1. Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong 999077, People’s Republic of China
2. School of Materials Science and Engineering, Harbin Institute of Technology (Shen Zhen), Shen Zhen 518055, People’s Republic of China
3. The Hong Kong Polytechnic University Shenzhen Research Institute, Shenzhen 518055, People’s Republic of China
4. School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana 47907, United States