1. Key Lab of Organic Optoelectronics and Molecular EngineeringDepartment of ChemistryTsinghua University Beijing 100084 China
2. Beijing National Laboratory for Condensed Matter PhysicsInstitute of PhysicsChinese Academy of Sciences Beijing 100190 China
3. School of Physical SciencesUniversity of Chinese Academy of Sciences Beijing 100049 China
4. National Synchrotron Radiation LaboratoryCAS Center for Excellence in NanoscienceUniversity of Science and Technology of China Hefei 230029 China