1. Beijing National Laboratory for Molecular Sciences, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China
2. University of Chinese Academy of Sciences, Beijing 100049, P. R. China
3. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P. R. China
4. Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, P. R. China