1. Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, 101400, P. R. China
2. Center on Nanoenergy Research, School of Physical Science and Technology, Guangxi University, Nanning, 530004, P. R. China
3. School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing, 100049, P. R. China
4. Georgia Institute of Technology, Atlanta, Georgia, 30332-0245, USA