Towards p-type conductivity in SnO2 nanocrystals through Li doping.

This paper examines electrical transport properties and Li doping in SnO(2) synthesized by the sol-gel method. Solid-state (7)Li-NMR lineshapes reveal that Li ions occupy two distinct sites with differing dynamic mobilities. The chemical exchange rate between the two sites is, however, too slow for detection on the NMR timescale. Compressed nanoparticulate films of this doped semiconductor exhibit a positive Seebeck coefficient implying a p-type conductivity. A variable-temperature direct current conductivity, over a 25-350 degrees C temperature range, follows an Efros-Shklovskii variable range hopping (ES-VRH) conduction mechanism (ln(rho) versus T(-1/2)) at temperatures below 100 degrees C with a crossover to 2D Mott variable range hopping (M-VRH) (ln(rho) versus T(-1/3)) conduction at temperatures above 250 degrees C. In a transition region between these two limiting behaviors, the dc resistivity exhibits an anomalous temperature-independent plateau. We suggest that its origin may lie in a carrier inversion phenomenon wherein the majority carriers switch from holes to electrons due to Li ion expulsion from the crystalline core and creation of oxygen vacancies generated by loss of oxygen at elevated temperatures.