1. Department of Physics, Sungkyunkwan University, Suwon 16419, Republic of Korea
2. School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea
3. Department of Physics, Sogang University, Seoul 04107, Republic of Korea
4. Department of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of Korea
5. School of Mechanical Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea
6. Department of Physics, Harvard University, Cambridge, Massachusetts 02138, United States
7. Samsung-SKKU Graphene Center and SAINT, Sungkyunkwan University, Suwon 16419, Republic of Korea