1. Department of Applied Physics; The Hong Kong Polytechnic University; Hong Kong SAR
2. Kunming Institute of Physics; Kunming 650223 P. R. China
3. Solar Energy Research Institute; Yunnan Normal University; Kunming 650092 P. R. China
4. Multidisciplinary Nanotechnology Center; School of Engineering, Swansea University; Singleton Park Swansea SA2 8PP UK