1. School of Materials Science and Engineering; Georgia Institute of Technology; Atlanta GA 30332-0245 USA
2. Key Laboratory for Optoelectronic Technology and Systems; Ministry of Education of China; Key Laboratory of Fundamental Science of Micro/Nano-Device and System Technology; Chongqing University; Chongqing 400044 China
3. State Key Laboratory of Polymer Materials Engineering; Polymer Research Institute; Sichuan University; Chengdu 610065 China
4. Beijing Institute of Nanoenergy and Nanosystems; Chinese Academy of Sciences; Beijing 100083 China