1. CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, and CAS Research/Education Center for Excellence in Molecular Sciences; Institute of Chemistry; Chinese Academy of Sciences (CAS); Beijing 100190 P. R. China
2. University of Chinese Academy of Sciences; Beijing 100049 P. R. China
3. Key Laboratory for Renewable Energy; Beijing Key Laboratory for New Energy Materials and Devices; National Laboratory for Condensed Matter Physics; Institute of Physics; Chinese Academy of Sciences; Beijing 100190 P. R. China
4. School of Physics and Electronics; Hunan University; Changsha 410082 China