Small polaron formation in porous WO3−x nanoparticle films

Porous tungsten oxide nanoparticle films were prepared by reactive gas evaporation. The structure was studied by x-ray diffraction and scanning electron microscopy, and the oxygen nonstoichiometry was inferred by x-ray photoelectron spectroscopy, elastic recoil detection analysis, and neutron scattering. Specifically, the films consisted of WO3−x with 0.25<x<0.4. The optical and electrical data were consistent with the formation of small polarons having a radius of 5–6Å. The infrared optical data, used to extract information on phonon energies, were instrumental to reach this conclusion. The polaron hopping energy was about half the polaron binding energy, as expected from the theory.