1. School of Materials Science and Engineering; Georgia Institute of Technology; Atlanta GA 30332-0245 USA
2. State Key Laboratory of Polymer Materials Engineering; Polymer Research Institute of Sichuan University; Chengdu 610065 P. R. China
3. Department of Optoelectronic Engineering; Chongqing University; Chongqing 400044 P. R. China
4. Beijing Institute of Nanoenergy and Nanosystems; Chinese Academy of Sciences; Beijing 100083 P. R. China