1. Department of Physics, National University of Singapore, Singapore 117551, Singapore
2. Centre for BioImaging Sciences, Department of Biological Sciences, National University of Singapore, Singapore 117557, Singapore
3. Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, Singapore 117546, Singapore
4. Institute for Materials Research and Engineering, Agency for Science, Technology and Research, Singapore 138634, Singapore
5. imec, Kapeldreef 75, Leuven B-3001, Belgium
6. Department of Materials Science and Engineering, National University of Singapore, Singapore 117575, Singapore