Seed/catalyst-free growth of zinc oxide on graphene by thermal evaporation: effects of substrate inclination angles and graphene thicknesses

A seed/catalyst-free growth of ZnO on graphene by thermal evaporation of Zn in the presence of O-2 gas was further studied. The effects of substrate positions and graphene thicknesses on the morphological, structural, and optical properties were found to be very pronounced. By setting the substrate to be inclined at 90 degrees, the growth of ZnO nanostructures, namely, nanoclusters and nanorods, on single-layer (SL) graphene was successfully realized at temperatures of 600 degrees C and 800 degrees C, respectively. For the growth on multilayer (ML) graphene at 600 degrees C with an inclination angle of 90 degrees, the grown structures show extremely thick and continuous cluster structures as compared to the growth with substrate's inclination angle of 45 degrees. Moreover, the base of nanorod structures grown at 800 degrees C with an inclination angle of 90 degrees also become thicker as compared to 45 degrees, even though their densities and aspect ratios were almost unchanged. Photoluminescence (PL) spectra of the grown ZnO structures were composed of the UV emission (378-386 nm) and the visible emission (517-550 nm), and the intensity ratio of the former emission (I-UV) to the latter emission (I-VIS) changed, depending on the temperature. The structures grown at a low temperature of 600 degrees C show the highest value of I-UV/I-VIS of 16.2, which is almost two times higher than the structures grown on SL graphene, indicating fewer structural defects. The possible growth mechanism was proposed and described which considered both the nucleation and oxidation processes. From the results obtained, it can be concluded that temperature below 800 degrees C, substrate position inclined at 90 degrees towards the gas flow, and ML graphene seems to be preferable parameters for the growth of ZnO structures by thermal evaporation because these factors can be used to overcome the problem of graphene's oxidation that takes place during the growth.