1. Department
of Physics, Centre
for Computational Science and Engineering, and Graphene Research Centre, National University of Singapore, Singapore 117542,
Singapore
2. Key Laboratory
for the Physics
and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing 100871, People’s
Republic of China
3. NUS-Tongji
Center for Phononics
and Thermal Energy Science, Department of Physics, Tongji University, Shanghai 200092, People’s
Republic of China