On the origin of enhanced photoconduction and photoluminescence from Au and Ti nanoparticles decorated aligned ZnO nanowire heterostructures

We have investigated the mechanism of photoinduced charge transport and origin of enhanced PC and PL from Au and Ti NPs decorated vertically aligned ZnO NWs arrays. Uniform decoration of metal NPs on the surface of the ZnO NWs was confirmed by high resolution electron microscopy imaging. Absorption spectra also indicate the presence of metal NPs layer with different thicknesses. At a fixed bias, the dark current of the Au/ZnO heterostructures decreases with the increase of Au coverage, while the Ti/ZnO heterostructures show very high dark current. The photocurrent (PC) spectra show a strong peak at the UV region due to the band-edge absorption followed by generation of the photocarriers and two other peaks in the visible region. For the Au/ZnO heterostructures, the PC increases with increase of Au coverage up to a certain thickness and then decreased. On the other hand, the PC in Ti/ZnO heterostructures increases with the increase of the Ti coverage. The PL spectra for both the system are consistent with the respective PC spectra, which show significant improvement in the band-edge related UV emission and quenching of green emission. The Ti/ZnO heterostructures shows much faster photoresponse compared to the Au/ZnO heterostructures. The changes in the PC and PL spectra with the size of the metal NPs are studied systematically and explained. A model based on energy band alignment is proposed to explain the results. (C) 2011 American Institute of Physics. [doi:10.1063/1.3671023]