Bimodal Grain-Size Scaling of Thermal Transport in Polycrystalline Graphene from Large-Scale Molecular Dynamics Simulations

Grain
boundaries in graphene are inherent in wafer-scale samples
prepared by chemical vapor deposition. They can strongly influence
the mechanical properties and electronic and heat transport in graphene.
In this work, we employ extensive molecular dynamics simulations to
study thermal transport in large suspended polycrystalline graphene
samples. Samples of different controlled grain sizes are prepared
by a recently developed efficient multiscale approach based on the
phase field crystal model. In contrast to previous works, our results
show that the scaling of the thermal conductivity with the grain size
implies bimodal behavior with two effective Kapitza lengths. The scaling
is dominated by the out-of-plane (flexural) phonons with a Kapitza
length that is an order of magnitude larger than that of the in-plane
phonons. We also show that, to get quantitative agreement with the
most recent experiments, quantum corrections need to be applied to
both the Kapitza conductance of grain boundaries and the thermal conductivity
of pristine graphene, and the corresponding Kapitza lengths must be
renormalized accordingly.