Sun et al. Nanoscale Research Letters 2011, 6:556 http://www.nanoscalereslett.com/content/6/1/556 for the NRAs with different Al2O3 shell-layer thickness, indicating that the increase of the Al2O3 shell-layer thickness enhances both the NBE and deep-level emis- sions due to the flat-band effect. To further investigate the effect of surface band bend- ing in ZnO nanorods, we conducted the PL measurement on ZnO/ZnO core-shell NRAs with different thicknesses of ZnO shell layers. Since the absorption coefficient of ZnO at l = 325 nm is about 1.5 × 105 cm-1 [40] and the estimated penetration depth is approximately 67 nm, both ZnO cores and ZnO shells could be excited by the He-Cd laser during the PL measurement. Figure 4 shows the PL spectra of the as-grown ZnO NRAs and ZnO/ ZnO core-shell NRAs at room temperature. As compared with as-grown ZnO NRAs, the NBE emission was enhanced and the deep-level band around 550 nm was suppressed after a 5-nm-thick ZnO shell layer was depos- ited. This can be realized that the ZnO shell layer could give rise to the increase of the flat-band region in the ZnO core and the reduction in the density of oxygen interstitials at the surface of ZnO core. Similar to the ZnO/Al2O3 core-shell NRAs, the residual deep-level band around l ≈ 550 nm of the NRAs coated with a 5- nm-thick ZnO shell layer can be attributed to light emis- sion from the oxygen interstitials inside the ZnO core. Figure 4 also presents the remarkable shift of the defect-related luminescence, from the yellow band Page 6 of 8 (approximately 550 nm) to the green band (approxi- mately 490 nm), as the thickness of the ZnO shell layer is greater than 10 nm. This green band can be also found in the PL spectrum of the ZnO seed layer grown by ALD, as shown in Figure 5, suggesting that the green band may originate from the ALD ZnO shell layer. It has been reported that the green band arises from the recombination of the electrons in the conduction band and the holes trapped by the V0+ center (one electron at the site of oxygen vacancy) [27,41]. As shown schemati- cally in Figure 6a, the photo-generated holes are accu- mulated near the surface of ZnO nanorods due to the surface band bending. As a 5-nm-thick ZnO shell layer was deposited by ALD, the V0+ centers in the ZnO shell layer trap the photo-generated holes and then convert to V0++ , as illustrated in Figure 6b. However, the band bending depletes the electrons near the surface so as to suppress the recombination of the electrons and the V++ centers. As a result, the green band associated with 0 V++ did not appear; instead, the yellow band from the 0 oxygen interstitials inside the ZnO core was observed in the PL spectrum. Figure 6c shows that the extension of flat-band region in the ZnO core can reach the ZnO/ ZnO core-shell interface as the ZnO shell layer is thick enough. Therefore, the V++ centers can recombine with 0 the electrons in the conduction band to yield the green Figure 4 PL spectra. Room-temperature PL spectra of as-grown ZnO NRAs and those coated with ZnO shell layers of different thicknesses.