1. Department of Mechanical Engineering, Columbia University, New York, New York 10027, United States
2. Samsung-SKKU Graphene Center (SSGC), Suwon, 440-746, Korea
3. School of Mechanical Engineering, Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon, Gyeonggi 440-746, Korea
4. Creative Research Center for Graphene Electronics, Electronics and Telecommunications Research Institute (ETRI), 218 Gajeong-ro, Yuseong-gu, Daejeon, 305-700, Korea
5. Department of Physics, Columbia University, New York, New York 10027, United States
6. Department of Chemistry, Columbia University, New York, New York 10027, United States
7. SKKU Advanced Institute of Nano-Technology (SAINT), Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon, Gyeonggi 440-746, Korea
8. National Institute for Materials Science, 1-1 Namiki, Tsukuba, 305-0044, Japan