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J . Phys. Chem. 1991, 95, 1572-1576
Additionally, it should be mentioped that an attempt was made to obtain the AH value by using the Raman spectrum of gaseous vinylsilyl chloride but without success. We found the results inconclusive because of the small change in relative intensities and broadening of the bands. Therefore, we conclude that the AH value is too small to be measured in the gas phase by the
Raman technique which is consistent with the value calculated from the potential function.
Acknowledgment. We gratefully acknowledge financial support of this study by the National Science Foundation through Grant CHE-83-11279.
Structure of Capped CdSe Clusters by EXAFS Matthew A. Marcus,* W. Flood, Michael Stiegerwald, Louis Brus, and Moungi Bawendi Bell Laboratories, Murray Hill, New Jersey 07974 (Received: May 30, 1990; In Final Form: August 15, 1990)
Clusters of CdSe and CdS capped with -SeC6HS or -SC6HS terminations were produced by synthesis in micellar solution. These clusters were studied by Se K-edge EXAFS. Substitution of S for Se in the bulk and cap were done in order to separate the structural information about the cluster interiors from that pertaining to their surfaces. We find that the cluster interiors have bulklike Cd coordinations about the Se, with distances reduced from the bulk by a marginally detectable amount. There is a static strain (dispersion of interatomic distances at low temperatures) of up to 0.0015 A2, deljending on the type of sample. The temperature dependance of the EXAFS Debye-Waller factor is the same as in the bulk, except for the addition of the static contribution. Se atoms on the surfaces are coordinated to interior Cd and chalcogen atoms, to other surface Se atoms, and to C atoms of the capping ligands.
Introduction
Semiconductor fine particles and clusters have been the subjects of much interest over the past several years because they show electronic and optical properties that can be tuned by varying their size.’-’ Quantum confinement of carriers has been shown to move bandgaps and excitons by more than an electronvolt from the corresponding bulk It is clearly of interest to know more about the structures of some of these clusters. The micelle-grown CdSe system has been used for a number of studies involving clusters of different sizes, compositions, and surface condition^.*-'^ This paper will deal exclusively with such clusters, so it is worth while to give a brief explanation of the high points of the synthesis. Detailed procedures will be found in the Experimental Section and in references. The procedure starts with an inverse-micellar solution of a Cd salt in water (inside phase) and heptane (outside phase). An organoselenium compound or a sodium sulfide solution is added ( I ) Yanagawa, T.; Sasaki, T.; Nakano, T. Appl. Pfiys. Lett. 1989,54,1495. (2) Fuyu, Y.; Parker, J. M. Mater. Len. 1988, 6, 233. (3) Bard, A. J. Ber. Bunsen-Ges. Pfiys. Cfiem. 1988, 92, 1187. (4) Wang, Y.; Herron, N. J. Pfiys. Cfiem. 1987, 91, 257. ( 5 ) Wang, Y.; Herron, N.; Mahler, W.; Suna, A. J. Opf. SOC.Am. 1989, 86. 808. (6) Brus, L. E. J . Cfiem. Pfiys. 1984, 80, 4403. (7) Kayanuma, Y. Phys. Reo. 1988, B38, 9197. (8) Alivisatos, A. P.;Harris, A. L.; Levinos, N. J.; Steigerwald, M. L.; Brus, L. E. J . Cfiem. Pfiys. 1988, 89, 4001. (9) Alivisatos, A. P.; Harris, T. D.; Caroll, P. J.; Steigerwald, M. L.; Brus, L. E. J . Cfiem. Pfiys. 1989, 90, 3463. (IO) Steigerwald, M. L.; Brus, L. E. In Sfructure and Reacriuify of Reuerse Micelles; Pileni, M. p., Ed.; Elsevier Science: New York, 1989; Vol. 1I . ( I I ) Steigerwald, M. L.; Alivisatos, A. P.;Gibson, J. M.;Harris, T. D.; Kortan, A. R.; Muller, A. J.; Thayer, A. M.; Duncan, T. M.; Douglass, D. C.; Brus, L. E. J. Am. Cfiem. SOC.1988, 110, 3046. (12) Thayer, A. M.; Steigerwald, M.L.; Duncan, T. M.; Douglass, D. C. Pfiys. Rev. Lett. 1988, 60, 2673. (13) Bawendi, M. G.;Kortan, A. R.; Steigerwald, M. L.; Brus, L. E. J. Chem. Pfiys. 1989, 91, 7282. (14) Kortan, A. R.; Hull, R.; Opila, R. L.; Bawendi, M.G.;Steigerwald, M. L.; Carroll, P. J.; Brus, L. E. J. Am. Cfiem. SOC.1990, 112, 1327. (15) Brennan, J.; Siegrist, T.; Carroll, P. J.; Stuczynski, S. M.; Brus, L. E.; Steigerwald, M. L. J . Am. Chem. SOC.1989, 1 1 1 , 4141.
to the micellar solution, depending on whether one wants CdSe or CdS. The solution quickly turns yellow or red as clusters form. Then, another quantity of Cd solution is added in order to make the cluster surfaces Cd-rich. Next, an excess of TMS-Se-Ph (TMS = trimethylsilyl, Ph = C,HJ is added. The TMS leaves, and the Se-Ph moiety adds to the Cd on the surface to form a capping layer. For a sulfur cap, H-S-Ph is used instead. The formation of this cap changes the solubility of the cluster, so the clusters precipitate out and can be collected. The cap protects them from coalescence and allows one to have macroscopic amounts in one place. The resulting powder can be redispersed in pyridine. The clusters at this point have been shown to have a disordered structure. Their crystallinity can be improved by annealing. Clusters of this sort have been characterized by optical spectroscopy, X-ray diffraction, TEM, and NMR ana lyse^.^-'^ The model is of a crystalline core of structure intermediate between zincblende and wurzite (for annealed samples), with a Se-rich surface layer. The optical properties suggest a bulklike band structure with changes due to quantum confinement. TEM studies show sizes ranging from 20 to 45 A, depending on preparation conditions, with a radius range of 5 A in all cases. By varying or eliminating the annealing step, one can change the crystallinity. One of the useful features of this type of sample is that there are several modifiable parts to the clusters. There is an interior region, defined by the metal salt and organochalcogen compound used in the initial reaction. The size of this region depends on the water and metal contents of the original micelles. Next, there is the “second metal”, which sits on the surface and is applied before the cap. Finally, the cap may be made of a different chalcogen than was used in the body of the cluster. Thus, one can use Se as a probe of a cluster that otherwise contains only S.
To understand the electronic properties of the particles, it is necessary to know something about their structural properties. Since clusters contain much surface area, surface structure becomes extremely important. Gibbs pressure (surface stress) at an irregular surface could cause strains in the interior. Also, the interior may not have an equilibrium bulk crystal structure, for kinetic and energetic reasons.
0022-3654/9 1/2095- 1572$02.50/0 0 199 1 American Chemical Society
Structure of Capped CdSe Clusters by EXAFS EXAFS is a useful tool for studying these questions. It is sensitive to local structure about one kind of atom, in this case Se. Coordination numbers and distances for at least the first shell can be inferred. The coordination number is related to the surface area, if one assumes a bulklike coordination number for the interiors. EXAFS does not require that the sample be crystalline. The EXAFS Debye-Waller factor can be used to deduce vibrational properties. This technique was used in combination with others by Herron et a1.I6 on tiny (
