1. Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA
2. Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, PA 16802, USA
3. Division of Physics, Engineering, Mathematics and Computer Sciences and Optical Science Center for Applied Research, Delaware State University, Dover, DE 19901, USA
4. Department of Physics, The Pennsylvania State University, University Park, PA 16802, USA
5. Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, Troy, NY 12180, USA
6. Institute of Materials Research, Tsinghua Shenzhen International Graduate School, Tsinghua University, Shenzhen 518055, China
7. Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, USA
8. Department of Physics, Binghamton University, Binghamton, NY 13902, USA