Role of an Individual Grain Boundary in Thermal Transport across Single Crystalline CVD Graphene

Graphene is a one atom thick sheet of carbon, joined together in a honeycomb structure. It is being considered as a way to overcome some of technological limitations of silicon in integrated circuit industry owing to its extraordinary electrical, mechanical and thermal properties. The most promising method to fabricate graphene in a large scale is through Chemical Vapor Deposition (CVD). However, CVD graphene is only available in polycrystalline form. In polycrystalline graphene, the single crystalline regions are joined together making grain boundaries which govern the physical properties of CVD graphene. It has been shown that the electrical properties of graphene are highly affected by the temperature. Therefore, it is important to understand physical phenomenon behind the thermal transport across polycrystalline graphene. In this respect, the role of grain boundaries on the thermal transport of graphene must be extensively explored. In this work, large area single crystalline graphene flakes with detectable grain boundaries are grown on copper substrate using atmospheric pressure CVD. An electrical thermometry platform is then design in order to simultaneously measure the thermal conductivity of a single crystalline graphene flake as well as an individual grain boundary formed between two graphene flakes. It was shown that an individual grain boundary acts as an effective 1.24 μm extension in length of the single crystalline graphene at room temperature which results in an average of 26% overall reduction of thermal conductivity. Also, the thermal conductivity of an isolated grain boundary is found to be around 3 orders of magnitude smaller than that of the graphene on Silicon Nitride substrate

Bimodal Phonon Scattering in Graphene Grain Boundaries

Graphene has served as the model 2D system for over a decade, and the effects of grain boundaries (GBs) on its electrical and mechanical properties are very well investigated. However, no direct measurement of the correlation between thermal transport and graphene GBs has been reported. Here, we report a simultaneous comparison of thermal transport in supported single crystalline graphene to thermal transport across an individual graphene GB. Our experiments show that thermal conductance (per unit area) through an isolated GB can be up to an order of magnitude lower than the theoretically anticipated values. Our measurements are supported by Boltzmann transport modeling which uncovers a new bimodal phonon scattering phenomenon initiated by the GB structure. In this novel scattering mechanism, boundary roughness scattering dominates the phonon transport in low-mismatch GBs, while for higher mismatch angles there is an additional resistance caused by the formation of a disordered region at the GB. Nonequilibrium molecular dynamics simulations verify that the amount of disorder in the GB region is the determining factor in impeding thermal transport across GBs