1. Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, P.R. China
2. College of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, P.R. China
3. Center on Nanoenergy Research, School of Physical Science and Technology, Guangxi University, Nanning 530004, P.R. China
4. School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States