1. School of Material Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States
2. Key Laboratory for the Physics and Chemistry of Nanodevices, and Department of Electronics, Peking University, Beijing 100871, China
3. Department of Optoelectronic Engineering, Chongqing University, Chongqing 400044, China
4. Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China