Photoconductivity in one dimensional metal oxides nanostructures.

Abstract

With recent development in individual nanowire (NW) characterization and device fabrication, study of photoconductivity of individual NWs has been proven to be an efficient approach in probing their electronic and surface related properties. In this work, systematic studies were carried out to investigate the photoconductivity of individual Nb2O5 and V2O5 NWs. The synthesized Nb2O5 and V2O5 NWs were characterized using various characterization techniques. Global and focused laser beam irradiation techniques were used as experimental approach for photoresponse study. The focused laser beam irradiation with spot size < 1 ?m had the advantage of probing the desired section of isolated NW along the NW-Pt interface. We observed, fast and prominent photoresponse from individual Nb2O5 NW towards visible and infrared laser irradiation under various conditions. The global irradiation on Nb2O5 NW showed multiple photocurrent contribution from defect level excitations, surface states and thermal heating effects. Significant photoresponse was observed in vacuum condition. The time characteristic of the observed photoresponse was further analysed and revealed characteristic response time in the photoresponse of the NW to laser irradiation. Interestingly, the photoresponse with focused laser beam showed large enhancement compared to global irradiation at relatively low applied bias. We found that NW-Pt contact played a major role in the photoresponse of the sample. This envisioned in developing better insight into the photoresponse of the NW, particularly along the metal-NW interface. The mechanisms to account for the observed photocurrent were proposed. We proposed that Schottky barrier formation and photo-induced thermoelectric effects are key carrier transport mechanisms for photocurrent generation, at the NW-Pt interface at zero bias. While at applied bias, the thermoelectric effect was observed to be less significant, and most photoresponse was likely from defect and surface state excitations. V2O5 NW showed rapid photoresponse at vacuum condition and very small photocurrent (~1 nA) in ambient condition at applied bias. The electrical properties were investigated at various pressure conditions and with varying laser power. From the time characteristics analysis, photocurrents in V2O5 NW were mostly attributed to thermal heating. The NW device was modelled as metal-semiconductor-metal structure composed of two Schottky diode connected back-to-back in series. Quantitative analysis was carried out and the carrier density and mobility of V2O5 NW were determined.