Effects of Interface States on Photoexcited Carriers in ZnO/Zn2SnO4 Type-II Radial Heterostructure Nanowires

Type-II band alignment of heterostructure contributes to spatially separate electrons and holes leading to an increase in minority carrier lifetime, which has much more advantages in photocatalytic activities and photovoltaic device applications. Here, Zn2SnO4-sheathed ZnO radial heterostructure nanowires were constructed to investigate systematically interfacial charge separation. The lattice mismatch between ZnO and Zn2SnO4 induces interface states to exist at their heterointerface. At low pump fluence, photoexcited charges are localized within the ZnO core rather than separated due to the large interface barrier. Correspondingly, only ZnO-related bandedge ultraviolet (UV) and green emissions are dominated in photoluminescence spectra. At high pump fluence, however, impurities are ionized and electrons trapped in interface states are excited, resulting in a decrease in interface barrier, which makes photogenerated charges efficiently separated at their heterointerface by direct tunneling, and, consequently, an additional blue-violet emission, attributed to the heterointerface recombination of electrons in Zn2SnO4 conduction band (CB) and holes in ZnO valence band. Additionally, the heterointerface can separate effectively photoexcited carriers and form a photovoltaic effect. Our results provide the localization/separation condition of photogenerated charges for the type-II band alignment of core/shell heterostructure, which should be very useful for the realization of underpinned mechanism of the developed optoelectronic devices.