1. Chinese Academy of Sciences (CAS) Key Laboratory of Molecular Nanostructure and Nanotechnology, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, CAS, Beijing 100190, P. R. China.
2. University of CAS, Beijing 100049, P. R. China.
3. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, CAS, Beijing 100190, P. R. China.