1. Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore
2. Chongqing 2D Materials Institute, Liangjiang New Area, Chongqing 400714, P. R. China
3. Institute of Microelectronics Technology and High Purity Materials, Russian Academy of Sciences, 6 Academician Ossipyan Str., Chernogolovka, 142432 Moscow, Russia