1. Van der Waals Materials Research Center, Yonsei University, Seoul 03722, Korea
2. Physics Department, Yonsei University, Seoul 03722, Korea
3. Korea Research Institute of Standards and Science, Daejeon 34113, Korea
4. LG Display R&D Center, 245 LG-ro, Wollong-myeon, Paju-si, Gyeonggi-do 413-811, Korea