, E. J. C. Dawnay, A. P. Kuczynski, D. E. Lynch, G. Yu and D. G. Bucknall, J. Muter. Chem., 1995, 5,975. 13 G. J. Ashwell, P.D. Jackson and W. A. Crossland, Nature, 1994, 368, 438; G. J. Ashwell, G. Yu, D. Lochun and P. D. Jackson, Polym. Prepr., 1994,35, 185. 14 G. J. Ashwell, D. Lochun, G. S. Bahra, C. R. Brown, I. R. Gentle and C. H. L. Kennard, Supramol. Sci., submitted. 15 G. J. Ashwell, P. D. Jackson, G. Jefferies, I. R. Gentle and C. H. L. Kennard, J. Muter. Chem., in press. 16 G. J. Ashwell, T. Handa, G. Jefferies and D. Hamilton, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1995, 102, 133. 17 G. J. Ashwell, W. A. Crossland, P. J. Martin, P. A. Thompson, A. T. Hewson and S. D. Marsden, Muter. Res. SOC. Symp. Proc., 1992,247,787. 18 E. Kretshmann, 2.Phys., 1971,241,438. 19 W. L. Barnes and J. R. Sambles, Surf. Sci., 1986, 177, 399; 1987, 183, 189. 20 K. Kajikawa, K. Kigata, H. Takezoe and A. Fukuda, Mol. Cryst. Liq. Cryst. A, 1990,182,91. 21 G. J. Ashwell, G. Jefferies, D. G. Hamilton, D. E. Lynch, J. Mater. Chem., 1996, 6(2), 131-136 135 M. P. S. Roberts, G. S. Bahra and C. R. Brown, Nature, 1995,375, 385; G. J. Ashwell, G. S. Bahra, C. R. Brown, D. G. Hamilton, D. E. Lynch and C. H. L. Kennard, J. Mater. Chem., in press. 22 K. P. Panajotov and A. Tz. Andreev, J. Opt. SOC. Am. B, 1994, 11, 826. 23 S. Zheng, L. N. Binh and G. P. Simon, IEEE Journal of Lightwave Technology, 1995, 13,244. 24 R. M. A. Azzam and N. M. Bashara, Ellipsometry and Polarised Light, North Holland, Amsterdam, 1st edn., 1977. 25 D. N. Payne, A. J. Barlow and J. J. Ramskov Hansen, IEEE Journal of Quantum Electronics, 1982, QE-18,477. 26 A. Yariv, Quantum Electronics, Wiley, New York, 3rd edn., 1989. 27 B. Broberg, B. S. Lindgren, M. G. Oberg and H. Jiang, IEEE Journal of Lightwave Technology, 1986,4, 196. Paper 5/05257B;Received 7th August, 1995 136 J. Mater. Chem., 1996, 6(2), 131-136

ISSN:0959-9428    

DOI:10.1039/JM9960600131

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Controlling the structure of transparent Langmuir-Blodgett films for nonlinear optical applications Geoffrey J. Ashwell,*' Paul D. Jackson,' Gary Jefferies,' Ian R. Gentleb and Colin H. L. Kennard* 'Centrefor Molecular Electronics, Cranjield University, Cranfeld, UK MK43 OAL bDepartment of Chemistry, The University of Queensland, Brisbane, Qld 4072, Australia Langmuir-Blodgett (LB) multilayers of (E)-N-alkyl-4-[ 24 4-docosyloxyphenyl)ethenyl]pyridinium bromide, C,H2,+ -Py+ -CH=CH-C6H,-O-C22H,,Br-(dye I), are centrosymmetric (Y-type) for n66 and non-centrosymmetric (Z-type) for 8 6n <20. The areas obtained from grazing incidence synchrotron X-ray diffraction indicate that the molecules adopt a 'stretched' rather than a 'U-shaped' configuration and, unlike conventional amphiphilic materials, the long chain homologues do not invert during deposition.The second-harmonic intensity from films of the higher homologues (n2 10) increases quadratically with the number of LB layers (120(N)=120(l)N2)whereas for the lower homologues (n<6) the intensity is negligible for even numbers of layers. The nonlinear optical properties of the sulfur (dye 11),selenium (dye 111) and quinolinium (dye IV) analogues are also reported and, in each case, quadratic enhancement of the second-harmonic intensity has been observed for suitable combinations of alkyl chain lengths. The second-order susceptibility, x(2)zzz, docosyloxyphenyl)ethenyl]quinolinium bromide (dye IV) is 20 pm V-'. of a 100 layer LB film of (E)-N-dodecyl-4-[ 2-(4- Interest in LB films for second-harmonic generation (SHG) stems from the requirement that the structure must be non- centrosymmetric and the fact that the LB technique offers control of the packing at the molecular level. However, as most conventional dyes form centrosymmetric Y-type struc- tures, in which the molecular layers pack head-to-head and tail-to-tail, it has been necessary to interleave with inactive For further improvement every layer should be active but to date, of the homomolecular films, only 2-docosyla- mino-5-nitropyridine, DCANP,' and the quinolinium zwit- terion, C16H33-Q3CNQ,6 have shown a quadratic increase in the SHG to thicknesses of a few hundred layers.Few optically nonlinear dyes are known to form Z-type (or X-type) films in which the molecular layers pack head-t~-tail.~-~ However, recent work at Cranfield has shown that unconventional materials with hydrophobic chains at opposite ends of a hydrophilic chromophore invariably form non-centrosym-metric Z-type structures providing that the alkyl groups are of an appropriate length.'0*'' For the majority of amphiphilic materials the layers tend to invert during deposition placing like ends together whereas this is resisted when each end is hydrophobic.In this work we report the nonlinear optical properties of four different dyes (I-IV) and show that Z-type film structures may be fabricated by careful consideration of the alkyl end-groups. Quadratic SHG enhancement has resulted and the applicability of the technique to different classes of dyes suggests that alternate-layer deposition is no longer necessary for the formation of non-centrosymmetric films. I IV Experimental Synthesis The optically nonlinear dye, (E)-N-alkyl-4-[ 2-( 4-docosyloxy- pheny1)ethenyll pyridinium bromide (dye I), was synthesised by the treatment of p-docosyloxybenzaldehyde (2 mmol) and the appropriate N-alkyl-4-picolinium bromide (2 mmol) in hot ethanol with piperidine (2 drops) as catalyst.The sulfur (dye 11) and selenium (dye 111) analogues and the quinolinium dye (IV) were synthesised in a similar manner by the treatment of the appropriate heterocyclic cation and a para-substituted benzaldehyde. The products were purified by column chroma- tography and the resultant pale yellow crystals characterised by their 'H NMR and 13C NMR spectra recorded on a Bruker AM300 spectrophotometer; J values are given inHz. The elemental data are consistent with incomplete combustion but an assessment of the I3C NMR spectra indicates purities better than 99.5%.(E)-N-Dodecyl-4[ 24 4docosyloxyphenyl) ethenyl ]pyridinium bromide (dye I, n= 12) 6,(360 MHz; CDC13) 9.04 (2 H, d, J 7), 8.05 (2 H, d, J 7), 7.72 (1 H, d, J 16), 7.60 (2 H, d, J 9), 7.01 (1 H, d, J 16), 6.90 (2 H, d, J 9), 4.68 (2 H, t, J 7), 3.96 (2 H, t, J 7), 1.28-1.19 (60 H, br m), 0.88-0.83 (6 H, m); 6,(91 MHz; CDC13) 161.78 (0), 153.85 (0), 144.01 (l), 142.10 (l), 130.46 (l), 127.19 (0), 123.80 (l), 119.61 (1), J. Muter.Chem., 1996, 6(2), 137-141 137 115.20 (l), 68.38 (2), 60.77 (2), 31.97 (2), 31.73 (2), 29.76 (2), 29.65 (2), 29.57 (2), 29.50 (2), 29.41 (2), 29.23 (2), 29.15 (2), 26.18 (2), 26.09 (2), 22.74 (2), 14.17 (3). (E)-N-Dodecyl-4-[ 2-(4-docosylsulfanylphenyl)ethenyl]-pyridinium bromide (dye 11) 6,(360 MHz; CDC1,) 9.08 (2 H, d, J 7), 8.08 (2 H, d, J 7), 7.73 (1 H, d, J 16), 7.55 (2 H, d, J 9), 7.35 (2 H, d, J 9), 7.13 (1 H, d, J 16), 4.73 (2 H, t, J 7), 2.96 (2 H, t, J 7), 1.29-1.20 (60 H, br m), 0.88-0.83 (6 H, m); 6,(91 MHz; CDC13) 153.57 (0), 144.18 (l), 142.62 (0), 141.71 (l), 131.28 (0), 128.96 (l), 127.26 (l), 124.13 (l), 121.20 (l), 60.97 (2), 39.34 (2), 32.36 (2), 32.02 (2), 31.99 (2), 31.80 (2), 29.79 (2), 29.69 (2),29.63 (2), 29.60 (2), 29.46 (2), 29.43 (2), 29.33 (2), 29.18 (2), 29.07 (2), 28.92 (2), 28.63 (2), 26.22 (2), 22.78 (2), 14.21 (3).(E)-N-Dodecyl-4-[244-docosylselanylphenyl)ethenyl]-pyridinium bromide (dye 111) 6,(360 MHz; CDC1,) 9.05 (2 H, d, J 7), 8.09 (2 H, d, J 7), 7.75 (1 H, d, J 16), 7.52 (2 H, d, J 9), 7.45 (2 H, d, J 9), 7.16 (1 H, d, J 16), 4.71 (2 H, t, J 7), 3.00 (2 H, t, J 7), 1.30-1.22 (60 H, br m), 0.89-0.84 (6 H, m); 6,(91 MHz; CDC13) 153.49 (0), 144.23 (l), 141.74 (l), 136.71 (0), 132.35 (0), 131.14 (l), 128.93 (l), 124.22 (l), 121.62 (l), 61.06 (2), 32.02 (2), 31.79 (2), 31.09 (2), 30.40 (2), 30.24 (2), 30.06 (2), 30.01 (2), 29.80 (2), 29.71 (2), 29.63 (2), 29.46 (2), 29.25 (2), 29.19 (2), 27.39 (2), 26.22 (2), 22.79 (2), 14.21 (3).(E)-N-Dodecyl-4-[2-(4-docosyloxyphenyl)ethenyl]-quinolinium bromide (dye IV ) 6, (360 MHz; CDCl,) 10.09 (1 H, d, J 7), 8.66 (1 H, d, J 9), 8.33 (1 H, d, J 7), 8.11 (2 H, m), 7.91 (1 H, m), 7.76 (2 H, d, J 17), 7.70 (2 H, d, J 9), 6.93 (2 H, d, J 9), 5.07 (2 H, t, J 8), 3.99 (2 H, t, J 7), 1.31-1.19 (60 H, br m), 0.87-0.82 (6 H, m); 6, (91 MHz; CDCl,) 161.98 (0), 153.33 (0), 148.78 (l), 143.92 (l), 137.78 (0), 134.89 (l), 130.80 (l), 129.13 (l), 127.64 (0), 126.97 (l), 126.61 (0), 118.31 (l), 116.79 (l), 116.07 (l), 115.26 (l), 68.44 (2), 57.30 (2), 31.98 (2), 31.95 (2), 30.16 (2), 29.76 (2), 29.64 (2), 29.57 (2), 29.47 (2), 29.44 (2), 29.41 (2), 29.37 (2), 29.24 (2), 26.62 (2), 26.09 (2), 22.73 (2), 14.16 (3). LB film deposition Dilute chloroform solutions of the two-legged dyes were spread onto the pure water subphase of one compartment of an alternate-layer LB trough (Nima Technology, model 622), left for 10 min and then compressed at 0.5 cm2 s-' (ca.0.1% s-' of compartment area). LB films were obtained by cycling a silicon wafer (for X-ray) or a glass substrate (for SHG) at 5 mm min-l via the second compartment of the LB trough to deposit on the upstroke only. Details of the deposition press- ures are provided in the legends to the Figures of the various dyes. SHG measurements The SHG was measured in transmission with the laser beam (Nd:YAG, A= 1064nm) at an angle of 45"to the LB film. The polarization of the fundamental beam was rotated using a half-wave plate and the p-polarized second-harmonic intensity was calibrated against the Maker fringes of a Y-cut quartz reference (dll =0.5 pm V-'). The data were analysed using the method described previously.2 Results and Discussion Pyridinium dyes The film-forming properties of dye I, CnH2n+1-Py' -CH=CH-C6H,-O-C22H,, Br-, are dependent upon the second hydrophobic group.The surface pressure uersus surface area isotherms are featureless for II <6 but show anoma!ous transitions for rn 2 8 (Fig. 1); the areas at collapse, 24 f4 A2 molecule-', are consistent with the molecular cross- sectiFn whereas the limiting areas at zero pressure, 110 to 140A2 molecule-' for 1126, match the face area of the chromophore (Fig. 2). The transitions may be associated with a change in the molecular geometry from 'U-shaped' at low pressures, whereby both alkyl chains point in the same direc- tion, to 'stretched' at high pressures with the chains pointing in opposite directions. The 'stretched' configuration has been confirmed by X-ray diffraction studies on both Langmuir and LB films (Table 1) and also, previously, from the layer thickness obtained from ellipsometry studies on deposited films." GrazinG incidence synchrotron X-ray diffraction studies at A= 1.488A were performed on wiggler beamline 16A at the Photon Factory, Tsukuba (Japan), using the procedure and apparatus described by Matsushita et X-Ray data were obtained for a Langmuir film of the tetradecyl homologue at 20.7 "C and 35 mN m-' with the film balance mounted on the sample stage of the diffractometer. The data are similar to 50 40 7 I 30z E1$ 3 20 Q 10 0 0 50 100 150 area per molecule/A2 Fig.1 Dependence of the surface pressure versus area (n-A) isotherms on the number of carbon atoms in the second alkyl chain of dye I at ca. 20°C: (a) n=20; (b) n=18; (c) n= 14. In each case the films were deposited at 35 mN m-'. F I t 1 8-00000000000 I...,r....r..,.l,.. . I I I 0 5 10 15 20 25 number of carbon atoms Fig. 2 Dependence of the compression data and nonlinear optical properties on the number of carbon atoms in the second alkyl chain of dye I: areas per molecule at collapse (0)and at zero pressure (0); SHG from LB monolayers of dye I (x).The region between the vertical lines corresponds to the alkyl chain lengths which give Z-type films. To the left of these lines the bilayer structure is Y-type whereas to the right there is a tendency towards antiparallel alignment within the layers. 138 J. Mater. Chem., 1996, 6(2), 137-141 Table 1 Areas per molecule of (E)-N-alkyl-4-[ 2-(4-docosyloxyphenyl) ethenyl] pyridinium bromide (dye I) obtained from the compression isotherms and from grazing incidence sychrotron X-ray diffraction isotherm" X-rayb alkyl group A,/A2 &/A2 Ad IA2 tetradec yl 22 27 23.83 octadec yl 23 31 (21.27) " A, and Ad correspond to the areas per molecule at collapse and at the deposition pressure of 35mN m-' respectively.bThe value in parentheses corresponds to the area per molecule from a deposited LB film and, in this case, the discrepancy probably results from a slight contraction upon deposition. All of the above values indicate that the molecule adopts a 'stretched' configuration with the hydrophobic chains pointing in opposite directions. those obtained for other Langmuir film^'^.'^ and are attributed to a hexagonal lattice with two-dimensional Miller indices (1,O). The scan shows a single X-ray peak at Q,= 1.383 A-l with a half width at half maximum of 0.024 A-l and these relate to a d-spaciqg of 4.543 A (dl,=2n/Q,) and to a correlation length of 42 A [Fig. 3(a)]. For comparison, the d-spacing and correlation length obtained for an LB film of the octadecyl homologue, deposited at 35mN m-', are 4.292 and 9.2A respectively [Fig.3(b)]. The correlation lengths are an estimate of the lower limit but clearly the crystallites are small, as is usual for such materials. The d-spacings correspond to areas of 23.83 42 for the tetradecyl homologue (Langmuir film) and 21.27 A2 for the octadecyl homologue (LB film). The areas are consistent with those obtained from the isotherms and confirm that the 200 h2.-c -200Ot 3$I,,,,,,]2-400 a-.-1.2 1.3 1.h 1.5 I coo I 200 0 I---I -200 1 I I I 1 1.0 1.2 1.4 1.6 1.8 Q*/A-' Fig. 3 X-Ray diffraction data for (a) a Langmuir film of the tetradecyl homologue of dye I at 20.7 "C and 35 mN m-' and (b)a ten layer LB film of the iodide salt of the octadecyl homologue.The solid line is a Lorentzian fit to the experimental data of the Qx resolved scan. molecules adopt a 'stretched' configuration (Table 1).The alkyl chains point in opposite directions and therefore the molecules must align on the subphase with one of the hydrophobic chains adjacent to the water surface. It is assumed that the shorter of the two alkyl chains is oriented downwards and, based upon the SHG data in Fig. 2, optimum alignment is realised when the chains differ by four or more carbons. The SHG decreases abruptly for n320, and the behaviour can be explained by assuming that the hydrophobic ends need to be sufficiently different for the chromophores to adopt a particular orientation (dipole up or down).The SHG from films comprising an even number of LB layers of the lower alkyl homologues of dye I (nd6) is negligible and indicates a centrosymmetric Y-type bilayer arrangement. Interestingly, the SHG from films of the higher homologues (10dn <18) increases quadratically with the number of layers and therefore the packing is Z-type. The sulfur (dye 11) and selenium (dye III) analogues also show a quadratic SHG dependence but the intensity is slightly reduced (see Fig. 4). The equivalent monolayer intensities, Izo(N)/N2where N is the number of layers, range from ca. 2% (dye 111) to 10% (dye I) of the signal from LB monolayers of the hemicyanine dye, (E)-4-[2-(4-dimeth ylaminophen yl)e thenyl ]-N-docos ylp yridinium bromide, first reported by Girling et a1." However, films of this dye are strongly absorbing at the harmonic wavelength =532 nm) whereas those of I to I11 are nearly transparent.The absorption maxima of I to I11 are at 360+5 nm in each case but the LB absorption band of the alkoxy dye is broader and has a very slight absorbance of 5 x layer-' at the harmonic wavelength. Thus, the nonlinear optical properties are probably influenced by minor variations in the resonant enhancement as the absorption band tails off. Quinolinium dye Homomolecular films of the quinolinium dye (IV) are rigid and difficult to deposit, but when spread in a 1 :1 ratio with stearic acid they become more manageable. As with the pyridinium analogues the isotherm shows a transitional region, only far more pronounced, and the areas per dye molecule are consistent with a transformation from a 'U-shaped' configur- ation at low pressures to a 'stretched' configuration above 28 mN m-' (Fig.5). An alternative explanation of collapse is far less likely; the strong SHG from films deposited in the high pressure regime and its quadratic dependence on the number lo ' 00 0*o 0 00 00 0 0 0 4-8 5U 2 0 10 20 30 60 number of layers Fig. 4 Variation of the square root of the second-harmonic intensity with the number of LB layers of the dodecyl homologue of dye I (x), dye I1 (a)and dye I11 (0) J. Mater. Chem., 1996, 6(2), 137-141 139 CO -I E 30z 4 5 20 h 10 0 area per molecuie/A* Fig.5 Surface pressure versus area (n-A) isotherm of dye IV and stearic acid (1:1 molar ratio) at ca.20 "C.LB films were deposited in the high pressure regime at 30 mN m- '. of layers leads us to firmly dismiss this idea. Thus, as with the pyridinium analogues the molecules probably align with a hydrophobic chain adjacent to the subphase at higher press- ures. This is contrary to notions currently perceived by the LB community but, for the series of two-legged dyes, all of the evidence (isotherms, X-ray diffraction, layer thickness) points to such an arrangement. LB films of dye IV are pale yellow and only weakly absorb at the harmonic wavelength with A2, =7 x layer-' (Fig. 6). The SHG increases quadratically with the number of LB layers (Fig.7) and the normalised intensity, 12,/N2, is 30% of the mean signal from monolayer films of Girling's hemicyan- ine dye." The hemicyanine has been extensively studied and has the drawback that it is coloured and forms centrosymmetric Y-type structures unless interleaved. In contrast, the quinolin- ium dye is nearly transparent at 532nm and it forms non- centrosymmetric Z-type structures. There is considerable interest in the efficiency/transparency tradeoff for second-order dyes and we have found that the peak absorbance of donor-(n-bridge)-acceptor materials may be finely tuned by varying the donor-acceptor combi-nation."." The greatly improved SHG from films of dye IV compared with the pyridinium dyes (I to 111) is attributed, in part, to increased resonant enhancement, the absorption maxi- mum being red-shifted from 360 to 410nm with the stronger quinolinium acceptor.Using the method of Kajikawa et the SHG polarization dependence, Iz,(p+p)/12,(s-+p), of the 100 layer film suggests that the charge transfer axis of the 0.4 Q) 0.3 as3 0.2 0.1 0 300 400 500 600 700 800 900 wavelengthhm Fig.6 Absorption spectrum of a 100 layer LB film of dye IV-stearic acid 140 J. Muter. Chew., 1996, 6(2), 137-141 number of layers Fig. 7 Variation of the square root of the second-harmonic intensity with the number of LB layers of dye IV-stearic acid molecule is inclined at an angle of 36" from the perpendicular to the substrate. However, as intermolecular charge transfer as well as intramolecular charge transfer can give rise to strong SHGi7 the validity of this technique is ambiguous.Nonetheless, the refractive index (at 670 nm) and thickness of the 100 layer film from ellipsometry are 1.42 and 0.46 pm respectively, the thickness being consistent with the value obtained from mol- ecular modelling when the chromophore tilt angle is taken into consideration. From these data the second-order suscepti- bility, x(2)zzz,of the 100 layer film is 20 pm V-l but significantly, the absorbance at the harmonic wavelength is very small. In addition, the SHG has shown no sign of deterioration through- out a period of about a year and we suggest that the Z-type film structure, in this case, is stabilised by the hydrophobic ends of the chromophore.Conclusions In this paper we have shown that two-legged molecules can adopt a 'stretched' configuration at the air-water interface and that Z-type structures may be obtained by careful consideration of the alkyl chain lengths. In fact, our investigations on chromophores of general formula: CnH2,,+ -A+ -(n-bridge)-Y -CmH2,+ where A+ is a heterocyclic cation (pyridinium, quinolinium, isoquinolinium or benzothiazolium) and Y is 0,S, Se, OC(O), N(H) or N(C,H,, + have resulted in Z-type structures."." Most combinations have shown SHG enhancement with film thickness and, in four cases, a quadratic dependence to 100 layers has been realised. In contrast, only two of the single- legged dyes, DCANP and C16H33-Q3CNQ, have shown quad- ratic enhancement to 100 although others have dem- onstrated such behaviour in alternate-layer fi1ms.I~~ Thus, we conclude that the addition of the second leg provides a convenient route to non-centrosymmetric LB structures for nonlinear optical applications.We acknowledge the EPSRC (UK) for funding the nonlinear optics programme at Cranfield, EPSRC and British Gas plc for providing a studentship to P.D.J., and the Australian Nuclear Science and Technology Organisation and the Australian National Beamline Facility for support. We also acknowledge Professor T. Matsushita for use of the facilities at Tsukuba. References 10 G. J. Ashwell, P. D. Jackson and W. A. Crossland, Nature, 1994, 1 2 G.J. Ashwell, E. J. C. Dawnay, A. P. Kuczynski and P. J. Martin, SPIE Int. Soc. Opt. Eng., 1991, 1361, 589. G. J. Ashwell, P. D. Jackson, D. Lochun, W. A. Crossland, 11 368,438. G. J. Ashwell, G. Yu, D. Lochun and P. D. Jackson, Polym. Prepr., 1994, 35, 185; G. J. Ashwell, T. Handa, G. Jefferies and D. G. Hamilton, Colloid Surf. A, 1995, 102, 133; G. J. Ashwell, P. A. Thompson, G. S. Bahra, C. R. Brown and C. Jasper, Proc. D. Lochun, G. S. Bahra, C. R. Brown, I. R. Gentle and Roy. Soc. Lond. A, 1994,445,385. P. Hodge, Z. Ali-Adib, D. West and T. A. King, Macromolecules, 1993,26,1789. M. Era, K. Nakamura, T. Tsutsui, S. Saito, H. Niino, K. Takehara, K. Isomura and H. Taniguchi, Jpn. J. Appl. Phys., 1990,29, L2261. G. Decher, B.Tieke, Ch. Bosshard and P. Gunter, J. Chem. Soc., 12 C. H. L. Kennard, Suprumol. Sci., submitted for publication; G. J. Ashwell, G.Jefferies, C. D. George, R. Ranjan, R. B. Charters and R. P. Tatam, J. Muter. Chem., 1996,6, 137. T. Matsushita, A. Iida, K. Takeshita, K. Saito, S. Kuroda, H. Oyanagi, M. Sugi and Y. Furukawa, Jpn. J. Appl. Phys., 1991, 30, L 1674. 6 Chem. Commun., 1988, 933; M. Kupfer, M. Florsheimer, Ch. Bosshard, H. Looser and P. Gunter, in Organic Materialsfor Non-linear Optics III, ed. G. J. Ashwell and D. Bloor, Special Publication No. 137, The Royal Society of Chemistry, Cambridge, UK, 1990,p. 68. G. J. Ashwell, E. J. C. Dawnay, A. P. Kuczynski, M. Szablewski, I. M. Sandy, M. R. Bryce, A. M. Grainger and M. Hasan, J. Chem. SOC., Faraday Trans., 1990, 86, 1117; G. J.Ashwell, G. Jefferies, E. J. C. Dawnay, A. P. Kuczynski, D. E. Lynch, G. Yu and 13 14 15 16 C. A. Helm, H. Mohwald, K. Kjaer and J. Als-Nielsen, Europhys. Lett., 1987,4,697;C. A. Helm, P. Tippmann-Krayer, H. Mohwald, J. Als-Nielsen and K. Kjaer, Biophys. J., 1991, 60, 1457. G. T. Barnes, 1. R. Girling, C. H. L. Kennard, J. B. Prey and 1. McL. Jamie, Langmuir, 1995, 11,281. 1. R. Girling, N. A. Cade, P. V. Kolinski, R. J. Jones, I. R. Peterson, M. M. Ahmad, D. B. Neal, M. C. Petty, G. G. Roberts and W. J. Feast, J. Opt. SOC. Am., 1987, B4, 950. K. Kajikawa, K. Kigata, H. Takezoe and A. Fukuda, Mol. Cryst. D. G. Bucknall, J. Muter. Chem., 1995,975. 0. A. Aktsipetrov, N. N. Akhmediev, I. M. Baranova, E. D. Mishina and V. R. Novak, Soviet Tech. Phys. Lett., 1985,11,249. T. Richardson, G. G. Roberts, M. E. C. Polywyka and S. G. Davies, Thin Solid Films, 1988,160, 231. 17 Liq. Cryst. A, 1990, 182,91. G. J. Ashwell, G. Jefferies, D. G. Hamilton, D. E. Lynch, M. P. S. Roberts, G. S. Bahra and C. R. Brown, Nature, 1995,375, 385; G. J. Ashwell, G. S. Bahra, C. R. Brown, D. G. Hamilton, D. E. Lynch and C. H. L. Kennard, J. Muter. Chem., 1996,6,23. R. Popovitz-Biro, K. Hill, E. M. Landau, M. Lahav, J. Leiserowitz and J. Sagiv, J. Am. Chem. SOC., 1988,110,2672. Paper 51047825; Receiued 20th July, 1995 J. Mater. Chem., 1996, 6(2), 137-141 141

ISSN:0959-9428    

DOI:10.1039/JM9960600137

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Chemical behaviour of oxotitanium( IV ) phthalocyanine (OTiPc) solutions associated with the preparation of OTiPc monolayers and multilayers Kimiya Ogawa, Jiachang Yao, Hisatomo Yonehara and Chyongjin Pac" Kawamura Institute of Chemical Research, 631 Sakado, Sakura-city, Chiba 285, Japan The chemical behaviour of oxotitanium(1v) phthalocyanine (OTiPc) in mixtures of dichloromethane and trihalogenoacetic acids was studied by means of spectroscopic measurements. In dichloromethane-trifluoroacetic acid mixtures, OTiPc was relatively unstable, undergoing gradual decomposition via the radical cation of OTiPc; phthalimide and 3-iminoisoindolin- 1-one were isolated. In dichloromethane-trichloroacetic acid mixed solvent, on the other hand, a stable solution of OTiPc was obtained with no formation of the radical cation.A monolayer of OTiPc was formed upon spreading this solution onto the water surface and was deposited on substrates with a horizontal lifting method to form a multilayer film. Polarized visible and near-IR absorption spectra and X-ray diffraction of the film indicated a specific order of molecular orientation. Phthalocyanine (Pc) compounds are widely known as diverse- functional materials, among which oxotitanium(1v) phthalo- cyanine (OTiPc, Fig. 1) is industrially utilized as a highly photoconductive material for photoreceptor devices in copiers and laser printers.' Preparation of thin solid films of OTiPc is of importance in view of spectroscopic, photoelectric and photoelectrochemical interests, and of potential device appli- cations.Depending on the method of film preparation, the molecular ordering and orientation in the film, which should affect its optical and photoelectric properties, may be con-trolled. Vacuum evaporation of thin films of OTiPc has been studied by several Evidence for preferential molecu- lar orientation is seen in some case^,^.^.^ though it may well be substrate-de~endent.~ On the other hand, a polymer-disper- sion coating method has been popularly employed in photo- receptor production.' However, a preferential orientation of molecules cannot be expected with this method. The Langmuir-Blodgett (LB) technique is a promising method for the fabrication of highly ordered ultrathin films of organic materials.8 The first example of LB films of Pc com- pounds was presented by Baker et al.for metal-free phthalo- cyanine and its tetra-tert-butyl-substituted deri~ative,~and since then numerous studies on LB films of Pcs have been reported." Because the preparation of sample solutions in volatile organic solvents is a requisite for the LB technique, the majority of the previous studies employed soluble Pcs with appropriate substituents at their peripheral benzene rings. On the other hand, the solubilities of most of the unsubstituted Pcs in common organic solvents are too low to apply the LB method. Exceptionally, LB films of metal-free PCS~*''*~~can be obtained by the hydrolysis of soluble dilithium Pc, whereas Fig. 1 Structural formula of OTiPc zinc Pc 13-' and rare-eart h-me tal bisp h t halocyanines "-" are known to be soluble in certain solvents.We previously reported a new method for the fabrication of LB films of OTiPc as mixtures with a soluble Pc derivative, based on the finding that the solubilities of OTiPc in chloroform are increased upon admixing with the Pc derivative." However, this method is not applicable, in principle, to the preparation of LB films of neat OTiPc, without any additives or matrices. A more general technique is therefore required for easy access to the fabrication of Langmuir monolayers and multilayers of various unsubsti- tuted Pcs. In a few patent specification^,^^^^^ it was reported that mixtures of trifluoroacetic acid (TFAA) with some common organic solvents, such as dichloromethane (DCM), can dissolve OTiPc in amounts large enough for crystal phase transform- ations by recrystallization.However, it is still left unexplored whether or not such mixtures of strong organic acids and common organic solvents can be used as good spreading solvents for the monolayer study of OTiPc and other unsubsti- tuted Pcs.'~ The present paper reports spectroscopic studies on the chemical behaviour of OTiPc solutions in mixtures of DCM with TFAA or trichloroacetic acid (TCAA), in connec- tion with their applicability as spreading solutions. We also report here the successful preparation of an OTiPc monolayer using an appropriate mixed solvent, and the characterization of a deposited multilayer.Experimenta1 General Pure OTiPc was synthesized according to a new method developed in our lab~ratory,'~ and sublimed twice prior to use. Trifluoroacetic acid (TFAA, > 98.0%), trichloroacetic acid (TCAA, > 99.0%), and dichloromethane (DCM, > 99.0%) were commercially obtained (Wako Pure Chemicals) and used as received. Solutions for spectroscopy and monolayer experi- ments were prepared by dissolving OTiPc in DCM containing various amounts of TFAA or TCAA. Stabilities of the solutions were examined by monitoring changes in visible and near-IR absorption spectra. Gas chromatography (GC) was performed on a Shimadzu GC-8A instrument equipped with an SE-54 capillary column. 'H NMR spectra were measured on a JEOL GSX-400 spectrometer, field desorption mass spectrometry (FDMS) on a Shimadzu GCMS 9100-MK instrument, and visible and near-IR (VIS-NIR) absorption spectra on a multichannel photodetector MCPD-1000 system (Otsuka Electronics).X-Ray diffraction (XRD) patterns were obtained J. Muter. Chem., 1996,6(2), 143-148 143 with an RAD-IIA diffractometer (Rigaku Co.) using Cu-Ka X-radiation. Reaction of OTiPc in DCM-TFAA To a mixture of DCM (20 ml) and TFAA (5 ml) was added OTiPc (OSOg) under nitrogen, and then the homogeneous solution thus obtained was allowed to stand in the dark for 9 days. After evaporation followed by the addition of 20ml of ethanol, the mixture was stirred for 2 h at room temperature and then filtered. The blue solid thus obtained was dried and subjected to FDMS measurement.The filtrate was condensed to dryness to give 0.2 g of colourless solid, which was analysed by GC and 'H NMR spectroscopy. arise from the protonation of OTiPc in equilibrium depending on TFAA concentrations [eqn. ( 1)]. OTiPc +H+ $OTiPc-H + (1) A further increase of the TFAA content (220%, Fig. 2b) brought about small blue shifts, ultimately giving a spectrum with almost unresolved maxima at 710-725 nm in neat TFAA. This change is not likely to be due to diprotonation of OTiPc, because each protonation step to Pcs usually causes red shifts of the Q-band.27 We tentatively suggest that this blue shift is caused by the greater solvation energy for polar 0TiPc.H' in its ground state in solvents with larger polarity.28 The spectrum of the OTiPc solution in conc.sulfuric acid showed a largely red-shifted%,,,at 815 nm (Fig. 3), which might be attributable Fabrication of monolayers and multilayers For the fabrication of monolayers and multilayers of OTiPc, DCM containing 0.1 mol dm-3 TCAA was preferably used as the spreading solvent, as will be described later. The sample solutions (typically cu. 2 x mol dm-3) were spread onto doubly distilled pure water (20°C) to form monolayers of OTiPc. Surface pressure (n)-area (A)isotherms were recorded with a commercial LB trough system (KSV-5000LB). Multilayers of OTiPc were deposited at a surface pressure of 9.5 mN m-' by means of a horizontal lifting method.26 Quartz or glass plates were used as substrates after their surfaces had been made hydrophobic with dimethyldichlorosilane.Results and Discussion Chemical behaviour of DCM-TFAA or TCAA solutions VIS absorption spectra of OTiPc in DCM-TFAA solvents at various mixing ratios are shown in Fig. 2. The solutions were prepared by adding TFAA, DCM or TFAA-DCM mixtures to a solution of OTiPc in neat TFAA. At very low TFAA contents up to 1% in volume, the absorption maximum (Amax) occurs at 700 nm (Fig. 2a), which is at slightly longer wave- length than that of a neat DCM solution (690 nm). At higher TFAA contents, the spectra showed a decrease in absorbance at 700 nm accompanied by a further red shift of Amax and the appearance of a new absorption peak at ca.740nm with an isosbestic point at 707 nm (Fig. 2a), giving a split spectral shape with the maxima at 711 nm and 743 nm at 5% TFAA in DCM. The separation (827 cm-') between 700nm and 743nm corresponds to an average red shift of the lowest energy Q-band (700+ 300 cm-') reported for the monoproton- ation of metal Pcs.~~ Therefore, the spectral changes should 0.2 s a42 n2 0.1 600 700 800 600 700 800 Alnm Fig. 2 Visible absorption spectra of OTiPc in TFAA-DCM mixed solvent at various mixing ratios: a, low TFAA contents of 0.5% (-), 1.0% (...), 2.0% (---) and 5.0% (---) v/v; b, higher TFAA contents of 20% (-), 50% (-..) and 100% (---) v/v ([OTiPc] z 3 x mol dm-3) 144 J. Mater. Chern., 1996, 6(2), 143-148 to the tetraprotonated form (OTiPc-4H+), based on the simi- larity to the case of a cobalt Pc deri~ative.~' The protonation equilibrium of OTiPc in DCM-TFAA solvents was also suggested from 'H NMR spectra for a solution of OTiPc in 4: 1 (v/v) CD,Cl2-CF,CO2D mixed solvent at various temperatures (Fig.4). At -30 "C, two 400 600 800 1000 A./nm Fig. 3 Visible and near IR absorption spectra of OTiPc for saturated DCM solution (-), for solutions (2x rnol dmP3) in TFAA(l%-DCM mixed solvent (...) and in neat TFAA (---), and for concentrated H,SO, solution (----, 1.5 x lop5mol dmp3) -20 I I 9 8 6 Fig. 4 'H NMR spectra of OTiPc in 1 :4 CF3C02D-CD2C12 measured at various temperatures resonances appear at 6 8.3 and 9.2, which are attributable to the protons at the 4,5 and 3,6 positions of the peripheral benzene rings, respectively.Upon raising the temperature, however, the signals become broader and coalesce at >0 "C; no signal can be observed at room temperature. This obser- vation strongly suggests that the protonation of OTiPc should be nearly complete in 4 : 1 DCM-TFAA at -30 "C but occurs in equilibrium at higher temperatures on the timescale of the nuclear magnetic transition. In D,S04, on the other hand, the distinct peaks at 6 8.7 and 9.7 can be observed even at room temperature, showing that the protonation equilibrium should be extremely biased toward the tetraprotonated form. Note that the VIS-NIR absorption spectra discussed above (Fig. 2 and 3) were recorded immediately after the preparation of the solutions.The VIS-NIR absorption spectra of OTiPc in 4 :1 (v/v) DCM-TFAA revealed interesting features follow- ing various storage periods. When OTiPc was dissolved (cu. mol dm-3) in a freshly prepared solvent, the absorp- tion bands at 650-740 nm attributable to OTiPc/OTiPcH+, which are green in colour, gradually disappeared with isosbestic points accompanied by the appearance of the absorption maxima of a purple species at 560nm and 880nm, as shown in Fig. 5(a). The same spectral change was observed also when the solution was kept in the dark. The spectrum of the purple species is typical for the monomeric cation radicals of metal Pcs.~~,~'The one-electron oxidation of bis-cyano( phthalocyan- inato)ferrates(~~~)~~and some aromatic molecule^^^.^^ is known to occur in DCM-TFAA.Thus, the spectral change in Fig. 5(a) indicates a stoichiometric transformation of OTiPc to its radical cation species [eqn. (2)]. OTiPc/OTiPc-H+ 2OTiPc' (2)+ 3 400 GOO 800 1000 400 600 800 1000 hlnm Fig. 5 Changes in visible and near IR absorption spectra of OTiPc in (a) freshly prepared and (b) aged 1:4 TFAA-DCM solvent, following various storage periods: immediately after dissolution (-) and after 1 h (...), 2 h (---) and 3 h (---); (a) [OTiPc]z2x lop5mol drnp3,(b) [OTiPc]z7 x lo-' mol dm-3 The one-electron oxidation of substituted terthiophenes in this solvent was discussed in terms of the photooxidation by molecular oxygen with the aid of catalysis by TFAA.32 In the case of OTiPc, however, this mechanism cannot reasonably explain the following observations.In contrast with the case of the fresh solvent [Fig. 5(a)], the purple species was almost instantaneously formed upon dissolving OTiPc into an 'aged' DCM-TFAA solvent that had been stored for 13 days after mixing [Fig. 5(b)]: the oxidation of OTiPc occurs much faster in aged solvents than in fresh solvents. This was again true with the aged solvent that had been bubbled with nitrogen for 20 min just before dissolution of OTiPc and also with the aged solvent stored for 13 days under an N, atmosphere. Presumably, an oxidant might be gradually generated and accumulated after the mixing of DCM and TFAA to induce the facile one-electron oxidation of OTiPc in the aged solvent.The bleaching of the OTiPc radical cation occurs within a few hours, as shown in Fig. 5(b), and was confirmed to be accelerated upon the addition of a few drops of water to the purple solution. This observation can be easily explained in terms of facile hydrolysis of the OTiPc radical cation, in agreement with the well known facts that radical cation species are unstable in the presence of water or methan01.~~'~' On the other hand, for a solution of OTiPc in a fresh DCM-TFAA (4: 1) solvent containing 1% (by volume) of water, the radical cation species was only slightly visible even after 3 h, and the consumption of OTiPc was suppressed to a significant extent compared with a solution in the absence of water.Thus, water added to fresh DCM-TFAA would retard the generation of the presumed 'oxidant' and/or the formation of the OTiPc radical cation, though it readily hydrolyses the radical cation once formed. These effects of water suggest that the radical cation formation should be a major pathway for the decomposition of OTiPc in DCM-TFAA solvents. Possible final products from the decomposition of OTiPc were analysed. After a solution of OTiPc in DCM-TFAA had been kept in the dark at room temperature for 9 days, ethanol- insoluble blue solids and ethanol-soluble colourless solids were separated. An FDMS for the blue solid showed an exclusive peak attributable to OTiPc (m/z 576) and a few small noises, but no signal of H,Pc at m/z 514, demonstrating that H,Pc contamination should be negligible or only minor.This was further confirmed by analysis of the IR absorption bands at 965 cm-' (OTiPc) and at 1007 cm-' (H,Pc); H,Pc would be present in d3%, if at all. Although the demetallation of OTiPc was reported to occur in TFAA,33 this reaction should be only a minor pathway in the present case. The colourless solid was confirmed by 'H NMR and GC to be a mixture of 3-iminoisoindolin-1-one (64%) and phthalimide (36%). Thus, the major pathway of the decomposition of OTiPc in DCM-TFAA solvent can be summarized as in eqn. (3). +[OTiPc +H $OTiPc.H + ]5OTiPc' + NH 0 From technical viewpoints of Langmuir monolayer study, TFAA-DCM mixtures could provide a possible spreading solvent for OTiPc, if preparation conditions can be carefully set to minimize the decomposition of OTiPc.We previously reported that monolayers of copper phthalocyanine (CuPc) can be successfully fabricated by the use of 10: 1 DCM-TFAA as spreading solvent, since the chemical change of CuPc is negligible in this solvent.24 However, with this solvent, minor decomposition of OTiPc is unavoidable before spreading or upon contact with water, and is clearly unfavourable for the fabrication of pure OTiPc monolayers. In contrast, a stable J. Muter. Chem., 1996,6(2), 143-148 145 and convenient spreading solution of OTiPc for the monolayer study was obtained by employing the weaker organic acid, TCAA, instead of TFAA. A sufficient amount of OTiPc (ca.2 x mol dmW3) is soluble in DCM containing 0.1 mol dm-3 TCAA. The solution showed a VIS absorption spectrum with Laxat 696nm, which is very similar to that of unpro- tonated OTiPc or to those in the presence of <1% TFAA shown in Fig. 2 and 3. Presumably, the protonation equilibrium of OTiPc is largely biased toward the unprotonated form in this solution. On standing the solution for 2 days under an ambient atmosphere, only a 5.6% decrease of the maximum absorbance was observed without any change in shape of the spectrum: the colour of the solution remained unchanged. This solution was used as the spreading solution within 24 h after preparation. Fabrication of monolayers and multilayers of OTiPc A surface pressure-area isotherm (20 "C) for the monolayer of OTiPc, formed on a pure water surface, i! shown in Fig.6. The limiting area for the isotherm was 62 A2, which suggests that the plane of the Pc macrocycle substantially stands up from the plane of the water surface, i.e., the 'edge-on' style molecular orientation of OTiPc. Hysteresis was observed on a compression-decompression (0-9.5 mN m -I) experiment, pre- sumably because of some irreversible aggregation that is likely to occur for monolayers of Pc compounds. However, the monolayer was fairly stable under the deposition conditions: the decrease of surface area at a constant surface pressure of 9.5 mN m-' was 5.3% in the first 10 min after compression, and was 2.4% in the next 10min.Fig. 7(a) shows polarized VIS-NIR absorption spectra for an as-deposited multilayer film of OTiPc (40 layers) at various incident angles of the p-polarized light beam with respect to the film plane. The solid line spectrum in Fig. 7(a) (incident angle 0") shows an absorption maximum at 720nm with a slight shoulder around 650 nm, a spectral feature characteristic of amorphous OT~PC.~,~ Nevertheless, molecular orientation in this film cannot be considered as entirely random as the term 'amorphous' may imply, because there is a clear depen- dence of the spectra [Fig. 7(a)] on the incident angle: the increase of the incident angle results in an increase of the absorbance at ca. 650 nm accompanied by a concomitant decrease of the absorbance at 720 nm with an isosbestic point at ca.685 nm. This clearly indicates that the transition moment for the 650 nm absorption stands nearly perpendicular to the film plane, i.e., OTiPc molecules in the multilayer film are preferentially arranged with such an orientation. An X-ray diffraction pattern for the 40-layer film of OTiPc [Fig. S(a)] showed a weak but distinct diffraction peak at 28=6.7" 20I 1 "0 20 40 60 80 100 120 area per moIecule/A* Fig. 6 Surface pressure-area isotherm (20"C) for OTiPc monolayer 146 J. Muter. Chem., 1996,6(2), 143-148 0.4 0.2 E n+? 400 600 800 1000 v v)ncd 0.4 0.2 400 600 800 1000 Alnm Fig.7 Polarized visible and near IR absorption spectra for (a) as-deposited and (b) CH,Cl,-treated OTiPc multilayers (40 layers) at various incident angles of p-polarized light beam.-, 0"; ..., 30"; ___ 45". ___ 600. _._._ ,70".39 >> 10 20 30 40 2O/degrees Fig. 8 XRD patterns for (a)as-deposited and (b)CH,Cl,-treated OTiPc multilayers (40 layers) (d= 13.2 A),but no other peaks in the higher 28 region except the halo from the substrate. These observations suggest that the OTiPc multilayer film may be amorphous but has a layered strycture in the molecular arrangement with a layer spacing of 13 A. This means that the interactions between neighbouring molecules are relatively weak to allow no severe regulation in their relative orientations, though OTiPc molecules adopt the preferential edge-on orientation with respect to the film plane.In other words, the preferential molecular orientation is afforded rather by the method of the film preparation, than by any self-organizing character of the molecule, in the present case. (In most LB film studies, ordering of molecules is considered to be afforded by both.) The edge-on arrangement of OTiPc is consistent with the limiting area in the surface pressure-area isotherm. On exposing the LB film to dichloromethane vapour for 2 h, a drastic change in the VIS-NIR absorption spectra occurred with a considerable bathochromic shift of the longest absorption maximum to 840 nm [Fig. 7(b)].This change is due to a phase transformation of amorphous OTiPc to an a-OTiPc ~rystal.~,~,~~ As seen in Fig. 7(b),the p-polarized absorp- tion spectra of the vapour-treated film again showed a clear dependence on the incident angles: the absorbance at ca.640nm increases with a decrease of that at 840nm as the incident angle increases. An X-ray diffraction pattern of the vapoFr-treated film [Fig. 8(b)]showed a peak at 28=7.5" (d= 11.8 A), but no other distinct peaks besides the halo from the plate. The observed peak is attributable to the diffraction from the (010) plane of a-OTiPc. Since other peaks reported for a-OTiPc were not detected, it can be inferred that the a-form crystals in the treated film are preferentially oriented with the (010) plane nearly paqllel to theofilm plane. The decrease in d-spacings from 13.2A to 11.8 A upon the DCM vapour treatment may imply that the OTiPc molecules in the treated film has a more 'inclined' molecular orientation than in the as-deposited film.This model of molecular orientation is illustrated in Fig. 9, based on the crystallographic data of Hiller et Note that the phase transformation by the vapour treatment occurs while the preferential orientation of the molecule is retained. Conclusions The spectroscopic investigation into the chemical behaviour of OTiPc in TFAA-DCM solvents demonstrated that the protonation of OTiPc occurs in equilibrium at room tempera- ture and that a major pathway for the decomposition of OTiPc in this solvent involves the one-electron oxidation of OTiPc followed by hydrolysis of the OTiPc radical cation. It was confirmed that the final products are mainly 3-iminoisoindolin- 1-one and phthalimide, whereas H,Pc is a minor product. On the other hand, TCAA (0.1mol dm-3)-DCM solvent gave a very stable solution with negligible decomposition of OTiPc.A monolayer of OTiPc on the water surface was successfully prepared from this solution. A deposited multilayer of OTiPc had a preferential orientation of the molecules with respect to C Fig. 9 Molecular orientation model for CH,Cl,-treated OTiPc multilayers the substrate plane. The molecular ordering was retained even after the crystal phase transition caused by the vapour treatment. The authors wish to thank the members of Analysis Group in Central Research Laboratories of Dainippon Ink & Chemicals, Inc., for their cooperation in the 'H NMR, FDMS and X-ray diffraction measurements.References 1 K-Y. Law, Chem. Rev., 1993,93,449. 2 T. J. Klofta, J. Danziger, P. Lee, J. Pankow, K. W. Nebesny and N. R. Armstrong, J. Phys. Chem., 1987,91,5646. 3 Y. Ueda, H. Yamaguchi and M. Ashida, in Chemistry of Functional Dyes, ed. Z. Yoshida and Y. Shirota, Mita Press, Tokyo, vol.2, 1993, p. 258. 4 A. Yamashita, T. Maruno and T. Hayashi, J. Phys. Chem., 1993, 97,4567. 5 P. Ghosez, R. Gate, L. Gastonguay, G. Veilleux, G. Denes and J. P. Dodelet, Chem. Muter., 1993,5, 1581. 6 H. Yonehara and C. Pac, Muter. Res. SOC. Symp. Proc., 1994,328, 301. 7 R. Aroca and A. Thedchanamoorthy, Chem. Muter., 1995,7,69. 8 G. Roberts, Langmuir-Blodgett Films, Plenum Press, New York, 1990.9 S. Baker, M. C. Petty, G. G. Roberts and M. V. Twigg, Thin Solid Films, 1983,99, 53. 10 A. Ulman, An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly, Academic Press, San Diego, 1991, p. 167. 11 H. Yamamoto, T. Sugiyama and M. Tanaka, Jpn. J. Appl. Phys., 1985,24, L305. 12 R. D. George, P. F. McMillan, V. A. Burrows and R. Hervig, Thin Solid Films, 1991,203, 303. 13 E. Kanezaki, Mol. Cryst. Liq. Cryst. Lett., 1988,5, 101. 14 E. Kanezaki, J. Coord. Chem., 1988,18,113. 15 E. Kanezaki, Y. Wada and Y. Egami, Arab. J. Sci. Eng., 1990, 15, 375. 16 J. Souto, R. Aroca and J. A. DeSaja, J. Raman Spectrosc., 1991, 22, 349. 17 J. Souto, R. Aroca and J. A. DeSaja, J. Raman Spectrosc., 1991, 22, 787.18 J. Souto, L. Tomilova, R. Aroca and J. A. DeSaja, Langmuir, 1992, 8, 942. 19 R. E. Clavijo, D. Battisti, R. Aroca, G. J. Kovacs and C. A. Jennings, Langmuir, 1992,8, 113. 20 R. Aroca, H. Bolourchi, D. Battisti and K. Najafi, Langmuir, 1993, 9, 3138. 21 K. Ogawa, H. Yonehara and C. Pac, Mol. Cryst. Liq. Cryst., 1995, 258,315. 22 J. M. Duff, J. D. Mayo, C-K. Hsiao, A-M. Hor, T. L. Bluhm, G. K. Hamer and P. M. Kazmaier, Eur. Pat. Appl., EP460565, 1991. 23 H. Ono, S. Otsuka and M. Hiroi, Jpn. Kokai Tokkyo Koho, JP 02 269 776 [90 269 7761,1990. 24 K. Ogawa, H. Yonehara and C. Pac, Langmuir, 1994,10,2068. 25 J. Yao, H. Yonehara and C. Pac, Bull. Chem. SOC. Jpn., 1995, 68, 1001. 26 K. Fukuda, H. Nakahara and T. Kato, J. Colloid Interface Sci., 1976,54,430. 27 P. A. Bernstein and A. B. P. Lever, Inorg. Chim. Acta, 1992, 198-200,543. 28 N. S. Bayliss and E. G.McRae, J. Phys. Chem., 1954,58,1002. 29 W. Kalz, H. Homborg, H. Kuppers, B. J. Kennedy and K. S. Murray, Z. Naturforsch., B Chem. Sci., 1984,39, 1478. 30 V. H. Homborg, Z. Anorg. Allg. Chem., 1983,507,35. 31 G. J. V. Berkel and K. G. Asano, Anal. Chem., 1994,66,2096. 32 B. Zinger, K. R. Mann, M. G. Hill and L. L. Miller, Chem. Mater., 1992,4, 1113. 33 T. Harazono and I. Takagishi, Bull. Chem. SOC. Jpn., 1993,66,1016. 34 T. Enokida and R. Hirohashi, Nippon Kagaku Kaishi, 1990,211. 35 W. Hiller, J. Strahle, W. Kobe1 and M. Hanack, 2. Kristallogr., 1982,159,173. Paper 5/060805;Received 14th September, 1995 J. Muter. Chem., 1996, 6(2), 143-148 147

ISSN:0959-9428    

DOI:10.1039/JM9960600143

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Self-assembled monolayers of phthalocyanine derivatives on glass and silicon Michael J. Cook,* Roxana Hersans, Jim McMurdo and David A. Russell School of Chemical Sciences, University of East Anglia, Norwich, UK NR4 7TJ The synthesis of three phthalocyanine derivatives functionalised with seven or eight substituents including either one or two trichlorosilylalkyl chains is described. Self-assembled monolayers of the derivatives covalently bound to silicon and glass have been formed and characterised by FTIR and visible region spectroscopies. The fabrication of thin films of phthalocyanine derivatives by oxyphthalonitriles 4 and 5 respectively. Phthalonitrile 4 was deposition from the vapour phase, by spin coating, and by prepared by acid-catalysed deprotection of 4,5-dicyanoisoprop- transfer of monolayers via the Langmuir-Blodgett method is well established1Y2 and provides formulations of potential value in displays, chemical sensors, and photoconducting devices.A difficulty with films deposited from the vapour phase and by spin coating is that there is often rather limited control over film thickness and crystallite size. The Langmuir-Blodgett technique, on the other hand, can produce films of precise thickness, with some degree of three dimensional ~rder.~.~ However, the films are often fragile; for device applications more robust and abrasion-resistant films are desirable. Techniques leading to self-assembled monolayer (SAM) films3 offer the prospect of obtaining ultrathin films chemically bound to the substrate surface.Such films are much more robust but compounds for this type of deposition require specific functionalisation. Recently, we described the formation of a phthalocyanine SAM film by depositing a disulfide derivative onto a gold surface.' In this paper, we report what we believe to be the first examples of phthalocyanine SAM films on glass and silicon. These have been obtained using three novel phthalocyanines bearing seven or eight substituents of which one or two are alkyl chains bearing a terminal trichlorosilyl group. The latter reacts with surface oxides and hydroxides to form films which have been characterised by both visible region and IR spectroscopy. Results and Discussion Phthalocyanine derivatives for SAM film formation Previous work from these laboratories has produced a number of isomerically pure octa-substituted phthalocyanine deriva- tives.6 Alkyl substituents confer a degree of solubility in organic solvents such as dichloromethane, toluene and tetrahydrofuran, a requirement for the SAM deposition method, as well as mesogenic behaviour,6-8 a potentially useful property that may encourage self organisation of the molecules during the mono- layer forming process.Accordingly, for the present work, attention was focussed on non-uniformly octa-substituted derivatives which contain a number of alkyl substituents and either one or two alkenyl chains. In principle, the latter can be hydrosilylated with trichlorosilane to give the desired derivatives for SAM deposition.A non-uniformly substituted compound described in recent work was the hydroxyalkyl phthalocyanine derivative la.9 In the present study, this compound was treated with methanesul- fonyl chloride and triethylamine in dichloromethane to give the methanesulfonate (mesylate) lb. The latter was treated with sodium but-3-en-1-olate in dry THF to give the monoalkenyl phthalocyanine lc. Two further alkenyl derivatives, 2a and 3a, were also synthesised. The immediate precursors to these were 3,6-di~ctylphthalonitrile~and the novel bis- and mono-butenyl- C0H17 N la X=H Ib X=SO&Ha IC X=(CH&CH=CH* Id X=(CH2)4SiCt3 2a X=(CH2),CH=CH2 2b X=(CH2),SiCJ3 -C0H17 3a X=(CH2)&H=CH2 3b X=(CH2)4SiC13 J.Mater. Chem., 1996, 6(2), 149-154 149 Scheme 1 Reagents: i, HCl,,-EtOH; ii, 4-bromobut-l-ene, K2C03, MEK Scheme 2 Reagents: i, but-3-en-1-01, K2C03, DMF ylidenecatechol" followed by base catalysed 0-alkylation with 4-bromobut-1-ene in methyl ethyl ketone (MEK), Scheme 1. Phthalonitrile 5 was prepared in one step from the commer- cially available 4-nitrophthalonitrile, Scheme 2. Base-catalysed crossed condensation of an excess of 3,6-dioctylphthalonitrile with 4 and with 5 afforded the required phthalocyanines 2a and 3a respectively. These were readily separated from the side-products of condensation, the main one being 1,4,8,11,15,18,22,25-octaoctylphthalocyanine,by routine chromatographic procedures. The three phthalocyanines lc, 2a and 3a were then converted into the corresponding trichlorosilylalkyl derivatives Id, 2b and 3b respectively.This was achieved by chloroplatinic acid- catalysed hydrosilylation of the terminal double bond with trichlorosilane in dry benzene or toluene under dry argon in a sealed tube at 100°C;ll there is no reason to suppose that side reactions occur involving the phthalocyanine nucleus because, under the same conditions, 1,4,8,11,15,18,22,25-octa-octylphthalocyanine was recovered intact. The required deriva- tives Id, 2b and 3b were recovered by evaporating the solution under argon and redissolving them in dry THF from which solutions they were deposited onto silicon or glass as described below. Characterisation of precursor phthalocyanines Compounds lb, lc, 2a and 3a each gave a satisfactory elemental analysis and a 'H NMR spectrum fully consistent with the structure of the compound.In particular lc, 2a and 3a each showed the signals characteristic of the protons for terminal alkene groups. They also showed aromatic proton signals expected for the ring substitution patterns of the compounds. For example, lc shows two well resolved AB patterns with signals at 6 7.64 and 7.73, and at 6 7.79 and 7.83. The remaining four protons are accidentally equivalent at 6 7.88. Compound 2a, the most symmetrical compound, shows a singlet at 6 8.19 for the protons at the C-1 and C-4 sites, a singlet at 6 7.96 assigned to the protons at the C-16 and C-17 positions and the remaining four protons give rise to an AB pattern, 6 7.52 and 7.63.The most complex set of aromatic proton signals arises from 3a. The spectrum shows the proton at C-4 to be the most deshielded, 6 8.8. The proton at C-1 appears at 6 8.39 and that at C-3 at 6 7.45. All show the appropriate splitting patterns. There are also two further AB patterns in the region 6 7.65-7.85 and a singlet for the remaining two protons at 6 7.93. Thermotropic mesophase behaviour among 1,4,8,11,15,18,22,25-octasubstitutedphthalocyanines is now well documented7.* and the four new compounds all proved to be enantiotropic liquid crystals, i.e. they exhibited one or more mesophases during both heating from the crystal state and on cooling from the isotropic liquid, I.Transition tempera- tures were monitored by polarised light microscopy and are collected in Table 1. The mesophases were identified by comparing their birefringence textures with those for other compounds whose mesophases have been characterised fully by X-ray diffraction methods.8 On cooling from the isotropic liquid all four compounds exhibited a mesophase, denoted as D,, which has a characteristic fan type texture. This type of birefringence is observed for discotic columnar mesophases of two dimensional hexagonal symmetry with disordering within the stacks, Dhd.Compounds lb, lc and 2a, on further cooling, exhibited a second mesophase, D2, having a needle texture. This is characteristic of the second, lower temperature, Dhd mesophase exhibited by some octaalkyl derivatives.' On further cooling, compounds lb and 2a crystallised but compound lc did not solidify even when cooled to -50°C.On cooling compound 3a from the D, mesophase, a mosaic texture of a further mesophase, D3, was observed. This texture is characteristic of a disordered discotic columnar mesophase of rectangular symmetry, Drd.' On further cooling, this gave way to the needle texture of the D, mesophase prior to crystallisation. SAM film preparation Silicon wafers and glass slides used as substrates were cleaned and rendered hydrophilic immediately prior to use, then rinsed with Millipore@ water, acetone, dry THF and dried in a stream of dry nitrogen, see Experimental section. The use of freshly cleaned wafers and slides proved to be essential for satisfactory film deposition; extended rinsing of the substrates in Millipore@' water for 30min proved to be detrimental to the subsequent film deposition process, giving inferior quality films.Attachment of the phthalocyanines Id, 2b and 3b to the wafer or slide surface was achieved simply by immersing the substrate in a ca. 3 x lop3mol dmp3 solution of the phthalocy- anine in THF for 24 h under an argon atmosphere. Extending the time to 72 h gave films showing the same spectral properties on subsequent analysis, see below. On withdrawal from the solution, the SAM coated substrates were washed immediately in fresh THF; failure to wash the substrate immediately on removal gave uneven films.We propose these arose from evaporation of the small amount of retained solution leaving behind non-covalently bound phthalocyanine material. This, in the presence of atmospheric moisture, could well form oligomers which would be difficult to remove. Spectroscopic characterisation of films Evidence for SAM film deposition onto silicon was obtained from transmission FTIR spectroscopy. The FTIR spectra for the SAM films obtained from all three compounds showed well defined bands in the C-H stretching region, with the spectrum of the film obtained using 3b, Fig. 1, giving rise to the highest intensity absorbance bands. The spectrum shows stretching modes at 2868 (CH2 symmetric stretch), 2933 (CH, asymmetric stretch, absorbance 0.02) and 2958 cm-' (CH, in- plane asymmetric stretch).Corresponding modes for the SAM Table 1 Compounds lb, lc, 2a and 3a, transition temperatures ("C) observed during heating (first set of data) and cooling" lb 51.5; 48.5 68.5; 67.5 170.6; 169.3 lb 66.0; 56.4 166.7; 164.0 2a 153.0; 129.0 170.0; 170.0 248.0; 248.0 3a 80.0; 50.0 128.0; 128.0 163.0 163.0 250.0; 250.0 a D1,fan texture, D,, needle texture, and D3,mosaic texture, are tentatively assigned as Dhd, Dhd, and D,, mesophases respectively, see text. 150 J. Mater. Chem., 1996, 6(2), 149-154 0 3200 3000 2800 2600 wavenum ber/cm-' Fig. 1 Part of the FTIR transmission spectrum of the SAM film obtained from 3b deposited onto the two sides of a silicon wafer.The spectrum shows the aliphatic C-H stretching modes. film from 2b appeared at 2870, 2928 (absorbance 0.005) and 2955 cm-'. The absorbance values were typically >50% more intense for films obtained from 3b over those from 2b which contains four more CH2 groups. The corresponding spectrum for the SAM film formed from Id gave bands at 2865, 2925 (absorbance 0.002) and 2960 cm-'. The absorbances in the films from Id were all lower than for the other films despite the marginally larger number of methylene groups. The absorp- tion intensities are not necessarily a linear measure of the number of CH2 groups present because the groups may well be anisotropically ordered within the film; the electric field component of the incident radiation interacts only with the component of the dynamic dipole moment of the C-H vibration which is parallel with the substrate surface.Visible region spectra were recorded of the SAM films deposited onto glass and each showed a Q-band broadened relative to that observed for the solution phase. For each phthalocyanine, the SAM film spectrum was reproducible from film to film and over different regions within the same film. The SAM derived from 3b shows the highest intensity Q-band absorbance, Fig. 2; in reality, the spectrum corresponds to a double SAM film (one either side of the glass slide), indicating an absorbance of ca. 0.005 per monolayer. The corresponding spectrum for the double SAM film derived from 2b, Fig. 3, I I I I 500 600 700 800 900 wavelengthhm Fig.2 The visible region spectrum of the SAM film derived from 3b deposited onto the two sides of a glass slide I I I I 500 600 700 800 900 wavelengthhm Fig.3 The visible region spectrum of the SAM film derived from 2b deposited onto the two sides of a glass slide shows an absorbance of ca.0.003 per monolayer. Both absorp- tion envelopes appear to show some degree of structure, particularly that in Fig. 3. In contrast the film derived from Id shows a low intensity absorption (absorbance ca. 0.001 per monolayer) and little structure. The band envelopes for the SAMs are different from those observed previously for LB12 and spin coated films13 of octa- alkylated phthalocyanines. Those bearing octyl chains show, relative to their solution phase spectra, a characteristic red and blue shifted band.This is illustrated in Fig. 4 where spectrum (a) is that for a spin coated film of compound 2a. Recent work has shown that mesogenic phthalocyanines for- mulated as spin coated films exhibit the same thermotropic liquid crystal behaviour as the bulk material^.'^ Spectra (b) and (c) in Fig. 4 show the spin coated film of 2a at temperatures corresponding to the D,, mesophase and the isotropic liquid, respectively. The absorption envelopes of the SAM films are most similar to spectra (b)and (c) indicating that the molecular packing within the SAM films may be comparable to the looser packing of either the liquid crystal phase or the liquid phase.The absorbance from the films decreases in the same order as for the FTIR bands observed for the corresponding films I I I I 1 500 600 700 800 900 wavelengthhm Fig. 4 The visible region spectra of a spin coated film of 2a. Spectrum (a), at room temperature and showing a broad band exciton split absorbance. Spectrum (b), at 200 "C corresponding to the Dhd mesophase and spectrum (c) at 260°C at which temperature the film has melted. J. Muter. Chem., 1996, 6(2), 149-154 151 on silicon. However, as with the FTIR results, it is difficult to use the absorption intensities to quantify the amount of material bonded to the substrate. Absorption coefficients, E, of metal-free phthalocyanines are normally quoted for conditions where aggregation is minimal and are of the order of 1.5 x lo5dm3 mol-l cm-l.For example, absorption coefficients for the precursor phthalocyanines, 2a and 3a, as dilute solutions in cyclohexane are 1.42 x lo5 (at 4.4 x mol dm-3) and 1.48 x lo5 (at 4.4 x mol drn-,), respect-ively. The crystal state molality of octa-substituted derivatives structurally related to the present compounds is ca. 0.94.105 These data predict a solid state absorbance of ca. 0.0014 per A of film thickness but clearly refer to a close packed film structure in which there are no exciton coupling interactions of the transition dipoles. It also refers to a film containing random organisation of the molecules, as in the solution phase, whereas in practice the molecules may be packed aniso-tropically.The two n-n* transitions contributing to the Q-band absorption are polarized orthogonally in the plane of the ring16 and therefore, quite apart from film thickness and packing density, the absorbance is dependent upon the mean orientation of the rings relative to the incident light beam. The absorbance arising when the rings are aligned with their planes perpendicular to the direction of the interrogating beam is twice that when they are aligned parallel. Nevertheless, despite these qualifications, the absorbance data do suggest that Id has deposited to give incomplete surface coverage. On the other hand, the absorbances from the films derived from 2a and 3a are essentially consistent with a monolayer film, e.g.one in which the rings are lying parallel to the substrate surface. While a number of other film structures cannot be precluded, it is interesting to note that this type of surface coverage was deduced from a combined reflection-absorption infrared spectroscopy (RAIRS) and transmission FTIR study of a SAM film of a sulfanylalkylphthalocyanine on gold.17 Finally, a film derived from 3b on glass was heated to 255 "C and then cooled to room temperature. The visible region spectrum of the heat treated film gave the same Q-band structure and absorbance intensity to within ca. 5% of that of the original film, clearly demonstrating the thermal stability of the SAM film. Conclusions The three examples of trichlorosilylalkylated phthalocyanines investigated in the present study, suitably substituted to render them soluble in toluene and benzene, react at the surface of hydrophilic silicon and glass to form SAM films.The films can be detected spectroscopically. Films on silicon examined by transmission FTIR spectroscopy show C-H stretching modes and films on glass show absorbance in the visible region spectrum. The intensities of the absorption bands for films from the three derivatives differ. These differences are reproduc- ible and may indicate that satisfactory SAM formation is dependent upon the substituents on the phthalocyanine ring. The highest visible region absorption intensity, cu. 0.005 per monolayer was obtained for the film derived from 3b. The thermal stability of this film was demonstrated by heating it to 255°C and cooling to room temperature after which the visible region absorption band was essentially unchanged.Experimental Equipment and measurements FTIR spectra were recorded on a €310-RAD FTS 165 spectro- photometer. 'H NMR spectra were measured at 60 MHz using a JEOL JNM-PMX 60 spectrometer and at 270 MHz using a JEOL EX 270 spectrometer. Routine mass spectra were recorded using a Kratos MS 25 mass spectrometer. UV-VIS spectra were recorded using a Hitachi U-3000 spectrophoto- meter. Visible region spectra of spin coated films of 2a on glass slides were recorded at various temperatures using a Mettler FP82 Hotstage adapted to fit inside the sample compartment. Melting points were measured and thermotropic mesophase behaviour monitored using an Olympus BH-2 polarising microscope in conjunction with a Linkam TMS 92 thermal analyser and a Linkam THM 600 cell.Materials Silica gel (Merck 7734) was used in chromatographic separa- tions. TLC was performed using silica gel (Merck 5554) supported on aluminium sheets. Solvents were dried, where appropriate, over sodium and distilled under an atmosphere of dry nitrogen. Phthalonitrile precursors 4,5-Dicyanoisopropylidenecatechol,mp 198-199 "C (lit.," 193°C) was prepared from catechol according to the route outlined in ref. 10. 3,6-Dioctylphthalonitrile was prepared from thiophene according to the route outlined in ref. 7. 4,5-Dihydroxyphthalonitrile. 4,5-Dicyanoisopropylidene-catechol (5.0g, 25mmol), 5 mol dmP3 hydrochloric acid (25 cm3) and ethanol (25 cm3) were heated under reflux with stirring for 3 h.The solvent was removed by distillation and the yellow residue recrystallised from water to give 4,5- dihydroxyphthalonitrile (4.0 g, 100%) as colourless needles, mp 285 "C, dH(60 MHz; [2H,]acetone) 7.40 (2 H, s). This was used as described below without further purification. 4,5-Bis(but-3-eny1oxy)phthalonitrile (4). 4,5-Dihydroxy-phthalonitrile (1.0g, 6.25 mmol) and potassium carbonate (5 equiv.) were stirred in MEK (100 cm3) for 10 min, after which 4-bromobut-1-ene (2.2 g, 16 mmol) was added. This mixture was stirred under reflux for 72 h after which time TLC analysis indicated that the reaction was part complete. A few drops of dicyclohexane-18-crown-6 were added and heating under reflux continued for 48 h after which time TLC analysis indicated that there was no remaining starting material.The reaction mixture was filtered and the solvent removed under reduced pressure to give ca. 2 g of a crude solid. Column chromatography (silica gel, eluent dichloromethane) followed by recrystallisation from light petroleum (bp, 100-120 "C)gave 4,5-bis(but-3-enyloxy)phthalonitrile ( 1.01 g, 60%) as colourless needles, mp 126 "c(Found: c, 71.4; H, 5.9; N, 10.4. C16H,6N,02 requires C, 71.6; H, 6.0; N, 10.4%); vrnax/cm-l 2225 (CN); dH(60 MHz; CDCl,) 2.7 (4 H, q), 4.2 (4 H, t), 5.2 (4 H, m), 6.0 (2 H, m), 7.2 (2 H, s); m/z 268. 4-( But-3-enyloxy)phthalonitrile (5). 4-Nitrophthalonitrile (2.0 g, 11.5 mmol) and but-3-en-1-01 (0.86 g, 12 mmol) in dimethylformamide (30 cm3) were heated, with stirring, to 100 "C.Freshly ground potassium carbonate (4.1 g, 30 mmol) was added in portions over 5 h and heating continued for a further 12h. The reaction mixture was allowed to cool to room temp., filtered and the filtrate washed with dichloro- methane (3 x 50 cm3). The combined organic extracts were washed with water (2 x 50 cm3), saturated brine (50 cm3), dried (MgSO,), filtered and the solvent removed under reduced pressure to give a pale yellow solid. The solid residue was dissolved in acetone and adsorbed onto silica gel (10g), loaded onto a silica gel column and eluted with light petroleum (bp 40-60 "C)-dichloromethane (1 :1).The pale yellow solid was chromatographed again (silica gel, carbon tetrachloride-dichloromethane, 2 :1 eluent) to afford 4-(but-3-enyloxy)ph- thalonitrile as a colourless solid (530 mg, 23%) mp 46-48 "C (Found: C, 72.5; H, 5.0; N, 14.1. Cl,Hl,N,O requires C, 72.7; H, 5.1; N, 14.1%); vrn,x/cm-l 2225 (CN); &(60 MHz; CDCl,) 152 J. Muter. Chem., 1996, 6(2), 149-154 2.7 (2 H, m), 4.4 (2 H, t), 5.2 (2 H, m), 6.0 (1 H, m) 7.8 (3 H, m); m/z 198. Phthaloc yanines 4-(5-Hydroxypentyl)-l-methyl-8,11,15,18,22,25-hexaoctyl-phthalocyanine, la, was prepared according to the route described in ref. 9. 1-Methyl-4-(5-rnethylsulfonyloxypentyl)-8,11,15,18,22,25-hexaoctylpht haloc yanine (1 b). The 5-hy drox ypen t ylp h thalocy- anine, la, from above (126 mg, 98 mmol) was dissolved in dichloromethane (10 cm3) and a large excess of triethylamine and methanesulfonyl chloride were added.The resultant mix- ture was heated under reflux for 2 h until TLC analysis indicated that the reaction had gone to completion. The solvent and excess reagents were removed under reduced pressure and the residue chromatographed (x 2) over silica gel [column one, eluent dichloromethane; column two, light petroleum (bp 40-60 "C)-dichloromethane, 2 : 11 to give 1-methyl-4-(5-methylsulfonyloxypentyl)-8,11,15,18,22,25-hexaoctylphthalocya-nine, lb (94.1 mg, 70%), mp 51.5 (K+Dhd), 68.5 (Dhd+Dhd), 170.5 "C (Dhd +I), (Found: C, 76.5; H, 9.4; N, 7.8. CgsH1,gNBOS requires C, 76.5; H, 9.4; N, 8.2); 6, (270 MHz; ['H6] benzene) -0.40 (2 H, s), 0.85 (18 H, m), 1.05-1.50 (64 H, m), 1.65-1.85 (8 H, m), 2.20 (2 H, q), 2.43 (6 H, m), 3.20 (2 H, t), 3.25 (2 H, t), 3.81 (3 H, s), 4.50 (2 H, t), 4.60-4.78 (12 H, m), 4.90-5.02 (2 H, m), 5.68-5.84 (1H, m), 7.64 (1H, d), 7.73 (1H, d), 7.79 (1H, d), 7.83 (1 H, d), 7.88 (4H, s).4-( 5-But -3-enyloxypenty1)- 1 -met hyl-8,1 1,15,18,22,25- hexa- octylphthalocyanine (lc). Sodium hydride (1 equiv.) was washed with dry light petroleum (bp 40-60°C) and then suspended in dry THF. The slurry was cooled to 10°C. But- 3-en-1-01 (1 equiv.) was added and the mixture allowed to warm to room temp. An excess of this mixture was added to 1-methyl-4-(5-methylsulfonyloxypentyl)-8,11,15,18,22,25-hexa-octylphthalocyanine, lb, (36.5 mg, 27 mmol) dissolved in dry THF.The solution was heated to reflux for 10 h when the reaction was complete as evidenced by TLC. The solvent was removed under reduced pressure and the residue chromato- graphed [silica gel, eluent light petroleum (bp 40-60 "C)-dichloromethane, 2 :11 to afford 4-( 5-but-3-enyloxy- pentyl )-l-methyl-8,11,15,18,22,25-hexaoctylphthalocyanine,lc (26.3 mg, 73%), mp 66.0 (Dhd+Dhd), 172 "c (Dhd+I), (Found: C, 80.2; H, 9.8; N, 8.1. C90H132N80 requires C, 80.55; H, 9.9; N, 8.35%); 6,(270 MHz; ['H6] benzene) -1.18 (2 H, s), 0.90 (18 H, m), 1.20-1.85 (64 H, m), 1.90-2.15 (6 H, m), 2.18 (3 H, s), 2.20-2.40 (8 H, m), 3.18 (3 H, s), 3.72 (2 H, t), 3.83 (2 H, t), 4.20 (4 H, t), 4.40 (4 H, t), 4.64 (4 H, t), 7.22 (1 H, d), 7.32 (1 H, d), 7.41 (1 H, d), 7.81 (4 H, dd).2,3-Bis( but-3-enyloxy)-8,11,15,18,22,25-hexaoctylphthalo-cyanine (2a). 4,5-Bis(but-3-eny1oxy)phthalonitrile (270 mg, 1 mmol) and 3,6-dioctylphthalonitrile (3.6 g, 9 mmol) in dry pentan-1-01 (30 cm3) were heated under reflux and lithium metal (0.4 g) was added in small pieces. Reflux was continued for 6 h and then the mixture was allowed to cool to room temp. Acetic acid (40 cm3) was added and the resultant slurry stirred for 1 h. The solvents were removed under reduced pressure and the residue triturated and washed with methanol to give a dark green solid which was dissolved into cyclo- hexane and then chromatographed over silica gel using first light petroleum (bp 40-60°C) as eluent to afford 1,4,8,11,15,18,22,25-octaoctylphthalocyanine,(1.30 g, 40%), identical by TLC to an authentic sample.The eluting solvent was then changed to light petroleum (bp 40-60 "C)-THF (9 :1) to give a second fraction that was further purified by column chromatography over silica gel (eluent cyclohexane-THF, 9 :1) to give a dark green solid that was recrystallised from THF- methanol to afford 2,3-bis(but-3-enyloxy)-8,11,15,18,22,25-hexaoctylphthalocyanine,2a (220 mg, 16%) as dark green crys- tals, mp 153 (K+Dhd), 170 (Dh,j-$Dhd), 248°C (Dhd+I) (Found: C, 79.6; H, 9.4; N, 8.6. CggH126N802 requires C, 79.6; H, 9.6; N, 8.4%); 6,( 270 MHz; C2H6] benzene) -2.00 (2 H, s), 0.90 (18 H, t), 1.40 (48 H, m), 1.90 (12 H, m), 2.23 (8 H, m), 2.43 (4 H, m), 2.90 (4 H, m), 3.92 (4 H, m), 4.32 (4 H, t), 4.40 (4 H, t), 4.62 (4 H, t), 5.40 (4 H, m), 5.25 (2 H, m), 7.52 (2 H, d), 7.63 (2 H, d), 7.96 (2 H, s), 8.19 (2 H, s); Ama,(4.4x lop6 mol dm-3 in cyclohexane)/nm 716.5 nm [&/dm3 mol-' cm-' 1.42x lo'], 679, 341.2-( But3-enyloxy)%, 11,15,18,22,25-hexaoctylphthalocyanine (3a). 4-(But-3-eny1oxy)phthalonitrile (200 mg, 1 mmol) and 3,6-dioctylphthalonitrile (3.6 g, 9 mmol) in dry pentan-1-01 (30 cm3) were heated under reflux. Lithium metal (0.4 g) was then added in small pieces and reflux continued for 6 h. The mixture was allowed to cool to room temp., acetic acid (40 cm3) was added and the resultant slurry stirred for 1 h. The solvents were removed under reduced pressure and the residue tritu- rated with methanol to give a dark green solid which was dissolved in cyclohexane and then chromatographed over silica gel using first light petroleum (bp 40-60 "C)as eluent to afford 1,4,8,11,15,18,22,25-octaoctylphthalocyanine,(1.31 g, 41 YO)and then light petroleum (bp 40-60 "C)-dichloromethane (3 :1) to give a second fraction.The latter was further purified by column chromatography over silica gel [eluent: light petroleum (bp 40-6OoC)-dichloromethane 3: 11 to give a dark green solid. Recrystallisation from THF-methanol followed by a further recrystallisation from THF-acetone afforded 2-(but-3- enyloxy)-8,11,15,18,22,25-hexaoctylphthalocyanine,3a (260 mg, 20%) as dark green Crystals mp 80 (K+Dhd), 128 (Dhd+Drd), 153 (Drd+Dlhd), 250°C (Dhd+I) (Found: c, 80.1; H, 9.5; N, 8.8.C84H120N80 requires c, 80.2; H, 9.6; N, 8.9%); 8H (270 MHz; ['H6] benzene) -1.40 (2 H, s), 0.90 (18 H, t), 1.40 (48 H, m), 1.90 (12 H, m), 2.30 (12 H, m), 2.70 (2 H, m), 4.20 (4 H, m), 4.60 (4 H, m), 4.70 (4 H, t), 5.30 (4 H, m), 6.20 (1 H, m), 7.45 (1 H, m), 7.65-7.85 (4H, m), 7.93 (2 H, s), 8.39 (1 H, d), 8.80 (1 H, d); Amax(4.4 x mol dm-3 in cyclohexane)/nm 718 (&/dm3 mol-' cm-l 1.48 x lo'), 680, 338. Hydrosilylation reactions. In a typical procedure, 4-( 5-but- 3-enyloxypenty1)-l-methy1-8,11,15,18,22,25-hexaoctylphthalo-cyanine, lc (25 mg), excess trichlorosilane, a catalytic amount of hexachloroplatinic acid (0.1 cm3, 0.1 mol dmp3 solution) and dry benzene or toluene were sealed in a glass tube under an argon atmosphere and then heated to 100°C for 48 h.Excess trichlorosilane and solvent were removed under reduced pressure and the residue dissolved in dry THF (4 cm3) and stored under a dry argon atmosphere. Depositionof self-assembled monolayer (SAM ) films Preparation of substrates. Silicon wafers and glass slides used as substrates were cleaned immediately prior to use in an ultrasonic bath using a mixture of 30% hydrogen peroxide in concentrated sulfuric acid, ('piranha' solution'') at 90 "C for 30 min or until they were judged to be completely hydrophilic. Substrates were then rinsed with Millipore@' water, acetone, dry THF and then dried in a stream of dry nitrogen. SAM deposition.In a typical procedure, a 3.2 x mol dm-3 solution of the trichlorosilylated phthalocyanine 3c (20 mg) dissolved in dry THF (4 cm3), prepared as above, was contained under an atmosphere of dry argon. The substrate was immersed in this solution for 24 h. Since the Si-C1 bond is susceptible to hydrolysis by atmospheric moisture each step was performed under a dry argon atmosphere. On withdrawal from the solution, the SAM film bearing substrate was washed immediately in fresh THF with sonication for 30 min. J. Muter. Chem., 1996, 6(2), 149-154 153 Spin coated films Spin coated films were prepared from solutions of 2a in THF (ca. 2.5 mg in 1.0 cm3) dropped onto a hydrophilic glass slide rotating at 2000 rpm using a Headway Spinner.Spinning was continued for 20 s by which time the solvent had evaporated. Comparisons of the visible region absorption intensities with those from LB films of known thicknesses prepared from comparably substituted phthalocyanines giving simil?r spectral features suggest that the film thickness was ca. 700 A.12 We thank the EPSRC (formerly SERC) and the EU HCM programme (grant no. CHRX CT94-0558) for financial sup- port. We also thank Mr. A. Jafari-Fini and Mr. T.R.E. Simpson for fruitful discussions. References A. W. Snow and W. R. Barger, in Phthalocyanines-Properties and Applications, eds. C. C. Leznoff and A. B. P. Lever, VCH, New York, 1989, p. 341. M. J. Cook in Spectroscopy of New Materials-Aduances in Spectroscopy Series, eds.R. J. H. Clark and R. E. Hester, Wiley, Chichester 1993 p. 87. A. Ulman, An Introduction to Ultrathin Films: from Langmuir- Blodgett to Self-Assembly, Academic Press, San Diego, London, 1991. M. J. Cook, Znt. J. Electron., 1994,76, 727. T. R. E. Simpson, D. A. Russell, I. Chambrier, M. J. Cook, A. B. Horn and S. C. Thorpe, Sens. Actuators, B: Chemical, 1995, 29, 353; I. Chambrier, M. J. Cook and D. A. Russell, Synthesis, 1995,1283. 6 M. J. Cook, J. Muter. Sci; Muter. Electron., 1994,5, 117. 7 I. Chambrier, M. J. Cook, S. J. Cracknell and J. McMurdo, J.Muter. Chem., 1993,3 841. 8 A. S. Cherodian, A. N. Davies, R. M. Richardson, M. J. Cook, N. B. McKeown, A. J. Thomson, J. Feijoo, G. Ungar and K. J. Harrison?Mol. Cryst. Liq. Cryst., 1991,196, 103. 9 G. C. Bryant, M. J. Cook, T. G. Ryan and A. J. Thorne, Tetrahedron, in the press. 10 I. Cho and Y. Lim, Chem. Lett., 1987,1,2107; I. Cho and Y. Lim, Mol. Cryst. Liq. Cryst. 1988,9, 154. 11 J. L. Speier, Advances in Organometallic Chemistry, Academic Press, New York, 1979, vol. 17, pp. 407-447. 12 M. J. Cook, J. McMurdo, D. A. Miles, R. H. Poynter, J. M. Simmons, S. D. Haslam, R. M. Richardson and K. Welford, J.Muter. Chem., 1994,4, 1205. 13 S. M. Critchley, M. R. Willis, M. J. Cook, J. McMurdo and Y. Maruyama, J. Muter. Chem., 1992,2, 157. 14 M. J. Cook, D. A. Mayes and R. H. Poynter, J. Muter. Chem., in the press. 15 I. Chambrier, M. J. Cook, M. Helliwell and A. K. Powell, J. Chem. SOC., Chem. Commun., 1992, 444 M. J. Cook, J. McMurdo and A. K. Powell, J. Chem. SOC.,Chem. Commun., 1993,903. 16 M. J. Stillman and T. Nyokong, in Phthalocyanines-Properties and Applications, eds. C. C. Leznoff and A. B. P. Lever, VCH, New York, 1989, p. 133. 17 D. A. Russell and T. R. E. Simpson, unpublished results. 18 A. H. Carim, M. M. Dovek, C. F. Quate, R. Sinclair and C. Vorst, Science, 1987,237,630. Paper 5105898H; Received 6th September 1995 154 J. Mater. Chem., 1996, 6(2), 149-154

ISSN:0959-9428    

DOI:10.1039/JM9960600149

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Laser-induced chemical vapour deposition of Si/C/H materials from monoorganylsilanes Josef Pola,at Zdenik Bast1,b Jan Subrt,E J. Rasika Abeysinghe" and Roger Taylor"" "School of Chemistry and Molecular Sciences, University of Sussex, Brighton, UK BN1 9QJ bJ. Heyrovskj Institute of Physical Chemistry, 182 23 Prague, Academy of Sciences of the Czech Republic, Czech Republic 'Institute of Inorganic Chemistry, 250 68 Rei, Academy of Sciences of the Czech Republic, Czech Republic Laser-induced photolysis of monoorganylsilanes RSiH, (R = H,C=CH, HC-C, ClHC=CH and H2C=CHCH2) produces thin Si/C/H films, thought to be saturated polycarbosilanes of low hydrogen content, and formed from unsaturated transients generated upon elimination of hydrogen and ethyne from the RSiH, molecules.The films incorporate oxygen upon exposure to the atmosphere. Chemical vapour deposition (CVD) induced by heat, plasma or photolysis is a well recognised technique for the production of Sic and Si/C/H materials which find use as advanced ceramics or in microelectronics. Various alternative volatile precursors containing both silicon and carbon in one molecule have been examined (see, e.g. refs. 1-12). Amorphous hydro- genated a-SiC:H films proved useful as wide band-gap materials and their deposition at low substrate temperatures is normally achieved by using plasma-enhanced decompositions (see, e.g. refs. 13,14) or UV laser phot~lysis.~ The latter has been carried out with chloromethylsilanes as precursors whose absorption cross-section for UV radiation is very low.Laser-induced photolysis of monalkylsilanes RSiH, (R = aliphatic group) is one promising CVD process and has been H2C=CHCH2) expanded in the reactor were recorded by a Philips PU 8720 UV-VIS scanning spectrometer. In order to evaluate the properties of the deposit by FTIR spectroscopy and by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) techniques, deposits were produced on a sheet of NaCl, quartz or copper placed in the reactor before irradiation. SEM studies of the deposits were performed on an ultra- high vacuum instrument (Tesla BS 350) and morphology of samples was investigated using an accelerating voltage of 16 kV. XPS measurements were performed on a VG ESCA 3 Mk I1 electron spectrometer.The spectra were recorded with A1-Ka radiation and an electrostatic hemispherical analyser operated in the fixed transmission mode. investigated so far using IR radiation of CO, lasers onl~.'~-~~ In a continuation of our studies on excimer laser photolysis of organosilicon compounds in the gas pha~e,"-'~ we present here our results on the ArF laser-induced photolytic decompo- sition of monoalkylsilanes and show this reaction to be a suitable method for low-temperature production of Si/C/H films. Experimental Laser photolysis experiments were carried out on gaseous samples of monoorganylsilanes RSiH, (R = H,C= CH, HC=C, ClHC=CH and H,C=CHCH,) using a Questek series 2000 excimer (ArF) laser operating at 193.3 nm with a repetition frequency of 10 Hz.The samples were irradiated in a glass reactor which consisted of two orthogonally positioned tubes (both 2 cm in diameter), one (15 cm long) fitted with NaCl windows and another (6 cm long) fitted with quartz plates. The unfocused beam (energy fluence typically 300 mJ) was spatially filtered by an aperture (1 cm in diameter) and entered the reactor through the quartz window. The progress of photolytic decompositions was monitored by FTIR using a Perkin-Elmer 1710 spectrometer with diag- nostic bands at 1398 cm-l (ethenylsilane), 2065 cm-' (ethynyl-silane), 1574, 1544 and 1299 cm- '(2-chloroethenylsilane) and 1643 cm-' (prop-2-enylsilane). Photolytic products, ethyne, hydrogen chloride and silane were determined by using the absorption at 3308 and 731,2821, and 2187 cm-', respectively. UV absorption spectra of gaseous monoorganylsilanes RSiH, (R =H,C=CH, HCEC, ClHC=CH and ton leave of absence from the Institute of Chemical Process Fundamentals, 165 02 Prague, ASCR, Czech Republic.Detailed spectral scans were taken over Si 2p, C Is, 0 1s and Si (KLZ3Lz3)regions. The latter spectra were excited by bremsstrahlung radiation. Core-level binding energies as well as Auger electron kinetic energies were corrected for static sample charging using the method of internal standard. The 0 1s binding energy was adopted in calibration to compensate for charging effects. The use of this method is based on our observation that 0 1s core-level binding energy of oxygen incorporated in the deposits is largely independent of the deposit composition, and is equal to 532.5k0.2 eV.The metal decoration technique was also employed to calibrate the spectra. The results from both methods were in good agreement. Furthermore, the value of the sum of the kinetic energy of Si (KL,,LZ3) Auger electrons and the binding energy of Si 2p electrons, defined as the modified Auger ~ararneter,,~ was calculated from the measured data. This value is independent of the static charge referencing procedure and is therefore obtained with higher accuracy than the energy of either type of electron alone. In addition, the magnitude of the Auger parameter chemical shift is often significantly greater than that of the core-electron binding energy chemical shift.Quantification of the element concentration ratios was accomplished by correcting the integral intensities of the photoelectron lines for their cross-se~tions,~~ allowing for the dependence of electron mean-free-paths on kinetic energy.26 Curve-fitting of overlapping spectra was carried out using lines of Gaussian-Lorentzian shape and damped non-linear least- squares te~hnique.'~ The monoorganylsilanes RSiH, (R = H,C=CH and H2C=CHCH2) were prepared by previously reported reductions of monoalkyltrichlorosilanes (Aldrich) with lithium J. Muter. Chem., 1996,6(2), 155-160 155 aluminium hydride in di-n-butyl ether.28 2-Chloroethynylsilane was prepared by the reduction of 2-chloroethenyltrichlorosil-ane with lithium aluminium hydride in diglyme; chloroethenyl- trichlorosilane was obtained using the thermal reaction between trichlorosilane and trans-l,2-di~hloroethene.~~~~~All the RSiH, compounds were distilled on a standard vacuum line and degassed prior to use.The identity of all the com- pounds was confirmed by their mass (GC/MS, Shimadzu, QP 1000 mass spectrometer) and FTIR (Perkin-Elmer 1710 spectrometer) spectra and their purity (>99%) was checked by gas chromatography (Shimadzu 14A, gas chromatograph, column packed with SE 30 silicon elastomer). Authentic samples of ethyne, silane and hydrogen chloride were from the laboratory stock. Results and Discussion In contrast to saturated analogues, the unsaturated mono-organylsilanes RSiH3 (R = H2C=CH, HCE C, ClHC=CH and H2C= CHCH,) show a significant absorption above 190nm and can be therefore photolysed by ArF laser radiation at 193.3 nm.Absorptivity (in kPa cm-') at this wavelength decreases in the order: ClCH=CHSiH, (300)>H2C=CHCH2SiH3 (290)>H2C=CHSiH, (280)> HC=CSiH3 ( 140)>H3CCH,SiH3 ( 1). The energy delivered by the photons corresponds to ca. 620 kJ mol-' and it is far in excess of the energy needed for cleavage of any of the molecular bonds. The ArF laser irradiation of all the RSiH3 compounds at pressures around 1kPa results in the formation of ethyne together with a white solid material which is deposited on the inside of the reactor. An uncondensable gas (hydrogen) is also formed.Silane was detected as a very minor product in the photolysis of ethenyl- and prop-2-enyl-silane. An increase in pressure from 0.6 kPa to 4kPa with 2-chloroethenylsilane does not affect the nature of the products. In contrast, the decompo- sition progress of ethynylsilane is affected by the pressure increase: a non-explosive decomposition was observed for 1 kPa, and an explosive one at pressures of 3.6 and 4.7 kPa. The (violent) explosive decomposition is induced with just a single laser pulse, and yields a mixture of methane, ethene and ethyne and a black soot-like fog. The gaseous products of the laser photolysis of the RSiH, compounds are given in Table 1. Probable steps of monoorganylsilane photolysis It is known that the major dissociation route with ethenyl- and ethynyl-~ilane~~.~~ is 1,l-H2 elimination [Eqn.( l)], RSiH, +RHSi: +H2 (1) and that this process is accompanied by 1,2-H2 and hydro- carbon elimination. The observation of an uncondensable gas, which can only be hydrogen, shows that the 1,l-elimination of hydrogen is operative with all the RSiH, compounds. Hydrocarbon elimination from ethenylsilane should yield ethene, that from ethynylsilane should give C2H2 +H2Si (when occurring via silirane) and C2H4 +Si (when occurring via silirene). The presence of ethyne, and no detection of ethene in the photolysis of ethenyl- and ethynyl-silane implies the occurrence of steps (2) and (3), while the absence of elemental Si in the deposits (see below) rules out step (4).H2C=CHSiH3 +H2C=CHSiH:+C2H2+H2Si: (2) HC=CSiH3 +C2H2 +H2Si: (3) HC=CSiH:+C2H2+ Si (4) The same assumptions also apply to the photolysis of 2- chloroethenylsilane, which undergoes dehydrochlorination and the reactions deduced for ethynylsilane. In contrast to the facile elimination of P-haloalkylsilanes into alkenes and chloro- silanes, P-haloalkenylsilanes are considered3' not to undergo the corresponding reaction giving alkynes. The observed dehydrochlorination of 2-chloroethenylsilane and no pro-duction of chlorosilane is in accord with this view. The formation of ethyne in the photolysis of 2-propenylsilane indicates cleavage of the C-C bond and shows that this channel contributes to an apparently major 1,l-H2 elimin- ation route.Silane, observed as a minor product in the photolysis of ethenyl- and prop-2-enyl-silane, indicates the formation of silylene and its reaction with dihydrogen. Properties of the solid deposits FTIR spectra of the deposits (Fig. 1, Table 2) show a pattern typical of a-SiC:H film^^',^^ and reveal characteristic absorp- tion of saturated moieties containing Si-H and C-H bonds. The spectra can be inter~reted~~,~~ in terms of the contributions from the vibrational modes as shown in Table2, and the relative content of C-H and Si-H bonds can be esti- mated38,39 using the Si-H and C-H per-bond oscillator strength in SiH, and CH,. Thin layers from ethenylsilane (Table 2, Fig. la) show domi- nant absorptions of Si-H bonds.The v(C-H) absorptivity is lower than that of v(Si-H) by a factor of ca. 5 which imp lie^^*.^^ that the concentration of H atoms bonded to carbon and those bonded to silicon is roughly equal. This 1 :1 relative occurrence of both types of hydrogen is in accord with the operation of both hydrogen and ethyne elimination. It is of interest to compare this spectrum to that of a white powderous poly(ethenylsi1ane) which we obtained from gas- eous ethenylsilane (with no hydrogen evolution) on storage in a glass ampoule. The spectrum of the polymer [708, 767, 824 Table 2 FTIR spectra of the Si/C/H deposits" wavenumber/cm-(absorptivity)b vibrational mode I I1 I11 IV v(Si-C) 801 (0.54) 803 (1.72) 801 (3.04) -S(Si-H)c 926 (1.0) -919 (0.47) 928 (1.60) S(C-H) 1061 (0.28) -1028 (0.70) -v(Si-Hj 2145 (1.0) 2122 (1.0) 2138 (1.0) 2131 (1.0) v(C-H) 2891 (0.21) 2917 (0.35) 2880 (0.01) 2923 (0.20) " I, 11, I11 and IV designate the deposits from H,C=CHSiH,, H2C= CHCH,SiH,, ClHC =CHSiH,, and HC= CSiH,, respectively.Normalised to the absorptivity of the v(Si-H) band. Also rocking and wagging CH,. Table 1 Typical product yield data of the RSiH, photolysis RSiH, pressure/kPa irradiation time/min conversion (%) H2C= CHSiH, 1.3 11.7 50 HCzCSiH, 1.2 6.3 42 3.6 C 78 ClHC=CHSiH, 1.1 1.5 87 4.0 7.5 73 H,C =CHCH,SiH, 1.3 6.7 31 gaseous products" (yieldb) C2H2(0.55), SiH, (trace) C2H2(0.41) C2H2 (0.45), CH4 (0.30), C2H4 (0.20) C2H2(0.55), HCl (0.96) C2H2(0.60), HCl (0.90j C2H2(0.50), SiH, (trace) " Detected by FTIR spectroscopy. In mol (mol RSiH, decomposed)- '.Single pulse. 156 J. Muter. Chem., 1996,6(2), 155-160 I' I I 400 1000 2000 3000 4000 wavenumber/cm-l Fig. 1 IR spectra of the solid deposits from RSiH,. a, R = H,C=CH; b, R=HC=C, white deposit; c, R=HC=C, black deposit; d, R= ClHC=CH; and e, R =H,C=CHCH,. (all s), 916 (vs), 999 (m), 1059, 1101, 1134 (all s), 1404 (m), 1462 (w), 21 12 (m), 2883,2937 (both w)] has a more complex pattern and it shows absorptivities [normalised to that of v(Si-H)] of 3.49 (at 824 cm-'), 4.64 (at 916 cm-'), 1.0 (at 2111 cm-') and 0.40 (at 2883 cm-I). The material photodeposited from prop-Zenylsilane (Table 2, Fig.le) possesses significant contributions from Si-C, Si-H and C-H bonds as reflected by absorptions at v(Si-C), v(Si-H) and v(C-H). The v(C-H) absorptivity is lower than that of v(Si-H) by less than a factor of ca. 5, which imp lie^^^,^^ that more H atoms are bonded to carbon than to silicon. Considering the partial cleavage of the C-C bond (and the escape of C2H, into the gas phase), this preference of H(C) atoms confirms the importance of the 1,l-elimination. The IR spectrum of the material from the photolysis of 2-chloroethenylsilane (Table 2, Fig. Id) is dominated by a strong absorption at v(Si-C) and shows a very low v(C-H)/v(Si-H) absorptivity ratio (ca. This indicate^^^,^^ the predomi- nance of hydrogen atoms bonded to silicon.We have observed that the IR spectrum, and consequently the nature, of the deposit does not depend on the pressure of 2-chloroethenyl- silane used. The prevalence of H(Si) over H(C) atoms in the deposit is consistent with an efficient expulsion of ethyne, this reaction being more facile than in the photolysis of 2-chloro- ethenylsilane. The nature of thin layers from the photolysis of ethynylsilane is strongly affected by the pressure at which the photolysis is carried out (Table 2, Fig. 1b and c). The white deposit obtained at 1.2 kPa shows strong absorptions at G(Si-H), v(Si-H) and v(C-H) with only a very small absorption at v(Si-C). In contrast, the black deposit formed during the explosive photolysis reveals solely a strong absorption at v(Si-C).The absence of v(Si-H) and v(C-H) absorptions and its black colour indicate the presence of particulate carbon. The absorp- Fig. 2 SEM images of the deposits from RSiH,. (a)R = H,C=CH; (b) R = HC=C, white deposit; (c) R = ClHC=CH and (d) R = H2C=CHCH,. J. Muter. Chern., 1996,6(2), 155-160 157 tivity v(C-H)/v(Si-H) ratio of the white material (0.2) is similar to that of the deposit from ethenylsilane and can be rati~nalised~~.~~in terms of equal numbers of hydrogen atoms bonded to Si and C. The low absorptivity at v(Si-C) suggests that the white material does not contain many Si-C bonds and it is composed of -(SiH,), -(CH,), moieties. Interestingly, none of the IR spectra of the deposits show any absorption of multiple bonds between C atoms, and all the v(C-H) bands belong to the region of saturated C-H bonds (<3000 cm-I).This indicates that all the intermediates containing unsaturated bonds are lost in a very efficient polymerization process. It is known4' that incorporation of carbon into the Si-Si framework shifts the v( Si- H) wavenumber to higher values and that a v(Si-H) absorption above 2110 cm-' corresponds to a-Si,-,C,:H films with carbon content x>0.8. The v(Si-H) absorptions observed with all the deposits are thus in line with a carbon content at least as high as that of silicon. IR spectra of the deposits which had been exposed to air (Table 3) reveal that the films incorporate oxygen: the v(Si-C) band remains almost unaltered, the v( Si-H) band diminishes, and a very strong v(Si-0) absorption band and a broad diffuse weak v(Si)OH band develop at 1060 and around 3600 cm-', respectively.XPS analysis of the superficial layers of the deposits exposed to air confirms that the films are composed of silicon, carbon and oxygen. The stoichiometry of the deposits obtained from H2C=CHSiH3, H2C=CHCH2SiH,, ClHC=CHSiH3, and HC=CSiH3 (after ion sputtering for 3min by Ar ions Of energy keV) are sil.00c0.9800.78, si1.00c1.5400.~1 Si1.00Co.6701.28 respectively. These Si/C and Si1~ooCo.7901~32, ratios reveal that the carbon content in superficial layers is somewhat lower than inferred for the bulk films from the IR spectra. This difference implies that partial removal of C from the superficial layers occurs upon their ion bombardment. SEM images of the deposits (Fig.2) reveal that the deposits have particulate structure and consist of agglomerates, the size of which is typically less than 1 pm. The morphology of the deposit makes parts of the surface unaccessible to the ion treatment and this, together with the high stability of the Si-0 bond, is a reason for the inability to remove oxygen. The reactivity of the films towards oxygen can be accounted for by two reactions: a fast addition of molecular oxygen to residual Si=C and/or Si=Si bond~,~',~~ or more probably by oxidation of Si-H bonds. The observed values of Si 2p core-level binding energies as well as the differences between the energies of the 0 1s and Si 2p electrons and particularly the values of the modified Auger parameter of deposits (Table 4) are consi~tent~~.~~ with the presence of an oxygen-containing organosilicon polymer.The difference in 0 1s and Si 2p binding energies for inorganic silicon compounds and in organosilicon polymers is in the range45 429.0-429.6 eV and 429.8-430.1 eV, respectively. The ion sputtering of the deposits results in a broadening of the Si 2p and Auger lines (Fig. 3 and 4), which is a consequence of the occurrence of another chemical state of Si. The split of Table 3 FTIR spectra of the deposits after exposure to air" absorptivityb vibrational mode I I1 I11 v(Si-C) 1.o 1.o 1.o v( Si-0) 1.8 1.4 1.2 v(Si-H) 0.7 0.3 0.07 v(C-H) 0.3 0.2 0 "I, TI and 111 designate the deposits from H,C=CHSiH,, H,C=CHCH,SiH, and ClHC=CHSiH,, respectively.Normalised to the absorptivity of the v(Si-C) band. 158 J. Muter. Chem., 1996,6(2), 155-160 Table 4 The Si 2p core-level binding energies, the differences between the energies of the 0 1s and Si 2p electrons and modified Auger parameters, M, in the deposits (in eV) material Si 2p 0 1s-Si 2p a deposit I after ion sputtering4 deposit I1 after ion sputtering" deposit 111 after ion sputtering" deposit IV after ion sputtering" poly(ethenylsilane)b Sicb 102.3 101.0/102.3 102.0 102.3 102.7 100.9/ 103.0 102.7 103.1 101.2 100.4 430.2 431.5/430.2 430.5 430.3 429.7 43 1.61429.5 429.8 429.4 431.3 431.1 1712.0 17 14.8/17 12.0 1711.7 1714.0 1711.5 17 14.5/17 12.2 1711.7 1712.7 1712.4 1714.1 " Sputtered by Ar ions (E =4 keV) for 3 min.Reference sample. 100 105 110 Fig.3 Si 2p core level spectra of the deposits produced from (1) ethynylsilane, (2) ethenylsilane, (3) prop-Zenylsilane and (4) 2- chloroethenylsilane: (a) deposits as produced, (b) deposits after sputtering by Ar ions with energy 4 keV for 3 min the C 1s line and particularly of the Si (KL23L23)Auger line into two inequivalent chemical states of carbon after the ion sputtering (Fig. 5) permits46 assignment of this state to silicon carbide, since the C 1s binding energy of the state produced by the ion sputtering (283.4 eV) is characteristic of Sic. With regard to the known chemical modifications of surfaces by ion we cannot differentiate whether the Sic is formed upon the ion bombardment, or it is present in the deeper layers of the deposit before the ion treatment.Conclusion The different features of the IR spectra of the deposits from all the unsaturated organylsilanes reflect the fact that tiny 1600 1622 kinetic energyleV Fig. 4 Si (KL,,L,,) Auger electron spectra of the deposits obtained by laser irradiation of (1) ethynylsilane, (2) ethenylsilane, (3) prop-2-enylsilane and (4) 2-chloroethenylsilane: (a) deposits as produced, (b)deposits after ion sputtering by Ar ions with energy 4 keV for 3 min 280 285 290 binding energy/eV Fig.5 C 1s core level spectra of the deposit obtained from 2-chloroethenylsilane. (1) sample as produced, (2) after heating in the spectrometer at 400 "C for 30 min, (3) after sputtering with Ar ions.differences in the structure of the RSiH3 precursor can lead to different structures of the deposited material. This convenient way of modifying the structure and properties of the deposited films, the ease in handling of all RSiH, precursors, and the fact that the technique can be used for the deposition of films at substrates kept at relatively low temperatures, makes the reported process promising for low-temperature deposition of amorphous Si/C/H films. We thank the Grant Agency of the Academy of Sciences of the Czech Republic (Grant no. A4072509) and EPSRC for support of this work. The technical assistance of I. Spirovova in the XPS measurements is gratefully acknowledged. References 1 W.Bocker and H. Hausner, Ber. Dt. Keram. Ges., 1978,233. 2 Y. Okabe, J. Hojo and A. Kato, J. Less-Common Met., 1979,68,29. 3 M. Endo, T. Sano, K. Mori, N. Urasato and M. Shiraishi, Yogyo Kyakaishi, 1987,95, 104. 4 J. A. O'Neil, M. Horsburg, J. Tann, K. J. Grant and G. L. Paul, J. Am. Ceram. Soc., 1989,72,1130. 5 A. E. Kaloyeras, J. W. Corbett, P. J. Toscano and R. B. Rizk, Muter. Res. SOC. Symp. Proc., 1990,192,601. 6 V. A. Zubov, M. Friedrich, H. D. Klotz, R. Fricke, R. Wesche and H. Prost, Zh. Neorg. Khim., 1990,35,2917. 7 D. E. Cagliostro and S. R. Riccitiello, J. Am. Ceram. Soc., 1990, 73, 607. 8 H. Schmidbaur, R. Hager and J. Zech, in Frontiers of Organosilicon Chemistry, ed. A. R. Bassindale and P.P. Gaspar, The Royal Society of Chemistry, Cambridge, 1991, p. 62. 9 Y. M. Li and B. F. Fieselmann, Appl. Phys. Lett., 1991,59, 1720. 10 A. Figueras, S. Garelik, J. Santiso, R. Rodriguez-Clemente, B. Armas and C. Combescure, Muter. Sci. Eng. B, 1992, 1183. 11 E. F. Hengge, in Abstracts of Lectures, Xth International Symposium on Organosilicon Chemistry, Poznan, Poland, 1993, p. 42. 12 J. M. Agullo, F. Fau-Canillac and F. Maury, J. Muter. Chem., 1994, 4, 695. 13 H. Y. Lu and M. A. Petrich, J. Appl. Phys., 1992,72,2054. 14 W. Beyer and G. Winterling, Appl. Phys. Lett., 1989,54, 1666. 15 J. S. Francisco, S. A. Joyce, J. I. Steinfeld and F. Walsh, J. Phys. Chem., 1984,88,3098. 16 D. M. Rayner, R. P. Steer, P. A. Hackett, C. L.Wilson and P. John, Chem. Phys. Lett., 1986,123,449. 17 J. V. Thoman and J. I. Steinfeld, Chem. Phys. Lett., 1986,124,35. 18 J. Pola, H. Beckers and H. Burger, Chem. Phys. Lett., 1991, 178, 192. 19 J. Pola, Z. Bastl, J. Tlaskal, H. Beckers, H. Burger and P. Moritz, Organometallics, 193, 12, 172. 20 Z. Bastl, H. Burger, R. Fajgar, D. Pokorna, M. Senzlober, J. hbrt, M. Urbanova and J. Pola, Appl. Organomet. Chem., in press. 21 J. Pola and R. Taylor, J. Organomet. Chem., 1993,446, 131. 22 J. Pola, Z. Bastl, J. Subrt and R. Taylor, J. Muter. Chem., 1995, 5, 1345. 23 J. Pola and R. Taylor, J. Organornet. Chem., 1995,489, C9. 24 C. D. Wagner, in Practical Surface Analysis, vol. I, Auger and X-ray Photoelectron Spectroscopy, ed. D. Briggs and M.P. Seah, Wiley, Chichester, 1994,p. 595. 25 J. H. Scofield, J. Electron Spectrosc. Relat. Phenom., 1976,8, 129. 26 M. P. Seah and W. A. Dench, Surface Interface Anal., 1979,1,1. 27 A. E. Hughes and B. A. Sexton, J. Electron Spectrosc. Relat. Phenom., 1988,48,31. 28 V. Baiant, V. Chvalovsky and J. Rathousky, Organosilicon Compounds, Academia, Prague, 1965. 29 E. A. Chernyshev and G. F. Pavelko, Bull. Acad. Sci. USSR, Chem. Sect., 1966, 12,2131. 30 R. F. Cunico, J. Org. Chem., 1971,36,929. 31 M. A. Ring, H. E. ONeal, S. F. Rickburn and B. A. Sawrey, Organometallics, 1983,2, 1891. 32 S. F. Rickburn, M. A. Ring, H. E. O'Neal and D. Coffey, Int. J. Chem. Kinet., 1984, 16,289. 33 D. S. Rogers, M. A. Ring and H. E. O'Neal, Organometallics, 1986, 5, 1521.34 J. J. W. McDouall, H. B. Schlegel and J. S. Francisco, J. Am. Chem. Soc., 1989,111,4622. 35 R. F. Cunico and E. M. Dexheimer, J. Am. Chem. Soc., 1972, 94, 2868, and refs. therein. 36 S. Ray, D. Das and A. K. Barna, Solar Energy Muter., 1987,15,45. 37 H. Rubel, B. Schroder, W. Fuhs, J. Krauskopf, T. Rupp and K. Bethge, Phys. Status Solidi, 1987,139, 131 and refs. therein. 38 H. C. Low and P. John, J. Organomet. Chem., 1980,201,363. 39 H. Shanks, C. J. Fung, L. Ley, M. Cardona, F. J. Demond and S. Kalbitzer, Phys. Status Solidi, 1980,100,43. 40 D. M. Bhusari and S. T. Kshirsagar, Muter. Lett., 1991,11,348. J. Muter. Chem., 1996, 6(2), 155-160 159 41 42 I. M. T. Davidson, C. E. Dean and F. T. Lawrence, J. Chem. SOC., Chem. Commun., 1981,52. R. West, H. B. Yokelson, G. R. Gillette and A. J. Millevolte, Silicon Chemistry, ed. E. R. Corey, J. Y. Corey and P. P. Gaspar, Ellis Horwood, Chichester, 1988, p. 269. 44 45 46 R. C. Gray, J. C. Carver and D. M. Hercules, J. Electron Spectrosc. Relat. Phenom., 1976,8, 343. C. D. Wagner, D. E. Passoja, H. F. Hillery,T. G. Kinisky, H. A. Six, W. T. Jansen and J. A. Taylor, J. Vac. Sci. Technol., 1982,21,933. K. L. Smith and K. M. Black, J. Vac. Sci.Technol., 1984, A2,744. 43 NIST X-ray Photoelectron Spectroscopy Database, US Dept. of 47 G. Marletta, S. M. Catalan0 and S. Pignataro, Surface Interface Commerce, Gaithersburg, 1989. Anal., 1990,16,407. Paper 5/04589D; Received 12th July, 1995 160 J. Muter. Chem., 1996,6(2), 155-160

ISSN:0959-9428    

DOI:10.1039/JM9960600155

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Growth of strongly orientated lead sulfide thin films by successive ionic layer adsorption and reaction (SILAR) technique Tapio Kanniainen," Seppo Lindroos, Jarkko Ihanus and Markku Leskela Department of Chemistry, P.O. Box 55, FIN-0001 4 University of Helsinki, Finland Lead sulfide thin films were grown at room temperature by the successive ionic layer adsorption and reaction (SILAR) technique on soda lime glass, IT0 and A1203 covered glass, SO2, ( 1OO)Si and ( 11 l)Si substrates. SILAR utilises sequential treatment of the substrate with aqueous precursor solutions. Dilute solutions of lead acetate and thioacetamide were used as precursors for Pb2+ and S2-, respectively. The lead precursor solution also contained triethanolamine (tea) as a complexing agent, with a Pb: tea mole ratio of 1 :2.On glass the growth rate was 0.12 nm per cycle with 0.2 mol dmP3 lead and 0.4 mol dm-3 thioacetamide solution. The appearance of the films was metallic. X-Ray diffraction studies revealed a strong [200) orientation of the films. According to the Rutherford back-scattering (RBS) and nuclear reaction analysis (NRA) results the films were stoichiometric PbS and contained small amounts of some lighter impurities, possibly 0 and H. Scanning electron microscope (SEM) images revealed that the films were rather rough and consisted of grains with a diameter approximately corresponding to the thickness of the film. Lead sulfide thin films have been used during recent years in photoconductive devices as detectors. Owing to its suitable bandgap of 0.41 eV, PbS can be used as an IR detector in various applications such as scientific instruments and indus- trial and military equipment. PbS thin films can be grown from solution either at ambient or at elevated temperatures and in the gas phase.Techniques utilizing aqueous conditions are chemical bath deposition (CBD),lP5 the 'thin liquid film' method, (which closely resembles CBD)6, successive ionic layer adsorption and reaction (SILAR)7*8 and electrodepo~ition.~ Gas phase methods used for the growth of PbS thin films are atomic layer epitaxy (ALE)"," and molecular beam epitaxy (MBE).I2 CBD is a widely used technique due to the simplicity of the procedure and the high quality of the produced films. Furthermore, the equipment is uncomplicated and the precur- sors are in most cases common laboratory chemicals.On the other hand, the procedure is a bulk reaction, where adjustment of the process is not an easy task. The advantages of the aqueous techniques like CBD and SILAR are: simple pro- cedure, low temperature and low cost. In addition, SILAR utilises a sequential growth mechanism, which makes process control straightforward. The SILAR technique for the deposition of thin films was introduced by Nicolau in the mid-1980s.I3 It is based on a heterogeneous reaction between adsorbed ions and solvated ions on the solid-liquid interface. The substrate is treated separately with each aqueous precursor solution so that the individual steps, adsorption and reaction, can take place.During the first step, cations are adsorbed onto the substrate, and in the next step all the excess unadsorbed cations are washed away by rinsing the substrate with purified water. The rinsing is followed by the reaction step, during which the substrate is immersed into a vessel containing the aqueous anion precursor solution. When the solvated anions enter the diffusion layer they react with the adsorbed cations, and a solid adsorbed compound is formed on the surface. Again the ions in the diffusion layer are washed away with a rinsing pulse. By repeating these deposition steps a thin smooth film can be grown layer by layer. The thickness of the growing film is controlled by the number of deposition cycles.So far the SILAR technique has been mainly used to grow 11-VI compound thin films on various Our previous study on the growth of PbS thin films by the SILAR technique from lead acetate and nitrate [Pb(CH,COO), and Pb(N03),] together with sodium sulfide and thioacetamide (Na,S and CH3CSNH2) precursors showed that this deposition method allows one to affect the crystallization and the crystal orientation, as well as the grain size of the films.7 The purpose of this work was to further investigate the deposition and properties of PbS thin films grown with the SILAR method using complexation of lead. Experimental For the deposition a new apparatus, shown schematically in Fig. 1, was used. The new system was based on a modified Gilson Model 232 XL automatic sample processor, which was controlled by a PC via an RS232 interface.A substrate holder was mounted in the place of the needle. The xyz robot allowed full control of the substrate positioning as well as the dipping and rinsing times. The reaction vessels were standard 250 cm3 glass laboratory beakers. The rinsing vessels were constructed in the same way as in the older eq~ipment.'~.'~ The rinsing was carried out in water with 18 MQcm resistivity in order to thoroughly rinse the diffusion layer. The water was provided by a Millipore equipment (Milli-Q) and the flow rate was 250 cm3 min-'. The immersion times for the cation adsorption and the anion reaction were 20 and 40 s, respectively. The rinsing time was 70s.During the immersion and the rinsing the substrates were continuously moved by 1mm side-to-side lN21 1 I[--I I I 1 GILSON x/z Fl REACTION VESSELS Fig. 1 Schematic diagram of the new SILAR deposition machine J. Mater. Chem., 1996, 6(2), 161-164 161 in order to enhance convection around the substrate. The total time required for one deposition cycle was 215 s; these cycles were repeated as many times as required. The depositions were carried out under an N, atmosphere. Aqueous solutions of lead acetate [Pb(CH,COO),, Merck p.a.1 complexed with tea [N(CH,CH,OH),, Riedel-deHaen p.a.1 for lead, and with taa (CH3CSNH2, Merck, p.a.) for the sulfide ion were used as precursors. The concentrations of the solutions were 0.05-0.60 and 0.10-1.20 mol drn-,, respectively (ie.Pb: S mole ratio 1 : 2). The Pb :tea mole ratio was 1 :2.l' The pH values were 7.0-7.8 for Pb(tea);+ and 4.7 for the taa solution. The substrate materials used were soda lime glass, ITO-covered ( 150 nm) soda lime glass, A1,03-covered ( 150 nm) soda lime glass, SiO,, ( 1OO)Si and ( 11 1)Si. The substrates were cleaned ultrasonically, rinsed with acetone and ethanol and dried overnight before deposition. The Si substrates were etched with hydrofluoric acid for 2min and washed with 18 MR cm water for 30 min inside the N2 gas chamber just before the deposition. The amount of lead in the films was determined by dissolving the film in concentrated nitric acid and measuring the amount of Pb2+ ions, either using a Techron atomic absorption spectro- photometer or in basic media with a Metrohm 626 polaro- graph. The nominal thickness of the film was calculated using the bulk density of PbS (7.5 g cm-,).The crystal structure, crystallite orientation and crystal size were determined using a Philips MPD 1880 X-ray powder diffractometer using Cu-Ka radiation. The surface morphology of the films was characterized with a Zeiss DSM 962 scanning electron microscope (SEM). The chemical composition and thickness of the films were also studied by Rutherford back-scattering spectrometry (RBS) of 2.0 MeV 4He+ ions from the 2.5 MV Van de Graaf acceler- ator of the Accelerator Laboratory, University of Helsinki.,' Hydrogen profiling was carried out by the nuclear reaction analysis (NRA) technique using a 15N2+ beam from the 5 MV tandem accelerator EGP-10-11 of the Accelerator Laboratory to excite the 6.385 MeV resonance of the 'H(15N, a, y)12C reaction.,' Results and Discussion Growth rate According to the principles of SILAR, film growth proceeds by the adsorption of lead ions and the consecutive reaction of the sulfide precursor with the adsorbed lead ions, hence the theoretical maximum growth rate would be one monolayer of lead sulfide per deposition cycle.On [2OO]PbS this can be defined as the interatomic distance of the atoms (cu. 0.30 nm). The maximum growth rate i.e. the nominal film thickness divided by the number of cycles, was 0.12 nm per cycle, corresponding to an average surface coverage of 40%.This value has already been achieved with the 0.2 mol dm-, lead and 0.4 mol dm-, sulfur precursor solutions. More concen- trated solutions did not increase the growth rate. On the other hand, smaller concentrations than these resulted in a slower growth rate. The growth rate may be limited sterically by both the size of the adsorbed Pb species and the size of the acetate counterion occupying the outer Helmholtz layer, and therefore the density of the smaller lead ions in the inner Helmholtz layer is limited. The growth is also limited by the dissolution of PbS in acidic solutions. The anionic precursor solution had a pH value of 4.7, which is low enough to dissolve some PbS.22v23 This was also indicated by some small black PbS precipitates in the taa solution. Furthermore, a longer dipping time than 40s in anionic solutions resulted in a lower growth rate.On IT0 and SiOz substrates the growth rate was roughly the same as that on the soda lime glass. The growth rate on 162 J. Muter. Chem., 1996,6(2), 161-164 both ( 1OO)Si and (1 11)Si was slightly lower than that on glass, similar to the results achieved for PbS films grown by CBD.24 On A1,03-covered glass the attachment of the film was poor. The growth rate observed using Pb(tea),,+ as the lead precursor was higher (0.12nm per cycle) than that from a solution without the complexation (0.04 nm per ~ycle).~ The pH value of the complexed precursor solution was 7.8, whereas that of the uncomplexed precursor was 6.5.7 On natural PbS (galena surface), the adsorption of the Pb(0H) + ion increased with increasing pH due to an increase in the number of negatively charged surface sites available for ad~orption.,~ Hence, the more effective adsorption of the lead complex from the precursor solution with the higher pH value is reasonable.Compared to CBD which is the method used most often to grow PbS thin films, the growth rate using the SILAR tech- nique was still lower. In CBD, all the reagents are present in the reaction vessel at the same time, hence a high growth rate is achieved. The average growth rate achieved by CBD has been up to several tens of nm min-l, independent of the precursors ~sed.'-~.~ On the other hand, in the gas phase analogy to SILAR, i.e.atomic layer epitaxy (ALE; both methods utilize sequential introduction of precursors), the overall growth rate varied between 0.01 and 0.09 nm per cycle depending on the precursor and the substrate temperature." X-ray diffraction As indicated in Fig. 2, where an XRD pattern of an as-grown 200nm thick PbS film on a soda lime glass substrate is depicted, the films were strongly [200] orientated. The intensit- ies of the other existing peaks in Fig. 2 were very small (Table 1). It has previously been stated that the lead counter- ion affects the orientation of the SILAR-grown PbS thin films.7 Compared to those results, it can be seen that the complexed lead precursor remarkably enhances the orientation. This obviously occurs by guiding the central lead atoms during adsorption to match the right distance of lead atoms on the PbS(200) plane.The Pb(tea)2+ complex is rather large I3' c.30. 9 .-25. v).c 8-C.-2e /degrees Fig.2 X-Ray diffraction pattern of a 200nm thick PbS thin film grown by SILAR on soda lime glass at room temperature Table 1 Six strongest reflections in the XRD patterns of the PbS thin films grown on glass, SO2, (100)Si, (1 11)Si and IT0 covered glass substrates (the pattern for PbS powder is presented for comparison26) substrate h k 1 glass SiO, (1OO)Si (111)Si IT0 powder 111 0 1 1 1 50-80 84 200 100 100 100 100 100 100 220 0 2 1 0 30 57 311 0 0 0 0 20 35 222 0 0 0 0 10 16 400 4 4 5 5 a- 10 a This reflection was masked by an IT0 reflection.0s OPb Fig. 3 Representation of the diagonal orientation of the Pb(tea),'+ complex on the (200)PbS surface. The PbS(200) plane is tilted through 66". (ca. 0.7 xO.5 nm2),27 and therefore it cannot bring its lead atom very close to the adjacent lead atom. The two tea groups are bound to the central lead atom via oxygen atoms. In an ideal case, i.e. the complex approaches an ideal PbS(200) plane, the ligands are parallel to the substrate surface and the oxygen atoms of the opposite ethanol groups may also affect the orientation of the complex as they are closest to the substrate surface and have an affinity to interact with the Pb atoms of the surface.On the PbS(200) plane this leads to a configuration where the complex is along the diagonal on the (200) plane, and the negative oxygen atoms, by reaching the positive lead atoms of the surface, guide the lead atom of the complex accurately on top of a sulfur on the surface. Hence the [2001 orientation is favourable. The possible approach of the Pb(tea)2f complex to the (200)PbS surface is illustrated in Fig. 3. The intensities of the six strongest reflections in the XRD patterns of the as-grown PbS thin films on various substrates are listed in Table 1. On amorphous glass and Si02 as well as on single-crystal Si substrates the [2001 direction was strongly dominating. One clear exception from this tendency was the XRD pattern of a film grown on polycrystalline ITO.Besides resembling the PbS powder pattern with the (111) reflection as the strongest peak, the intensities of the peaks were very low compared to those observed for films on other substrates. This indicates a lower crystallinity of PbS films on ITO. Annealing for 2 h at 120°C in air had no effect on the XRD patterns of the films. The CBD method employs aqueous solutions as precursors and is carried out at room temperature, similarly to the SILAR method. However, the XRD pattern of the PbS thin films grown on amorphous glass by CBD are in general powder- like, regardless of the lead precursors used, and no orientation has been found,2 even with the same precursors as used in this st~dy.~The elevated deposition temperature (up to 50 "C) using a 'thin liquid film' method was found to enhance crystallinity, but the diffraction pattern was still powder-like.6 On the other hand, PbS thin films grown by CBD on (1OO)Si were (100) ~rientated,~~.~*whereas on ( 11 1)Si the 220 and 31 1 reflections dominated.24 The sequential introduction of reactants in the SILAR method involves heterogeneous reactions between the adsorbed species and the solvated ions in the solution.This layer-by-layer growth affects the crystal structure of the grow- ing film. Similarly to SILAR, the ALE method also utilizes sequential reactant pulses followed by inert rinsing/purging. Although ALE utilizes higher temperatures (300-350 "C for PbS) and reduced pressure, the XRD pattern of the PbS films grown on glass closely resembled the powder pattern. However, by suitable choice of the lead precursor, orientated films were achieved on alumina covered glass, where the (200) orientation was prevailing.lO*ll Electrodeposition of PbS thin films from an acidic solution containing Pb(N03)2 and Na2S203 on titanium sheets resulted in a powder-like XRD at tern.^ The crystallite size derived from the Scherrer equation for a 200 nm thick film was ca.100 nm. By comparison, CBD-grown PbS thin films on glass (thickness also ca. 200 nm)2 and electrodeposited PbS on a titanium sheetg resulted in crystallite sizes of 40 and 13 nm, respectively. However, larger crystallite sizes have also been observed in CBD films, uiz.crystallite sizes of 200nm in 400 nm thick films.24 The ratio of film thickness :crystallite size is in this case similar to that observed in this work.SEM studies The topography of an as-grown PbS thin film deposited on a soda lime glass is presented in Fig. 4(a), where the top view of a SEM image is depicted. The appearance of the film is rather rough and inhomogeneous, however, no cracks or voids can be detected. The roughness is caused by the preferred crystalline growth and the cubic crystallization is clearly seen in some of the particles. A side view [Fig. 4(b)] reveals that the particles are columnar and reach down to the substrate, and that the particles are well separated. The sizes of these particles, as estimated from Fig.4, were ca. 200 nm, which is similar to the thickness of the film. The grain size is approximately the same as those produced from PbS thin films grown with SILAR using uncomplexed lead acetate solution as the lead precursor. Fig. 4 SEM image of a 200 nm thick PbS film grown on soda lime glass. (a) Top view, (b)tilted view (45"). J. Mater. Chem., 1996, 6(2), 161-164 163 The grain sizes are much smaller than the 1 pm found in thicker PbS films grown by ALE'' and CBD3 with the same precursors, i.e. lead acetate, tea and taa precursors. When lead nitrate, NaOH and thiourea were used to grow PbS thin films by CBD, the grain size varied from values close to our results' up to 1-10 ~m.'~ RBS and NRA According to RBS analysis, the films contained equal amounts of lead and sulfide.In addition, 6-8 atom% of lighter impurities distributed evenly throughout the film could be detected. These elements (H, C, N or 0)could not be detected individually due to the limited resolution of the RBS method and the roughness of the film, which makes the fitting of the RBS results difficult. The amount of impurities present is roughtly the same as that in ZnS thin films grown by SILAR, which contained water and possibly zinc hydr0~ide.l~ Accordingly, because in NRA 6-8 atom% of hydrogen was found, it is likely that the PbS films contain water. Natural PbS (galena) was found to react with air to give lead carbonate and/or lead hydroxycarbonate species on the galena surface,23 and after several days of exposure to air sulfate species were also dete~ted.~'Hence, as the films were stored in air it is possible that the impurities are of atmospheric origin.Annealing for 2 h at 120 "C in air did not change the Pb :S ratio, nor did it affect the amount of lighter impurities. According to NRA, the amount of hydrogen decreased by 2-5 atom% after the annealing. Auger analysis of CBD-grown PbS thin films revealed that the films also contain oxygen.24 However, in contrast to this work, oxygen was detected on the surface, down to 20nm thickness, as well as on the interface between the film and the Si(100) substrate. Conclusions High quality PbSthin films were synthesized at room tempera- ture and normal pressure by using the SILAR technique.The film surfaces were homogeneous, smooth and metallic. A complexed lead ion [bis(triethanolamine)lead(II), Pb(tea),'+ 3 was used as the lead precursor. The complexation of the lead precursor enhanced the growth rate. Furthermore, the orien- tation of the films was strongly affected by the precursor. The PbS thin films grown on several different substrates were found to be cubic and showed remarkable [200] orientation. The films also exhibited a rather large crystallite size. On the other hand, the morphology and the grain size of the film were not affected by the complexation of the lead precursor when compared to the films deposited from the uncomplexed lead acetate precursor.The stoichiometry of the films was PbS, but according to RBS the films contained some light-atom impurities. Dr. E. Rauhala and Mr. P. Haussalo (the Accelerator Laboratory of the University of Helsinki) are acknowledged for the RBS and NRA measurements. Facilities provided by the Department of Electron Microscopy at the University of Helsinki were exploited for SEM characterization. The work was supported financially by the Academy of Finland and Technology Development Center, TEKES, Helsinki, Finland. References 1 M. Isshiki, T. Endo, K. Masumoto and Y. Usui, J. Electrochem. SOC.,1990, 137,2697. 2 Y. S. Sarma, N. K. Misra and H. N. Acharya, Znd. J. Phys. A, 1989, 63,445. 3 P. K. Basu, T. K. Chaudhuri, K. C. Nandi, R. S. Saraswat and H.N. Acharya, J. Muter. Sci., 1990,25,4014. 4 P. K. Nair, V. M. Garcia, A. B. Hernandez and M. T. S. Nair, J. Phys. D: Appl. Phys., 1991,24, 1466. 5 P. K. Nair, M. T. S. Nair, A. Fernanadez and M. Combo, J. Phys. D: Appl. Phys., 1989,22,829. 6 K. Ito and S. Tamaki, Tech. Char. Synth. Inorg. Muter. Lett., 1991, 10, 1395. 7 T. Kanniainen, S. Lindroos and M. Leskela, in Advances in Inorganic Films and Coatings, vol. 5, Advances in Science and Technology. Proceedings of the Topical Symposium 1 of the Forum on New Materials of the 8th CIMTEC World Ceramic Congress and Forum on New Materials, ed. P. Vincenzini, Techna Srl, Faenza, 1995, p. 291. 8 V. P. Tolstoi, Russ.Chem. Rev. (Engl. Transl.), 1993,62,237. 9 M. Takahashi, Y. Ohshima, K.Nagata and S. Furuta, J. Electroanal. Chem., 1993,359,291. 10 M. Leskela, L. Niinisto, P. Niemela, E. Nykanen, P. Soininen, M. Tiita and J. Vahakangas, Vacuum, 1990,41,1457. 11 E. Nykanen, J. Laine-Ylijoki, P. Soininen, L. Niinisto, M. Leskela and L. G. Hubert-Pfalzgraf, J. Muter. Chem., 1994,4, 1409. 12 H. Zogg, C. Maissen, J. Masek, T. Hoshino, S. Blunier and A. N. Tiwari, Semicond. Sci. Technol., 1991,6, C36. 13 Y. F. Nicolau, Appl. Surf. Sci., 1985,22/23, 1061. 14 S. Lindroos, T. Kanniainen and M. Leskela, Appl. Surf. Sci., 1994, 75,70. 15 Y. F. Nicolau and J. C. Menard, J. Cryst. Growth, 1988,92, 128. 16 Y. F. Nicolau and J. C. Menard, J. Appl. Electrochem., 1990, 20, 1063. 17 Y. F. Nicolau, US Pat., 4675207, 1987. 18 V. V. Klechkovskaya, V. M. Maslov, M. B. Muradov and S. A. Semiletov, Sov. Phys. Crystallogr. (Engl. Transl.), 1989, 34, 105. 19 K. Singh and M. Singh, Ind. J. Chem. A, 1982,21, 595. 20 E. Rauhala, J. Keinonen, K. Rakennus and M. Pessa, Appl. Phys. Lett., 1987,51,973. 21 H. J. Whitlow, J. Keinonen, M. Hautala and A. Hautojarvi, Nucl. Instr. Methods Phys. Res; Sect. B, 1984,5, 505. 22 Z. X. Sun, W. Forsling, L. Ronngren, S. Sjoberg and P. W. Schindler, Colloids Surf., 1991,59, 243. 23 D. Fornasiero, F. Li, J. Ralston and R. St. C. Smart, J. Colloid Interface Sci., 1994, 164,333. 24 H. Elabd and A. J. Steckl, J. Appl. Phys., 1980,51, 726. 25 D. Fornasiero, F. Li and J. Ralston, J. Colloid Znterface Sci., 1994, 164, 345. 26 Joint Committee on Powder Diffraction Standards, Card 5-592 27 V. Kokozay and A. Sienkiewicz,J. Coord. Chem., 1993,30,245. 28 H. Rahmanai, H. J. Gray and J. N. Zemel, Thin Solid Films, 1980, 69, 347. 29 A. N. Buckley and R. Woods, Appl. Surf. Sci., 1984,17,401. Paper 5/04104J;Received 26th June, 1995 164 J. Muter. Chem., 1996, 6(2), 161-164

ISSN:0959-9428    

DOI:10.1039/JM9960600161

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Microstructure of superconducting copper oxycarbonate thin films of the Ba-Ca-Cu-C-0 system Maryvonne Hervieu, Bernard Mercey," Wilfred Prellier, Jan L. Allen, Jean-Franqois Hamet and Bernard Raveau Laboratoire de Cristallographie et Sciences des Matkriaux, ISMRA, Universitk de Caen, Boulevard du Markchal Juin, 14050 Caen Cedex, France The microstructural state of a-axis-oriented superconducting oxycarbonate films, grown by pulsed laser ablation, has been studied by high resolution electron microscopy (HREM). The member rn =3 of the structural family (CaCuO,),( Ba2Cu02C03), is preferentially stabilized, in agreement with the nominal composition of the target, but rn' members ranging between rn' =1 and 11 have been identified. Different types of domain boundaries, mainly parallel to {loo}, or { 1 lo},, involve local distortions of the framework.These non-stoichiometry features were analysed, and were found to involve complex structural mechanisms such as variations of the cationic distribution within the different types of layers, intergrowths perpendicular to the layers, and intercalations of [CaCu,O,] and [Ca,CuO,]-type layers. Laser ablation has been revealed as a very promising method for the stabilization of new phases which cannot be pre- pared according to the usual solid-state chemical processes. Sm, -xSrxCu02.5 -x/2+d and La,BaCu,O,,+, perovskite-related phases have been synthesized recently using this The deposition of unusual species is an interesting challenge for conceiving new materials and for understanding the con- ditions of their stabilization.Recently, superconducting thin films of copper oxycarbon- ates have been synthesized for the first time, using laser ablati~n.~The film consists of an intergrowth of several members of an original structural family with the general composition (CaCuO,),( Ba,CuO,CO,),; the main member of the family which forms the matrix has rn= 3 and n= 1, which corresponds to Ba,Ca3Cu408C0,. This species can be described as a double layer of Cu04 groups sandwiched by pyramidal CuO, layers and interconnected with carbonate layers (Fig. 1). The characterization of these new materials, especially their microstructural states compared to those of the bulk samples prepared by solid-state reactions, is important for understanding the structural mechanisms governing the formation of copper oxycarbonates, which are as yet ill-known.This paper presents the microstructural features which have been observed in superconducting oxycarbonate thin films. Experimental The target preparation and the conditions of the film deposition have been previously described., The substrate was a (001) single crystal of LaAlO,; the film was deposited at 625"C, at a pressure of 0.2mbar with a gas mixture of 6% co2-94% 0, followed by slow cooling in 500 mbar of 02.The high resolution electron microscopy (HREM) was performed with a TOPCON 002B instrument, with a point resolution of 1.8 A; the thin film was observed along a direction perpendicular to the substrate. Energy dispersive scattering (EDS) analyses were systematically performed. The resistance measurements showed a broad transition with the T,onset ranging from 80 to 100 K and a T,(R=0)of 58 K.Results A first investigation3 of the thin film showed that the material is characterized by a structure directly related to an oxygen- deficient perovskite: slices of variable thicknesses of the so-called 'infinite layer structure' CaCuO, are stacked along c; in the electron diffraction (ED) patterns, streaky lines are observed along c* due to the variations of the slice thickness, but nodes are observed, generated by the frequent stabilization of one of the members of the structural family. For a nominal composition Ba2Ca,Cu,0g of the target, the mean rn value is 3.The formation of 90" oriented domains is also a characteristic of the material. This is clearly evidenced in the ED patterns recorded along the direction perpendicular to the substrate (Fig. 2); the patterns systematically exhibit a perovskite-type subcell with two perpendicular streaky lines, corresponding to the c* axes of the two 90" oriented components. The reconstruc- tion of the reciprocal space as well as the XRD patterns confirm that the parameter along tFe direction perpendicular to the substrate, a, is close to 3.86 A. The contrast of the enlarged HREM images was inter-pretated, in a first step, on the basis of experimental images previously reported for copper oxycarbonate~~ and infinite layer structure^;^ it allowed a structural model to be proposed3 based on the intergrowth of CaCu0,-type slices separated by one layer of carbonate groups sandwiched between two barium layers.The formula (CaCuO,),( Ba,Cu02C03), has been pro- posed to express the structure of these oxycarbonates. The idealized structure of the [m=3, n =11 member, which is the most frequently observed, is given in Fig. 1. Image interpretation In order to interpret the numerous features which were observed for this film, simulated images were calculated, using the MacTempas program and varying the focus values and the thickness of the crystal. The positional parameters which were used were deduced from the theoretical model proposed in Fig. 1; they are given in Table 1.The microscope parameters are V=200 kV and the coefficient of spherical aberration is Cs =0.4 mm. The focus series calculated for a crystal thickness of 31 A ds given in Fig. 3 (a)for focus v!lues ranging from 200 to -950 A. For a focus value of -250 A, close to the Scherzer value, the cations positions are imaged as dark spots and the rows of brightest spots correspond to the positions of the oxygen vacancies within the [Ca], layers. For a focus value close to -550 A, the positions of the cations appear as bright spots, the two brightest ones being correlated to the barium positions. Note that, as previously rep~rted,~ the carbonate layers exhibit a typical contrast !or different focus values,f, close to 100, 0, -150 and -850 A; it consists mainly of single rows of very J.Muter. Chern., 1996, 6(2), 165-173 165 suhst rat e 'substratef+substrate Fig. 1 Idealized drawing of the Ba,Ca,Cu,O,CO, structure, deposited on an (001) single crystal of LaAlO,. The film is a-axis oricnted. Fig.2 Enlargcd [OOl] ED pattern of the oxycarbonate film; the intense reflections are those of the perovskite subcell. The streaked lincs are correlated to the existence of different periodicities along c; the nodes correspond to the 001 reflections of the m=3 member. Only one set of reflections has been indexed; the other set is oricntccl at 90 to this set. bright spots and is maintained for higher thickness values [Fig. 3(h)]. Two typical images are given as examples in Fig.3(c) and (d), with the m values indicated by white numbers. In Fig. 3(c), the cation: are imaged as white spots for a focus value,j, close to -550 A: for m= 3, double barium layers (large white spots) Table 1 Ba,Ca,Cu,O,CO, : pptional prameters of the idealid tetragonal structure," a=3.86 A, c= 17.5 A atom x J' 0.0 0.0 0.389 0.0 0.0 0.0 0.0 0.0 0.189 0.5 0.5 0.094 0.5 0.5 0.289 0.5 0.0 0.094 0.5 0.0 0.289 0.5 0.5 0.389 0.747 0.5 0.5 0.5 0.5 0.5 Space group P4,mmm, oxygen atoms located in ideal positions. Occupancy factors have been fixed to 1, except for O(4) which is supposed to be in a split position (z=0.25). are intercalated between quadruple copper layers (small white spots). In Fig. 3(d),the cations are imaged as dark spots for a focus value close to -250 A.so that the C03groups are easily identified as rows of small grey spots surrounded by four bright spots [see Fig. 3(a)]. Thus, the good fit between the experimental and calculated images confirms the structural model (Fig. 1) according to the following sequence of layers 166 J. Murer-. C~W/H..1996, 6(2), 165-173 Fig. 3 Cdlculdted imdges of Ba,Ca,Cu,O,CO, The positional parameters are taken from Table 1 (a) focuo seriec for d thickness, t. of 31 4 (the projected potential is indicated beneath each image, (h) thickness oeries for AfzO. Experimentdl imagex recorded for (0A/= -550 A and (d)A/= -250 A stacked along c: Ba0-CO-Ba0-Cu0,-Ca-Cu0,-Ca-CuO,-Ca-CuO,-BaO. The [CuO,] layers with adjacent [BaO] layers form pyrami- dal CuO, layers, whereas the [CuO,] layers sandwiched by calcium layers consist of CuO, square planar groups.The CO, groups are intercalated between two successive [BaO], layers. The parameters of the different members can, therefore, be easily estimated in a tetragonal cell, according to the fcllowing relations: 11 2h =up;c' 2 +rn x cCcz(7.9+y12 x 3.3)A where(aBCC cBCCis the c parameter of Ba,CuO,CO,, which is close to 2 x ap, and ccc is that of the infinite layer structure CaCuO,, which is close to 3.3 A. Film morphology a-Axis orientation of the film. The a axis orientation of the film implies that the carbonate layers and the copper layers are perpendicular to the substrate (Fig.1). Referring to the infinite layer structure films" where the copper layers have been observed parallel to the substrate (in agreement with the good fit of the a and b parameters of the film and the substrate), this can be considered a rather surprising point. Such an orientation may simply originate in the deposition temperature. which has been observed to be a highly critical factor; such behaviour could be compared to that previously observed for the 123 superc~nductor;~ however, deposition experiments show that they differ at higher temperatures since complete destruction of the oxycarbonate film is observed. A second factor could play an important role; i.e., the CO, partial pressure, which would favour the presence of carbonate groups in every structural level, i.t..in every perovskite-type layer parallel to the substrate plane. Mosaic-like morphology. The overall images show a mosaic-like film morphology which results from the existence of 90" oriented domains. An example is given in Fig. 4. The formation of 90' oriented domains is a frequent feature in ordered perovskites, resulting from the establishment of the structural mechanism along one of the equivalent directions J. Meter. Clienz., 1996, 6(2), 165 173 167 Fig.4 Overall [OOl] image of the film. The boundaries are mainly parallel to {loo}, or { llO},. Three types of boundary are observed, depending on the orientation and of the way the junction between the different layers is formed; they are marked by black numbers on white arrows.of the perovskite subcell, provided that the structure does not involve a strong distortion of the framework. Two ex- amples are well known in the layered cuprate compounds i.e. the twinning domains in YB~,CU~O~-~and Bi,Sr,Ca, -1C~,02m+ ,. This phenomenon is rarely observed in the thallium- and mercury-based copper oxycarbonates [1201][S2CC], but it is a systematic feature for the 123-type oxycarbonates, YCaBa,Cu,(N0,),(C03), -,Oil and (Y, -xCax)(Ba,Sr)2,Cu3,-107n-3C03.4'8In both compounds, the orientation of the domain boundaries was observed to be mainly parallel either to the { 110}, or to the {loo}, planes of the perovskite subcell. In the present film, three types of boundaries are observed (Fig.4). Boundaries 1 and 3 are parallel to {loo},, i.e. parallel or perpendicular to the carbon- ate layers, while boundary 2 is roughly parallel to { 1 i.e. oriented at approximately 45". The atomic arrangement at the level of {loo), junction is rather simple. For boundary 1, the 90" oriented copper layers are directly connected to one another as shown schematically in Fig. 5(u),whereas for boundary 3 the junction consists of a triple [BaO-CO-BaO], layer built up from one CO layer sandwiched between two BaO layers [Fig. 5(b)]. In fact, such junctions involve local distortions due to the mismatch between the layer spacings of the two adjacent domains. This is illustrated in the enlarged image of the type 1boundary (Fig. 6) which evidences a significant buckling of the layers at the junctions; this allcws the mismatch between the Cu-Cu distance: (ca.3.3 A along c,) and Cu-0-Cu distances (ca. 3.9 A along b,) to be compensated. For the { 110}, junctions (labelled 3 in Fig. 4), the situation is more complex and depends on the members which are connected on both sides of the boundary. In contrast to the {loo}, junctions, the { 1lo}, are not really planar; their nature depends on the thickness of the copper layers (m value) which may differ from one domain to the other. An example is shown in Fig. 6(u), where the { 1lo}, boundary (indicated by the dashed line) separates two domains, A and B. Starting from the upper left part of the image, the sequences of the different members, marked by black numbers, are: domain A, 5-6-6-2; domain B, 3-2-2-2-3-5.The [BaO-CO-BaO] triple layers (marked by white arrow- heads) intersect at the level of the boundary; no strong local distortion is needed for such a configuration. A model can thus be proposed to illustrate the way the different layers are connected through the boundary [Fig. 6(b)]: the [Ba0lm layers are 90" oriented and the junction between the 90" 168 J. Muter. Chem., 1996, 6(2), 165-173 Fig. 5 Idealized drawing of the layers junctions between {loo}, boundaries: (a) boundary marked 1: copper layers are connected to copper layers; (b) boundary marked 3: one [Ba0-CO-BaO] triple layer borders on the boundary oriented square-planar CuO, group layers can be effected through copper-oxygen octahedra and pyramids.Note that for this ideal {110}, boundary, the interlayer distances of domains A and B coincide so that one does not observe any mismatch at the junction, in contrast to that observed at the {100}, boundary. The only distortion would result from the existence of one CuO, pyramid or one CuO, octahedron to effect the change of the direction of the copper layers. When the [Ba0-CO-BaO] triple layers intersect closely, but not exactly at the same level, one observes shifting and bending of the layers in order to ensure a direct intersection. Two examples are indicated in Fig. 6(a). In the bottom part of the image, one m =2 and one rn =6 member of domain A must connect with one m=2 and one rn=3 member of domain B.In the upper part, one m =5 and one m =6 member of A have to be connected to two rn =2 members and one m =3 member of B. The differences between the m values of the two domains imply that the [BaO-CO-BaO] triple layers cannot intersect at the level of the boundary. In such a configuration, complex phenomena like shifting, bending and interruption of the layers facilitate a direct connection. In the upper part, the [BaO-CO-BaO] triple layers are shifted so that the rn=2 and rn= 3 members of B are transformed into m= 1 and m=4 members, whereas in the bottom part of the image, the layers are bent (curved arrow). Lastly, when the m values of the A and B domains are very different, e.g. m=6 with m=l and m=2, the [BaO-CO-BaO], triple layer is simply interrupted at the level of the boundary (marked by a black arrowhead).The distortion of the matrix due to the translations and bending of the [BaO-CO-BaO] triple layers, results very often in small mis-orientations of the adjacent domains. In this example, the c axes of domains A and B are no longer perpendicular, the angle is close to 94" and area B is slightly tilted. Fig. 6 (u)[OOl] image illustrating the connection of the layers through one { 1l0lp boundary (type 1, marked in a white arrow) and one (loo), boundary (type 2, marked in a white arrow). The {110}, boundary is drawn as a dashed line and the 111 values of the different members are indicated as black numbers. The domains are labelled A (left) and B (right).(h)Idealized drawing of the layer junctions through a { 1lo), boundary.Different members of the Ba-Ca-Cu-C-0 system As observed in Fig. 3 and 4, for example, the origin of the streaks observed along the c* axis is undoubtedly correlated to the existence of coherent intergrowths along c, of different m and ii members. The average /nand n values of the matrix, determined from the images of different areas of the film, are 3 and 1, respectively [Fig. 3(c) and (d)]. Members with /TI ranging from 1 to 12 and ti= 1 have been identified; the two limiting members, /?I= 1 and nz= 11, are shown in Fig. 7(u) (the white numbers in the black circles correspond to the IZI values). Note that the high-m members often arise from a structural mechanism which simply corre- sponds to the replacement of one [BaO-CO-BaO] triple layer with a [Ca-Cu0,-Ca] layer.This is illustrated in Fig. 7(u) where the carbonate layers are imaged as rows of very bright spots. It can be seen (white arrowheads) that most of the larger members result from the replacement of three adjacent [BaO-CO -BaO] layers by three [Ca-Cu02-Ca] layers; in this way, two adjacent members, wzl and m2,are replaced by a larger member, 111 =IZZ~+tn2+2. ((I) 1Fig. 7 Overall image showing the existence of I and I?? 111 members (Lvhite numbers). The white arrowheads indicate the formation of large 111 values as ;I result of the substitution of a [BaO CO BaO] triple layer by a [Ca-CuOz-Ca] triple layer. (h) Enlarged image of such a substitution, with one HI=1 and one 111=3 mcmber repliiced bj one IH =6 member.((,) Idealized drawing of the mechanism. Such a replacement poses the problem of the carbonatei copper layer interconnection because two [BaO] layers ate 3.9 A apart whereas two [Ca] rows are separated by 3.3 A. The way this replacement is effected is shown in the enlarged image of Fig. 7(h).where the cation positions are imaged as bright spots: one IH =1 and one rn=3 member are transformed into an nz=6 member. It can be seen that the rather important difference of interlayer distances is easily accommodated within the matrix by a smooth buckling of the adjacent layers. The idealized arrangement is shown in Fig. 7(c).Such an accommo- dation illustrates the great flexibility of the framework.This remarkable adaptability of the framework originates other types of non-stoichiometry phenomena. Tu7o such phen- omena are shown in Fig. X(rr), indicated by white arrowheads and small arrows, where carbonate layers are subjected to 1ocal interruption and/or trans la t ion mec h a11isms. First. .I. ,‘Lltrter. Chm., 1996, 6(2), 165-I73 169 Fig. 8 ((I) [OOl] image where the carbonate layers are interrupted (white arrowheads) andlor translatcd (small whitc arrows). Idealized drawing of the two types of defects: (h)interruption; and (c) shifting. [BaO-CO-BaO] triple layers are replaced, along b, by [Ca-Cu0,-Ca] layers according to the mechanism described above, but the event extends over a segment which is only a few polyhedra long [see white arrowheads in Fig.X(u)]. It involves the local formation of a mixed carbonkopper layer [Fig. 9(h)]; this point will be discussed further. For the second mechanism [small white arrows in Fig. 8(a)]. the [BaO-CO-BaO] triple layers are shifted by upalong c so that one barium layer out of two remains unchanged whereas the other is connected to a calcium layer and the carbon layer is connected to a copper layer [Fig. 8(c)]. In a number of places. the two barium laycrs surrounding a carbon layer, are translated simultaneously in opposite directions; an example is observed in the bottom part of the micrograph [curved arrow in Fig. 8(u)];such shirtings and interconnections of the layers often involves the formation of ill-ordered slices.Cationic distribution Owing to the nature of the different layers. as seen above, cationic exchanges between Ba and Ca on one hand, or between Cu and C in the layer located between the two [BaO] layers, on the other hand. can be expected. Such points are important if one takes into account the fact that supercon- ducting properties could be relatcd to copper-for-carbon substi- tution or carbon deficiency. as was discussed previously.3 Two examples of variations of the cationic distribution are shown in Fig. ~(LI)and (h)which correspond to the same area recorded with two difl'erent focus values (-550 and -250 A, respect-ively); in the first one, the cations are imaged as bright spots [Fig.9(u)] whereas in thc second one.they appear as dark spots [Fig. 9(h)]. In the bottom part of the micrographs, the contrast observed Fig. 9 [OOl] images of-an areit where cation suhctitutions are observed: ((I) Af? -550 A and (h)Af. -250 A. (c) Calculated images comparing a carhonatc layer [CO] with a copper layer [CumO] intercalated between the two [BaO] layers (crystal thickness =31 A). at the level of the [CO] layer varies locally (white arrowheads). In Fig. 9(u), the small grey spots sandwiched between the barium rows (brightest spots) corresponds to carbon positions; at the level of the defect. two grey spots are replaced by brighter spots (white arrowheads). This suggests that carbon is locally replaced by a heavier atom, which is supposed to be copper.In Fig. 9(b), the phenomenon is reversed and two darker spots (white arrowheads) are observed at this level. Such an observation is consistent with the existence of mixed 'C-Cu' layers. 'Theoretical images. calculated on the basis of a full copper and a full carbon occupancy, keeping the BaO-BaO interlayer distance constant indeed allows the two species to be differentiated as shown for two focus values Af= -250 and -550 A [Fig. 9(c)],in agreement with previous studies of 123-oxycarbonates.' Therefore. this defect can be interpreted as the replacement of two adjacent rows of carbonate groups by two rows of CuO, pyramids [Fig. 10(tr)]. Fig. 10 ((11 Tdealrmi model of the defect correspondiilg to 'I local Cu for C submtution 01) ~ded~l/Kd model of the [RaZCuO,CO,], [Bn('uo,]2 defect Fig.11 ([I) [OOl] images. The curved arrow indicates an area where a regular Cu for C substitution is observed; the new periodicity, 3 x tip, is observed between the small white arrows. (h)Idealized model. In the areas indicated by curved arrows in Fig. 9(a) and (h), one observes a variation of contrast at the level of the calcium atoms; the small dots correlated to the Ca positions are replaced by very bright dots in Fig. 9(a) and by very dark dots in Fig. 9(b) (both similar to those observed for barium positions); simultpeously, the interlayer distance is increased from 3.3 to 3.8 A and the contrast at the level of copper atoms is not modified. These observations suggest that the sequence Ba0-CuO2-Ca-CuO,-Ca-CuO2-Ba0-CO is re-placed by Ba0-Cu0,-Ba-CuO2-Ba-CuO2-BaO-CO i.e., the [Ba,CuO,CO,], [BaCuO,], member of the structural family is formed.Thus. at this level, the double oxygen-deficient perovskite layer [CaCuO,], built up from one [CuO,], layer of square-planar groups sandwiched by two pyramidal copper layers is interrupted. and replaced by a double perovskite layer [BaCuO,],, which exhibits an infinite layer type-structure,8 as schematized in Fig. lO(h). The local substitution of copper for carbonate groups is also evidenced in Fig. 11(a). In this figure, where the cations are imaged as dark spots and the carbonate rows as rows of bright spots, one can observe a zone where the variation of contrast at the level of the carbonate row is more frequent, and consists of the disappearance of the bright spots which are replaced by grey spots.At the level of the curved arrow. this substitution is periodic and corresponds to a tripling of the h parameter, i.e. 3 x ap along b (between the two small white arrows): two grey spots alternate with one bright spot, so that locally two carbonate rows out of three are replaced by two rows of CuO, groups. The corresponding idealized model is shown in Fig. 11(h).The junction between one carbonate group and one copper square-planar group involves the formation of one CuOs pyramid without any variation of the oxygen content, as previously shown in the 123 Such a feature corresponds to the local stabilization, ordered or not, of [Ba,Cu, +x02(C03)1 ,][CaCuO,],,, members, which, from the ~ charge-balance point of view, involves an increase of the copper valency.Intergrowth with other structural units The forlnation of extended defects resulting from the insertion of a structural unit which is different from that of the mother structure, has been observed in the matrix. ‘CaCu,O,’ layer. The first example is shown in Fig. 12(a), in which the position: of the cations are imaged as bright spots (Af’close to -550 A). In the regular matrix, the rows of bright spots running alorg c (corresponding to copper layers) are separated by 3.86 A, and the two successive rows of grey spots Fig. 12 (ti) [OOl] HREM image: the formation of double copper layers appears as elongated bright spots (indicated by curved arrows).(h)Projection of CalCuO,, perpendicular to the square-planar CuO, groups. (c)Idealized model of the defect. viewed along a. (corresponding to calcium layers) are 3.86 A apart. At the level of the defect (curved arrows), one observes the formation of large bright rows running along c; the variation of contrast arises at the level of ;i [CuO,] layer. with elongated bright spots (correlated to copper positions) coupled with a significant increase of the interlayer distances: two successive rows of grey spots (Ca layers) whith sandwich a brighto row (copper) are now separated by 5.7 A. instead of by 3.86 A. Such geometrical characteristics suggest the existence of a double [(CuO)-] layer built up from edge-sharing CuO, square-planar groups; this structural unit corresponds to a CaCu,O, layer;” the idealized model of the defect viewed perpendicularly to the square-planar groups is shown in Fig.12(h). Taking into account the orientation of the infinite J. hlater. C’hcw., 1996. 6(3). 165-173 171 layer structure in the film, one [C~CU,O,] layer will b: observed along that projection as two adjacent Cu rows, 1.9 A apart [Fig. 12(c)]. In order to confirm the interpretation of the contrast at the level of the Ca and Cu layers, images calculations have been performed with positional parameters deduced from the theoretical model proposed in Fig. 12(c);the projection of the cations is shown in Fig.13(u). This structure is built up from the intergrowth of one CaCu,O,-type layer with two [CaCuO,] layers ('infinite layer' structure). The image calculated for similar focus (close to -550 A) and thickness values [Fig. 13(b)] is in agreement with the exper- imental contrast. The formation of such double copper layers within the oxygen-deficient perovskite matrix can be compared to what happens in the 123 compounds, where similar features were originally observed in the form of defects" and, until they were characterized in the form of a single phase, the '124' and related compounds." One can observe that shiftings or interruptions of the [BaO -CO-BaO] triple units, similar to those described in the previous section. are also observed through the [CaCu,O,] triple layer.[Ca,CuO,] layers. A second example of additional layers is shown in Fig. 14(u), where the cations are imaged as dark spots; the carbonate rows appear as rows of very bright spots and the smaller bright dots are correlated to the zones of weak potential located between the calcium atoms. At the level of the defects (curved arrows), one observes the formaiion of double rows of bright sppts, with spacings of about 2.2 A along band translated by 1.9 A along c,which suggests the existence of double calcium rows; such a contrast is usually observed in Fig. 13 ((I) Projection of the structure built up from the intergrowth of one [CaCu,O,] unit with [CaCuO,], units. The doublc copper layers areo indicated byo curved arrows.(h) Calculated image (thick- ness = 31 A, Afz~ 550 A). 172 J. Muter. Chenz., 1996, 6(2), 165-173 Fig. 14 (a) [OOl] HREM image: the forination of double calcium layers appears as two rows of staggered bright dots. (h) Idealized model of the defect. thin films of perovskite-related phases' and is easily interpret- able. It is correlated to the local formation of rock-salt-type layers within a perovskite-type matrix, as shown in the idealized model of Fig. 14(h). Thus, locally the structure may consist either of Ca,CuO,-type ribbons built up from CuO, groups intergrown with CaO rock-salt layers, or of La,CuO,-type ribbons built up from octahedral [CuO,] layers intergrown with CaO rock-salt 1a~ers.l~ To effect the formation of rock- salt-type layers, one can assume that oxygen is located in the two adjacent calcium layers [(CaO),] and, therefore, that the adjacent square groups are tilted 90" with respect to the CaCuO, matrix.Here again, the defect is associated with shiftings or interruptions of the [BaO-CO-BaO] triple layers. The formation of rock-salt-type layers as extended defects has been previously observed in perovskite-like matrices. In bulk material^,'^ the defective rock-salt layers form generally infinite straight or broken (with 90'' angles) lines. In thin films, the rock-salt units are generally limited to segments of a few nanometers in length; such limited lengths imply that one perovskite layer is directly connected to one rock-salt layer, in spite of the structural differences (interatomic distances and atomic coordination); however, the stabilization of copper- based 'collapsed' phases, which have been recently i~olated,'~ l7 shows that such connections are accommodated without any problem, owing to the great flexibility of the perovskite framework. Concluding Remarks This study of a superconducting oxycarbonate thin film deposited on an LaAlO, single crystal has allowed the structure and microstructure of the new material to be understood; it shows that the tetragonal phase, whose parameters can be easily related to the perovskite and infinite-layer struc-ture accprding to a z h z up and c 2cBCC+m x ccc z (7.9+ m x 3.3) A, is deposited with an a-axis orientation and with b and c parallel to the [1001 direction of the LaAlO, perovskite.The alignment of the [loo] and [OOl] directions of the oxycarbonate with the [loo] direction of the substrate is ensured in spite of the mismatch between the parameters (3.78!A for aLaMO3compared to am=,=3.86A and c,,,=~= 17.8A for the film); however, as shown in the case of the a-axis 123-type films deposited on an MgO substrate," the relative orientations of the film and the substrate (bfil,parallel to bsubs) cannot be considered as the result of an epitaxy mechanism, but only as a way to minimize the energy. The a- axis orientation of the oxycarbonate film could be correlated to several factors: the thermal conditions of deposition, the C02 partial pressure or, possibly, a problem of ptting of the parametys of the substrate and of the film, 3.86 A and 17.8 5 x 3.56 A being, respectively, longer and shorter than 3.788 A.The easy formation of oriented domains could be also a consequence of this problem of mismatch between the latt- ices. The oxycarbonates such as S~,CUO~CO~'~ or the 123- related oxycarbonate~~*~and oxycarbonates of the system Ba-Ca-Cu-C-0 prepared under high as bulk materials, do not exhibit such behaviour. Further investigations of thin-film oxycarbonates deposited on various substrates will be necessary to answer this question. The numerous structural features which occur in the super- conducting oxycarbonate thin film have been analysed. They involve various structural mechanisms, such as coherent intergrowths of different members of the (CaCuO,),( Ba,CuO,CO,), family or of different structural units parallel or perpendicular to the carbonate planes, and variations of the cationic distributions within the different types of layers.The formation of intergrowths is a classical mechanism observed in the layered cuprates and is a direct consequence of the flexibility of the framework which allows, in other respects, the stabilization of such phases. The vari- ations in the cationic distribution are favoured by the ability of copper and carbonate groups to form mixed layers on one hand, and by the ability of Ca and Ba to be located between copper layers on the other hand. The defects observed in such films may significantly influence the superconducting properties of these materials.There is no References A. Gupta, B. Mercey, M. Hervieu and B. Raveau, Chem. Muter., 1994,6, 101 1. R. Desfeux, J. F. Hamet, B. Mercey, C. Simon, M. Hervieu and B. Raveau, Physica C, 1994,221,205. J. L. Allen, B. Mercey, W. Prellier, J. F. Hamet, M. Hervieu and B. Raveau, Physicu C, 1995,241,158. M. Hervieu, C. Michel, M. Huve, C. Martin, A. Maignan and B. Raveau, Microsc. Microanal. Microstruct., 1993,4,41. B. Mercey, A. Gupta, M. Hervieu and B. Raveau, J. Solid State Chem., 1995,116,37. N. Sugii, M. Ichiharu, K. Hayashi, K. Kubo, K. Yamamoto and H. Yamauchi, Physica C, 1993,213,345. J. B. Barner, C. T. Rogers, A. Inam, R. Ramesch and S.Bersey, Appl. Phys. Lett., 1991,59,742;J. F. Hamet, B. Mercey, M. Hervieu and B. Raveau, Physica C, 1992,193,465. 8 M. Yoshimoto, M. Nagata, J. P. Gong, H. Ohkubo and H. Koinuma, Physicu C, 1991,185-189,2085. 9 Ph. Boullay, B. Domenges, M. Hervieu and B. Raveau, Chem. Muter., 1993,5, 1683. 10 C. L. Teske and H. K. Muller-Buschbaum, 2.Anorg. Allg. Chem., 1969,370,134. 11 B. Domenges, M. Hervieu, C. Michel and B. Raveau, Europhysics Lett., 1987,4, 211. 12 P. Bordet, C. Chaillout, J. Chenavas, J. L. Hodeau, M. Marezio, J. Karpinski and E. Kaldis, Nature, 1988,334,796. 13 C. L. Teske, H. K. Muller-Buschbaum, 2. Anorg. Allg. Chem., 1969, 371, 325. 14 V. Caignaert, M. Hervieu, N. Nguyen and B. Raveau, J. Solid State. Chem., 1986,62, 281.15 M. Hervieu, C. Michel, A. Q. Pham and B. Raveau, J. Solid State Chem., 1993,104,289. 16 Y. Ieda, H. Ito, S. Shimomura, Y. Oue, Inaba, Z. Hiroi and M. Takano, Physicu C, 1989,159,93. 17 M. Hervieu, M. T. Caldes, D. Pelloquin, C. Michel, S. Cabrera and B. Raveau, J. Solid State Chem., in press. 18 J. F. Hamet, B. Mercey, M. Hervieu, G. Poullain and B. Raveau, Physicu C, 1992,198,293. 19 D. V. Fomichev, A. L. Kharlanov, E. V. Antipov and L. M. Kovba, Superconductivity, 1990,3,216. 20 T. Kawashima, Y. Matsui and E. Takayama-Muromachi, Physicudoubt that the ideal formula (B~,CUO~CO~),(C~CUO,)~ C, 1993,224,69. should not lead to superconductivity owing to the absence of mixed valence for copper and of hole reservoirs in the [BaO-CO-BaO] triple layers. Thus, the replacement of C03 groups by copper polyhedra, in the form of isolated defects or ordered domains as evidenced in this study, could explain the superconducting properties of these films. 21 M. Alario Franco, P. Bordet, J. J. Capponi, C. Chaillout, J. Chenavas, T. Fournier, M. Marezio, B. Souletie, A. Sulpice, J. L. Tholence, C. Colliex, R. Argoud, J. L. Baldenedo, M. F. Gorius and M. Perroux, Physica C, 1994,231,103. Paper 5/040461; Received 22nd June, 1995 J. Mater. Chem., 1996,6(2), 165-173 173

ISSN:0959-9428    

DOI:10.1039/JM9960600165

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Shearing mechanism in the Bi-Sr-Cu oxycarbonates: HREM study of a new collapsed phase Bi,,Sr2&u,, (C03),056 Maryvonne Hervieu," Maria T. Cald6s,b Denis Pelloquin," Claude Michel," Saul Cabrerac and Bernard Raveau" "Laboratoire CRISMAT, ISMRA et Universiti de Caen, Bd du Mardchal Juin, 14050 Caen Cedex, France bInstitut de CienCa de Materials de Barcelona Campus UAB, 08193, Bellaterra, Spain 'Institute de Investigaciones Quirnicas, UMSA, La Paz, Bolivia A new oxgcarbonate, Bil~Sr,9Cu12(C03)7056, has been synthesized. It crystallizes in a monoclinic cell, u =22.139(7) A,b = 5.498(2) A, c= 39.82(1)A and p= 119.45(2)", with the possible space groups A2/m, Am or A2. Its structure, determined by an HREM investigation, is derived from that of the single intergrowth (Bi,Sr2Cu06) [Sr,Cu(CO,)O,] by a shearing mechanism along the (010) plane, so that it can be described as an assembly of [2201], [S,CC], ribbons which are m= 7 CuO, octahedra wide and shifted by 12 A with respect to each other.In fact, local substitutions exist such that the phase cannot be classified as a true shear structure, and for this reason it is referred to as 'collapsed'. The detailed microstructural study allows other collapsed members (m=8) oriented domains with chemical twins to be identified. New extended defects corresponding to the intergrowth of 2201-type ribbons in the oxycarbonate matrix along [OOl] and [?Ol], are also observed. The correlation between the shearing mechanism in this structure and the amplitude of the modulation in the single intergrowth [2201],[S2CCll compounds is discussed.Two key factors are unanimously recognized as essential for high-T, superconductivity in the layered cuprates, (AO),(A'CuO,-,),: a mixed valence for copper, which can be controlled through the synthesis process or the chemical composition, and the two-dimensional character of the struc- ture which is directly related to the existence of infinite [CuO,], layers. These factors are why every event able to modify such a state, i.e. which would occur at right angles to these layers, is of fundamental importance to the understanding of the mechanisms of superconductivity, no matter how the properties (T,and J,) evolve; creation of columnar defects by heavy ion irradiation,' intergrowths of perovskite-related slice in the tubular and shearing phenomena in layer structures are three mechanisms which induce transversal modifications of the structure.Several copper-based materials have been recently disco- vered, whose structures result from (or are likened to) the periodic arrangement of shear planes in a layered mother structure. These new structures have been termed 'collapsed structures' since some of them are not strictly 'shear structures' due to complex double translation^.^,^ In this respect, the 'collapsed' cuprates and copper oxycarbonates can be classified into two large families whose parent structures exhibit single and double rock-salt layers, respectively. The first family deals with the oxycarbonates corresponding to the general formula (A,M),(Ba,Sr),Cu,(C0,)07 with A = T1, Hg and M=V, Cr,10-12 which derive from the single intergrowth of the '1201' and Sr,Cu(CO,O,)( S2CC) represented by Tlo~5Pbo.5Sr4Cu2(C0,)07'3by the application of a shearing mechanism along (100).In such compounds, two successive [1201]l[S,CC]1 blocks are simply shifted by c/2 with respect to each other, so that they can be considered as true 'shear structures'. An important characteristic of these shear-like oxycarbonates deals with the fact that then [CuO,], layers are not interrupted by the shearing mechanism. Consequently, the shear-like (100) or (110) oxycarbonates, like their parent structures [120111 [S2CC],, remain superconducting. The second family is represented by the cuprates of general -x012formula (Bi2A2Cu06),-2( Bi4+xA4C~2 +x,2) with A =Ba, Sr.8,14,15It results from shear operations along (OlO), in the 2201 structure of Bi2Sr2Cu06 + Nevertheless, the mechanism is more complex than for the first series, due to the fact that double [Bi,O,] layers replace single [TlO] oo layers.Consequently, the copper-oxygen layers are interrupted, and a variation of composition is involved, as well as a rearrange- ment of the polyhedra at the boundary between two successive '2201' blocks so that the structures can no longer be described as pure shear structures, and are better referred to as 2201- collapsed structures. A second consequence is that these col- lapsed bismuth cuprates are not superconductors.A similar mechanism applied to the 2212 cuprate Bi,Sr,CaCu,O, allows a new collapsed phase Bi16Sr28CU1706g to be generated, resulting from a double shearing operation.' In order to understand these complex shearing mechanisms and their influence upon superconducting properties, new compounds must be generated. In this respect, nothing is yet known about the possibility of creating new phases by applying shearing mechanisms to the oxycarbonates (Bi2Sr2Cu06), [S~,CU(CO~)O,]~.'~-'~ This paper is devoted to the first 'collapsed' bismuth oxycar- bonate Bi,,Sr,gC~,2(C03)7056, which derives from the (Bi2Sr2Cu06)[ Sr2Cu(C03)02] single intergrowth by a shear- ing mechanism. Results Synthesis The previous results on the 2201-and 2212-collapsed phases8,9,14.'5 show that there exists, at the level of the shearing boundaries, a local variation of the cation distribution with regard to the 2201 and 2212 mother structures. This results from the direct connection of layers of different natures; consequently, the collapsed phases are stabilized for actual compositions which differ slightly from the ideal ones calcu- lated on the basis of ideal models; they are mainly copper deficient.Starting from the composition of the mother struc- ture, Bi2Sr4Cu,(CO3)Og, we have examined different composi- tions corresponding to the cationic ratios Bi, +3r4+,Cu2-=. The samples were synthesized from mixtures of Bi203, SrCO, and CuO, pressed in the form of bars, wrapped in a gold foil and heated at 800°C in air according to a very short thermal treatment process in order to avoid complete carbon- ate decomposition.The bars were introduced at 915"C, then J. Muter. Chem., 1996,6(2), 175-181 175 after 30 min the temperature was decreased to 800 "C at a rate of 200 "C h-I, after which the samples were quenched to room temperature. The best results were obtained for the nominal composition Bi2~,Sr4~1Cu,~7(C03)08. Nevertheless, SrCO, and the oxycarbonates (Bi2Sr2Cu06),[Sr2Cu( C0,)02], were always detected in trace amounts. XRD and ED characterisation Electron diffraction (ED) was carried out with a JEOL 200 CX electron microscope fitted with an eucentric goniometer (f60"). Energy-dispersive X-ray (EDX) analyses were system- atically performed on numerous crystals with a Kevex analyser; these allowed the nominal cationic ratios to be confirmed.The reconstruction of the reciprocal space from the ED patterns indicates a monoclinic symmetry. The [0101 electron diffraction pattern is given in Fig. 1. Note that some crystals exhibit streaks along a* which have been correlated to the existence of local intergrowth defects. The X-ray powder diffraction (XRPD) studies were per- formed with a Philips vertical goniometer, using Cu-Ka radi- ation in the range 3"<28,<70" by step scanning with increments of 28 =0.02". Lattice constants were determined using the profile refinement computer program FULLPROF.I9 On the bases of the ED results, the cell parameters were :efined from theo XRPD patttrn (Fig. 2) to a=22.139(7)A, b= 5.498( 2) A, c =39.82( 1)A and p =119.45(2)",with the possible space groups A2/rn, Am and A2.Fig. 1 [OlO] Electron diffraction pattern ~.'......l'.'"'"'r.~.--.'l.~.~~~~,.I.'..'....I.."'..'.I'..'.....4000-0N0 0-r.0 HREM study The layer stacking mode along c and the structural mechanism which originates the superstructure along a have been studied by high-resolution electron microscopy (HREM). The grains which were selected for such an investigation indeed exhibited [OlO] orientation. The grains were crushed in alcohol and deposited on a holey carbon film; the study was carried out with a Topcon 002B microscope, yorking at 200 kV and with a point-to-point resolution of 1.8 A.The interpretation of the experimental images was carried out by comparing with images recorded for the 2201, 2212 and bismuth oxycarbonate phases;16-18,20-23the simulated images were calculated using the Mac Tempas program. The comparison of the overall [OlO] HREM image of B~,,S~,,CU~~(CO~)~O~~[Fig. 3(u)] with that of the oxycarbon- ate (Bi2Sr2Cu06)[ S~,CU(CO,)O~]~~ [Fig. 3(b)] which is also [2201], [S2CC], and corresponds to a single intergrowth of the 2201 and S2CC structures, shows the close relationship between the two structures. One can observe [2201], [S,CC],-type ribbons parallel to (100) that are seven octahedra wide on average, and are shifted along c with respect to each other [Fig. 3(a)]. Within these ribbons, the contrast characteristic of the [2201], [SzCC] intergrowth is clearly seen.First, a typical contrast similar to that observed in the layered bismuth c~prates~'-~~indicates the existence of double bismuth layers (black arrowheads). Secondly, a typical contrast indicates the presence of carbonate groups (curved arrow~).~~-~~ According to the different HREM studies devoted to the oxycarbonates, 176 J. Muter. Chern., 1996, 6(2), 175-181 it was indeed shown that the presence of carbonante groups (the cation positions are given in Table 1 and the oxygen in a matrix, in the form of isolated groups or complete rows, atoms were located in ideal positions with regard to the cation positions) for different focus values and different thicknesses can be easily evidenced, but only for a few focus val~es.~~-~~ The enlarged images of Fig.4 illustrate these points. In Fig.4(u), the bright spots are correlated to the heavy atom positions; the contrast can be described as two groups of rows of bright dots: the first one is a group of four rows, oval shaped, correlated to the sequence (SrO),-(BiO)T (BiO),-(SrO), along c (the [BiO] layers are indicated by small arrowheads); the second is a group of two rows correlated to two (SrO), rows. The nature of the layers located between these rows were determined by changing the focus value. In Fig. 4(b), it can be observed that the row located between the two (SrO), rows exhibits a very bright contrast (curved arrow), similar to that observed in the oxycarbonates, and correlated to the row of carbonate groups; the layers located between the two groups of strontium and bismuth rows appear as grey dots correlated to the [CuO,] layers.Then, the layer stacking along c would be (SrO)(BiO)(BiO)(SrO)(CuO,)-(SrO)(CO,)(SrO)(CuO,), which is that observed in the [2201]1 [S2CCI1 oxytarbonates. Two successive (100) ribbons are shifted by cu. 12 A (i.e. ca. 1/2 c~zzol))with respect to each other so that one 'bismuth' layer is directly connected to a 'carbonate' layer through the shearing plane [Fig. 4(b)]. In order to confirm this interpretation of the layer stacking mode within a (100) ribbon, simulated images were calculated. To determine the positional parameters, we began from a theoreti- cal model presented in Fig.5(b) which takes into account the nature of the different layers (space group A2/m) and the way they are shifted. Only the positional parameters of the cations were refined from X-ray diffraction data; in order to limit the number of variables, some x values were constrained. The theoretical images were calculated from these refined values Fig. 4 Enlarged [OlO] HREM image of Bil,Sr29Cu12(C03)7056 recorded for focus values of (a)ca. -55 nm and (b)ca. -20 nm. The calculated images are compared in the left parts of the images for the positional parameters of Table 1. (cTTion of the model used for image calculations; the refined cation positions are given in Table 1; the anion positions have been arbitrarily fixed by considering appropriate interatomic distances.of the crystal; the corresponding projected structure is given in Fig. 5(c). The images c$lculated for focus values of -55 and -20 nm (thicknessx 30 A) are shown on the left-hand side of Fig. 4(u) and (b),respectively. They confirm the layer stacking mode of each (100) ribbon which is characteristic of the [(2201)],[(SzCC)], structure, and the way they are shifted [Fig. 5(u)]. Note that such a structure was previously observed in the form of a defectz4 which originated this work. Starting from the [(2201)]1[(SzCC)]l structure [Fig. 5(a)] which is built up from the intergrowth of the BiZSrzCuO6 (2201) and SrzCuC030z( SzCC) structures, the new oxycarbonate Bi15Srz9C~1z(C03)7056[Fig.5(b)] can be described as a '[( 2201)11[( S,CC)] shear structure' characterized by (010) shearing operations arising, on average, every seven octahedra. The c parameter is similar to c(2201)(s2cc~,i.e. it roughly corre- sponds to [c~zz01)+2 The junction of the (010) ribbons, C(~&J. through the crystallographic shear plane (CSP) involves the connection of layers of different natures and the formation of 'segments' instead of infinite layers as observed in the parent structure, [2201][ S,CC]; in this way, if we consider ribbons of seven octahedra wide: (i) a (BiO), segment is connected to a (CO,), segment on one side and to a (SrO)l4 segment on the other side; (ii) every (CuOZ), segment is connected to another (CuO2), segment, leading to the formation of (CUO~),~ segments; the (CUOZ)~~ segments are connected to a (BiO), segment on one side and to a (SrO),, segment on the other side; (iii) in the same way, every (SrO), segment is systematically connected to another one, leading to the formation of (SrO)14 segments; the (SrO),, segments are connected to a (BiO), segment on one side and to a (CUOZ)~~ segment on the other side.Such an ideal structure would correspond to the composition Bil4SrZ8Cul4; however, as previously mentioned for the 2201 Table 1 Cation positional parameters used for the image calculations, space group A2/m atom site X Y Z Bi 0.0 0.0 0.211 Bi 0.146" 0.5 0.246 Bi 0.146" 0.0 0.326 Bi 0.287" 0.0 0.280 Bi 0.287" 0.5 0.361 Bi 0.4 14" 0.5 0.323 Bi 0.414" 0.0 0.375 Sr 0.293" 0.5 0.026 Sr 0.425" 0.0 0.049 Sr 0.0 0.5 0.044 Sr 0.138" 0.0 0.085 Sr 0.293" 0.5 0.126 Sr 0.425" 0.0 0.166 Sr 0.0 0.5 0.139 Sr 0.138" 0.0 0.180 Sr 0.293" 0.5 0.218 Sr 0.425" 0.0 0.258 Sr 0.138" 0.5 0.383 Sr 0.293" 0.0 0.414 Sr 0.425" 0.5 0.436 Sr 0.138" 0.5 0.487 cu 0.425" 0.5 0.496 cu 0.0 0.0 0.096 cu 0.138" 0.5 0.131 cu 0.293" 0.0 0.169 cu 0.425" 0.5 0.211 cu 0.138" 0.0 0.438 cu 0.293" 0.5 0.470 C 0.0 0.0 0.0 C 0.138" 0.5 0.035 C 0.293" 0.0 0.070 C 0.425" 0.5 0.105 " Constrained values. J.Muter. Chem., 1996,6(2), 175-181 177 Fig.5 Idealized drawing of (a) the [2201][S2CC] parent structure and (b) the Bi,5Sr29C~,2(C03),056collapsed structure, which is directly correlated to the former via a periodic shearing mechanism every seven octahedra and 2212 collapsed c~prates,'-~*'~a strong variation in contrast is observed at the level of the CSP [large arrows in Fig. 4(a)]; this suggests the existence of local cationic substitutions, such as the replacement of copper by Bi or Sr atoms at the extremities of the copper segments, and local variations of the cation environment; for this reason, the polyhedra which ensure the junction at the level of the shear plane are not well defined and are drawn in dotted lines in the idealized model of Fig.5(b).This observation is in agreement with the actual composition, which corresponds to the cationic ratio Bi15Sr2,Cu12 instead of Bi14Sr2,Cu14 for the theoretical composition. The oval shape of the bismuth segments, (BiO)7, clearly observed in the images [Fig.4(a)], may be correlated to an effect of the stereoactivity of the 6s2 lone pair of Bi3+,similar The average width of the (100) ribbons is seven octahedra, but the local stabilisation of ribbons with different widths is not a rare event; an example of an isolated rn=8 octahedra wide ribbon is shown in Fig.4(a) (see rn values in the upper part of the image). An example of crystal where the m=8 members are frequently stabilized is shown in Fig. 6(a).The schematized structure of the rn = 8 member is shown in Fig.6(b); the (100) ribbons are eight octahedra wide; the b and c parameters remain unchanged, whereas a and p are modified. For a member m of this new family, the theoretical parameters of the monoclinic cell can be royghly calculated: a ~os(fl-90)~xm x u,,/2/2; b xap,/2 z 5.4 A; c - C(2201HS2CC) = 39.5 A; tan (p-90)FZ 3 + ,/2/m. In general, the defective mem-bers rn' we have observed in the crystals differ by only one octahedron with regard to the nominal composition, i.e. rn'= to that observed in the layered 2201,2212and 2223 c~prates.~'-~~7fl. In fact, the different layers undulate with a rather large ampli-tude, due on one hand to this oval shape of the bismuth segments and on the other hand to the distortion induced by the direct junction of layers of different natures through the shearing plane; as an example, BiO, octahedra and carbpnate layers, whi$h exhibit different apical distances (dBi4 z 2.1 A and dc4 x1.3 A), are connected.This marked undulation of the layers and the complex microstructural features (see below) hinder structural refinement by the use of X-ray data. Microstructural study Three types of nonstoichiometry features have been systemati-cally observed in the crystals of this new collapsed bismuth oxycarbonate. 178 J. Mater. Chem., 1996,6(2), 175-181 The existence of oriented domains is a rather frequent feature; an example is shown in Fig. 7 (area 1). The c axes of the two domains are at ca. 93" to each other and the domain boundary varies within the matrix while remaining along an average direction which is roughly the [Sol] direction of the [( 2201)], [(S2CC)], parent structure.Although the coincidence is not perfect due to intergrowth defects, the shearing planes generally intersect at the level of the domain boundary. In this image, a second type of oriented domain is observed, in area 2. The (100) plane acts as a role mirror plane; this feature is in fact a chemical twin due to the monoclinic symmetry of the structure; an idealized model is proposed in Fig. 7(b) where it can be seen that the number of bismuth rows of each ribbon is not modified but, through the twin plane, a copper segment, 21 octahedra in width, is formed. Fig. 6 Example of an m=8 member of the family of the collapsed bismuth oxycarbonate: (a) [OlO] HREM image and electron diffraction pattern, (b)idealized structure Crystals are often striated by extended defects running along segments.At the level of the defects, indicated by white arrow- [Ool], [1001 and [ZOl]; an example is shown in Fig. 8(a). In heads labelled ‘a’ and ‘c’, one can observe the disappearance of this image, the carbonate groups are imaged as bright dots so the white segments, suggesting that carbonate groups are locally that the contrast in the thicker part of the crystal consists deleted. The [lOo] and [Ool] defects are generally connected mainly of small white segments associated with the ‘carbonate’ to each other and correspond to the disappearance of the J.Mater. Chem., 1996,6(2), 175-181 179 Fig. 7 Examples of oriented domains observed in the B~,,S~,,CU,~(CO~)~O,~crystals: (a) [OlO] image; in the area labelled 1, the two c axes are at 93” to each other and the shearing planes intersect at the level of the boundary. In area 2, the twin boundary is parallel to (100). (b) Idealized drawing of the twinning domains in area 2. carbonate groups, Pividing the crystal into Jwo parts, which are translated by 12A along c and by 5.4A along [loll with respect to each other. Taking into consideration the positions of the carbonate and bismuth layers, and these translations, a structural model of this defect can be proposed [Fig. 8(b)]. The [loo] defect corresponds to the introduction of a 2201-type layer along the (001) plane, { [1001-oriented arrowhead labelled ‘a’ in Fig.8(b));the COO11 defect corresponds to the substitution of carbonate groups by copper octahedra and the formation of a 2223-type slice parallel to (100) (arrowhead ‘c’ whose width may vary. Thus, the defective [1001 regions of dark contrast in the crystals correspond in fact to the replacement of oxycarbon- ate regions by pure bismuth cuprate-type regions as shown schematically in Fig. 8(b). The defects running along [?Ol] can be explained easily on the same basis, since due to the symmetry of the structure, similar staircase-like [(2201)], [(S,CC)], slices are observed parallelly to (102). Thus, the defects running along [ZOl] correspond to the intergrowth of additional [(2201)], slices parallel to (102).Note that the formation of such defects resulting from the intergrowth of additional [( 2201)], slices has been reported in a collapsed ferrite, Bi13Ba2Sr2sFe,306~3 whose structure results from a similar mechanism; the structure of this iron oxide can indeed be described as a double collapsed structure derived from the [(2201)], [(0201)], parent structure. Discussion The first important observation deals with the fact that shear- ing mechanisms applied to the bismuth oxycarbonate lead 180 J. Muter. Chern., 1996, 6(2), 175-181 Fig. 8 Examples of defects striating the B~,,S~,,CU,,(CO~)~O,, crys-tals. (a) Overall [OlO] image; defects running along (100) are labelled ‘a’ and those running along (001) are labelled ‘c’.They result mainly from the replacing of oxycarbonate regions by pure bismuth copper oxide regions. (b)Idealized drawing of the defects. systematically, as previously observed for the ‘2201’ and ‘2212’ collapsed phases, to a rupture of the [CuO,], layers so that no superconductivity should be expected. One indeed observes that this phase does not superconduct, in contrast to the mother structure Bi2+xSr4Cu2(C03)0816 which exhibits a T, of 30K. The rupture of the [CuO,], layers involves the formation of ‘copper tapes’ running along [OlO]. The width of these tapes, which corresponds tq 14 octahedra for the collapsed 2201 oxycarbonate (ca. 35 A) is significantly larger than that observed for the collapsed 2201 yprate Bi17Sr16CU704814 which is eight octahedra (cu.21.6 A) and close to that observed in the 2212 collapsed phase (17 polyhedra). In this respect, these new structures are intermedi- ate between the collapsed 2201 cuprates and the [22011, [S2CC], oxycarbonates16-18 which exhibit infinite copper layers instead of ‘tapes’. A second interesting characteristic of these collapsed phases deals with the oval-shaped segments observed in all the bismuth compounds. This property may be correlated to the stereoactiv- ity of the 6s2 lone pair of Bi3+, which controls the undulation of the bismuth-oxygen layers in the mother structures. It is necessary to determine whether there exists any correlation between the amplitude of the modulation in the mother structures and the periodicity of the shearing mechanism in the collapsed oxycarbonate Bil,Sr2,Cu12(C03)7056. For this purpose, we can compare the four bismuth oxycarbonates which have been prepared so far: Bi,,5Pb0.5Sr3.5C~2(C03)08;27 Bi2Sr3.5C~2(C03)08;27 Bi2+xSr4Cu2(C03)1 and-x08+s16 Bi15Sr29C~12(C03),056(this work), which are shown sche- matically in Fig.9 (where only the bismuth and carbonate layers are represented). Comparing the two oxycarbonates Bil,,Pb0.,Sr,,sCU,(C03)08and Bi2Sr3.,Cu2(C03)08 that both exhibit the same cationic ration Bi +Pb +Sr:Cu =2.75, one can 000000000 0 0 00000oo 00000 000000000 DDDDDDDDD * ooooooo~oo~ooooooo DDDDDDDDDDDDDDDDDD 000000000 0000ooooo~ooooo0000 000000000 0 0 ooooooo00ooooooo0 <-- 9.5 --> (Bi+Sr)/Cu = 3.1 ooooooo ooooooo DDVDDDD (d1 DDDDDDDooooooo ooooooo ooooooo o~ooo~oDDVDDDD <--7 --> 0000000 ooooooo Fig.9 The four bismuth-based oxycarbonates schematically drawn through the double bismuth layers (as dark spots) and the carbonate groups (as triangles) see that the first compound exhibits a non-modulated structure, whereas the second has a modulated structure with a period- icity b=9.5upJ2. When the (Bi +Sr) content increases, the amplitude of the modulation decreases as shown for Bi,+,Sr,Cu,(CO,), -x08fd,whose periodicity is only b= 8.8.uPJ2. An important feature deals with the fact that for lower (Bi +Sr) contents such as Bi2Sr3.,Cu,(C03)08 the bis- muth bilayers undulate in phase opposition [Fig.9(b)], whereas for higher (Bi+Sr) contents such as Bi,+xSr,Cu,(CO,), -x08f6,they undulate in phase [Fig. 9(c)]. This can be understood easily if we consider that when larger cations occupy some sites of the carbonate layers, the minimiz- ation of the strain between the different layers stacked along cwill occur for a centred configuration between the undulating bismuth layers and the mixed C/Bi layers. When the (Bi +Sr) content is increased further, the strains can longer more be minimized and the structure collapses as shown for B~,,S~,,CU~~O~~(CO~)~[Fig. 9(d)]; nevertheless, the bismuth and carbonate segments remain connected as in the Bi,+,Sr,Cu2(C0,), -,08,sstructure [Fig.9(c)]. Note that a similar effect is observed in the iron-based bismuth oxides (2201)(0201) where shearing mechanisms are observed with decreasing iron content.33 From the comparison of these phases which all exhibit double bismuth layers, it appears clear that the composition of every layer through the different cationic ratio, (Bi+Sr):Cu and Bi:Cu, plays an important role in the stabilisation of the different structures. The great ability of the rock-salt and perovskite layers to accommodate various cat- ions and nonstoichiometry allows large homogeneity ranges to be observed for one given structure, through the formation of mixed layers; however, the strains and the variation of the oxygen content which are then involved are accommodated up to a certain point and, beyond this point, lead to the formation of new structures.The authors are grateful to the ISC Programme of the European Commission for financial support. References 1 V. Hardy, A. Ruyter, Ch. Simon, J. Provost, D. Groult, M. Hervieu and B. Raveau, Proc VIIth Int. Workshop on Critical Currents in Superconductors, IWCC, Alpbach, Austria, 1994, pp. 1-6. 2 A. Fuertes, C. Miravitlles, J. Gonzales-Calbet, M. Vallet-Regi, X. Obradors and J. Rodriguez-Carjaval, Physica C, 1989,157,529. 3 M. T. Caldes, M. Hervieu, A. Fuertes and B. Raveau, J. Solid State Chem., 1992,97,48. 4 B. Domenges, M. T. Caldes, M. Hervieu, A. Fuertes and B. Raveau, Microsc. Microanal. Microstruct., 1992,3,415. 5 M. T. Caldes, M.Hervieu, B. Raveau and A. Fuertes, J. Solid State Chem., 1992,98,301. 6 M. T. Caldes, A. Fuertes, P. Gomez, X. Obradors and J. Rodriguez, Physica C, 1992,185-189,681. 7 M. T. Caldes, M. Hervieu, A. Fuertes and B. Raveau, J. Solid State Chem., 1992,98,48. 8 M. Hervieu, C. Michel, A. Q. Pham and B. Raveau, J. Solid State Chem., 1993,104,289. 9 M. Hervieu, M. T. Caldes, S. Cabrera, C. Michel, D. Pelloquin and B. Raveau, J. Solid State Chem., 1995,119, 169. 10 F. Goutenoire, M. Hervieu, A. Maignan, C. Michel, C. Martin and B. Raveau, Physica, 1993,210,359. 11 M. Uehara, S. Sahoda, H. Nakata, J. Akimitsu and Y. Matsui, Physica C, 1994,222,27. 12 A. Maignan, D. Pelloquin, S. Malo, C. Michel, M. Hervieu and B. Raveau, Physica C, 1995,249,220.13 M. Huvt, C. Michel, A. Maignan, M. Hervieu, C. Martin and B. Raveau, Physica C, 1993,205,219. 14 Y. Ikeda, H. Ito, S. Shimomura, Y. Oue, K. Inaba, Z. Hiroi and M. Takano, Physica C, 1989,159,93. 15 M. Hervieu, C. Michel, M. T. Caldes, A. Q. Pham and B. Raveau, J. Solid State Chem., 1993,107, 117. 16 D. Pelloquin, M. T. Caldes, A. Maignan, C. Michel, M. Hervieu and B. Raveau, Physica C, 1993,208,121. 17 D. Pelloquin, A. Maignan, M. T. Caldks, M. Hervieu and B. Raveau, Physica C, 1993,212, 199. 18 D. Pelloquin, M. Hervieu, A. Maignan, C. Michel, M. T. Caldes and B. Raveau, J. Solid State Chem., 1994,116,53. 19 J. Rodriguez-Carjaval, Proc. Satellite Meeting on Powder Diflraction of the XVth Congress of Int. Union of Crystallography, Toulouse, France, 1990, p.127. 20 H. W. Zandbergen, W. A. Groen, F. C. Mulhoff, G. Van Tendeloo and S. Amelinckx, Physica C, 1988,156, 325. 21 0.M. Kwei, J. B. Shi and M. C. Ku, Physica C 1991,174,180. 22 0.Eibl, Physica C, 1990,168,215. 23 Y. Matsui, S. Takekawa, H. Nozaki, A. Umezono, E. Takayama-Muromachi and S. Moruichi, Jpn. J. Appl. Phys., 1988,27, L1241. 24 M. Hervieu, M. T. Caldes, C. Michel, D. Pelloquin and B. Raveau, J. Solid State Chem., 1993, 108, 346. 25 M. Hervieu, C. Michel, M. Huve, C. Martin, A. Maignan and B. Raveau, Microsc. Microanal. Microstruct., 1993,441. 26 B. Domenges, M. Hervieu and B. Raveau, Physica C, 1993,207,65. 27 X. F. Zhang, G. Van Tendeloo, S. Amelinckx, D. Pelloquin, C. Michel, M. Hervieu and B. Raveau, J. Solid State Chem., 1994, 113, 327. 28 H. W. Zandbergen, W. P. Groen, F. C. Mijhoff, G. Van Tendeloo and S. Amelinckx, Physica C, 1988,156, 325. 29 Y.Gao, P. Lee, D. Coppens, M. A. Subramanian and A. W. Sleight, Science, 1988,241,954. 30 A. Yamamoto, M. Onada, E. Takayama-Muromachi, F. Izumi, T. Ishigaki and H. Asano, Phys. Rev. B, 1990,42,4228. 31 C. Michel, C. Martin, M. Hervieu, D. Groult, D. Bourgault, J. Provost and B. Raveau, Proc. Int. Symp. on Superconductivity, Hsinshu, Taiwan, 1989. 32 H. Leligny, S. Durcok, P. Labbe, M. Ledesert and B. Raveau, Acta Crystallogr., Sect. B, 1992,48,407. 33 M. Hervieu, M. T. Caldes, D. Pelloquin, C. Michel and B. Raveau, J. Solid State Chem., 1995,118, 357. Paper 5/04982B; Received 27th July, 1995 J. Muter. Chem., 1996, 6(2), 175-181 181

ISSN:0959-9428    

DOI:10.1039/JM9960600175

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Electrodeposi tion of thin-film rare-eart h-me ta1 oxocupra tes ~~ ~~ Robert Janes, Paul M. S. Monk,* Robert D. Partridge and Simon B. Hall Department of Chemistry, Manchester Metropolitan University, Chester Street, Manchester, UK M1 5GD Cathodic deposition from an aqueous solution containing the simple nitrate salts of Nd3 + and Cu2+ yields insoluble thin-film oxyhydroxide layers. Variation of the electrode substrate, solute concentrations (absolute and relative, one to another), applied cathodic potential and cell geometry allows the production of films with a wide range of precursor compositions. Conditions were optimised to allow production of a solid thin film which, on firing at 940 "C,yielded Nd,CuO,. Since the electrodeposited material comprises intimate mixtures of oxide, subsequent firing to yield the desired semiconductor requires a relatively low firing temperature.The compositions of the products formed are accounted for in terms of solubility constants and a reaction front between the electrogenerated material and cations in the bulk solution. There is presently great intere~tl-~ in fabricating high-quality superconducting thin films. A much-quoted potential appli- cation is the manufacture of passive microwave devices, e.g. for use as filters, resonators or antennae.',, Conventional techniq~es',~of forming such films include laser ablation' and cathodic or electron-beam sputtering.6 A major problem with this method is the requirement for expensive volatile precur- sors. Alternative methods include ion-beam sputtering and laser ablation, both of which require solid targets of the starting material: in some cases this is an unattainable goal for the materials of interest in this study.Relatively few workers have prepared thin-film superconduc- tors using electrodeposition. Weston et uL7 prepared Y:Ba :Cu :0 with dimethylformamide (DMF) as solvent. Their work has apparently not been developed further. Wieckowski and coworker^^*^ used aqueous solutions for electrodeposition of Y :Ba :Cu: 0 but quote no XRD data. These authors suggest that the films are of poor quality and it may be for this reason that no other workers use water as solvent. Noufi et uL1'-12 prepared T1: Ba :Ca :Cu :0 and Y :Ba :Cu :0 using dimethyl sulfoxide (Me,SO) as solvent but the films were of poor quality, contained sulfur from the Me,SO, were 'difficult to fire' and did not allow for high critical currents.Finally Abolmaali and Talbot13 prepared YBa2Cu307-, by using isopropyl alcohol and and applying a potential of -423 V to the working electrode. The form of electrodeposition used in this present study yields insoluble mixtures of oxide and hydroxide (usually of indeterminate ratio and hence are termed 'oxyhydroxide' here) following electron-transfer reactions to the nitrate counter ion. Following high-temperature firing, the solid products on the electrode are potentially superconducting. One mechanism postulated for the electrodeposition of nickel o~yhydroxide'~ involves the following series of reactions as a mechanism for the electrodeposition: SNi(OH), +5Ni0 +5H20 (3) Oxyhydroxide products will ensue if eqn.(3) does not occur quantitatively. Also, a stoichiometry of eight electrons yielding 5 NiO units will only be achieved in full if convective control is maintained. The technique of electrodeposition as a means of composi- tional control in d-block oxide materials is well established, and has been successfully employed to prepare films of mate- rials such as tungsten trioxide," cobalt oxide,16 and molyb- denum oxides17 as well as a large range of mixed-metal oxides." Recently, a series of novel (electrochromic) films have been produced comprising as many as four transition metal oxides, e.g.W :Co :Cr :Mo or W :Co :Ni :Zn oxides, either series of mixture being deposited cathodically on platinum." The compositions of the electrodeposited product are con- trolled by the deposition voltage, V,, and ratios of solute in solution. Other variables include temperature, ionic strength, solvent permittivity (adjusted by addition of organic solvent to the deposition solution) and ligation. For example, the compositions of the tungsten-nickel oxide formed by elec- trodeposition varies between W :Co =1:2.10 (when V,= -0.7 V) to 1 :1.05 (at V,= -1.3 V)." The oxyhydroxide products of electrodeposition are usually amorphous and are therefore assumed to possess a greater homogeneity, e.g. than do materials formed using bulk pro- cedures such as grinding of solid-state components, since all the precursors are wholly soluble.This follows since the mixing of solutes occurs on an atomic scale. In this work, we describe the formation of neodymium- copper phases, and show how the adjustment of physicochemi- cal parameters allows for the preparation of well-known mate- rials such as neodymium cuprate, Nd2Cu0,. Experimental Cu( NO3), and Nd(N03)3 precursors were Aldrich reagents and used in aqueous solution at a concentration of 0.010 mol dm -3 unless stated otherwise. All water was doubly distilled. Solutions were purged with N, gas prior to electrochemistry. Only results using potentiostatic (three-electrode) deposition are quoted here. The working electrode was silver and the counter electrode was platinum (both had an area of cu.2 cm2; the counter electrode was the larger). The reference electrode was always a saturated calomel electrode, SCE. All applied potentials, V,, here are cited with respect to SCE. The depos- ition current was in the range 50-100 mAcm-,, the value depending on the voltage applied. All electrochemical pro- cedures were performed on a PAR 273A potentiostat interfaced to an IBM PC-clone computer operating PAR 270 electro-chemistry software. Solutions were agitated gently using the nitrogen gas flow during deposition to maintain convective control. In all cases the cathodic limit employed represents the formation of molecular hydrogen at the cathode. No results are quoted here in which hydrogen gas did form at the cathode.Films were rinsed with water following deposition. Still solu- tions did not allow solid films to form: flocculent metal oxyhydroxide products were observed sedimenting from the electrode. J. Muter. Chern., 1996, 6(2), 183-186 183 UV-VIS spectroscopy used indium-tin oxide-coated glass (Pilkington 20 SZ sq-') as the optically transparent electrode substrate. Films were dried in a desiccator prior to firing, and then warmed in an oven at cu. 110 "C to remove occluded moisture. Such films were then fired in air in a conventional furnace for 10 min at 940 "C. X-Ray powder diffractograms (XRDs) were obtained using a Philips PW 3020 diffractometer operating with Cu-Kcr radiation. Measurements of energy dispersion by X-rays (EDX) were obtained using a Cambridge 250 electron microscope.The elemental compositions of deposited (and fired) films were determined using a Philips PU 7450 inductively coupled plasma (ICP) instrument. Films were usually dissolved in 1.0 mol dm-3 nitric acid before analysis, although concentrated acid was used sometimes to effect complete dissolution. ICP standards were matrix matched, i.e. Nd3+ standards were prepared in solutions containing Cu2+. Repeated determi- nation of composition using known solutions showed the reproducibility of the ICP technique was satisfactory (to & 1.0%), provided the ionic strength and deposition tempera- ture remained constant. ICP also showed that the film composi- tions were remarkably invariant at fixed deposition potential, V,.Results Film formation and appearance XRD analysis indicated that all electrodeposited precursor films were either amorphous or insufficiently crystalline for XRD diffraction peaks to be obtainable. Precursor films did not form during electrochemistry with still solutions, i.e. any solid generated did not adhere to the electrode. By stirring the solution, and thus increasing the availability of the electroactive species at the electrode-solution interface, electrodeposition occurred much closer to the elec- trode substrate and gave denser and thus more robust films, as discussed below. Films appeared smooth and even under standard laboratory illumination. The films often looked slightly gelatinous prior to firing.For this reason, it is possible to term the film formation process 'electrochemical gel formation' rather than electrodeposition. Electron-beam microscopy of dried and/or fired films revealed a relatively uneven texture but without any pinholes or significant discontinuities. The material chosen as the working electrode substrate also affected the adhesivity of deposited films, both before and after firing. The range of possible substrates is limited since high- temperature firing is necessary following deposition. Films on steel, platinum and IT0 were fragile and only weakly attached. In contrast, films electrodeposited on silver were considerably more robust. Film composition Initially, the composition of deposited films were determined as a function of deposition voltage, V,, whilst using a con- stant deposition-solution composition ([Nd3+] =[Cu" ]= 10mmol dm-3).The mole fractions of Nd and Cu in the deposited film changed with deposition voltage as shown in Fig. 1: as voltage (V,)becomes more negative, the mole fraction of Nd increases. Next, films were electrodeposited from solutions made with various concentration ratios of Nd and Cu, i.e. keeping [Cu2+]= 10 mmol dm-3 but increasing the Nd3+ concen- tration to a limit of 0.1 mol dmP3 (Fig. 2). Increasing the concentration of the Nd3+ ion increases its availability at the electrode-solution interface and thus enhances its chance of 184 J. Muter. Chem., 1996,6(2), 183-186 0.2 i 00 0,.a IIIIIIIIII 0!4 Ow T ' l!2 ' 1!4 ' 1!6 I l!S I 2' ' deposition voltageN Fig. 1 Plot of mole fraction against deposition voltage for solid mixed- metal oxide films. Copper and neodymium ions were both used as aqueous nitrate salts at a concentration of 10mmol dm-3. No additional inert electrolyte was employed. The electrode substrate was silver metal. 1.0 0.8 rnz 3.-0.6 8-0 c 0.4 *8 0)-0.2 0.0. l~l[l~l]l 2 4 6 8' molar ratio of Nd:Cu Fig. 2 Plot of mole fraction of Nd3+/Cu2+ in solution against the mole ratio within solid mixed-metal oxide films: solution-phase [Cu2+] was 10mmol dm-3 throughout while [Nd3+] was varied. No additional inert electrolyte was employed. The electrode substrate was silver metal.-, 1.4; ---, 1.6; -.-.-, 1.8 V. being in the mixture of oxides electrodeposited, so the mole fraction of Nd3+ increases even at constant V,. Further analyses involved electrodeposition from solutions with constant Nd :Cu mole ratios but with differing absolute concentrations. Results showed that as absolute concentration increases the mole fraction of deposited Cu increases and the mole fraction of Nd decreases (Fig. 3). By controlling all the deposition variables which affect film composition, it is possible to 'tailor' films to have the required stoichiometry. The target material, 'neodymium cuprate', of composition Nd2Cu0, may be prepared using a silver electrode held at a potential of -1.6 V while immersed in an aqueous solution comprising [Nd(NO,),] =0.125 mol dm-3 and [Cu(NO,),] =0.025 mol dm-, at 25 "C.[In fact, while the ideal composition of x(Nd) is 0.66, the mole fraction normally lies in the range 0.66 f0.005; the extremes of composition with these conditions may be considered to be 0.660<x(Nd)<0.670.1 Deposition time All films described so far were deposited for a fixed length of time (120 s). It is possible that a finite iR drop existed across 0.9 0.8 '" 0.7 8 8 0.6 c0.-c.8t 0.5 0) -0.4 f ', Nd 0.3 0 0 0.2 0.1 10 0.02 0.04 0.06 0.08 0.10 concentration/mmol dm3 Fig. 3 Plot of mole fraction against concentration for solid mixed- metal oxide films. Copper and neodymium ions were both used as aqueous nitrate salts.No additional inert electrolyte was employed. The electrode substrate was silver metal and the applied potential was -1.8 V US. SCE. the incipient thin film. The value of R will slowly increase since films grow thicker with time, thus altering the actual potential across the the solution-film interface. Accordingly, films were deposited for varying lengths of time (in the range 5 s to 10min) and the ratio of neodymium to copper ascer- tained. The variation in this ratio with deposition time was within experimental error in all cases, i.e. variation in the product iR was always small implying that films were gel-like prior to firing, rather than compact oxide. Film firing Films were fired in air at 940 "C for 10min. EDX (Fig.4) and ICP showed the post-annealed Nd :Cu ratio was maintained at 2: 1. The XRD pattern (Fig. 5) was identical to traces obtained for bulk samples of neodymium cuprate, NdzCu04, prepared using conventional sol-gel techniques. Investigation of mechanism The model of electrochemical gel formation employed here is given by eqn. (1) and (2) and uses nickel as an example. Solid Nd li cu Fig. 4 EDX spectrum of electrodeposited neodymium cuprate (Nd2Cu04) on silver after drying i1 0.20 ' 215 ' 30 ' 35 ' do ' 45 ' ! 0' 2Wegrees Fig. 5 XRD of electrodeposited neodymium cuprate (Nd2Cu0,) on silver metal. *indicates peaks due to the silver substrate. Inset: XRD of a Nd2Cu0, sample prepared using a sol-gel procedure.films were not produced when the component salts were sulfate or halide; only in the presence of nitrate were these films formed. Films could be formed in the presence of both C1- and NO,-. When ammonium ion was added to solution (as the nitrate salt) then the resultant films were much darker, occasionally appearing pink or purple. Fig. 6 shows the differ- ences by means of the (transmission) electronic spectra of films deposited with and without ammonium ions in solution: a new peak appears at ca. 400 nm which we ascribe to the formation of an underlayer (the 'ringing' effects are caused by thin-layer interference). Solutions containing NH4+ but no nitrate did not allow for film formation. These results are taken to support the idea that adsorption of ammonium ion can play a part in the electrodeposition mechanism here.There was little perceptible change in the visual appearance of the films when the ionic strength of the solution was increased by addition of extra nitrate ion, as KNO,. That nitrite can be electroreduced to form ammonium ion is well established;" and the electroreduction of nitrate to nitrite is readily achieved." Ammonium ion has not been positively identified as the product of reaction (1). The poor sensitivity of standard analytical tests is thought to be the cause of this. For example, no NH4+ was detected chemically, even after the deliberate addition of small amounts of NH4N0, to the solution. NH4+ complexation with cations is thought 300 500 700 Ilnm Fig.6 UV-VIS spectra of neodymium cuprate thin films on ITO. The deposition solution contained [Nd(NO,),] =0.05 mol dm-, and [Cu(NO,),] =0.025 mol drn-,; (a) no NH,NO, in solution; [NH,NO,] =0.1 (b),0.3 (c),0.5 mol dm-, (d). J. Muter. Chem., 1996,6(2), 183-186 185 to be unlikely since no new spectroscopic bands were formed after deliberate additions of ammonium ion to the solution. That hydroxide ion was electrochemically generated at the electrode was demonstrated by electrolysing KN03 solution in the presence of universal indicator. The indicator suggests that the pH at the solution-electrode interface was ca. 10-11. Discussion The pH at the electrode-solution interface is very high since hydroxide is electrogenerated.If efficient mixing within the depletion layer is assumed to occur but without any precipi- tation (i.e. without product loss), then a simple calculation reveals a pH of ca. 13 at the electrode-solution interface within the ‘depletion region’. (This simple model assumes a typical depletion region thickness of 50 pm and a current of 100 mA cm-2; higher currents will have the effect of raising the pH.) However, the solubility constants of the metal hydroxide species used here are small (ref. 20 cites K, {Nd(OH),} = and K, {Cu(OH),) = lo-”) so the electrode reaction (1) is assumed to be followed promptly by reaction (2),with precipi- tation of (mixtures of) solid oxyhydroxide phase(s) which, following firing, represent mixed-metal oxide product.K, for each hydroxide is so small that the amounts of solute ion remaining in solution will be quite small and with the pH remaining high. Conversely, the pH in the bulk solution is always low (in the range 3-6) since Cu2+ cations hydrolyse coordinated water, leading ultimately to increased proton concentrations. The concentration of solute in the solution bulk is not particularly changed from that used initially because the deposited films do not comprise much material. So, after generation of the first modicum of solid oxyhydroxide, the reaction between OH- and M”+ occurs at a front which falls at an indeterminate distance d from the film-solution interface (and probably the value of d increases from zero during deposition).The actual position of d depends on many physicochemical parameters, as follows. The pH derives from the concentration of hydroxide generated, cOH-,itself the quotient of the amount of OH- formed (obtained via the current flowing, I) and the volume of the depletion region around the electrode (d x electrochemical electrode area). The thickness of the depletion region depends on the rate of solution stirring and will follow an approximate form of the Levich equation. This explains why superior films are formed if the solution is stirred: the reaction front resides nearer to the electrode than when using a still solution; and the solid formed at the front has a greater chance of adhering when d is small. The Faradaic current flowing will be 8F(d [NO,-]/dt); the factor of 8 arising from the number of electrons involved in equation (1).Additionally, the current depends in a compli- cated way on the nature of the electrode-solution interface. One measure of the facility of electron transfer across the double layer is the so-called exchange current, i,, which may be treated as the rate constant of charge uptake by electroactive materials residing at the electrode-solution interface. Compendia of exchange-current data at silver (or other sub- strates) do not exist for these solutions. i, may depend markedly on the electrolyte concentration. In the absence of published data, we have measured i, (for comparative purposes) using silver and then platinum as the electrode substrate, and using identical electrolyte solutions.i, for reaction (1) was measured as follows: i, (Ag)=7.0 A m-2 and i, (Pt)=6.5 x lop2A m-2. The hundred-fold increase in i, accounts for the observation that a silver substrate produces more compact (mechanically robust) films than does platinum. It is important to deduce why the ratio of ions within the solid product is a function of the voltage applied for deposition, 186 J. Mater. Chem., 1996, 6(2), 183-186 V,. The rate of reaction (1) depends strongly on potential (following an exponential law from Butler-Volmer consider- ations) so current follows a exp(K)cci relationship; cOH-is directly proportional to i. Both Cu2+ and Nd3+ in the depletion region react with electrogenerated OH- and it will be assumed that the rates of hydroxide formation and precipitation are the same for each, being ‘instantaneous’.The amount of each ion in solution is independent of V,and K,, the solubility constant, is constant for all the solutions, but K, values for each solid differ significantly. So, the amount of hydroxide available (which depends on V,) changes and the OH-is partitioned between the two ions. Clearly, at low concentrations of OH- (less negative V,),more of the relatively soluble Nd (as aquo ion) will remain in solution, i.e. the mole fraction of copper in the solid will be larger at more positive potentials. This result is in accord with that found experimentally. It follows from the above that adjustments to the ratios of solute ions present in solution will affect the ratios of ions in the solid product. Also, if the relative amounts of each ion in solution remain the same but their overall concentrations increase, the precipitation of the least soluble metal hydroxide (in this case Cu2+)will be preferred.Thus by suitable choice of experimental variables, it is possible to prepare films of Nd,Cu,,-O having any composi- tion. We have shown that electrodeposition can yield well known and previously studied phases such as Nd2Cu0,, which are identical to samples made by other, more familiar routes. We wish to thank the EPSRC for a research studentship (R.D.P.). References 1 Science and Technology of Thin-Film Superconductors 2, ed. R. D. McConnell and R.Noufi, Plenum Press, New York, 1990. 2 M. Cyrot and D. Pavuna, Introduction to Superconductivity and High-T,Materials, World Scientific, Singapore, 1992. 3 D. B. Chrisey and G. K. Hubler, Pulsed Laser Deposition of Thin Films, Wiley, Chichester, 1994. 4 R. R. Romanovsky, NASA/Lewis Centre, Cleveland, OH, 1990. 5 D. Dijkamp, T. Venkatesin, X. D. Wu, S. A. Shaheen, N. Jisrawi, Y. H. Min-Li, W. L. McLean and M. Croft, Appl. Phys. Lett., 1987, 51,619. 6 K. Wasa, H. Adachi, Y. Ichikawa, K. Hirochi and T. Matsushima, in M. R. Beasley and the Stamford Thin-Film Group, Physica B, 1987,148,191. 7 A. Weston, S. Lalvani and N. Ali, J. Mater Sci., 1991,2, 129. 8 D. J. Zurawski, P. J. Kulesza and A. Wieckowski, J. Electrochem. SOC.,1988, 136, 1607. 9 P. Slezak and A. Wieckowski, J. Electrochem. SOC., 1991,138,1038. 10 R. N. Bhattacharya, P. A. Parilla, R. Noufi, A. Arendt and N. Elliott, J. Electrochem. SOC., 1992,139,67. 11 R. N. Bhattacharya, R. Noufi, L. L. Roybal and R. K. Ahrenkiel, J. Electrochem. SOC., 1991,138, 1643. 12 R. N. Bhattacharya, P. A. Parilla, A. Mason, L. L. Roybal, R. K. Ahrenkiel, R. Noufi, R. P. Hellmer, J. F. Kwak and D. S. Ginley, J. Mater. Res., 1991,6, 1389. 13 S. B. Abolmaali and J. B. Talbot, J. Electrochem. SOC., 1993, 140, 443 14 D. A. Corrigan and M. K. Carpenter, SPIE Institute Series, 1990, IS4,298. 15 P. M. S. Monk and S. L. Chester, Electrochim. Acta, 1993,38,1521. 16 P. M. S. Monk, S. L. Chester and D. S. Higham, Proc. Electrochem. SOC.,1994,942, 100. 17 P. M. S. Monk, T. Ali and R. D. Partridge, Solid State Zonics, 1995, 80, 75. 18 P. M. S. Monk, R. D. Partridge, R. Janes and M. J. Parker, J. Mater. Chem., 1994,4, 1071. 19 K. Ogura and H. Ishikawa, J. Chem. SOC., Faraday Trans., 1984, 1, 80. 20 R. M. Smith and A. E. Martell, Critical Stability Constants, uol. ZV: Inorganic Constants, Plenum Press, New York, 1976. Paper 5106524K; Received 3rd October, 1995

ISSN:0959-9428    

DOI:10.1039/JM9960600183

出版商:RSC    

年代:1996

数据来源: RSC