1. Department of Electronics and Nanoengineering Aalto University Tietotie 3 FI‐02150 Espoo Finland
2. Institute of Physics and Beijing National Laboratory for Condensed Matter Physics Chinese Academy of Sciences Beijing 100190 China
3. Songshan‐Lake Materials Laboratory Dongguan Guangdong Province 523808 China
4. School of Physical Sciences University of Chinese Academy of Science Beijing 100190 China
5. National Institute for Materials Science 1‐1 Namiki Tsukuba 305‐0044 Japan
6. Cambridge Graphene Centre University of Cambridge Cambridge CB3 0FA UK
7. QTF Centre of Excellence Department of Applied Physics Aalto University FI‐00076 Aalto Finland