1. Beijing Key Laboratory of Energy Conversion and Storage Materials College of Chemistry Beijing Normal University Beijing 100875 P. R. China
2. College of Engineering Peking University Beijing 100871 China
3. Department of Physics Beijing Normal University Beijing 100875 P. R. China
4. Center for Nanomedicine Institute for Basic Science Seoul 03722 South Korea
5. Department of Physics Yonsei University Seoul 03722 South Korea
6. College of Design and Engineering National University of Singapore 9 Engineering Drive 1, #07‐26, EA Singapore 117575 Singapore