1. Center on Nanoenergy Research, School of Physical Science and Technology, Guangxi University, Nanning 530004, P. R. China
2. Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 101400, P. R. China
3. School of Electronic and Information Engineering, Beijing Jiaotong University, Beijing 100044, P. R. China
4. Georgia Institute of Technology, Atlanta, Georgia 30332, United States