Thermal transport in nanocrystalline graphene: the role of grain boundaries

Abstract Single grain boundaries of crystalline graphene with varying mismatch angles from 3° to 16° have been investigated using molecular dynamics simula- tions. Four- to eight-atomic rings are found to be the most abundant non-hexagonal polygons in the grain boundary for all mismatch angles. Tetra- and octagons are predominant for mismatch angles of 4.1° and 6.6° in contrast to nanocrystalline samples where penta- and heptagons are dominating. Out-of-plane buckling at the grain boundary is most pronounced for a mismatch angle of 3.0° and it tends to decrease with increasing mismatch angle. At 16.1°, the out-of-plane buckling vanishes. Analysis of the vibrational density of states of boundary atoms revealed a significant decrease of the main peak of optical vibrations and the evolution of secondary peaks below and above the major frequency attributed to vibrations of non-hexagonal rings. The thermal boundary resistance in single graphene interfaces has been approximated. It tends to increase with increasing mismatch angle, indi- cating reduced thermal conductivity when such interfaces are present in crystalline graphene. In nanocrystalline graphene samples, the thermal conductivity is signif- icantly reduced with respect to crystalline graphene and it decreases with decreasing grain size according to an increasing number of single boundaries.