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 simulations. 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, indicating reduced thermal conductivity when such interfaces are present in crystalline graphene. In nanocrystalline graphene samples, the thermal conductivity is significantly reduced with respect to crystalline graphene and it decreases with decreasing grain size according to an increasing number of single boundaries.