1. Department of Electrical Engineering, Columbia University, New York, New York 10027-6902, United States
2. Department of Mechanical Engineering, Columbia University, New York, New York 10027-6902, United States
3. SKKU Advanced Institute of Nano Technology, Sungkyunkwan University, Suwon, Gyeonggi-do 16419, Korea
4. Department of Physics, Graduate Center of the City University of New York, New York, New York 10031-9101, United States
5. Department of Applied Physics and Mathematics, Columbia University, New York, New York 10027-6902, United States