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 Materials Science and Engineering; National Chiao Tung University; Hsin-chu 30010 Taiwan
3. Science and Technology on Monolithic Integrated Circuits and Modules Laboratory; Nanjing Electronic Device Institute; Nanjing China
4. WUT-Harvard Joint Nano Key Laboratory; School of Materials Science and Engineering; Wuhan University of Technology; Wuhan China
5. Department of Physics and Materials Science; City University of Hong Kong; Tat Chee Avenue Kowloon Hong Kong SAR China