Pure Exciton UV Emission of Colloidal ZnO Nanocrystals and Evidence of Surface Origin of Green Luminescence

A semiconductor-molecule interface acts as a quasi-two dimensional system with fruitful physics. Up to date, how the surface or interface affects the photophysical properties of the ZnO nanocrystals (NCs) is still under debate. Here, we fabricate the ideally surface-passivated colloidal ZnO NCs, which exhibit nearly pure exciton-stemmed UV emission. The nitrogen atoms of amino-moiety of the long-chain oleylamine groups passivate the ZnO NC surfaces and cause complete vanishing of the green emission exhibited in the un-passivated ZnO NCs and usually observed in various kinds of ZnO nanostructures. It is found that the arising and disappearance of the green emission are reversible by breaking and recovering the optimal surface passivation. The green emission of these ZnO NCs originates from the electronic transition from the conduction-band delocalized state to surface-localized state.