Transition in the Optical Emission Polarization of ZnO Nanorods

We study the electronic and optical properties of ZnO nanorods as a function of the aspect ratio using atomistic empirical pseudopotentials and configuration interaction for the excitonic states. We find a sharp crossover for the optical polarization, from an in-plane polarized luminescence, for an aspect ratio smaller than 3, to an out-of-plane (along c-axis) polarization for an aspect ratio larger than 3. This suggests the possibility to tailor the optical polarization by a change in morphology. We elucidate the underlying mechanism from an analysis of the valence band structure and highlight the importance of exciton correlations, which shift the crossover to a larger aspect ratio.