1. Materials Sciences Division Lawrence Berkeley National Laboratory Berkeley CA 94720 USA
2. Department of Materials Science and Engineering University of California Berkeley CA 94720 USA
3. Berkeley Education Alliance for Research in Singapore Singapore 138602 Singapore
4. Centre for BioImaging Sciences Department of Biological Sciences National University of Singapore Singapore 117557 Singapore
5. Department of Physics National University of Singapore Singapore 117551 Singapore
6. Centre for Advanced 2D Materials and Graphene Research Centre National University of Singapore Singapore 117546 Singapore