1. Department of Physics University of South Florida Tampa FL 33620 USA
2. Department of Physics The Pennsylvania State University University Park PA 16802 USA
3. Department of Materials Science and Engineering The Pennsylvania State University University Park PA 16802 USA
4. Center for Two Dimensional and Layered Materials The Pennsylvania State University University Park PA 16802 USA
5. Center for Integrated Nanostructure Physics (CINAP) Institute for Basic Science (IBS) Suwon 16419 Republic of Korea
6. Department of Energy Science Sungkyunkwan University Suwon 16419 Republic of Korea
7. Department of Chemistry The Pennsylvania State University University Park PA 16802 USA