1. Key Lab of Organic Optoelectronics and Molecular Engineering; Department of Chemistry; Tsinghua University; Beijing 100084 P. R. China
2. National Synchrotron Radiation Laboratory; Hefei Science Center CAS; University of Science and Technology of China; Hefei Anhui 230029 P. R. China
3. School of Medicine and Tsinghua-Peking Joint Centers for Life Science; Tsinghua University; Beijing 100084 P. R. China
4. Beijing National Center for Electron Microscopy; School of Materials Science and Engineering; The State Key Laboratory of New Ceramics and Fine Processing; Key Laboratory of Advanced Materials (MOE); Tsinghua University; Beijing 100084 P. R. China