Investigation of charge transport and photoexcitation effects in metal oxide nanonets.
Date of Issue2012
School of Materials Science and Engineering
Tin oxide based, highly oriented metal oxide nanowire networks or ‘nanonets’ based devices are fabricated through photolithography free techniques. These nanonets studied at submillimeter scales in thin film transistors for macroelectronics & optoelectronics yielded good transparency and excellent electrical characteristics in vacuum. However, exposure of the SnO2 nanonet transistors to the ambient atmospheres resulted in positive threshold voltage shifts due to electron trapping by oxygen at the nanowire surface, thus introducing barriers for both intra- and inter- nanowire electrons transportation. Device performance in air was improved by applying passivation layers such as polystyrene or SiO2. Electrical and photophysical studies confirmed that the top few atomic layers of the oxide nanostructure surface critically influenced device performance showing high photo-response and strong influence of ambient atmosphere, especially oxygen and water vapor. Doping of tin oxide nanowires with antimony allowed modulation of electrical properties, with 5% Sb yielding highly conducting nanonets whereas doping with 0.5% Sb resulted in excellent semiconducting properties as well as stability in ambient atmospheres. In addition to conventional electrical probe techniques, non-contact optical methods were also used to confirm the semiconducting behavior of the nanonets; whereas surface analysis indicated that antimony is likely to segregate to the surface thus providing a degree of passivation and thus environmental stability. The significant role played by nanowire-nanowire junctions in the nanonet devices is investigated through a series of temperature-dependent charge transport studies and the existence of localized/trapped states at the junction sites is demonstrated as a function of changes in channel lengths and thus the number of nanojunctions present. These states aredetrimental to the semiconducting nature of the nanonets at low carrier densities caused by low conductivity or low wire density of the nanonets. However, semiconducting behavior will be greatly enhanced under the assistance of localized/trapped states at high nanowire densities or with higher carrier concentrations. Photoexcitation effects are enhanced by coating the nanonet with absorbers such as poly 3 hexylthiophene, copper indium selenide, or metallorganic dyes. The immediate transfer of free electrons from the photo-sensitive material to the nanonet upon light illumination ensures a promising photo-detecting function and potential photovoltaic applications in either organic-inorganic hybrid, thin-film or DSC format. In addition, ambipolar devices could also be realized by combining p-type conjugated polymers and nanonet devices. In summary, this study paves ways to an intensive understanding of the role of nanowire junctions in the nanonet-type electron transportation through precise control of the nanowire alignment, array density, the nanowire surface defects, the testing environment, and dopant concentration. Large-area uniformity testing demonstrates a pathway towards macro-electronic device integration. Together with photoexcitation study including the photovoltaic fabrication and characterization, these results suggest that SnO2 nanonets that offer fault tolerance, flexibility, high transparency and stable electrical properties could form a very attractive material set with properties that can find applications in both electronics and photovoltaic energy harvesting.