Room temperature and elevated temperature characterization of nanocrystalline SnO2 surfaces by scanning tunneling microscopy and spectroscopy

Semiconductor gas sensors based on nanocrystalline SnO2 offer many advantages over current technologies for detecting reducing gases, such as low cost, long lifetime, and high selectivity and sensitivity. However, the local surface properties on the nanoscale of SnO2 nanocrystals are not fully understood, which impedes the exploitation of the full potential of SnO2 for gas sensing applications. In this paper, we present a scanning tunneling microscopy and spectroscopy (STM/STS) study of nanocrystalline SnO2 at room temperature, and under standard sensing conditions at 120°C. STS data indicate that the electronic surface properties change with nanoparticle size, temperature and exposure to gas. The surface density of states in the band gap is shown to increase with temperature while CO exposures induce a large drop in the density of band gap states as the CO molecules react with chemisorbed oxygen species.