1. Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 101400, P. R. China
2. School of Nanoscience and Engineering, University of Chinese Academy of Sciences, Beijing 101400, P. R. China
3. School of Advanced Materials, Shenzhen Graduate School, Peking University, Shenzhen 518055, P. R. China
4. Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, Beijing 100084, P. R. China
5. Center on Nanoenergy Research, School of Physical Science and Technology, Guangxi University, Nanning 530004, Guangxi, P. R. China
6. Department of Information Engineering, University of Brescia, Brescia 25123, Italy