1. Beijing National Laboratory for Condensed Matter Physics Institute of Physics Chinese Academy of Sciences Beijing 100190 China
2. School of Physical Sciences University of Chinese Academy of Sciences Beijing 100049 China
3. Department of Physics North Carolina State University Raleigh NC 27695 USA
4. Department of Materials Science and Engineering North Carolina State University Raleigh NC 27695 USA
5. Key Laboratory of Structure and Functional Regulation of Hybrid Materials of Ministry of Education Institutes of Physical Science and Information Technology Anhui University Hefei 230601 China
6. Organic and Carbon Electronics Lab (ORaCEL) North Carolina State University Raleigh NC 27695 USA
7. Songshan Lake Materials Laboratory Dongguan 523808 China