1. Department of Mechanical Engineering, The University of Tokyo, Tokyo 113-8656, Japan
2. Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
3. Institute of Nanotechnology, Karlsruhe Institute of Technology, Karlsruhe 76021, Germany
4. Institute of Materials Science, Technische Universität Darmstadt, Darmstadt 64287, Germany
5. Institute for Quantum Materials and Technologies, Karlsruhe Institute of Technology, Karlsruhe 76021, Germany
6. Center for Carbon-Based Electronics, Peking University, Beijing 100871, China
7. Key Laboratory for the Physics and Chemistry of Nanodevices, Department of Electronics, Peking University, Beijing 100871, China
8. Shanxi Institute for Carbon-Based Thin Film Electronics, Peking University (SICTFE-PKU), Taiyuan 030012, China