1. Department of Physics; The Chinese University of Hong Kong; Shatin Hong Kong SAR China
2. Department of Electronic Engineering and Materials Science and Technology Research Center; The Chinese University of Hong Kong; Shatin Hong Kong SAR China
3. Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics; Peking University; Beijing 100871 China
4. Beijing Computational Science Research Center; Beijing 100084 China