1. Department of Applied Physics; The Hong Kong Polytechnic University; Hung Hom, Kowloon Hong Kong P. R. China
2. The Hong Kong Polytechnic University Shenzhen Research Institute; Shenzhen 518057 P. R. China
3. Department of Physics; Renmin University of China; Beijing 100872 P. R. China
4. Beijing Key Laboratory of Optoelectronic Functional Materials & Micro-Nano Devices; Renmin University of China; Beijing 100872 P. R. China
5. School of Materials Science and Engineering; Nanyang Technological University; Singapore 639798