J. CHEM. SOC. FARADAY TRANS., 1994, 9012), 349-354 Stepwise Growth of Size-confined CdS in the Two-Dimensional Hydrophilic lnterlayers of Langmuir-Blodgett Films by the Repeated Sulfidation-Intercalation Technique lsamu Moriguichi, Katsuhiko Hosoi, Hidenori Nagaoka, lchiro Tanaka, Yasutake Teraoka and Shuichi Kagawa" Department of Applied Chemistry, Faculty of Engineering, Nagasaki University, Nagasaki 852,Japan When a CdS-bearing stearic acid multilayer produced by exposing a cadmium stearate Langmuir-Blodgett (LB) film to H,S gas is immersed in aqueous CdCI,, Cd ions are intercalated to regenerate the cadmium stearate multilayer without the escape of CdS. The repetition of sulfidation-intercalation cycles allowed the size-quantized CdS to grow in a stepwise fashion in the hydrophilic interlayers.The layered structure was main- tained throughout the repeated cycles, although the multilayer-constructing stearate molecules became less oriented by the formation and growth of CdS. The formation of a two-dimensional CdS plane was suggested. Nanosized inorganic semiconductor materials have been attracting much attention owing to their structural, chemical and physical properties which are somewhat different from those of the corresponding bulk materials. The most promi- nent feature of nanosized semiconductors as compared with the bulk materials is revealed in the electronic structure, that is, the blue-shifted energy gap and the discrete electronic levels.*-3 Therefore, the photophysical, photochemical and photocatalytic applications of nanosized semiconductors have been studied e~tensively.~-~ Since nanosized particles are inherently liable to aggregate or grow in order to reduce the surface energy, careful and controlled synthetic methods are required.The methodology applied so far can be conveniently classified into the follow- ing three categories: (1) Arrested precipitation in solutions by controlling solvents, concentration and temperat~re'~~ or by using stabilizers and growth-terminating reagents."?' ' (2) Stabilization of small particles in or by aggregation-preventing matrices such as polymers,'2,'3 gla~ses,'~.' sur- faces of monolayer thickness at the air/water interfa~e'~.'' and bilayer lipid membranes.I8 (3) In situ synthesis in the q2,confined spaces of zeolites,' '3,' clays,21 organized sur-factant aggregates (reverse mi~elles,~~-~' vesicle^^^.^^ and Experimental Materials Stearic acid, cadmium chloride, sodium hydrogen carbonate, benzene (Kishida Chemical Co., Ltd.), cadmium stearate (Shimakyu's Pure Chemicals Co., Ltd.) and hydrogen sulfide gas (>99.9%, Sumitomo Seika Co., Ltd.) were used as received.The water used for subphase solutions was purified by a Milli-Q system (Millipore Corp., resistivity > 14 MR cm). CaF, plates [20 mm (diameter) x 2 mm, Japan Spectro- scopic Co., Ltd.], quartz plates (10 mm x 45 mm x 1.25 mm, Fujiwara Co., Ltd.), borosilicate glass plates (76 mn x 26 mm x 1.5 mm, Matsunami Glass Ind., Ltd.) and gold plates (30 mm x 30 mm x 0.2 mm, Nilaco Corp., purity >99.95%) were used as substrates on which the LB film was deposited.The surfaces of gold plates were polished with an alumina abrasive (0.05 pm, Buehler) and were washed with pure water and then methanol (Kishida Chemical Co., Ltd.). Surfaces of other plates were washed with a dilute aqueous solution of HF (Wako Pure Chemical Industries, Ltd.), pure water and then methanol. Four substrates were used properly to meet the condition of instrumental analysis. bilayer membranes28), protein cages29 and LB filrn~.~'-~~Preparation of LB FilmsThese matrices play an aggregation-preventing role as well. Recently, some groups (including ourselves) reported the synthesis of metal chalcogenides, especially sulfides, in the hydrophilic interlayers of LB films of fatty acids and their Since the interlayers are the two-dimensional reac- tion field and the arrangement and the amount of precursor metal ions are well controlled by metal ion-binding head groups which are highly ordered and assembled, the inter- layers serve as the well restricted reaction field for the in situ synthesis of nanosized materials.The present invention relates to a new method for increas- ing in a stepwise manner the dimensions of size-quantized semiconductor particles or films in the interlayer of LB films. The method has a potential for wide applications to the prep- aration of size-quantized semiconductors with arbitrary dimensions. We have found and reported the in situ growth of size-constrained CdS in the LB film by the repeated sulfidation-intercalation te~hnique.,~In the present paper, a detailed investigation will be described on the synthesis and growth of CdS in the hydrophilic interlayers of LB films and the accompanied structure change of the LB matrix.A benzene solution of stearic acid (1 g dm-') was spread at 20°C on the surface of 3 x mol dmP3 aqueous CdCl, which was adjusted to pH 5.8 by adding aqueous NaHCO,. A separate experiment confirmed that under these conditions cadmium stearate was formed at the air/water interface after reaction between Cd2+ and stearic acid. The LB deposition was performed at a surface pressure of 30 mN m- ',at which the monolayer was in the solid condensed state, in the verti- cal mode with a combination of a film balance (Sanesu Keisoku Co.Ltd., Model FSD-20) and a lifter (Sanesu Keisoku Co. Ltd., Model FSD-23). Monolayers of cadmium stearate were transferred at a deposition rate of 10 mm min-', and the Y-type LB film of cadmium stearate was suc- cessfully built up with a transfer ratio of unity on well cleaned borosilicate glass, quartz, CaF, and gold plates. Production of CdS and Intercalation of Cd Ions The production of CdS was performed by exposure of a cadmium stearate LB film to a flow of H,S gas (105 cm3 3 50 min-') at room temperature, which is referred to as the sul- fidation (S) process. The sulfidized LB film was then immersed in 3 x lo-, mol dm-3 aqueous CdCl, which was adjusted to pH 5.8 with aqueous NaHCO,, followed by rinsing with pure water for 5 min [intercalation (I) process].Thereafter, the film was subjected alternately to sulfidation and intercalation processes. In this paper, the samples after the sulfidation and intercalation processes are denoted as S(n) and I(n) films, respectively, where n is the number of specified processes undergone by the film. It is natural that the S(n) film underwent the sulfidation process n times and the inter- calation process n -1 times, and the I(n) film underwent n sets of S-I cycles. Instrumental Analysis UV-VIS spectra of quartz-supported films were recorded on a Shimazu UV-3 100 spectrometer in the transmission mode, and the absorption onset was determined by the second derivative of the spectrum.IR spectra of CaF2-supported films were measured using a Nihon Bunko-IR-180 instrument or a Perkin-Elmer 1650 FTIR spectrometer in the transmis- sion mode. The reflection-absorption (RA) FTIR spectra were taken for Au-supported films with the FTIR spectrom- eter equipped with a specular reflectance accessory (Spectra- Tech Inc. Model 501, 85" incident angle) using p-polarized light. The sample chamber of the FTIR spectrometer was purged with a flow of dry air so as to minimize background H,O. X-Ray photoelectron spectra (XPS) of quartz-supported films were recorded on a Shimadzu ESCA-850M instrument with an Mg-Ka source (1253.6 eV). The binding energies (E,,) were calibrated with reference to the C Is line of the aliphatic carbon of stearate molecules (285.0 eV).The atomic ratios, Cd : C, S : C and Cd :S, were determined by using integrated areas, photoelectron cross-sections, and inelastic mean free paths of the C Is, S 2p and Cd 3d,,2 photoelectron lines. X-Ray diffraction (XRD) patterns of borosilicate glass-supported films were taken with a Rigaku 2034 diffractometer using Cu-Ka radiation. Results and Discussion Chemical Change during the Sulfidation and Intercalation Processes Changes of the carboxylate groups during the sulfidation and intercalation processes were followed by monitoring the IR absorption band of the C=O stretching vibration, and the formation of CdS was measured using UV-VIS spectroscopy.As shown in Fig. l(a), the original LB film of cadmium stearate (19 layers) showed an antisymmetric CO, stretching band of carboxylate ions (RCO;) at 1548 cm-'. Upon contact with H,S gas for 5 min, the band of RCO; was totally replaced by that of protonated carboxylic acids (RC0,H) at 1702 cm-', and the IR spectrum did not change on prolonged exposure to H,S [Fig. l(b)]. The change in the carboxylate groups from RCO, to RCO,H suggests that the cadmium stearate multilayer reacts with H2S to yield the stearic acid multilayer and CdS (acid-form composite film), [(RCOy),Cd]L, + H2S +[2RCO2H]L, + CdS (1) where the subscript LB refers to constituent molecules of the mu1 tilayer. The exposure to H,S gas gave rise to the appearance of an optical absorption due to CdS.The wavelength of the absorption onset (Aos) of the 19-layer film increased on increasing the exposure time to H2S (sulfidation time) and then reached a constant value after 20 rnin (Fig. 2). This J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 wavenumber/cm-' Fig. 1 Transmission IR spectra of a cadmium stearate LB film (19 layers) after exposure to H,S gas and immersion in aqueous CdCI, : (a) original LB film, (6) after sulfidation, (c) after immersion for 1 h, (d)after immersion for 3 h means that an exposure to H,S for at least 20 rnin is required to form the CdS in an equilibrium state under the present experimental conditions. As stated above, reaction (1) was completed within 5 rnin in view of the change of carboxylate groups.Accordingly, the observed spectral changes between 5 and 20 rnin are due to the growth of CdS rather than concen- tration effects; the CdS produced in the initial stages of the reaction may be in the form of small clusters (or even molecules), which grow with time. This result indicates that the dimensions of the CdS particles can be controlled by the sulfidation conditions. We reported previously34 that when the LB film of cadmium stearate was sulfidized in a flow of H2S gas (25 cm3 min-') for 15 min, the CdS formed had an absorption onset at 370 nm. In the present study, exposure to a flow of H,S gas (105 cm3rnin -') for 20 min was adopted as the sulfidation process in order to examine the equilibrium state.Note that the production and growth of CdS described below is realized even when sulfidation is stopped before reaching the equilibrium state.34 Fig. 3 shows UV-VIS spectra of 19-, 29-and 39-layer films sulfidized for 20 min. The fact that the absorption onset, that is, the dimension of CdS formed, was identical irrespective of 450 440E 278 430 .-cE $ 9 420 410 0 10 20 30 sulfidation time/rnin Fig. 2 Onset of absorption in the cadmium stearate LB film (19 layers) as a function of sulfidation time J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0.6, number of layers 1 0.1 J O'350. ' ' ' 400' ' ' ' ' 450' ' ' ' "500 wavelength/nrn Fig. 3 UV-VIS spectra of (a) 19-, (b)29-and (c) 39-layer cadmium stearate LB films after the sulfidation process.Inset : neat absorption of CdS (AA, see text) as a function of the number of layers. These spectra were obtained by using a quartz plate as a reference on each size of which a five-layer cadmium stearate LB film was deposited. the number of layers demonstrates that controlled formation of CdS takes place within each hydrophilic interlayer and the CdS formed remains in the interlayers. Because the hydro- philic interlayers of a cadmium stearate LB film are spatially confined and well separated from each other by the hydro- phobic organic layers and the amount of Cd2+ ions therein is regulated exclusively by the complexation with carboxylate groups, the hydrophilic interlayers would serve as a restricted reaction field for the in situ synthesis of CdS.In the present case the size of the CdS particles is constant and therefore the size-dependent absorption coefficient at a given wavelength should not be taken into account. The absorption spectra exhibited Beer-Lambert behaviour, as shown in the inset in Fig. 3. In order to eliminate from the UV-VIS spectra the influence of an increase in baseline absorbance (possibly due to scattering or reflection of light), the difference in absorb- ance at 370 and 600 nm (AA) was taken as the neat absorp- tion of CdS. The subsequent immersion of the sulfidized film into aqueous CdCl, caused the transformation of C0,H back to COY, and immersion for 3 h was necessary to complete the transformation, as shown in Fig.l(c) and (4. During the treatment, the intensities of the C-H stretching IR bands (Fig. 2) and UV-VIS absorption remained unchanged. Accordingly, it can be concluded that during the immersion process Cd ions are stoichiometrically intercalated into the acid-form composite film to give the salt-form composite film of CdS and cadmium stearate without the escape of CdS and stearate molecules from the film. Hereafter, the intercalation of Cd2+ ions was carried out by immersion into aqueous CdCl, for 3 h. Growth of CdS CdS coexists with its precursor ions (Cd2') in the hydrophilic interlayers of the salt-form composite films obtained in this way, and it is conceivable that CdS can be grown in a well controlled manner by the repetition of the sulfidation (S) and intercalation (I) processes.Changes in the IR and UV spectra of the 19-layer film caused by the S and I treatments are shown in Fig. 4 and 5, respectively. Stoichiometric and reversible transformation of carboxylate groups between the ionized and protonated forms was observed by the repetition of the S-I cycle while keeping the C-H stretching bands intact. This transformation was confirmed in up to six cycles of I-S treatments, though the spectra before the third S treat-ment are shown in Fig. 4. Repetition of the I-S cycle gave rise to an increase in A,,, which provides direct evidence for the growth of CdS in an LB matrix. Note that CdS formed after 35 1 IIII I LBISIiSI S1 1800 1600 1400 wavenumber/crn-' Fig.4 IR spectra of the C-0 stretching vibration of (a) a cadmium stearate LB film, and (b) S(1), (c) I(l), (6)S(2), (e) I(2) and cf) S(3) films; 19-layer films deposited on each side of a CaF, substrate the sixth sulfidation process even has A,, at 486 nm which is blue-shifted from that of bulk CdS (520 nm) and therefore is still size-quantized. As shown in the inset in Fig. 5, an increase in Aos became moderate after the fourth sulfidation process. This is because Aos gradually approaches the limiting bulk value with the growth of CdS. The optical absorption due to CdS (AA) increased with repetition of I-S cycles. This suggests an increasing amount of CdS, though the change in the absorption coefficient with size should be taken into account.19-Layer films were analysed by XPS. C 1s Spectra were composed of a main peak due to aliphatic carbon (285.0 eV) and a small additional peak due to carbonyl carbon (288.5 eV); the binding energy (EJ of the latter was the same in both the acid- and salt-form multilayers. The Cd 3d,,, signal was observed at 405.6 eV in the original LB film, the CdS- 0.6 490 480 .. 0.0 ' ' ' ' I ' ' ' " " 400 " ' 500' ' 600 wavelength/nrn Fig. 5 UV-VIS spectra of (a) a cadmium stearate LB film and (b)-(f)S(n) composite films of stearic acid multilayers and CdS. (b)n = 1, (c)n = 2, (d) n = 3, (e)n = 4, (f) n = 6. The onsets of absorption (A,) of S(n)films are plotted in the inset.These spectra were recorded with 19-layer films deposited on each side of a quartz substrate using uncoated quartz as a reference Table 1 Quantitative XPS analysis of the 19-layer film atomic ratio sample Cd : C s:c Cd : S S( 1) film S(5) film S( 10)film 0.069 : 1 0.23 : 1 0.33 :1 0.073 : 1 0.23 :1 0.29 :1 0.95 :1 1.0 : 1 1.0 :1 bearing multilayers in both the acid- and salt-form composite films and the reference materials (cadmium stearate and CdS powder), indicating that the divalent Cd ions in cadmium stearate and CdS cannot be distinguished. The S 2p signal was observed at 162.0 eV in the composite films after the first S treatment. The results of quantitative analysis are shown in Table 1. The amounts of cadmium and sulfur relative to carbon (Cd : C, S : C) increased steadily with repeating I-S cycle, and the Cd : S atomic ratio is nearly unity after each sulfidation process.These results confirm the increase in the amount of CdS in the LB film by repeating the I-S cycle. Layered Structure The layered structures of the cadmium stearate LB film and the CdS-bearing composite films were investigated by XRD (Fig. 6). The original LB film, which gave distinct X-ray 001 Bragg peaks [Fig. qa)], had a well organized layered struc- ture with a basal-plane spacing (d) of 50.2 A as reported by Matsuda et After the sulfidation process [Fig. 6(b)],the XRD peaks weakened considerably. However, the XRD pattern characteristic of the layered structure reappeared again after the intercalation process [Fig.qc)], though the peak intensity was weaker than that of the original LB film. Through further repetition of S-I cycles, a weakening in the intensity or almost complete disappearance of the peaks was observed during the S treatment, while reappearance of the peaks occurred during the I treatment [Fig. 6(d)-(f)]. The full width at half maximum of the (002), (003) or (004) peaks of the CdS-bearing composite films was 0.19",which is com- (f)Ik5 10 15 5 10 15 : 2O/degrees 20fdegrees Fig. 6 XRD patterns of (a) a cadmium stearate LB film (lo00 counts s-l) and (b)-cf) CdS-bearing composite films (400 counts s-l): (b)W),(4W),(4I(2), (4 I(3),(S)I(6) J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 parable to that of the original cadmium stearate LB film (0.20'). These results indicate that the layered structure is maintained through repetition of the S-I cycle. Particulate CdS, the strongest XRD peak of which appears at 28 = 26.5", was not detected even in the S(6)film. Orientation of the Stearate Molecules The change in orientation of the stearate molecules was investigated by RA and transmission IR spectroscopies. It is well known that the RA and transmission spectra are selec- tive to bands having transition moments perpendicular and parallel to the substrate surface, respectively. In addition, comparison of the RA and transmission intensities reportedly gave quantitative information about the molecular orienta- ti~n.~~-~'Fig. 7 shows RA spectra of nine-layer films sup- ported on a gold substrate. For the original cadmium stearate LB film, the symmetric COY stretching band at 1433 cm-',the transition moment of which is perpendicular to the surface, was exclusively observed in the RA spectrum (Fig.7). In the transmission spectrum (Fig. 4), on the other hand, the following strong bands with transition moments parallel to the surface appear: the antisymmetric and symmetric CH, stretching bands at 2916 and 2849 cm-', respectively, and the antisymmetric CO, stretching band at 1542 cm-l. These results mean that the stearate molecules in the original LB film are nearly perpendicular to the substrate surface. The RA intensities of the symmetric and antisymmetric CH, stretching bands of the CdS-bearing composite films increased gradually with repetition of the S-I cycles.More- over, the antisymmetric CO, band was observed for the 1(1) and 1(2) films in the RA spectra. These results give clear evi- dence that stearate molecules in CdS-bearing composite films become inclined gradually with repeated S-I cycles. The pres- ervation of the all-trans configuration and the uniaxial orien- tation of the alkyl chains through the spectra in Fig. 7(a)-(f) is suggested by the identical wavenumber of the C-H stretching vibration band^^'-^^ and the appearance of the band progression due to the CH, wagging mode between 1400 and 1200 cm- 1,44respectively. The molecular orientation of the nine-layer film is quanti- tatively evaluated by comparison of the RA absorbance (AR) of the Au-supported film (Fig.7) and the transmission absorbance (AT) of the CaF,-supported film (not shown). The A, value is naturally one half of the observed absorbance, because the CaF, plate carried nine layers on each side LB 3000 2800 Fig. 7 Change in the RA FTIR spectra on repeating the sulfidation-intercalation cycle; nine-layer films deposited on a gold plate J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 353 (totally 18 layers). When stearate molecules are uniaxially orientated and the in-plane contribution of the electric field to the RA spectra can be neglected, the orientation angles (0, and 0,) between the surface normal and transition moments of the symmetric and asymmetric CH, stretching modes are given by ei = tan-'J[rnXA,/A,),]; i = s or a (1) where m is the enhancement factor for the RA intensity on the Au substrate with respect to the transmission intensity on the CaF, substrate. Because the rn value of Au is the same as that for Ag, values of rn, = 9.16 and rn, = 9.05 are used in this study, which were reported for the nine-layer LB film of cadmium stearate on Ag and CaF, substrate^.^' The tilt angle (y) of the alkyl chain axis from the surface normal can be obtained by the orthogonality relation among 8,, 8, and y: cos2 e, + cos2 e, + cos2 = 1 (11) For the original cadmium stearate film, y is calculated to be 10".This value agrees well with the reported tilt angle of cadmium salts of long-chain fatty and the angle estimated from the X-ray diffraction res~lt.~~,~' The calcu- lated tilt angles of the original, sulfidized [S(n)] and inter- calation [I(n)] films are shown in Fig.8, from which two characteristic features are observed. The tilt angles of the salt- form films (open circles) increase on going from the original to 1(2) films and are almost constant from 1(2) to 1(4) films. The same tendency is also observed for the acid-form films (closed circles). This suggests that the formation and growth of CdS up to the second cycle cause the stearate molecules to be less oriented. Another characteristic observed from Fig. 8 is that the stearate molecules become less oriented during the S treatment and rearrange to recover the orientation during the I treatment.This may be due to the weaker molecular interaction in the acid-form films than in the salt-form films.50*51As described above, the acid-form, S(n) films showed no distinct XRD peaks. This is presumably due to the poor orientation of stearate molecules in the acid-form films and to Cd not forming a well arranged plane in the hydrophilic interlayers. Consideration of the Form of CIS The basal-plane spacings (d)of the salt-form films are calcu- lated from XRD results and plotted in Fig. 9. d increased steadily, but gently, with growth of CdS. It is clear that the variation of d includes changes due to both the formation of CdS and the molecular orientation. Thus the net change due to the formation of CdS (Ad,) is estimated by the following 54*01 n in I(n)film Fig.9 Basal-plane spacing (6)and estimated thickness of the CdS layers (Ad, see text) of I(n) films equation which takes the change in molecular orientation into account: d = d, -do[sin(90 -yi)/sin(90-yo)] (111) Here, do and yo are the basal-plane spacing and tilt angle of the original film, and d, and y, are those of the I(n) films. As shown in Fig. 9, Ad increases more steeply than d. Note that the Ad value of the 1(3) film (3.5 A) is close to the ionic diam- eter of sulfide ion (3.4 A). The area per stearate molecule in the original LB film was estimated to be ca. 21 81' from the surface pressure (n)-area (A) isotherm of a calcium stearate monolayer at the air/water interface.Since Cd2+ ions in Y-type cadmium stearate LB films are reportedly arranged in the same plane,37 the area per Cd2+ ion is also 21 A'. If we assume a close-packed planar arrangement of sulfide ions in the hydrophilic layers, then the area per sulfide ion is 10 A'. Therefore two or three repetitions of the I-S cycles are required to form a continuous CdS sheet at a monolayer level. The coincidence between Ad of the 1(3) film and the ionic diameter of the sulfide ions may imply the formation of such a monolayer sheet of CdS. If the reported correlation between band gap and particle size of spherical CdS'*3 is applied to the present cases, the diameters of CdS particles in S(n) and I(n) films are estimated to be 48 (n = l), 66 (n= 2), 70 (n = 3) and >100 A (n = 4).It seems that the spherical CdS particles, whose diameters are comparable to or larger than the basal-plane spacing, are too large to be accommo- dated in the hydrophilic interlayers without destroying the layered structure. Judging from the too large diameter of the imaginary spherical CdS particle and the observed magnitude of Ad, we consider that the CdS material formed is two- dimensional in nature, although a more detailed study will be .~~necessary to confirm this. Smotkin et ~1 and Grieser et .~~~1reported the formation of disc-shaped CdS in LB films. Conclusion301 " o S(l) i(1) S(2) l(2) S(3) i(3) S(4) l(4) Fig. 8 Tilt angles of the alkyl chain axis in original, sulfidized [S(n)] and intercalated [I@)] films The exposure of a cadmium stearate LB film to H,S gas yielded size-quantized CdS in the hydrophilic interlayers with concomitant conversion of the cadmium stearate multilayer to a stearic acid one.By immersion into aqueous CdCl, ,Cd ions are intercalated into the hydrophilic interlayers of the CdS-bearing stearic acid multilayer without the escape of CdS to form the CdS-bearing cadmium stearate multilayer. Further repetition of the sulfidation-intercalation cycle makes it possible to grow CdS in the hydrophilic interlayers. The layered structure is maintained during the repetition of sulfidation-intercalation cycles, though stearate molecules tend to become less orientated with the formation and growth of CdS.The onset of absorption of CdS and thickness 354 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 of the CdS layer, coupled with simple geometric consider- ations, suggest that the CdS produced in the hydrophilic interlayers might be in the form of a two-dimensional plane. In this study, we have developed a novel and simple tech- nique by which size-quantized CdS is formed and grown in the hydrophilic interlayers of LB films without destroying the 21 22 23 24 25 R. D. Stramel, T. Nakamura and J. K. Thomas, J. Chem. SOC., Faraday Trans. 1, 1988,84, 1287. H.Miyoshi, H. Mori and H. Yoneyama, Langmuir, 1991,7,503. M. Meyer, C. Wallberg, K. Kurihara and J. H. Fendler, J. Chem. SOC.Chem. Commun., 1984,90. C. Petit and M. P. Pileni, J. Phys.Chem., 1988,92,2282. M. L. Steigerwald, A. P. Alivisatos, J. M. Gibson, T. D. Harris, layered structure. The present results also suggest that the hydrophilic interlayers of metal-containing LB films serve as a restricted reaction field for the well controlled, in situ synthesis of inorganic materials, CdS in the present case. This is because the interlayers are spatially constrained and the amount of precursor metal cations in the interlayers is regu- 26 27 28 R. Kortan, A. J. Muller, A. M. Thayer, T. M. Duncan, D. C. Douglass and L. E. Brus, J. Am. Chem. SOC., 1988,110,3046. Y. M. Tricot and J. H. Fendler, J. Am. Chem. SOC., 1984, 106, 2475. R. Rafaeloff, Y. M. Tricot, F. Nome and J. H. Fendler, J. Phys. Chem., 1985,89,533. N. Kimizuka, T. Miyoshi, I.Ichinose and T. Kunitake, Chem. lated exclusively by complexation with the hydrophilic moiety of the multilayer-constructing molecules. 29 Lett., 1991, 2039. F. C. Meldrum, V. J. Wade, D. L. Nimmo, B. R. Heywood and S. Mann, Nature (London), 1991,349,684. We are indebted to the Cooperative Research Center, Naga- saki University and Mr. H. Furukawa of the Faculty of Engineering, Nagasaki University for the XPS analysis. 30 31 A. R. Teixier, J. Leloup and A. Barraud, Mol. Cryst. Liq. Cryst., 1986,134,347. C. Zylberajch, A. R. Teixier and A. Barraud, Synth. Met. B, 1988, 27, 609. 32 E. S. Smotkin, C. Lee, A. J. Bard, A. Campion, M. A. Fox, T. E. References Mallouk, S. E. Webbwe and J. M. White, Chem. Phys. Lett., 1988,152,265. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 L.E. Brus, J. Chem. Phys., 1984,80,4403. P. E. Lippens and M. Lannoo, Phys. Rev. B, 1989,39, 10935. M. G. Bawendi, M. L. Steigerwald and L. E. Brus, Annu. Rev. Phys. Chem., 1990,41,477. Y. Nosaka and M. A. Fox, J. Phys. Chem., 1988,92,1893. X. K. Zhao, S. Baral, and H. J. Fendler, J. Phys. Chem., 1990,94, 2043. Y. Wang, Acc. Chem. Res., 1991,24, 133. M. W. Peterson, 0. I. Micic and A. J. Nozik, J. Phys. Chem., 1988,92,4160. C. J. Sandroff, S. P. Kelty and D. H. Hwang, J. Chem. Phys., 1986,855337. J. N. Nedeljkovic, R. Herak and 0.I. Micic, Langmuir, 1992, 8, 299. A. Fojik, H. Weller, U. Kock and A. Hengline, Ber. Bunsenges. Phys. Chem., 1984,88,969. N. Herron, Y. Wang and C. Eckert, J. Am. Chem. SOC., 1990, 112, 1322. Y. Wang, A.Suna, M. Mahler and R. Kasowski, J. Chem. Phys., 1987,87,7315. M. E. Wozniak, A. Sen and A. L. Rheingold, Chem. Muter., 1992,4, 753. T. Rajh, M. I. Vucemilovic, N. M. Dimitrijevic, 0. I. Micic and A. J. Nozik,Chem. Phys. Lett., 1988,143,305. T. Arai, H.Fujimure, I. Umezu, T. Ogawa and A. Fujii, Jpn. J. Appl. Phys., 1989,244,484. X. K. Zhao and J. H. Fendler, Chem. Muter., 1991,3, 168. X. K. Zhao, L. D. McCormik and J. H. Fendler, Adv. Muter., 1992,4,93. S. Baral, X.K. Zhao, R. Rolandi and J. €3. Fendler, J. Phys. Chem., 1987,91,2701. N. Herron, Y. Wang, M. M. Eddy, G. D. Stucky, D. E. Cox, K. 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 D. J. Scoberg, F. Grieser and D. N. Furlong, J. Chem. SOC., Chem. Commun., 199 1,5 15. I. Moriguchi, I. Tanaka, Y.Teraoka and S. Kagawa, J. Chem. SOC.,Chem. Commun., 1991, 1401. F. Grieser, F. N. Furlong, D. Scoberg, I. Ichinose, N. Kimizuka and T. Kunitake, J. Chem. SOC.,Faraday Trans., 1992,88,2207. I. Morigichi, I. Tanaka, Y. Teraoka and S. Kagawa, Nippon Kagaku Kaishi, 1991, 1392. A. Matsuda, M. Sugi, T. Fukui, S. Iizima, M. Miyahara and Y. Otsubo, J. Appl. Phys., 1977,48,771. P. A. Chollet, J. Messier and C. Rosilio, J. Chem. Phys., 1976,64, 1042. D. L. Allara and R. G. Nuzzo, Langmuir, 1985,1,52. C. Naselli, J. F. Rabolt and J. D. Swalen, J. Chem. Phys., 1985, 82, 2136. J. Umemura, T. Kamata, T. Kawai and T. Takenaka, J. Phys. Chem., 1990,94,62. H. Sapper, D. G. Cameron and H. H. Mantsch, Can. J. Chem., 1981,59,2543. M. Kubota, Y. Ozaki, T. Araki, S. Ohki and K. Iriyama, Lang-muir, 1991, 7, 774. J. Hayashi and J. Umemura, J. Chem. Phys., 1975,63, 1732. J. Umemura, Hyomen, 1988,26, 180. J. F. Rabolt, F. C. Bums, N. E. Schlotter and J. D. Swalen, J. Chem. Phys., 1983,78,946. D. L. Allara and J. D. Swalen, J. Phys. Chem., 1982,86,2700. D. Duschl and W. Knoll, J. Chem. Phys., 1988,88,4062. M. Sugi, T. Fukui, S. Iijima and K. Iriyama, Bull. Electrotech. Lab., 1979,43, 825. P. A. Chollet, Thin Solid Films, 1978,52, 343. A. Bonnerot, P. A. Chollet, H. Frisby and M. Hoclet, Chem. Phys., 1985,97, 365. Moller and T. Bein, J. Am. Chem. SOC.,1989, 111, 530. 20 G. A. Ozin and S. Ozkar, Adv. Muter., 1992,4, 11. Paper 3/03693F; Received 28th June, 1993