Tungsten oxide nanorod growth by pulsed laser deposition: influence of substrate and process conditions.

Tungsten oxide nanorods (NRs) have been grown on W, Ta and Cu substrates following 193 nm pulsed laser ablation of a WO3 target in a low background pressure of oxygen. The deposited materials were analysed by scanning and (high resolution) transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), X-ray diffraction, Raman and X-ray photoemission spectroscopy, and tested for field emission. In each case, HRTEM analysis shows NR growth along the [100] direction, and clear stacking faults running along this direction (which are also revealed by streaking in the SAED pattern perpendicular to the growth axis). The NR composition in each case is thus determined as sub-stoichiometric WO(3-δ), but the NR morphologies are very different. NRs grown on W or Ta are short (hundreds of nm in length) and have a uniform cross-section, whereas those grown on a Cu substrate are typically an order of magnitude larger, tapered, and display a branched, dendritic microstructure. Only these latter NRs give significant field emission.