1. Institute of Physics & University of Chinese Academy of Sciences, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, People’s Republic of China
2. Beijing Key Laboratory for Nanomaterials and Nanodevices, Beijing 100190, People’s Republic of China
3. School
of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China
4. Beijing
National Laboratory for Molecular Sciences, Key Laboratory of Organic
Solids, Institute of Chemistry, Chinese Academy of Science, Beijing 100190, People’s Republic of China
5. Department
of Physics and Astronomy and Department of Electrical Engineering
and Computer Science, Vanderbilt University, Nashville, Tennessee 37235, United States