Sun et al. Nanoscale Research Letters 2011, 6:556 http://www.nanoscalereslett.com/content/6/1/556 Page 5 of 8 A B Figure 3 Band diagrams. Schematic band diagrams of (a) as-grown ZnO NRAs and (b) ZnO/Al2O3 core-shell NRAs. the carrier recombination through the deep-level states inside the ZnO core. As illustrated in Figure 3a,b, the flat-band effect may also enhance the deep-level emis- sion around l ≈ 550 nm originating from the oxygen interstitials inside the ZnO core due to the increase of the wavefunction overlap. Accordingly, as shown in Fig- ure 2b, the normalized PL spectra present almost the same ratio of the deep-level band to the NBE emission