Strain engineering of thermal conductivity in graphene sheets and nanoribbons: a demonstration of magic flexibility.

Graphene is an outstanding material with ultrahigh thermal conductivity. Its thermal transfer properties under various strains are studied by reverse nonequilibrium molecular dynamics. Based on the unique two-dimensional structure of graphene, the distinctive geometries of graphene sheets and graphene nanoribbons with large flexibility and their intriguing thermal properties are demonstrated under strains. For example, the corrugation under uniaxial compression and helical structure under light torsion, as well as tube-like structure under strong torsion, exhibit enormously different thermal conductivity. The important robustness of thermal conductivity is found in the corrugated and helical configurations of graphene nanoribbons. Nevertheless, thermal conductivity of graphene is very sensitive to tensile strain. The relationship among phonon frequency, strain and thermal conductivity are analyzed. A similar trend line of phonon frequency dependence of thermal conductivity is observed for armchair graphene nanoribbons and zigzag graphene nanoribbons. The unique thermal properties of graphene nanoribbons under strains suggest their great potentials for nanoscale thermal managements and thermoelectric applications.