Phononic Thermal Transport along Graphene Grain Boundaries: A Hidden Vulnerability

While graphene grain boundaries (GBs) are well characterized experimentally, their influence on transport properties is less understood. As revealed here, phononic thermal transport is vulnerable to GBs even when they are ultra‐narrow and aligned along the temperature gradient direction. Non‐equilibrium molecular dynamics simulations uncover large reductions in the phononic thermal conductivity (κp) along linear GBs comprising periodically repeating pentagon‐heptagon dislocations. Green's function calculations and spectral energy density analysis indicate that the origin of the κp reduction is hidden in the periodic GB strain field, which behaves as a reflective diffraction grating with either diffuse or specular phonon reflections, and represents a source of anharmonic phonon–phonon scattering. The non‐monotonic dependence with dislocation density of κp uncovered here is unaccounted for by the classical Klemens theory. It can help identify GB structures that can best preserve the integrity of the phononic transport. Contrary to conventional wisdom, phononic transport in graphene is vulnerable to grain boundaries comprising 5‐7 defects aligned along the thermal gradient. Atomistic simulations uncover a dual role of the strain field as a reflective phononic diffraction grating, showing either specular (left) or diffuse (right) phonon reflections (depending on the 5‐7 density), and as a source of anharmonic phonon–phonon scattering.