Tight-binding theory of quantum-dot quantum wells: Single-particle effects and near-band-edge structure

Electron and hole states of multishell CdS/HgS/CdS quantum-dot quantum-well nanocrystals are determined by use of atomistic tight-binding theory. Single-particle energies, symmetries and charge densities, trapping in the internal quantum well, transition energies, and optical spectra are determined. Comparison with experiment shows that tight-binding theory provides a good description of nanosystems with monolayer variations in composition. Tight-binding theory correctly predicts the Stokes shift between the dim ground state and the lowest optically active transition. The energy splitting between the lowest two optically active transitions is also correctly described. The Stokes shift is determined by the level splitting between the lowest two hole states. The splitting between the lowest two optically active transitions is determined by the Stokes shift and the splitting between the lowest two electron states. Comparison with multiband effective mass theories shows that both theories provide a similar picture for the single-particle states but that the tight-binding theory provides a better description of observed transition energies. Calculations are done for different nanocrystal shapes, different positions and sizes of the internal quantum well, different values for the surface passivation, the spin-orbit coupling, the HgS bulk band gap, and the band offset. Results are robust to these variations indicating that the physical character of states in quantum-dot quantum wells is determined by the effects of global confinement in the dot and local confinement in the internal well rather than by the specific details of the quantum-dot quantum-well size, shape, or geometry or by special, fortuitous choices for material parameters in the models.