Variation of the optical properties with size and composition of small, isolated CdxSey+ clusters

Global energy minimum structures and optoelectronic properties are presented for isolated CdxSey+ clusters with x + y ≤ 26. The compositional‐ and size‐dependent variation of optical, electronic and geometric properties is systematically studied within the framework of ground state and time‐dependent density functional theory. The applied methods are justified by benchmarks with experimental data. It is shown that the optical gap can be tuned by more than 2 eV by only changing the composition for a fixed number of atoms. The stoichiometric species reveal an unexpected size‐dependent behavior in comparison to larger colloidal CdSe quantum dots, that is, a redshift of the optical gap was observed with decreasing cluster size in contrast to predictions by quantum‐size effects. This unexpected result is discussed in detail taking the positive charge of the clusters into account.