Optical properties of ZnO nanostructures: a hybrid DFT/TDDFT investigation

We report on the first principles computational modeling of the electronic and optical properties of ZnO nanosystems. 1D, 2D and 3D ZnO nanostructures with different characteristic size are examined and their lowest optical transition energies are calculated by hybrid TDDFT to investigate the effect of quantum confinement on the optical properties of the systems. For a realistic 3D nanoparticle model we evaluate the influence of oxygen vacancies, including relaxation of the excited states, on the photoluminescence process. The results are in quantitative agreement with experimental data, indicating that neutral oxygen vacancies are likely at the origin of green emission in the ZnO nanostructure. The calculated emission process corresponds to radiative decay from a long-living triplet state, in agreement with the experimental evidence of ∼μs emission lifetime and with the results of optically detected magnetic resonance experiments.