Sun et al. Nanoscale Research Letters 2011, 6:556 http://www.nanoscalereslett.com/content/6/1/556 Page 7 of 8 Figure 5 PL spectrum. Room-temperature PL spectrum of the ZnO seed layer grown by ALD. luminescence. As a result, the green band dominates over the yellow band as the ZnO shell-layer thickness is greater than 10 nm, as shown in the PL spectra in Figure 4. Conclusion In summary, the ZnO/Al2O3 and ZnO/ZnO core-shell NRAs have been prepared using the aqueous chemical synthesis and the conformal ALD technique. The deep-level emission around l ≈ 550 nm from the oxy- gen interstitials at the surface of ZnO cores was sup- pressed by the Al2O3 and ZnO shell layers. The shell layers also reduce the surface band bending, leading to the increase in overlap of the wavefunctions of elec- trons and holes in the ZnO core. Therefore, the NBE emission at l ≈ 380 nm and the deep-level band around l ≈ 550 nm from the oxygen interstitials inside Figure 6 Band diagrams. Schematic band diagram of ZnO/ZnO core-shell structures with ZnO shell layers of different thicknesses. the core were enhanced by the shell layers. Further- more, the shift of defect-related emissions from the ZnO/ZnO core-shell NRAs was observed due to the competition between light emissions from the oxygen interstitials inside the ZnO core and the oxygen vacan- cies in the ZnO shell. As the thickness of the ZnO shell layer increased, the green luminescence (l ≈ 490 nm) originating from the oxygen vacancies in the shell dominated over the yellow band (l ≈ 550 nm) asso- ciated with the oxygen interstitials inside the ZnO core due to the flat-band effect. The results indicate that the shell layers prepared by ALD have significant influ- ence both on the NBE and defect-related emissions of the ZnO NRAs. Acknowledgements This work was financially supported by the National Science Council in Taiwan under contract number NSC98-2112-M-002-018-MY2 and NSC100- 3113-E002-011. Author details 1Department of Materials Science and Engineering, National Taiwan University, Taipei 10617, Taiwan 2Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 10617, Taiwan Authors’ contributions All the authors contributed to the writing of the manuscript. WCS and YCY carried out the experiments under the instruction of MJC. CTK performed the TEM measurement. All authors read and approved the final manuscript. Competing interests The authors declare that they have no competing interests.