Optical and electrical properties of vanadium pentoxide xerogel films: modification in electric field and the role of ion transport

Abstract

Reversible modification of the optical and electrical properties of V2O5×nH2O gel films under the action of an electric field is studied. It is shown that under cathodic polarization (I = 10-6-10-5 A, t~10min, film thickness ~10 µm), the films turn from brownish-yellow to red. This internal electrochromic (EC)effect is caused by the redistribution of hydrogen ions inside the film, instead of insertion from the outside (from an electrolyte). A local increase in the hydrogen concentration occurs near the cathode, and this in turn results in modification of the optical properties. The rise of transmittance in the long-wavelength region of the spectrum, as well as the shift of the absorption edge near hν~2.5eV towards longer wavelengths, is observed. In addition, according to the infrared data, some increase in water content also contributes to the process of colouration. The change in the optical properties is accompanied by a change in the electrical properties, namely, an increase in ionic conductivity from ~4×10-5 to 10-4Ω-1 cm-1. At higher currents, electroforming resulting from the transport of oxygen ions occurs. This process leads to the formation of a channel consisting of vanadium dioxide due to reduction of V2O5 to VO2. Electrical switching with the S-type negative resistance, associated with an electrothermally driven metal-insulator transition in the channel, is observed in the sandwich M/V2O5×nH2O/M devices (unlike the planar devices described in the literature). Finally, applied potentialities of these phenomena for micro- and opto-electronics (EC devices, sensors) are discussed.