1. Department of Chemistry, National University of Singapore, 3 Science
Drive 3, Singapore 117543
2. Solar Energy Research Institute of Singapore (SERIS), 7 Engineering Drive 1, Singapore 117574
3. Materials
Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
4. Center for Advanced 2D Materials and Graphene Research Centre, 2 Science Drive
2, Singapore 117526