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 and Micro-nano Devices; Renmin University of China; Beijing 100872 P. R. China
5. National Laboratory of Solid State Microstructures; School of Electronic Science and Engineering, and; Collaborative Innovation Center of Advanced Microstructures; Nanjing University; Nanjing 210093 P. R. China
6. School of Materials Science and Engineering; Nanyang Technological University; 639798 Singapore
7. Materials Science and Technology Division; Oak Ridge National Laboratory; Oak Ridge TN 37831 USA