1. Beijing Institute of Nanoenergy and Nanosystems Chinese Academy of Sciences Beijing 101400 P. R. China
2. Key Laboratory of Metallurgical Equipment and Control Technology Ministry of Education Wuhan University of Science and Technology Wuhan Hubei 430081 P. R. China
3. School of Nanoscience and Engineering University of Chinese Academy of Sciences Beijing 100049 P. R. China
4. College of Materials Science and Opto‐Electronics Technology University of Chinese Academy of Sciences Beijing 101408 P. R. China
5. Faculty of Electrical Engineering and Computer Science Ningbo University Ningbo 315211 P. R. China
6. School of Materials Science and Engineering Georgia Institute of Technology Atlanta GA 30332‐0245 USA