Optical absorption edge evolution of vanadium pentoxide films during lithium intercalation

Abstract

The influence of lithium intercalation on the optical absorption of vanadium pentoxide films is reported in this paper. Thin films were prepared by thermal evaporation followed by annealing in air. Lithium was inserted electrochemically so that Li x V 2O 5 (0 ≤ x ≤ 0.54) was obtained. The pristine and lithium inserted films were characterized by X-ray diffraction (XRD), optical spectroscopy, and ellipsometry. The optical absorption coefficient was calculated from measured normal transmittance and reflectance spectra. XRD analysis has shown that the as-grown V 2O 5 films were amorphous, becoming microcrystal after annealing at 350 °C. The optical absorption spectra of all the samples exhibited two distinct regions of behavior: at high-energy where ( αhν) 1/2 varied linearly with the photonic energy hν, and a low-energy tail. The blue shift of the optical absorption edge is attributed to the Fermi level going into the conduction band after lithium insertion, and the relation between the change of the optical gap and the inserted electron concentration is interpreted in terms of a Burstein–Moss shift. The results confirm that the anodic electrochromism of V 2O 5 thin films is due to the blue shift of the absorption edge, and the near-infrared cathodic electrochromism arises from absorption of small polarons in V 2O 5.