Controlling The Growth Direction of ZnO Nanowires on c-Plane Sapphire

Well oriented vertical ZnO nanowires (NWs) are grown on c-plane sapphire via a vaporphase transport process using an Au thin film as a catalyst. This new finding is unexpected due to the fact that the lattice mismatch between the zinc oxide and the underlying substrate is 18%. Xray diffraction (XRD) analysis shows that single-crystal, wurtzite NWs grow in the [0001] direction normal to the basal sapphire plane, which proves that a-plane sapphire is not essential for growth of vertical ZnO NWs, as has been previously stated.[1] We have found that by controlling the thickness of the Au-film and pre-growth annealing of the Au/sapphire substrate NWs can be grown either tilted or vertical. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) studies on Au films with thicknesses ranging from 1 to 10 nm show that in the absence of film annealing, NWs can be grown 32o tilted from the surface normal, whereas pre-annealed Au films result in growth of NWs in the surface normal direction. We attribute the formation of the normal and tilted growth directions to the surface concentration of O and Al ions on sapphire.