Nanostructured tungsten oxide with controlled properties: Synthesis and Raman characterization

Abstract

Tungsten oxide thin films are interesting for their electrochromic and gas sensing properties. For technological applications, the control of morphology (effective surface, porosity) and structure (crystallinity, stoichiometry) at the nanoscale is of paramount interest to deliver distinctive properties. Nanostructured tungsten and tungsten oxide films have been synthesized by nanosecond pulsed laser deposition (PLD) starting from a W metallic target. Both inert and reactive buffer gases (He, Ar, dry air) have been exploited to induce cluster formation and/or oxidation and to vary the deposition energies of ablated species. A wide range of morphologies extending from compact and smooth, up to extremely porous and spongy-like structures have been obtained. The structure, the oxide phase and the degree of crystallinity of the films have been investigated by Raman spectroscopy. Films deposited in the presence of inert buffer gases reveal a spontaneous ex-situ oxidation, when exposed to ambient atmosphere, which is strongly related to the nanostructure. Instead, deposition in a varying pressure of dry air permits to grow tungsten oxide films and to tune their structure from amorphous to nano- and microcrystalline with different coexisting oxide phases.