1. School of Materials Science and Engineering; Georgia Institute of Technology; Atlanta GA 30332 USA
2. State Key Laboratory on Integrated Optoelectronics; College of Electronic Science and Engineering; Jilin University; Changchun 130012 P. R. China
3. George W. Woodruff School of Mechanical Engineering; Georgia Institute of Technology; Atlanta GA 30332 USA
4. Institute of Functional Nano and Soft Materials; Soochow University; Suzhou, Jiangsu 215123 P. R. China
5. Beijing Institute of Nanoenergy and Nanosystems; Chinese Academy of Sciences; National Center for Nanoscience and Technology (NCNST); Beijing 100083 P. R. China