1. Department
of Physics, National University of Singapore, 117542, Singapore
2. Graphene
Research Center, National University of Singapore, 117542, Singapore
3. NanoCore, National University of Singapore, 117576, Singapore
4. Department
of Chemistry, National University of Singapore, 117542, Singapore
5. Center
for Phononics and Thermal Energy Science, School of Physics Science
and Engineering, Tongji University, 200092 Shanghai, China