Synthesis of ≈ 10 Å thiophenolate-capped CdS clusters. Observation of a sharp absorption peak

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Abstract
The controlled growth of semiconductor clusters from a precursor phenyl-capped CdS molecule, [Cd10S4(SPh)16]4–, by the addition of stoichiometric amounts of sulfide ion has led to a material having a sharp (full width at half-maximum at room temperature ≈ 800 cm–1) excitonic feature at 351 nm (3.53 eV) in its absorption spectrum. This narrow spectral feature, coupled with the synthetic conditions necessary to produce it and the chemical stoichiometry, suggests a very narrowly dispersed, anionically charged, pyramidal-shaped cluster of around 10 Å diameter and composed of ≈55 atoms in a tetrahedral (sphalerite) arrangement (by analogy with the starting material structure). Computer simulation shows that the X-ray powder diffraction data are consistent with this hypothetical pyramidal structure. A simple tight-binding (Hückel) model, fitted to the bulk band structure, is used to describe the energy levels in such a cluster; the lowest calculated transition, after perturbation by the Coulomb interaction, displays an isolated peak at 3.66 eV, close to the observed value. The wavefunctions of the hole and electron calculated from the model reveal the hole largely located at the central S atom of the cluster and the electron delocalized over the surrounding 16 Cd atoms.