1. School of Electrical Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea
2. Graphene/2D Materials Research Center, KAIST, Daehakro, Yuseong-gu, Daejeon 34141, Republic of Korea
3. Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS), Pohang 790-784, Republic of Korea
4. Department of Physics, Pohang University of Science and Technology, Jigokro, Nam-gu, Pohang 37673, Republic of Korea