1. Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, USA
2. Department of Physics, Columbia University, New York, New York 10027, USA
3. Graphene Labs, Istituto Italiano di Tecnologia, Via Morego 30, I-16163 Genova, Italy
4. NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore, I-56126 Pisa, Italy
5. Department of Physics and Astronomy, Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, USA
6. School of Materials Engineering, Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, USA
7. School of Electrical and Computer Engineering, Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, USA
8. Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544, USA