Sun et al. Nanoscale Research Letters 2011, 6:556 http://www.nanoscalereslett.com/content/6/1/556 temperature. Therefore, highly conformal Al2O3 and ZnO shell layers could be deposited upon the surface of ZnO nanorods by ALD to form the ZnO/Al2O3 and ZnO/ ZnO core-shell NRAs in this study. PL measurements were conducted to investigate the optical characteristics of ZnO/Al2O3 and ZnO/ZnO core-shell NRAs. The near- band-edge (NBE) emission was significantly enhanced, and the deep-level band was suppressed by the Al2O3 and ZnO shells due to the flat-band effect and the reduction in the surface defect density. In addition, the shift of deep-level emissions from the yellow band to the green band in ZnO/ZnO core-shell structure was reported. The mechan- isms of flat-band effect and the shift of deep-level emis- sions were elucidated in detail. Experimental details The ZnO NRAs were synthesized on (100) Si wafers by aqueous chemical growth. Before the synthesis, a 50- nm-thick ZnO seed layer was deposited on the wafer at a temperature of 180°C by ALD. Diethylzinc and H2O vapors were used as the precursors for zinc and oxygen, respectively. After the ALD deposition, the seed layer was treated by rapid thermal annealing at 950°C for 5 min in nitrogen atmosphere to improve its crystal qual- ity. Afterwards, the ZnO NRAs were grown in 320 ml aqueous solution, containing 10 mM zinc nitrate hexa- hydrate and 5 ml ammonia solution, at 95°C for 2 h. More details of ZnO NRA synthesis have been described elsewhere [31,32]. Finally, Al2O3 and ZnO shell layers were prepared by the ALD on the as-grown ZnO NRAs to fabricate ZnO/Al2O3 and ZnO/ZnO core-shell NRAs. The precursors for Al2O3 deposition were trimethylaluminum and H2O vapors, and the deposition temperature was 180°C. The Al2O3 shell layers were 2, 5, and 10 nm in thickness. The ALD con- dition of ZnO shell layers was the same as that of the ZnO seed layer. The thicknesses of ZnO shell layers were 5, 10, and 15 nm, respectively. The details of ZnO and Al2O3 ALD parameters can be found in our pre- vious studies [33-35]. The structural characterization of ZnO NRAs was examined by Germini LEO 1530 field emission scanning electron microscopy (SEM) (Carl Zeiss Microscopy, Carl-Zeiss-Straße 56, 73447 Oberkochen, Germany) and FEI Tecnai G2 T20 transmission electron microscopy (TEM) (FEI Company, 5350 NE Dawson Creek Drive, Hillsboro, Oregon 97124 USA). X-ray diffraction (XRD) measurement was used to characterize the crystallinity and crystal orientation of ZnO NRAs. PL spectroscopy was measured in a standard backscattering configuration where the light emission from top surface of the ZnO NRAs was collected, using a continuous-wave He-Cd laser (l = 325 nm) as the excitation source. Page 2 of 8 Results and discussion Top-viewed and cross-sectional SEM images of as- grown ZnO NRAs are shown in Figure 1a,b, respec- tively. The diameter of ZnO nanorods is in the range of 90 to 100 nm, and the length is about 1 μm. The sub- strate-bound NRAs were mechanically scraped, soni- cated in ethanol, and deposited on carbon-coated copper grids for TEM characterization. Figure 1c,d shows low-magnification TEM images of ZnO/Al2O3 and ZnO/ZnO core-shell nanorods, indicating the uni- formity in both of the core and shell layers. It can be seen that about 5 nm Al2O3 and 10 nm ZnO shell layers were deposited upon the surface of ZnO nanorods, demonstrating high conformality of the ALD technique. XRD pattern of as-grown ZnO NRAs is shown in Figure 1e, and the only dominant peak corresponding to (0002) plane was observed in the spectrum, revealing that ZnO nanorods are highly c-axis orientated. Moreover, it was noted that ZnO NRAs cannot be synthesized on (100) Si wafers without the ZnO seed layer. Figure 2a shows the room-temperature PL spectra of as-grown ZnO NRAs and those coated with the Al2O3 shell layers. Both the NBE emission (l ≈ 380 nm) and deep-level band associated with the oxygen interstitials (Oi) (l ≈ 550 nm, yellow band) [22] were observed in the as-grown ZnO NRAs and ZnO/Al2O3 core-shell NRAs. As compared with as-grown ZnO NRAs, the NBE emission was significantly enhanced and the deep- level band was suppressed for the samples coated with Al2O3 shell layers. The intensity of NBE emission grows along with the increase of the Al2O3 shell-layer thick- ness. The deep-level band also increases slightly with the thickness of the Al2O3 shell layer. The PL spectra normalized to the peak intensity of each NBE emission are shown in Figure 2b. It can be seen that the ratio of the deep-level band to the NBE emission of the samples coated with Al2O3 shell layers is much smaller than that of as-grown ZnO NRAs. It may be also noted that the ratio of deep-level band to the NBE emission is almost identical for the ZnO/Al2O3 core-shell NRAs with dif- ferent shell-layer thickness, suggesting that the same mechanism governs the increase of the NBE and deep- level emissions with the Al2O3 shell-layer thickness. As compared with the deep-level band of as-grown ZnO NRAs, the considerable suppression of the deep- level luminescence by the deposition of Al2O3 shell layers, as shown in Figure 2a,b, can be ascribed to the decrease in the density of oxygen interstitials at the sur- face of ZnO core [36]. The residual deep-level emission from the ZnO/Al2O3 core-shell NRAs may mainly origi- nate from the oxygen interstitials inside the ZnO core. On the other hand, the remarkable enhancement of the ZnO NBE emission by depositing Al2O3 shell layers can