Tight-binding theory of ZnS/CdS nanoscale heterostructures:: Role of strain and d orbitals

The electronic and optical properties of colloidal multishell ZnS/CdS nanoscale heterostructures have been studied in the framework of empirical tight-binding models. Our approach takes into account the effects of the strain caused by the large lattice mismatch at the interface between the two materials. We show that the inclusion of d orbitals into a minimal basis set is necessary to provide a correct description of the ZnS and CdS clads that are only a few monolayers (MLs) thick. The role of strain is also important. Strain shifts of the electron energy levels are highly dependent on the thickness of the core and shell materials. The effects of strain in the valence band are more complex. In CdS/ZnS structures, strain can change the symmetry of the ground hole state. In ZnS/CdS systems, strain lowers the energy of the ground hole state of P symmetry with respect to the first S symmetry state. Lattice relaxation also redistributes the charge densities of electron and hole states. The predicted absorption onsets resulting from our model are in good agreement with the experimental data and in better agreement with data than previous theories without strain and d orbitals.