1. Center of Materials Science and Optoelectronics EngineeringCollege of Materials Science and Opto-Electronic Technology & CAS Center for Excellence in Topological Quantum Computation & CAS Key Laboratory of Vacuum PhysicUniversity of Chinese Academy of Sciences Beijing 100049 P. R. China
2. Key Laboratory of Organic SolidsBeijing National Laboratory for Molecular ScienceInstitute of ChemistryChinese Academy of Sciences Beijing 100190 P. R. China
3. Department of ChemistryThe Hong Kong University of Science and Technology, Clear Water Bay, Kowloon Hong Kong China
4. School of Materials Science and EngineeringSouth China University of Technology Guangzhou 510640 China