Spectroscopic Studies of Ligand-Capped Nanoclusters of Group II-VI Semiconductors

Abstract

Vibrational spectrometry and electrospray mass spectrometry (ESMS) have been applied to the characterisation and investigation of the fundamental chemistry of thiolate-capped nanoclusters of group II-VI semiconductors, with emphasis on CdS clusters. Thiophenolate-capped nanoclusters of cadmium sulfide and the cadmium-thiophenolate compounds Cd(SPh)2, [Cd(SPh)4](Me4N)4, [Cd4(SPh)10](Me4N)2 and [Cd4X4(SPh)4](Et4N)4 (X = Cl, Br, I) were studied by low frequency FTIR 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 far-IR ν(Cd-S) core band showed a progressive broadening and shift from 288 cm-l towards the peak position of 241 cm-1 in bulk (wurtzite) CdS with increasing cluster size up to ca. 40 Å. The study has developed the technique of low-frequency FTIR spectroscopy for identifying the size and structural elements of group II-VI nanoclusters. Electrospray mass spectra were recorded for the thiophenolate-capped clusters [E4M10(SPh)16](Me4N)4, and [S4M17(SPh)28](Me4N)2 (M = Cd, Zn; E = S, Se), and for the metal thiophenolate complexes M(SPh)4](Me4N)2, [M4(SPh)10](Me4N)2 and [Cd4X4(SPh)6]Et4N)2 (X=Cl, Br, I). The fragmentation processes, and the exchange of M, E and X which occur in various mixtures of these clusters and complexes were investigated. The study demonstrates the power of ESMS for the study of metal thiolate complexes and metal chalcogenide thiolate clusters. The method is particularly attractive for the fundamental cluster chemistry which it reveals. On the basis of the fundamental chemistry which was revealed by ESMS, a synthetic method was discovered for the controlled growth of a CdS cluster. Oxidation and removal of the surface-capping SPh- ligands of the CdS cluster [S4cdl0(SPh)16]4- in solution by reaction with iodine resulted in the growth of a novel CdS cluster of about l5 Å diameter. The capping and functionalisation of a CdS nanocluster was investigated by electrospray mass spectrometry using ligand exchange reactions with a range of thiol ligands. Information about the extent of exchange and bonding modes of the ligands was obtained. By using sodium ethoxide as an ionisation agent it was shown that ESMS is applicable to the study of transition-metal alkoxide compounds and the complexes which form between metal alkoxides and functionalised capping ligands.