Overcoming the Lattice Mismatch Barrier for Atomic Reconstruction in MoSe ₂ /MoS ₂ Heterobilayers

Twisted transition metal dichalcogenide (TMD) bilayers have garnered significant attention due to the emergence of unconventional quantum phenomena, such as sliding ferroelectricity in multidomain TMD bilayers with domain walls (DWs). Thus, understanding their atomic reconstruction is essential for elucidating the origin of such properties. While atomic reconstruction has been observed in twisted bilayers with small lattice mismatch, large-mismatch systems have generally been thought to retain incommensurate moiré superlattices as the substantial bond deformation required for reconstruction renders it energetically unfavorable. Here, we demonstrate that MoSe₂/MoS₂ heterobilayers with a large lattice mismatch can reconstruct into commensurate domain structures by encapsulation annealing at a high temperature of >1100 °C. This process supplied sufficient thermal energy and vertical compression to enable local atomic rearrangements and facilitates vacancy-assisted chalcogen exchange across the interface, where Se and S atoms migrate between the two layers through chalcogen vacancies. This interlayer atomic diffusion results in the formation of the MoSe_2-<i>x</i>S_<i>x</i>/MoS_2-<i>x</i>Se_<i>x</i> alloy, effectively reducing the lattice mismatch and enabling the formation of commensurate AB/BA domains separated by tensile saddle point (SP) boundaries. These tensile boundaries reflect residual in-plane strain from the reduced lattice mismatch, distinct from the shear-type boundaries observed in lattice-matched systems. Spectroscopic analysis further reveals enhanced interlayer coupling and room temperature valley polarization in the alloyed heterobilayer. Our findings establish a conceptual framework for structural reconstruction in large-lattice-mismatched van der Waals (vdW) heterostructures, where chalcogen alloying enables domain formation previously thought inaccessible, thereby offering additional pathways to engineer the structural and optoelectronic properties of 2D materials.