1. College of Engineering, Zhejiang Normal University, Zhejiang 321000, P. R. China
2. Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, P. R. China
3. School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, P. R. China
4. Center on Nanoenergy Research, School of Physical Science and Technology, Guangxi University, Nanning, Guangxi 530004, P. R. China
5. Georgia Institute of Technology, Atlanta, GA 30332-0245, USA