^a State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Advanced Functional Polymer Composites, Beijing University of Chemical Technology, Beijing 100029, China

^b Faculty of Physics and Astronomy, Friedrich Schiller University Jena, Jena 07743, Germany

^c Institute of Applied Physics, Karlsruhe Institute of Technology(KIT), Karlsruhe 76128, Germany

^d Institute of Nanotechnology, Karlsruhe Institute of Technology(KIT), Eggenstein-Leopoldshafen 76344, Germany

^e Institute of Toxicology and Genetics, Karlsruhe Institute of Technology(KIT), Eggenstein-Leopoldshafen 76344, Germany

^f Department of Physics, University of Illinois at Urbana-Champaign, Urbana, IL 61801, United States

^g State Key Laboratory of Solidification Processing, Center for Nano Energy Materials, School of Materials Science and Engineering, Northwestern Polytechnical University, Shaanxi Joint Laboratory of Graphene(NPU), Xi'an 710072, China