1. Department of Physics, University of California at Berkeley, Berkeley, California 94720, United States
2. Materials
Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
3. State Key Laboratory of Low Dimensional
Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China
4. Advanced
Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
5. Stanford Institute for Materials and Energy
Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States
6. Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore
7. Geballe
Laboratory for Advanced Materials, Departments of Physics and Applied
Physics, Stanford University, Stanford, California 94305, United States
8. Kavli Energy NanoSciences Institute, University of California Berkeley and the Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States