1. Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China
2. Institute of Microelectronics, Agency for Science, Technology and Research (A*STAR), Singapore 117685, Singapore
3. Defense Key Disciplines Lab of Novel Micro-nano Devices and System Technology, International R & D center of Micro-nano Systems and New Materials Technology, Key Laboratory of Optoelectronic Technology & Systems, Ministry of Education, Chongqing University, Chongqing 400044, China
4. School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States