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. School of Physics and Electronics Central South University Changsha 410083 P. R. China
4. Beijing Institute of Fashion Technology Beijing 100029 P. R. China
5. Center on Nanoenergy Research School of Physical Science and Technology Guangxi University Nanning 530004 P. R. China
6. School of Materials Science and Engineering Georgia Institute of Technology Atlanta GA 30332 USA