1. Department of Chemistry, National University of Singapore, Singapore 117543, Singapore
2. Department of Physics, National University of Singapore, Singapore 117542, Singapore
3. Centre for Advanced 2D Materials, National University of Singapore, Singapore 117546, Singapore
4. Bruker Nano Surface Division, 30 Biopolis Street 09-01, The Matrix, Singapore 138671
5. Department of Mechanical Engineering, National University of Singapore, Singapore 117575, Singapore