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.

Metal-insulator transitions and other electronic transitions) 72.80.Ga (Transition-metal compounds)Abstract. Effect of doping with H and W on the properties of V2O5 and VO2 derived from V2O5 gel has been studied. It is shown that the treatment of V2O5 in low-temperature RF hydrogen plasma for 1 to 10 min. leads to either hydration of vanadium pentoxide or its reduction (depending on the treatment conditions) to lower vanadium oxides. For some samples, which are subject to plasma treatment in the discharge active zone, a non-ordinary temperature dependence of resistance, with a maximum at T ~ 100 K, is observed. For W-doped VO2 films, it is shown that substitution of V 4+ with W 6+ results in a decrease of the temperature of metal-insulator transition.Also, it has been shown that the doping of the initial films with ~3 at.% of W reduces the statistical scatter in the threshold parameters of the switching devices with S-shaped I-V characteristics on the basis of V2O5 gel films.

We report on the experimental study of electrical instabilities in thin film structures on the basis of molybdenum oxides. Thin films of molybdenum oxide are obtained by thermal vacuum evaporation and anodic oxidation. The results of X-ray structural analysis, investigation of optical and electrical properties, are presented. It is shown that the initial vacuum-deposited oxide represents amorphous MoO 3 . In the MOM (metal-oxide-metal) structures with Mo oxide films obtained by the two methods, the effect of electrical switching with an S-shaped current-voltage characteristic is found. We put forward a hypothesis according to which the switching mechanism is associated with the development of electrical instability caused by the insulator-to-metal transition in Mo 8 O 23 . The switching channel, comprising this lower valence oxide, emerges in the initial film during the process of electrical forming of the MOM structure. The obtained results indicate the possibility of application of these structures in oxide micro-and nanoelectronics as electronic switches and other electronic devices.

The non-equilibrium (photoinduced) phase transition from a metal (M) state to a semiconducting (S) state is discussed using the example of samarium monosulfide. An expression for the critical concentration, analogous to the Mott criterion, is obtained. It is shown theoretically that the critical electron density for such a M → S transition should be of the order of, or little less than, the equilibrium density in the metallic phase beyond the transition point. Some relevant experimental data concerning the metal-insulator transitions in SmS and VO 2 are discussed.

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