Thermal Transport across Grain Boundaries in Graphene by Molecular Dynamics Simulations
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The superior mechanical, electronic and thermal properties of graphene make it an exceptional material with many potential applications in thermal management, energy, and electronics technology. However, the presence of grain boundaries in polycrystalline graphene during chemical vapor deposition growth processes can dramatically impact these properties. This dissertation presents an atomistic study of the thermal transport across grain boundaries in graphene using molecular dynamics simulations. We first employ non-equilibrium molecular dynamics simulations to investigate the effect of strain on the thermal conductance of grain boundaries in graphene. The thermal boundary conductance is found to decrease significantly under biaxial tension as expected due to the softening of the phonons with increasing lattice spacing. In contrast, under biaxial compression, the thermal boundary conductance is strongly affected by the dimensions of the graphene monolayer, increasing with strain for specimen with length-to-width ratio of less than 20 and being insensitive to strain for length-to-width ratio above 20. This rather unexpected size-dependence under biaxial compression is found to be a result of geometric instabilities. We further perform phonon wave-packet dynamics simulations to determine the contribution of different phonon modes to the thermal boundary conductance in graphene based on lattice dynamics. We consider three grain boundaries, two of which are flat, while the third shows significant out-of-plane buckling to accommodate the strain due to lattice mismatch. Our simulations reveal that the in-plane acoustic modes make the dominant contribution to the Kapitza conductance of grain boundaries. This is in sharp contrast to the thermal conductivity of graphene which is dominated by out-of-plane acoustic phonons. Finally, we extend this work to study the effect of tensile strain on the scattering of phonons at grain boundaries. We find that biaxial tension has little influence on the transmission coefficients of the LA and TA branches but can affect the overall fluctuations of the transmission coefficient of the ZA mode which tends to increase and approaches 1 as the tensile strain increases. Thus, our study on the interplay of size and strain effects on phonon scattering provides atomistic insights into the role of grain boundaries in thermal transport in two-dimensional materials.