1. Beijing Institute of Nanoenergy and Nanosystems Chinese Academy of Sciences Beijing 100083 P. R. China
2. College of Nanoscience and Technology University of Chinese Academy of Sciences Beijing 100049 P. R. China
3. State Key Laboratory for Superlattices and Microstructures Institute of Semiconductors Chinese Academy of Sciences Beijing 100083 P. R. China
4. School of Chemistry and Chemical Engineering Guangxi University Nanning 530004 P. R. China
5. CUSPEA Institute of Technology Wenzhou Zhejiang 325024 P. R. China
6. School of Materials Science and Engineering Georgia Institute of Technology Atlanta GA 30332 USA