Grain rotation and growth in two-dimensional hexagonal materials

Two-dimensional (2D) materials such as graphene or hexagonal boron nitride (h-BN) form polycrystalline structures during large-scale fabrication, with individual domains separated by grain boundaries which evolve with time. We study this dynamic process via Phase Field Crystal modeling for 2D hexagonal materials. By tracking the motion of circular grains misoriented with respect to the surrounding crystalline matrix we analyze the angle dependence of grain boundary dynamics and its implications for grain growth. Due to the lattice planes' continuity across the boundary, the grain is expected to rotate towards a larger misorientation angle, as governed by the Cahn-Taylor mechanism for the coupled normal-tangential boundary motion. This is found in our simulations of 2D graphene at small enough angles, while at large angles the grain rotation slows down and ceases near 30 degrees. However, this slowing-down and stagnation behavior does not occur for a binary hexagonal material like h-BN, where grains still rotate at high angles. It indicates the important role played by the lattice inversion symmetry breaking in binary materials, causing the change of detailed structures and dynamics of dislocation defects at grain boundaries as compared to single-component materials like graphene.