1. Department of Applied Physics, State Key Laboratory of Power Transmission Equipment and System Security and New Technology, Chongqing University, Chongqing 400044, P. R. China.
2. Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, P. R. China.
3. School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA.
4. Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan.