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 101400 P. R. China
3. Center on Nanoenergy Research Guangxi University Nanning 530004 China
4. School of Materials Science and Engineering Georgia Institute of Technology Atlanta GA 30332‐0245 USA