Enhanced near band edge luminescence of Ti/ZnO nanorod heterostructures due to the surface diffusion of Ti

Information on the mechanistic differences in the luminescence properties of Ti/ZnO nanorods (NRs) has been obtained through the preparation of heterostructures by (a) varying the thickness of Ti from 1 nm to 20 nm keeping the substrate temperature at 400 degrees C, (b) varying the substrate temperature from room temperature (RT) to 500 degrees C while keeping the metal thickness constant at 10 nm and (c) annealing the RT Ti sputtered NRs at temperatures of 400 degrees C and 500 degrees C. The photoluminescence (PL) spectra show that the near band edge luminescence of ZnO in the ultraviolet (UV) region is enhanced as the thickness of Ti increases up to 5 nm and, thereafter, it falls. Sputtering of Ti on ZnO NRs at RT does not cause any UV enhancement but when sputtered at and above 400 degrees C, the UV intensity is enhanced. Annealing of RT Ti sputtered NRs at and above 400 degrees C also results in the enhancement of the UV peak, although with a lesser magnitude. Analysis of the PL results, supported by X-ray diffraction, field emission scanning electron microscopy, elemental mapping, high resolution transmission electron microscopy, Fourier transform infrared spectroscopy and electrical I-V measurement results, show a clear indication that the surface diffusion of Ti causes a reduction in the surface defects.