Transitions in ZnS and CdSe quantum dots and wave-function symmetry

Abstract

Excitation energies for wurtzite spherical ZnS and CdSe quantum dots in the range of 40–4000 atoms were calculated using empirical pseudopotentials and a real-space basis. The energies are compared to experiments and other pseudopotential models. For ZnS quantum dots, squared transition dipole sums were computed efficiently, without the need for full wave functions of the excited states; and some transition dipole calculations include the effects of an approximate electron-hole Coulomb potential. Squared transition dipole sums from the highest energy linear dipole like valence states to the lowest excited state were computed as a function of dot size. The model predicts that the per atom dipole transition sum decreases with quantum dot size for those transitions. The mixing of even and odd angular components and charge asymmetry of the wave functions affect the dipole transition strengths. The total oscillator strength for the lowest energy transition region increases with size at small radii, resembling the pattern recently observed experimentally for CdSe quantum dots. We examined the role of wave-function angular momentum for transitions to conduction band surface states.