1. Department of Physics, Bernal Institute, University of Limerick, Limerick V94 T9PX, Ireland
2. Department of Physics, University of Central Florida, Orlando, Florida 32816, USA
3. Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543
4. Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546