1. Beijing Institute of Nanoenergy and Nanosystems; Chinese Academy of Sciences; Beijing 100083 P. R. China
2. CAS Center for Excellence in Nanoscience; National Center for Nanoscience and Technology (NCNST); Beijing 100190 P. R. China
3. University of Chinese Academy of Sciences; Beijing 100049 P. R. China
4. State Key Lab Elect Thin Films and Integrated Devices; University of Electronic Science and Technology of China; Chengdu 610054 P. R. China
5. School of Materials Science and Engineering; Georgia Institute of Technology; Atlanta GA 30332 USA