Structural and electronic properties of bare and capped CdnSen/CdnTen nanoparticles (n = 6, 9)

Relationships between structures and properties (energy gaps, vertical ionization potentials (IP$_{v})$, vertical electron affinities (EA$_{v})$, and ligand binding energies) in small capped CdSe/CdTe nanoparticles (NPs) are poorly understood. We have performed the first systematic density functional theory study of the structures and electronic properties of Cd$_{n}$Se$_{n}$/Cd$_{n}$Te$_{n}$ NPs (n = 6, 9), both bare and capped with NH$_{3}$-, SCH$_{3}$, and OPH$_{3}$-ligands. NH$_{3}$- and OPH$_{3}$-ligands cause HOMO/LUMO energy \textit{destabilization} in capped NPs, more pronounced for the LUMOs than for the HOMOs. Orbital destabilization drastically reduces both the IP$_{v}$ and EA$_{v}$ of the NPs compared with the bare NPs. For SCH$_{3}$-capped Cd$_{6}$X$_{6}$ NPs, formation of expanded structures was found to be preferable to crystal-like structures. SCH$_{3}$-groups cause \textit{destabilization} of the HOMOs of the capped NPs and \textit{stabilization} of their LUMOs, which indicates a reduction of the IP$_{v}$ of the capped NPs compared with the bare NPs. For the Cd$_{9}$X$_{9}$ NPs, similar trends in stabilization/destabilization of frontier orbitals were observed.