1. Centre for Advanced 2D Materials & Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore
2. Physics of Nanodevices, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands
3. National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
4. Nanocore, National University of Singapore, 4 Engineering Drive 3, Singapore 117576, Singapore