Metal-to-insulator transition induced by UV illumination in a single SnO₂ nanobelt

An individual tin oxide (SnO₂) nanobelt was connected in a back-gate field-effect transistor configuration and the conductivity of the nanobelt was measured at different temperatures from 400 K to 4 K, in darkness and under UV illumination. In darkness, the SnO₂ nanobelts showed semiconductor behavior for the whole temperature range measured. However, when subjected to UV illumination the photoinduced carriers were high enough to lead to a metal-to-insulator transition (MIT), near room temperature, at T _MIT = 240 K. By measuring the current versus gate voltage curves, and considering the electrostatic properties of a non-ideal conductor, for the SnO₂ nanobelt on top of a gate-oxide substrate, we estimated the capacitance per unit length, the mobility and the density of carriers. In darkness, the density was estimated to be 5-10 × 10¹⁸ cm^-3, in agreement with our previously reported result (Phys. Status Solid. RRL 6, 262-4 (2012)). However, under UV illumination the density of carriers was estimated to be 0.2-3.8 × 10¹⁹ cm^-3 near T _MIT, which exceeded the critical Mott density estimated to be 2.8 × 10¹⁹ cm^-3 above 240 K. These results showed that the electrical properties of the SnO₂ nanobelts can be drastically modified and easily tuned from semiconducting to metallic states as a function of temperature and light.