Sun et al. Nanoscale Research Letters 2011, 6:556 http://www.nanoscalereslett.com/content/6/1/556 Page 4 of 8 Figure 2 PL spectra. (a) Room-temperature PL spectra of as-grown ZnO NRAs and those coated with Al2O3 shell layers of different thicknesses. (b) Normalized PL spectra of (a). The PL spectra were normalized to the peak intensity of the NBE emission. overlap between the wavefunctions of electrons and holes in the ZnO core is increased, leading to the enhancement of NBE emission. The increase of the Al2O3 shell-layer thickness from 2 to 10 nm may further lower the band bending near the interface and thus enhance the wavefunction overlap, resulting in the increase in NBE emission with the thickness of the Al2O3 shell layer. The same argument also holds for