1. Department of Electronic Engineering; Chinese University of Hong Kong; Shatin New Territories Hong Kong
2. College of Materials Science and Engineering; Shenzhen University; Shenzhen 518060 P. R. China
3. Shenzhen Institute of Advanced Technology; Chinese Academy of Science; Shenzhen 518055 P. R. China
4. Instrumental Analysis and Research Center; Sun Yat-sen University; Guangzhou 510275 P. R. China
5. Department of Physics; Hong Kong University of Science & Technology; Clear Water Bay Kowloon Hong Kong