Chem. Mater. 1997, 9, 967-975
967
Vibrational Spectroscopic Study of Thiophenolate-Capped Nanoclusters of CdS and of Cadmium Thiophenolate Complexes Thomas Løver, Graham A. Bowmaker,* John M. Seakins, and Ralph P. Cooney Department of Chemistry, University of Auckland, Private Bag 92019, Auckland, New Zealand Received October 14, 1996. Revised Manuscript Received January 30, 1997X
Thiophenolate-capped nanoclusters of cadmium sulfide and the cadmium thiophenolate compounds Cd(SPh)2, [Cd(SPh)4](Me4N)2, [Cd4(SPh)10](Me4N)2, and [Cd4X4(SPh)6](Et4N)2 (X ) Cl, Br, I) have been studied by low-frequency FT-IR and Raman spectroscopy. In the spectrum of the cluster [S4Cd10(SPh)16](Me4N)4 a band at 288 cm-1 was assigned to the ν(Cd-S) stretching modes of the (Cd-S) bonds of the S4Cd6 cluster core. The ν(Cd-S) stretching frequencies of the bridging and terminal SPh- ligands which cap the cluster core were assigned to bands in the region 150-187 cm-1. The ν(Cd-S) core band for [S4Cd17(SPh)28](Me4N)2 with tetracoordinated sulfides, as in bulk CdS, was observed at 270 cm-1. The far-infrared ν(Cd-S) core band showed a progressive broadening and shift from 270 cm-1 toward the peak position of 241 cm-1 in bulk (wurtzite) CdS with increasing cluster size up to ca. 40 Å. This shift is interpreted in terms of an increase in the average Cd-S bond length. Far-infrared spectroscopy has been shown to be a valuable technique for the characterization of thiolate-capped metal chalcogenide clusters.
Introduction Nanometer-sized semiconductor clusters are a class of materials of considerable current interest. In this intermediate size regime (10-60 Å, 15-4000 atoms for CdS) a gradual transition from solid-state to molecular structure occurs as the particle size decreases. A splitting of the energy bands into discrete, quantized levels occurs. The optical, electronic, and catalytic properties of such nanoparticles (quantum dots) differ markedly from the corresponding macrocrystalline substance,1-6 and studies have shown a strong size and dimensionality dependence of these properties.7 With the development of methods that permit uniform clusters to be prepared, a new trend of research is evolving with the aim of arranging individual particles in twoand three-dimensional superlattices and investigating the collective properties resulting from the interactions of the particles.8 One method that has produced highly monodisperse samples involves the use of ligands that cap the particle surface by direct coordination to surface atoms. This has proved effective for clusters of metal chalcogenides (ME; M ) Cd, Zn; E ) S, Se) using mainly thiolates and in particular thiophenolate (SPh-) as the capping ligand.9-19 The process involves precipitation Abstract published in Advance ACS Abstracts, March 15, 1997. (1) Brus, L. E. Appl. Phys. A 1991, 53, 465. (2) Wang, Y.; Herron, N. J. Phys. Chem. 1991, 95, 525. (3) Bawendi, M. G.; Steigerwald, M. L.; Brus, L. E. Annu. Rev. Phys. Chem. 1990, 41, 477. (4) Hengelein, A. Top. Curr. Chem. 1988, 143, 113. (5) Hengelein, A. Chem. Rev. 1989, 89, 1861. (6) Wang, Y.; Herron, N. Phys. Rev. B 1990, 42, 7253. (7) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41. (8) Weller, H. Angew. Chem., Int. Ed. Engl. 1996, 35, 1079. (9) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (10) Noglik, H.; Pietro, W. J. Chem. Mater. 1994, 6, 1593. (11) Nosaka, Y.; Shigeno, H.; Ikeuchi, T. J. Phys. Chem. 1995, 99, 8317. (12) Hayes, D.; Micic, O. I.; Nenadovic, M. T.; Swayambunathan V.; Meisel, D. J. Phys. Chem. 1989, 93, 4603.
of the semiconductor material in the presence of the capping species, or growth of larger clusters from molecular precursors which already incorporate the capping ligands. Steigerwald et al.16 discovered that micelle-encapsulated CdS particles capped with SPhions could be grown in a controlled manner by additions of S2- ions to the microemulsion. The particles increased in size by an inorganic polymer-like process where added sulfide displaces the caps and builds up a new CdS layer, followed by the reattachment of caps to the particle surface. Following this, Herron et al.17 mixed a solution of Cd2+ ions with a solution containing a mixture of S2-and SPh-. Adjustment of the sulfideto-thiophenolate ratio caused the competitive reaction rates of these species with Cd2+ ions to control the eventual cluster size. The clusters reported ranged in size from
