1. Beijing Institute of Nanoenergy and Nanosystems Chinese Academy of Sciences Beijing 101400 P. R. China
2. School of Nanoscience and Engineering University of Chinese Academy of Sciences Beijing 100049 P. R. China
3. Institute of Microscale Optoelectronics Shenzhen University Shenzhen 518060 P. R. China
4. Department of Applied Physics The Hong Kong Polytechnic University Hong Kong 999077 P. R. China
5. Institute of Atomic Manufacturing Beihang University Beijing 100191 P. R. China