1. School of Electronics and Information Engineering Southwest University Chongqing 400715 China
2. Key Laboratory of Advanced Technologies of Materials (Ministry of Education) School of Materials Science and Engineering Southwest Jiaotong University Chengdu 610031 China
3. Beijing Institute of Nanoenergy and Nanosystems Chinese Academy of Sciences Beijing 100083 China
4. Key Laboratory of Optoelectronic Technology & Systems (Ministry of Education) Department of Optoelectronic Engineering Chongqing University Chongqing 400044 China
5. School of Materials Science and Engineering Georgia Institute of Technology Atlanta GA 30332‐0245 USA