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