1. Department of Physics, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon 999077 Hong Kong
2. Paul Drude Institute for Solid State Electronics, Leibniz Institute in the Research Association Berlin e.V., Hausvogteiplatz 5-7, Berlin 10117, Germany
3. Department of Materials Science and Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon 999077 Hong Kong