Sun et al. Nanoscale Research Letters 2011, 6:556 http://www.nanoscalereslett.com/content/6/1/556 Page 3 of 8 Figure 1 SEM images, TEM images, and XRD pattern. (a) Top-viewed and (b) cross-sectional SEM images of as-grown ZnO NRAs, (c) TEM image of the ZnO core with approximately 5 nm Al2O3 shell, (d) TEM image of the ZnO core with approximately 10 nm ZnO shell, and (e) XRD pattern of as-grown ZnO NRAs. be attributed to the flat-band effect [27,37]. Negatively charged oxygen ions may adsorb on the surface of as- grown ZnO nanorods, resulting in a depletion region near the surface [38]. Weber et al. have reported that the width of depletion region is about 20 nm [39], which is smaller than the diameter of the ZnO nanorods (approximately 100 nm) prepared in this study. This depletion region can be regarded as an upward band bending toward the surface as presented in the band diagram shown in Figure 3a. When the ZnO NRAs are irradiated by the pumping laser beam, the photo-gener- ated holes are inclined to accumulate near the surface, and the photo-generated electrons tend to reside inside the core. As a result, the wavefunctions of electrons and holes are separated from each other, lowering the prob- ability of radiative recombination to yield NBE emission. However, as plotted schematically in Figure 3b, the Al2O3 shell layer would eliminate the oxygen ions adsorbed on the ZnO surface and hence reduce the band bending near the interface [27]. Therefore, the