Modulating Roughness and Ripple Dynamics in Hexagonal Boron Nitride via Defect Engineering.

Two-dimensional materials like hexagonal boron nitride (hBN) display intrinsic roughness and thermal rippling that profoundly affect their mechanical, electronic, and thermal properties; yet, the influence of atomic defects on these dynamics is poorly understood. In this work, we introduce a machine-learning-based Atomic Cluster Expansion (ACE) potential that accurately models hBN and captures defect energies and dynamicsa limitation of existing hBN potentials. Our simulations predict a new defect structure consisting of a triplet of square-octagon (4|8) defects in an otherwise pristine sheet that is more favorable than a previously observed isolated single 4|8 defect that terminates a dislocation. Similar to graphene, we uncover a defect concentration-dependent crossover from spatially random rippling in pristine hBN to diminished fluctuations around a fixed corrugation at elevated defect densities with larger surface roughness. Unlike graphene, where divacancy defects most strongly modulate sheet rippling, hBN vacancies (N-monovacancies, B-monovacancies, and divacancies) and N-interstitials exert negligible influence on corrugation and rippling; instead, the local stress induced by Stone-Wales (5|7) defects dominates this crossover, followed by square-octagon (4|8) and B-interstitial defects. The differences with graphene presumably arise from the heterogeneous composition of hBN and its polar-covalent bonding character, contrasting with graphene's homogeneous nature and purely covalent bonding. These results underscore material-specific sensitivity of rippling to defects and open avenues for targeted defect engineering to control hBN's flexibility in nanoelectronics, membranes, and beyond.