1. Center on Nanoenergy Research, School of Physical Science and Technology, Guangxi University, Nanning, Guangxi 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. Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Physics and Optoelectronic Engineering, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, P. R. China
4. School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, P. R. China