Seedless and catalyst-free growth of zinc oxide nanostructures on graphene by thermal evaporation

Metal-oxide, namely zinc oxide (ZnO) nanostructures and thin films on graphene is interesting because these structures can offer additional functionality to graphene for realizing advanced electronic and optoelectronic applications. Graphene has a great potential for novel electronic devices because of its extraordinary electrical mobility exceeding 104 cm2/Vs and a thermal conductivity of 103 W/mK. Therefore, with the excellent electrical and thermal characteristics of graphene layers, the hybrid ZnO/graphene structure is expected to offer many sophisticated device applications such as sensing devices. In this study, the seed/catalyst-free growth of ZnO on single layer (SL) and multilayer (ML) graphene by thermal evaporation of Zn in the presence of oxygen (O2) gas was performed. The effects of substrate temperatures, substrate positions and graphene thicknesses on the morphological, structural, and optical properties were found to be very pronounced. The grown ZnO structures exhibit three different structures, i.e., nanoclusters, nanorods, and thin films at 600°C, 800°C, and 1,000°C, respectively. By setting the substrate to be inclined at 90°, the growth of ZnO nanostructures, namely nanoclusters and nanorods, on SL graphene was successfully realized at temperatures of 600°C and 800°C, respectively. However, no growth was achieved at 1,000°C due to the possible severe oxidation of graphene. For the growth on ML graphene at 600°C with an inclination angle of 90°, the grown structures show extremely thick and continuous cluster structures as compared to the growth with substrate’s inclination angle of 45°. Moreover, the base of nanorod structures grown at 800°C with an inclination angle of 90° also become thicker as compared to 45°, even though their densities and aspect ratios were almost unchanged. The morphologies of grown structures at 1,000°C with an inclination angle of 90° do not show significant difference with 45°. The intensity ratio of UV emission (IUV) and visible emission (IVIS) was changed, depending on the temperature. The structures grown at a low temperature of 600°C show the highest value of IUV/IVIS of 16.2, which is almost two times higher than the structures grown on SL graphene, indicating fewer structural defects. From the results obtained, the temperature below 800°C, substrate position inclined at 90° 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 overcome the problem of graphene’s oxidation that takes place during the growth