1. Key Laboratory of Advanced Materials (MOE); School of Materials Science and Engineering; Tsinghua University; Beijing 100084 China
2. School of Instrumentation Science and Opto-electronics Engineering; Beihang University; Beijing 100191 China
3. Department of Materials Science and Engineering; The Pennsylvania State University; University Park PA 16802 USA
4. Department of Chemistry; Department of Physics and Center for 2-Dimensional and Layered Materials; The Pennsylvania State University; University Park PA 16802 USA
5. Institute of Carbon Science and Technology; Shinshu University; Wakasato 4-17-1 Nagano 380-853 Japan