Phonon scattering and thermal conductivity of pillared graphene structures with carbon nanotube-graphene intramolecular junctions

Abstract

We present results of a reverse non-equilibrium molecular dynamics study of thermal transport in single-walled carbon nanotube (SWCNT)-graphene junctions comprised of carbon-carbon (C-C) bonds with either sp2 or mixed sp2/sp3 hybridization. In both cases, a finite interfacial thermal resistance is observed at the SWCNT-graphene junctions for thermal transport in the out-of-plane direction. The interfacial thermal resistance at the junctions is attributed to the combined effects of scattering of the phonons at the SWCNT-graphene junctions due to the presence of distorted sp2 bonds in the junction region and the change in dimensionality of the medium along the phonon transport path as the phonons propagate from SWCNT pillars (quasi-1D) to graphene sheet (2D) and then again to SWCNTs. Moreover, the thermal resistance is found to depend on the C-C bond hybridization at the intramolecular junctions with mixed sp2/sp3 hybridization showing a higher interfacial resistance when compared to pure sp2 bonding. Thermal conductivity of typical SWCNT-graphene unit cells was observed to increase nearly linearly with an increase in cell dimensions, and then reaches a plateau as the pillar height and the inter-pillar distance approach the critical length for ballistic thermal transport in SWCNT and single layer graphene. These results indicate that the thermal transport characteristics of the three-dimensional SWCNT-graphene (hybrid) structures can be tuned by controlling the unit cell size.