1. Department of Physics and Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education; Wuhan University; Wuhan 430072 China
2. Department of Physics and Lab Nanoscale Condense Matter Physics; Xiamen University; Xiamen 361005 China
3. Department of Materials Science and Engineering; National Chiao Tung University; Hsin-chu 30010 Taiwan
4. Department of Electronic and Computer Engineering; The Hong Kong University of Science and Technology; Clear Water Bay; Kowloon Hong Kong SAR China
5. Suzhou Institute of Nano-tech and Nano-bionics; Chinese Academy of Sciences; Suzhou 215000 China