1. Beijing Institute of Nanoenergy and Nanosystems; Chinese Academy of Sciences; Beijing 100083 P. R. China
2. Institute of Semiconductors; Chinese Academy of Sciences; Beijing 100083 P. R. China
3. University of Chinese Academy of Sciences; Beijing 100049 P. R. China
4. CAS Center for Excellence in Nanoscience; National Center for Nanoscience and Technology (NCNST); Beijing 100083 P. R. China
5. School of Materials Science and Engineering; Georgia Institute of Technology; Atlanta GA 30332 USA