Evidenece for surface states in ZnO nanostructures using non-linear optical spectroscopy

An unexpected presence of ferromagnetic (FM) ordering in nanostructured ZnO has been reported previously. Recently, from our detailed magnetization studies and \textit{ab initio} calculations, we attributed this FM ordering in nanostructured ZnO to the presence of surface states, and a direct correlation between the magnetic properties and crystallinity of ZnO was observed. Such defect induced surface states appear as green/yellow emission in the photoluminescence spectrum of ZnO nanostructures. In this study, through a systematic sample preparation of both pristine and Co-doped ZnO nanostructures, and detailed PL and nonlinear optical measurements, we confirm that the observed FM ordering is due to the presence of surface states.