1. School of Chemistry, Chemical Engineering and Biotechnology, and School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore
2. Cambridge Graphene Centre, University of Cambridge, Cambridge CB3 0FA, U.K.
3. College of Chemistry, Key Laboratory of Theoretical and Computational Photochemistry, Beijing Normal University, Beijing 100875, People’s Republic of China
4. Institute of Solid State Physics, Friedrich Schiller University Jena, Max-Wien-Platz 1, 07743 Jena, Germany
5. School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona 85287, United States
6. Department of Physics, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133 Milano, Italy
7. IFN-CNR, Piazza Leonardo da Vinci 32, I-20133 Milano, Italy
8. Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States