Enhanced optical luminescence in ZnO nanostructures following O 1s to p$_{z}$ excitation*

Room temperature ultraviolet ($\sim $385 nm) lasing in ZnO nanostructures has recently been demonstrated.$^{1}$ This phenomenon is thought to arise from the natural cavity formed by the wurtzite nanostructure and its faceted ends. X-ray excited optical luminescence (XEOL) provides the capability to determine the nature of the sites responsible for producing low energy (1-6 eV) fluorescence. We will present XEOL excitation curves taken at the Zn L and O K edge obtained using both the defect ($\sim $510 nm) and bandgap ($\sim $370 nm) transitions as signals. Results obtained at the Zn L edge resemble the x-ray absorption curve of the nanostructure. However, striking differences are observed at the O K edge. Excitation to states of p$_{z}$ symmetry (along the c axis) leads to enhanced luminescence while excitation to p$_{x,y}$ states (lying in the basal plane) decreases the yield. We interpret this phenomenon as resulting from the lower probability of quenching by near surface defects for states excited along the c-axis as opposed to those excited perpendicular to it. 1. M.H. Huang, \textit{et al}., Science \textbf{292}, 1897 (2001).