1. Key Laboratory of Optoelectronic Technology & Systems, Ministry of Education and International R & D Center of Micro-Nano Systems and New Materials Technology, Chongqing University, Chongqing 400044, P. R. China
2. Institute of Electronic Engineering, China Academy of Engineering Physics, Mianyang 621900, Sichuan, P. R. China
3. Beijing
Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, P. R. China