Fast-Response Single-Nanowire Photodetector Based on ZnO/WS Core/Shell Heterostructures.

The surface plays an exceptionally important role in nanoscale materials, exerting a strong influence on their properties. Consequently, even a very thin coating can greatly improve the optoelectronic properties of nanostructures by modifying the light absorption and spatial distribution of charge carriers. To use these advantages, 1D/1D heterostructures of ZnO/WS core/shell nanowires with a-few-layers-thick WS shell were fabricated. These heterostructures were thoroughly characterized by scanning and transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. Then, a single-nanowire photoresistive device was assembled by mechanically positioning ZnO/WS core/shell nanowires onto gold electrodes inside a scanning electron microscope. The results show that a few layers of WS significantly enhance the photosensitivity in the short wavelength range and drastically (almost 2 orders of magnitude) improve the photoresponse time of pure ZnO nanowires. The fast response time of ZnO/WS core/shell nanowire was explained by electrons and holes sinking from ZnO nanowire into WS shell, which serves as a charge carrier channel in the ZnO/WS heterostructure. First-principles calculations suggest that the interface layer i-WS, bridging ZnO nanowire surface and WS shell, might play a role of energy barrier, preventing the backward diffusion of charge carriers into ZnO nanowire.