Single-Site-Directed Unidirectional Epitaxy of Large-Scale 2D Materials

Large-scale synthesis of van der Waals (vdW) layered 2D materials remain a major challenge for their integration into high-performance electronics and optoelectronics. Although the multi-grain coalescence method provides a promising route to large-area growth, its success critically depends on achieving uniform crystallographic orientation across all nucleated domains, a challenge that has not yet been fully resolved. Here, we present an effective strategy for realizing unidirectional epitaxial growth of 2D materials by controllably introducing single active sites on flat terraces. Using the heteroepitaxy of MoSe₂ on Au(111) as a model system, we combine molecular beam epitaxy with first-principles calculations to validate this mechanism. We show that Se adsorption disrupts the intrinsic herringbone reconstruction of Au(111), releasing surface Au atoms that aggregate into stable dimers. These dimers act as symmetry-breaking nucleation centers, guiding the unidirectional alignment of MoSe₂ domains. From the perspective of interfacial interactions, we further clarify how substrate surface activation enhances the epitaxial quality of 2D materials. This work establishes a scalable pathway toward large-scale single-crystal 2D films and provides a conceptual framework for advancing the epitaxial growth of 2D materials.