1. Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong SAR
2. Kunming Institute of Physics, Kunming, 650223, Yunnan Province, P.R. China
3. Department of Electrical and Computer Engineering, Texas Tech University, Lubbock, Texas 79409, United States
4. Solar Energy Research Institute, Yunnan Normal University, Kunming 650092, P.R. China
5. Multidisciplinary Nanotechnology Center, College of Engineering, University of Swansea, Singleton Park, Swansea SA2 8PP, United Kingdom