Electronic excitation energies in copper selenide clusters

Abstract

We report calculated singlet and triplet excitation energies in bare copper selenide clusters (Cu2Se)n up to n = 18 and ligand-covered clusters (Cu2Se)n(PH3)m up to n= 10 (m≤8). Various approaches for the calculation of excitation energies (including coupled cluster methods) are compared for small clusters which suggests the method of configuration interaction restricted to single substitutions (CIS) to give reasonable results with an error in the predicted energies of roughly +1 eV. The lowest CIS excitation energies show a steep increase with the cluster size up to about n = 6 and a slow decrease for larger clusters, for which a rationalization is given. Even for the largest of the treated clusters the computed excitation energies are higher than 3.5 eV which suggests that these clusters are still far from showing β-Cu2Se bulk properties (energy gap 1 eV). The electronic excitations are found to be essentially of charge transfer character (Se→Cu). © 1996 American Institute of Physics.