Growth of aligned ZnO nanorod arrays by catalyst-free pulsed laser deposition methods

Arrays of well-aligned ZnO nanorods were synthesized on a Si substrate at 600°C by 193 nm pulsed laser ablation of a ZnO target in low pressures of oxygen. Electron microscopy and X-ray diffraction revealed highly c-axis aligned, ultra-thin, needle-like nanorods with tip diameters <50 nm growing vertically from the substrate. Many of the rods appear to be capped by particles, which high-resolution transmission electron microscopy shows to be ZnO also. It is suggested that these derive from post-growth crystallisation of oxygen rich molten zinc droplets that cap the nanorods during growth - indicating a vapour-liquid-solid growth mechanism. The room temperature photoluminescence spectra of these samples show strong, narrow bandwidth UV emission ∼380 nm and only weak visible emission.