1. Department of Physics, City University of Hong Kong, Kowloon, Hong Kong
2. Department of Physics, Comilla University, Kotbari, Cumilla 3506, Bangladesh
3. Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, CA 94720, USA
4. Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon, Hong Kong
5. Department of Materials Science and Engineering, City University of Hong Kong, Kowloon, Hong Kong