1. School of Applied and Engineering Physics, Cornell University 1 , Ithaca, New York 14853, USA
2. Department of Electrical and Computer Engineering, Cornell University 2 , Ithaca, New York 14853, USA
3. Department of Materials Science and Engineering, Cornell University 3 , Ithaca, New York 14853, USA
4. Research Laboratory of Electronics, Massachusetts Institute of Technology 4 , Cambridge, Massachusetts 02139, USA
5. Kavli Institute at Cornell for Nanoscale Science, Cornell University 5 , Ithaca, New York 14853, USA
6. Paul-Drude-Institut für Festkörperelektronik, Leibniz-Institut im Forschungsverbund Berlin e.V. 6 , 10117 Berlin, Germany