1. Department of Electrical and Computer Engineering, Duke University, Durham, North Carolina 27708, United States
2. Sensors and Electron Devices Directorate, United States Army Research Laboratory, Adelphi, Maryland 20783, United States
3. Department of Physics, Lehigh University, Bethlehem, Pennsylvania 18015, United States
4. Department of Chemistry, Duke University, Durham, North Carolina 27708, United States