Using non-equilibrium molecular dynamics simulations, we study the decrease in thermal conductivity (k) in isotopically impure carbon nanotubes (CNTs) and graphene nanoribbons. We observe that the decrease in k for the doped CNTs, developed from the zone-folding approach of graphene sheets, is more than that of the doped graphene nanoribbons since the edge scattering effects due to development of dangling bonds in graphene decreases the participation of phonon modes there. We analyze the vibrational density of states (DOS) both along and perpendicular to the direction of heat transfer. We find that the high energy modes of vibration shift to lower wave numbers and the highest peaks of the vibrational modes become shorter reflecting the strong influence of mass disorder that impedes formation of delocalized modes in impure materials. We analyse that in the lower as well as the higher wave number range, both the pure and doped graphene nanoribbons possess enhanced number of vibrational modes than the corresponding nanostructures. At lower wave numbers, the out-of-plane flexural acoustic modes in graphene nanoribbons and the delocalized transverse modes in CNTs play a more prominent role in the heat transfer process. The presence of isotopic dopants perturbs the DOS as well as the phonon dispersion curves thereby decreasing the overall phonon group velocity and reducing k.
Iowa State University of Science and Technology
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