1. Department of Electrical and Electronic Engineering, The University of Hong Kong, Pok Fu Lam Road, Hong Kong SAR, China
2. Department of Physics, The Hong Kong Baptist University, Kowloon Tong, Hong Kong, P. R. China
3. Department of Materials Science & Engineering, University of Washington, Seattle, Washington 98195, United States
4. Department of Materials Science & Engineering, City University of Hong Kong, Kowloon Tong, Hong Kong, P. R. China