1. Center on Nanoenergy Research, School of Physical Science & Technology, Guangxi University, Nanning 530004, P. R. China
2. CAS Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro-nano Energy and Sensor, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 101400, P. R. China
3. School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, P. R. China
4. School of Material Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States