1. Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 101400, P. R. China
2. College of Materials Science and Engineering, Changsha University of Science & Technology, Changsha 410114, P. R. China
3. School of Physics and Electronics, Central South University, Changsha 410083, P. R. China
4. School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, P. R. China
5. Center on Nanoenergy Research, School of Physical Science and Technology, Guangxi University, Nanning 530004, P. R. China
6. CUSTech Institute, Wenzhou 325024, Zhejiang, P. R. China