Theoretical analysis of structures and electronic spectra in molecular cadmium chalcogenide clusters

We present calculated structural and optical properties of molecular cadmium chalcogenide nonstoichiometric clusters with a size range of less than 1 nm to more than 2 nm with well-defined chemical compositions and structures in comparison to experimental characterization and previous theoretical work. A unified treatment of these clusters to obtain a fundamental understanding of the size, ligand, and solvation effects on their optical properties has not been heretofore presented. The clusters belong to three topological classes, specifically supertetrahedral (Tn), penta-supertetrahedral (Pn), and capped supertetrahedral (Cn), where n is the number of metal layers in each cluster. The tetrahedrally shaped Tn clusters examined in this work are Cd(ER)4(2-) (T1), Cd4(ER)10(2-) (T2), and Cd10E4 (')(ER)16(4-) (T3), where R is an organic group, E and E' are chalcogen atoms (sulfur or selenium). The first member of the Pn series considered is M8E'(ER)16(2-). For the Cn series, we consider the first three members, M17E4 (')(ER)28(2-), M32E14 (')(ER)36L4, and M54E32 (')(ER)48L4(4-) (L = neutral ligand). Mixed ligand clusters with capping ER groups replaced by halogen or neutral ligands were also considered. Ligands and solvent were found to have a large influence on the color and intensity of the electronic absorption spectra of small clusters. Their effects are generally reduced with increasing cluster sizes. Blueshifts were observed for the first electronic transition with reduced size for both cadmium sulfide and cadmium selenide series. Due to weakly absorbing and forbidden transitions underlying the one-photon spectra, more care is needed in interpreting the quantum confinement from the clusters' lowest-energy absorption bands.