1. School of Physics, School of Electronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, China
2. Multidisciplinary Nanotechnology Centre, College of Engineering, Swansea University, Swansea SA1 8EN, United Kingdom
3. Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China
4. College of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, China