1. Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
2. University of Chinese Academy of Sciences, Beijing 100049, China
3. Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA
4. Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011, USA
5. Ames Laboratory, U.S. Department of Energy, Ames, Iowa 50011, USA
6. Department of Physics and Astronomy and Department of Materials Science and Engineering, Iowa State University and Ames Lab, Ames, IA 50011, USA