Epitaxially Constrained Grain Boundary Structures in an Oxide Honeycomb Monolayer

Grain boundaries (GBs) are ubiquitous in solids. Their description is critical for understanding polycrystalline materials and explaining their mechanical and electrical properties. A GB in a 2D material can be described as a line defect and its atomic structures have been intensively studied in materials such as graphene. These GBs accommodate the relative rotation of two neighboring grains by incorporating periodic units consisting of nonhexagonal rings along the boundary. Zero-degree GBs, called domain boundaries (DBs), where there is only a lattice offset between two grains without any rotation, are rare in 2D van-der-Waals (vdW) bonded materials where the grains can easily move. However, this movement is not possible in 2D materials that have a strong epitaxial relationship with their substrate such as the M2O3 (2 x 2) honeycomb monolayers on noble metal (111) supports. Involving experimental and theoretical investigations, four main DBs are observed here in a monolayer of Ti2O3 supported on Au(111) and their atomic structures are solved. The DB formation energies explain why some DBs are more frequently observed than others. The strong epitaxial constraint from the Au(111) substrate stabilizes some unique Ti2O3 monolayer DB structures that are not observed in vdW-bonded 2D materials.

Automated summary

Grain boundaries are important to understand polycrystalline materials and their properties. In 2D materials, grain boundaries can be described as line defects, and their atomic structures have been extensively studied. Zero-degree grain boundaries, called domain boundaries, where there is only a lattice offset between two grains without any rotation, are rare in 2D materials. By combining experimental and theoretical investigations, researchers observed and solved the atomic structures of four main domain boundaries in a monolayer of Ti2O3 supported on Au(111). The formation energies of these domain boundaries explain their different frequencies of occurrence. The strong epitaxial constraint from the Au(111) substrate stabilizes unique domain boundary structures that are not observed in van-der-Waals bonded 2D materials.