1. Department of Physics, The University of Texas at Austin, Austin, Texas 78712, United States
2. 2D Crystal Consortium, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
3. Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
4. Department of Physics, National Tsing Hua University, 300044 Hsinchu, Taiwan
5. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States