1. State Key
Laboratory of Optoelectronic
Materials and Technologies, School of Physics and Engineering, Sun Yat-sen University, Guangzhou 510275, P. R. China
2. Department of Materials Science
and Engineering, College of Engineering, Peking University, Beijing 100871, P. R. China
3. Department of Physics, Hong Kong University of Science and Technology, Clear
Water Bay, Kowloon, Hong Kong, China