1. Department of Physics, Columbia University, 538 W. 120th St., 704 Pupin Hall MC 5255, New York, New York 10027
2. Graphene Labs, Istituto Italiano di Tecnologia, Via Morego 30, I-16163 Genova, Italy and NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore, Piazza San Silvestro 12, I-56127 Pisa, Italy