Optical spectroscopy of cadmium-chalcogenide clusters of the type [Cd ₁₀E₄(E′Ph)₁₂(PR₃) ₄], (E = Te, Se; E′ = Se, S)

The optical properties of a group of cadmium-chalcogenide cluster-molecules with the general formula [Cd₁₀E ₄(E′Ph)₁₂(PR₃)₄], (E = Te, Se; E′ = Se, S) were studied to investigate the effect of substitution of the chalcogen atoms in positions E and E′. The cluster-molecules were prepared by an organometallic synthesis route and their structures were determined by single crystal X-ray diffraction. The clusters display visible emission only at low temperatures, and the peak position and lifetime of the photoluminescence (PL) depends significantly on the E′ chalcogen atom type. The PL of clusters with outer selenophenyl ligands is centered at 500 nm while for clusters with thiophenyl ligands the emission is shifted to 420 nm, and the emission lifetime is significantly longer. This provides strong support for assigning the emission to forbidden transitions related with the cluster capping ligands. The onset of the absorption and the PL excitation spectra display a systematic blue-shift upon variation of the E and E′ atoms from Te to Se to S. This behavior resembles that of the band gap shift in the respective bulk semiconductors demonstrating that such cluster-molecules manifest a molecular limit of the solid state.