1. Department of Physics, The University of Hong Kong, Hong Kong 999077, People's Republic of China
2. Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, People's Republic of China
3. Institute of Applied Physics and Materials Engineering, University of Macau, Macau 999078, People's Republic of China