STM and XPS characterisation of vacuum annealed nanocrystalline WO3 films

Scanning tunnelling microscopy and X-ray Photoelectron Spectroscopy were conducted on magnetron sputtered WO3 thin films, following a sequence of ultra high vacuum anneals from 100 degrees C to 900 degrees C. Annealing from 100 degrees C to 400 degrees C induced an upward surface band bending of about 0.3 eV, attributed to the oxygen migration from the bulk to the surface, but no changes in the surface topography. Chemical changes occurred from 600 degrees C to 800 degrees C, associated with the formation of secondary oxide species. STM imaging showed that the film surface consists of amorphous particles 35 nm in size up to 600 degrees C, while higher temperatures resulted in an increase in particle size. Crystallisation of the nanoparticles started to occur after annealing at 600 degrees C. The implications in terms of gas sensing are discussed. (c) 2007 Elsevier B.V. All rights reserved.