1. Department of Physics, The Pennsylvania State University, University Park, PA 16802, USA.
2. Research Center for Electronic and Optical Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan.
3. Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan.
4. Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, PA 16802, USA.