Growth mechanisms for ZnO nanorods formed by pulsed laser deposition

Arrays of well-aligned ZnO nanorods have been grown on Si substrates at 600 ∘C using a catalyst-free pulsed laser deposition (PLD) method, with and without pre-deposition of a thin ZnO layer at a lower substrate temperature. Deposited products were analyzed and characterized by scanning and transmission electron microscopy (SEM and TEM), energy dispersive X-ray analysis, X-ray diffraction and photoluminescence (PL) measurements. EM revealed that rods grown directly on Si exhibit needle-like morphologies, with diameters, d, typically in the range 20-60 nm, lengths ∼200-800 nm and, in most cases, capping particles of similar cross-section at their tips. HRTEM images show that these nanoparticles are ZnO also, suggesting that these derive from post-growth crystallisation of oxygen rich molten zinc droplets that cap the nanorods during growth. PLD of ZnO onto Si substrates that have been pre-coated with a thin ZnO film deposited at 300 ∘C yields denser, more uniform arrays of longer (∼1-1.2 μm), thinner (d∼6-20nm) nanorods, without any obvious capping particle. This suggests that a ZnO buffer layer can play a useful role in providing a high density of nucleation sites for subsequent growth of smaller diameter nanorods. The respective product arrays are most readily understood in terms of vapor-liquid-solid (V-L-S) and vapor-solid (V-S) growth models, respectively. Comparative studies of the room temperature PL from the respective samples reveal much higher UV emission intensity from nanorod arrays grown on the ZnO pre-coated Si substrates.