1. Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
2. Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
3. Department of Chemistry and Department of Materials Science & Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
4. Faculty of Engineering, Shinshu University, Wakasato 4-17-1, Nagano 380-8553, Japan