Synthesis of CdS and CdSe nanoparticles by thermolysis of diethyldithio-or diethyldiseleno-carbamates of cadmium? Tito Trindade and Paul O'Brien*S Department of Chemistry, Queen Mary and Westjield College, Mile End Road, London, UK El 4NS Cadmium sulfide and cadmium selenide nanoparticles have been synthesised by a novel route involving the thermal decomposition of the bisdiethyldithio- or bisdiethyldiseleno-carbamates of cadmium in refluxing 4-ethylpyridine solutions. The nanodispersed materials were studied by electronic spectroscopy and bandgaps were blue shifted. Transmission electron microscopy of the samples showed material to be in the nanosize range and crystalline. There has been considerable interest in the synthesis and characterisation of semiconductor nanoparticle~.'-~ Nanoparticles, also known as nanocrystallites, Q-particles or quantum dots, are particles with a high surface : volume ratio and diameters of up to 10-20 nm, their opto-electronic proper- ties are different from the bulk counterparts, and new techno- logical applications have been proposed for this type of The prospects for devices are now more immediate and a number of recent papers have reported on either the photoluminescent properties of nanodispersed II-VI materials' or photoluminescent devices based on such II-VI materiakg Nanoparticles are also important in fundamental research because they represent a state of matter in which the transition from molecular to the bulk (macrocrystalline) level can be investigated e~perimentally.'-~ The preparations of nanoparticles of many semiconductors have been reported, these include: PbS,lo,l' CdS,',1'-'6 CdSe,16-'9 CdTe,16 ZnS,20-22 ZnO,', Ti02,24 InP,25 G~As,'~.'~ Zn3PZ2' and Cd3P2.20*21 More recently, the synthesis of nano- composites has been subject of intense research as well.Some examples of nanocomposite materials described in the literature are ZnS/CdSe," CdS/PbS,'1*2' SiO,/CdSZ9 and CdS/ZnS.30 There are other reports of studies concerning the preparation of nanoparticulate systems including elemental Ag,31 Ge,32 Pd33 and Pt34or metal halides such as HgIZ3' and PbI,.36 Theoretical models predicting the optical properties of semi- conductor nanoparticles are a~ailable,~-~' but the properties of nanoparticles obtained by any new synthetic procedure are hard to anticipate. The following characteristics are desirable in the final nanodispersed system: high purity, monodispersity and an ability to control surface derivatization. Nanoparticles with these properties have been prepared by several synthetic methods and/or separation technique^.^^^ The chemical methods used for the preparation of semiconductor nanopart- icles involve reactions in various media including: aqueous solution, microemulsions, zeolites, gels, polymers and glasses.Steigerwald et al." prepared CdSe nanoparticles using the solution-phase thermolysis of Cd [Se(C,H,)], . The method involved" refluxing the precursor in 4-ethylpyridine, a high boiling point solvent, to give optically clear solutions contain- ing the nanoparticles.The high reflux temperature promotes the decomposition of the precursor producing the semicon- ductor nanoparticulate material. Such single-molecule precur- sors contain the metal and non-metal (chalcogenide) within the same molecule and are therefore attractive sources for ?Presented at the Second International Conference on Materials Chemistry, MC', University of Kent at Canterbury, 17-21 July 1995. 1Present address: Department of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington, London, UK SW7 2AZ. the one-step preparations of nanoparticles containing those elements. Such an approach avoids the use of toxic and pyrophoric compounds such as Cd( CH,), , which is commonly used for preparing nanodispersed cadmium chalcogens.l6 Solid cadmium diethyldithiocarbamate (Cddtc) and cad- mium diethyldiselenocarbamate (Cddsc) are dimeric com-pounds of molecular formula (Cd [E,CN(C,H,),], ), (E = S, Se). Their crystal structures have been rep~rted~',~~ and show distorted square-pyramidal coordination at the metal. The bisdiethyldithio- or bisdiethyldiseleno-carbamates of cadmium have been used in chemical vapour deposition experiments to prepare II-VI semiconductor film^.^',^^ In this work, solutions of these compounds in 4-ethylpyridine were used to produce CdS and CdSe nanoparticles. This solvent has a high boiling point (168 "C, 1 atm) and dilute solutions of Cddtc and Cddsc in 4-ethylpyridine remain optically clear for more than 24 h.Nanoparticulate material with a derivatized surface has pre- viously been obtained by using 4-ethylpyridine as the solvent;I8 however, metal thiocarbamates/selenocarbamates have never been used as precursors for semiconductor nanoparticles. Experimental Chemicals CdC1,(99 + %, Aldrich), NaS2CN(C,HS),.3H,0 (98%, Aldrich), 4-ethylpyridine (98Y0, Aldrich), pyridine (99 + YO, Aldrich, CH2Cl, (99Y0, BDH) and light petroleum (bp 6O-8O0C, BDH) were all used as received except 4-eothylpyri- dine which was dried with molecular sieves (type 3 A, BDH) and deoxygenated under a nitrogen flow. Synthesis of molecular precursors Cddtc was synthesized by adding stoichiometric quantities of aqueous equimolar (0.1mol drn-,) solutions of CdC1, and NaS2CN(C,H,),~3H,0.The white solid obtained was filtered off and washed thoroughly with deionised water. This solid was purified by recrystallization from hot CH2Clz. Cddsc was synthesized by the method described in the literature4' by treating N,N-diethyldiselenocarbamate, as the diethylam-monium salt, with an aqueous solution containing a stoichio- metric amount of CdC1,. The compounds were identified by 'H NMR (CDC1,) and IR spectroscopy. Synthesis of the CdS and CdSe nanoparticles Solutions (5-50 mmol dm-, in the precursor) were prepared by dissolving the required amount of the compound in 4- ethylpyridine at room temperature. The solutions were filtered and then heated at the reflux temperature of 4-ethylpyridine J.Mater. Chem., 1996, 6(3),343-347 343 (168°C). The reflux was performed both under the ambient atmosphere and an N2 atmosphere. The formation of the nanoparticles as a function of the time of heating was moni- tored by extracting an aliquot of the refluxing solutions and transferring it to a vial immersed in an ice-bath and recording the optical absorption spectrum. The addition of light pet- roleum to the final cooled and optically clear solutions resulted in the flocculation of a precipitate which was collected by centrifugation. These solids were washed with dichloro-methane-light petroleum and then dried under vacuum to give powders which were stored under N,. The material gave optically clear solutions when redissolved in either pyridine or 4-ethylpyridine; any insoluble material in these redispersions was isolated by centrifugation and discarded.Material characterisation and instrumentation IR spectroscopy of the powders was performed using CsI (99.9%, Aldrich) pellets and a Perkin-Elmer 1720X FTIR spectrometer. Optical absorption spectra were recorded at room temperature with a Philips PU 8710 spectrophotometer. Silica cells (1 cm) were used and the starting solution for each precursor was used as reference. The pyridine solutions were analysed using pure pyridine as reference. The 'H NMR spectra were recorded in a Bruker 250 AM pulsed Fourier transform instrument. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDAX) spectroscopy were performed with a JEOL JSM35CF microscope operating at 25 kV.The samples for analysis were prepared by putting one drop of the sample solution onto pure aluminium plates and letting the solvent evaporate at room temperature. X-Ray powder diffraction (XRPD) patterns were measured using a Guinier camera and a Philips 1130 X-ray generator (Cu-Kol radiation). Samples for X-ray diffraction were prepared by placing the powder onto adhesive tape. Conventional transmission electron microscopy (TEM) results were obtained using a JEOL-JEM 1200 EX I1 scanning and transmission electron microscope operating at 100 kV; high resolution transmission electron microscopy (HRTEM) was performed with a JEOL 2000 FX electron microscope operating at 200kV.A sample for TEM was prepared by placing an aliquot of pyridine solution containing the nanopar- ticles onto an amorphous carbon surface on a copper grid and wicking away the solvent with a paper tip. Particle sizes were determined by measuring the diameter of around one thousand particles on the TEM images. Results and Discussion Optical properties of nanodispersed CdS and CdSe The optical properties of solutions of Cddtc and Cddsc in 4- ethylpyridine change with heating. Typical changes, as a func- tion of time of heating, using the starting solution as reference, are reported in Fig. 1 and 2 for the Cddtc and Cddsc precursors, respectively. With longer heating times the optical homogeneity of the solutions is not maintained and scattering perturbs the absorption spectra.In both cases the absorption edge is blue- shifted in relation to the bulk bandgap value, (the absorption edge is taken as the intersection of the base line with the tangent drawn to the band shoulder). Such shifts in the absorption edge of semiconducting materials have been associ- ated'-5 with particles having sizes comparable to the de Broglie wavelength of the electron and hole. The alterations observed in the band profiles (Fig. 1 and 2) are associated with a chemical transformation of the molecular precursors, since for both cases the starting solution was used as reference. A blank consisting of pure 4-ethylpyridine, refluxed over 6 h under ambient atmosphere, showed an increase in the intensity of the absorption band at 325 nm.If 344 J. Muter. Chem., 1996, 6(3), 343-347 0.30 w avelength/nm 0.40 r 0.30-,Q> \ \0 \ \c \(d \ 0.20 -\\\ \% \ bulk band gap\ \n 6 \ 0.10--.-.------._.-0.00 Fig. 2 Optical absorption spectra of Cddsc in 4-ethylpyridine solutions at different reflux times: ---, 2; ......, 4;-, 6 h the reflux is carried out under N2 the 4-ethylpyridine shows no such changes in its optical characteristics. However, the changes associated with the formation of nanodispersed mate- rial are similar if heating is performed in the presence or absence of N,. The absorption characteristics for CdS (Fig. 1) and CdSe (Fig. 2) are in agreement with the initial formation of CdS or CdSe nanoparticles which agglomerate to form particles of larger dimensions.Other authors'' have reported similar optical spectra for CdSe particles grown from Cd [Se(C6H5)I2 in 4-ethylpyridine solutions. The band at 420 nm was assigned'* to electronic transitions occurring in small CdSe clusters dispersed in 4-ethylpyridine. In this work an absorption band at around 413 nm was also observed in both 4-ethylpyridine and pyridine solutions containing the CdSe species (Fig. 3). The optical absorption spectrum of CdS nanoparticles have been p~blished'~*~~ and are similar to those shown in Fig. 1, even though media as diverse as ze01ites'~ and water" have been used in the preparation. The species responsible for the absorption were isolated as powders by addition of light petroleum to the cooled solutions followed by centrifugation.The solids obtained are readily dispersed in pure 4-ethylpyridine or pyridine. The optical absorption spectra for the pyridine solutions were recorded, and showed that the shifts in the edges to higher energies compared to the bulk values are still observed (Fig. 3). The 1.60 4.501 / ... 400 450 500 550 600 650 700 750 800 wavelengthlnm Fig. 3 Optical absorption spectra of the pyridine solutions containing the soluble powders obtained from Cddsc in 4-ethylpyridine at different reflux times: ---,0.25; ---, 0.5; ---, 2; .-.-.+,4; -.-.-, 5; -, 6 h observed band shifts, on refluxing, suggest an increase in mean particle diameters for the CdSe clusters with time.I6 It was found that the pyridine solutions containing the nanoparticles were unstable and the optical properties changed with time.A broadening of the sharp band at 413nm, after 48 h, can be seen clearly (Fig. 4) for the sample obtained from Cddsc after 15 min reflux. The 4-ethylpyridine growth solutions also shows broadening of the absorption bands when kept standing. Such broadening is to be expected if an agglomeration process occurs because the polydispersity of the particulate system will be increased. The spectrum of a pyridine solution containing the powder obtained from the addition of light petroleum to the 4- ethylpyridine solution of Cddtc after 6 h reflux is shown in Fig.5. The 'bandgap' of the CdS nanoparticles was determined using the direct transition method4' by fitting the absorption data to eqn. (1) (Fig. 5, inset): a(hv)cc (hv -E,)1'2 (1) where a is the absorption coefficient of the semiconductor material, hv is the photon energy and E, is the optical bandgap. The optical bandgap obtained by using this method is 2.63 eV, which is slightly blue-shifted from that of bulk CdS (2.53 eV). CdS particles begin to present4 the charactoeristic bandgap of bulk material at a diameter of around 80A, i.e. for particles within the nanosize range. 420 470 520 570 620 670 720 waveleng t h/nm Fig. 4 Optical absorption spectrum of the pyridine solution after 48 h (t=0.25 h in Fig. 3) 350 400 450 500 550 wavelength/nm Fig. 5 Optical absorption spectrum of the pyridine solution containing the soluble powder obtained from Cddtc in 4-ethylpyridine (6 h reflux).Inset shows the fit of the absorption edge by the direct transition method. Characterization of solid phases Prolonged times of heating led to 4-ethylpyridine solutions containing solid material in suspension and/or fixed to the walls of the flask. With the dithiocarbamate the suspended material is dark yellow and the XRPD pattern consists of broad lines typical of hexagonal CdS. The bulk material obtained from the selenocarbamate precursor is brown-grey and adhered firmly to the walls of the flask forming a specular film. The XRPD pattern showed evidence for elemental hexag- onal Se and hexagonal CdSe; for the latter case the SEM showed well defined spherical particles within the submicro- metric range (Fig.6). EDAX on a single particle showed the presence of both Se and Cd. These results suggest that Cddtc and Cddsc are thermally decomposed in 4-ethylpyridine solu- tions. Prolonged heating times lead to bulk material but the solutions still contain nanosized particles of CdS and CdSe, as indicated by their optical absorption spectra. This hypothesis is also supported by the results obtained from the characteris- ation of the solid phases obtained from the syntheses. The powders isolated from the solutions during the growth of nanodispersed material were characterised by IR, EDAX and XRPD. The IR spectra of the powders do not show the characteristic bands of the molecular precursors.The ease of dissolution of these powders in pyridine and 4-ethylpyridine suggests the binding of solvent molecules to the nanoparticles surface. However, the characteristic bands of the 4-ethylpyri- dine were not found in the IR spectra (e.g. the sharp and strong bands due to the ring stretching of 4-ethylpyridine around 1602 and 1560cm-'). It is probable that surface coverage has occurred to an extent below the detection limits of the IR experiment. The low surface coverage of the nanopart- Fig. 6 SEM image of CdSe particles J. Mater. Chem., 1996, 6(3), 343-347 345 icles by the solvent molecules could also explain their relatively Fig. 7 CdSe nanoparticles obtained from Cddsc in 4-ethylpyndine after refluxing for 5min (0)conventional TEM image.(b) HRTEM" \I "l\, image (bar =10 nm) facile agglomeration The elements detected by EDAX were Cd and the chalcogen- ide element (S or Se) Si and C1 were also detected as contaminants, probably from vacuum grease and/or precursor Unlike the bulk solids obtained in both syntheses these powders did not show XRPD patterns in our equipment, a result that does not preclude some crystallinity in the samples Particle size assessment The material contained in the optically clear pyridine solutions containing CdSe was subjected to further analysis by TEM The TEM analysis was performed for samples obtained after 15min [Fig 7(4, Fig 7(b) shows the HRTEM image] and 360min reflux The particle size distribution for the shorty time is shown in Fig 8, in which the mean diameter is 48 A The TEM of a sample refluxed for longer time shows larger particles with some agglomeration having occurred The results obtained by TEM confirm that the agglomer- ation is a favourable process in the pyridine solutions contain- ing the CdSe nanoparticles The agglomeration leads to some spread on the particle size distribution (Fig 8) In ,Fig 7(b)the lattice fringes of particles with diameters up to 50 A are clearly observed, confirming the presence of dispersed nanocrystallites in the pyndine solution Analysis of the patterns for several different particles was most consistent with a predominance of the hexagonal phase This result shows that the CdSe nanopart- icles have the bulk unit-cell structure, despite their markedly different optical properties, in agreement with reports made by other authors This work showed that single-molecule precursors such as Cddtc and Cddsc can be used for the preparation of soluble nanosized CdS and CdSe particles, respectively Work is in progress in our laboratories on the use of alk~l of the precursors used in the work described here Our main concern is to overcome some of the limitations found in this work, such as the practical manipulation of significant quantities of semiconductor nanoparticles and the inherent Fig.8 Particle size distnbution of the CdSe nanoparticles obtained from Cddsc in 4-ethylpyndine after refluxing for 15min and redispersion in pyndine 346 J Muter Chem , 1996,6(3), 343-347 instability of the solutions containing the nanoparticulate materials.21 22 H. 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