1. Department of Physics; Umeå University; 90187 Umeå Sweden
2. Department of Materials Science and Engineering; The Pennsylvania State University; University Park; PA 16802 USA
3. Department of Chemistry; Department of Physics; Center for Atomically Thin Multifunctional Materials (ATOMIC), and Center for 2-Dimensional and Layered Materials; The Pennsylvania State University; University Park; PA 16802 USA
4. Institute of Carbon Science and Technology; Shinshu University; 4-17-1 Wakasato Nagano 380-8553 Japan