1. School of Electrical and Computer Engineering, Cornell University, Ithaca, New York 14853, USA
2. Department of Physics, Cornell University, Ithaca, New York 14853, USA
3. School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA
4. Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, USA
5. Crystal IS, Green Island, New York 12183, USA
6. Kavli Institute for Nanoscale Science, Cornell University, Ithaca, New York 14853, USA