STEM-Cathodoluminescence of SnO2 nanowires and powders

For the first time, ultra-high spatial resolution STEM-cathodoluminescence has been applied to SnO2 nanowires and nanoparticles in order to study the spatial distribution of point defects that form deep levels in the band gap and often dictate their gas-sensor performance. SnO2 nanowires consistently had emission signals centered at 1.76, 1.99, and 2.45 eV. Emission at 1.99 eV was found to be more intense with the STEM probe at the edges of nanowires and in higher surface-area nanoparticles compared to emission most intense at 2.45 eV with the probe in the center of a nanowire or particle. This result is contrary to recent studies proposing that both emission energies are associated with two different surface oxygen vacancy types. The ability to spatially map the luminescence of a single nanostructure also facilitated much less-ambiguous spectral deconvolution consistent across many particles which gives confidence in properly assigning the energies of each defect state. Individually-probed SnO2 nanowires were found to have much more consistent and reproducible defect states compared to commercial nanoparticles which has implications in stability as gas sensors and efforts to model interactions between analyte gases and the oxide surface.