1. Beijing Institute of Nanoenergy and Nanosystems Chinese Academy of Sciences Beijing 101400 P. R. China
2. School of Nanoscience and Technology University of Chinese Academy of Sciences Beijing 100049 P. R. China
3. Institute of Applied Nanotechnology Jiaxing Zhejiang 314031 P. R. China
4. Inner Mongolia Key Laboratory for Physics and Chemistry of Functional Materials College of Physics and Electronic Information Inner Mongolia Normal University Hohhot 010022 P. R. China
5. School of Materials Science and Engineering Georgia Institute of Technology Atlanta GA 30332‐0245 USA