Evidence of Unintentional n-Doping in ZnO Nanorods

ZnO nanorods were synthesized by using a simple two-step hydrothermal approach. The nanorod diameter ranged from 40 to 60 nm and their length was about 2 mu m. X-ray diffraction shows good crystalline quality with an axial alignment of the nanorods along the wurtzite c axis. Raman and visible photoluminescence demonstrate that thermal annealing suppresses structural defaults. Photoluminescence experiments performed between 3 and 200 K close to the exciton resonances evidence the presence of optical transitions related to electron-bound exciton scattering. To our knowledge, this is the first time that this feature is identified in ZnO nanorods. The occurrence of such phenomena is an indication that, despite a good crystalline quality, the samples exhibit a strong native n-type character.