Interstitially Bridged van der Waals Interface Enabling Stacking-Fault-Free, Layer-by-Layer Epitaxy

van der Waals (vdW) crystals are prone to twisting, sliding, and buckling due to inherently weak interlayer interactions. While thickness-controlled vdW structures have attracted considerable attention as ultrathin semiconducting channels, the deterministic synthesis of stacking-fault-free multilayers remains a persistent challenge. Here, we report the epitaxial growth of single-crystalline hexagonal bilayer MoS2, enabled by the incorporation of Mo interstitials between layers during layer-by-layer deposition. The resulting bilayers exhibit exceptional structural robustness, maintaining their crystallinity and suppressing both rotational and translational interlayer misalignments even after transfer processes. Atomic-resolution analysis reveals that the Mo interstitials are located at a single sublattice site within the hexagonal lattice, where they form tetrahedral bonds with sulfur atoms from both MoS2 layers, effectively anchoring the interlayer registry. Density functional theory calculations further indicate that these Mo atoms act as nucleation centers, promoting the selective formation of the hexagonal bilayer phase. This approach offers a robust strategy for the deterministic growth of multilayer vdW crystals with precisely controlled stacking order and enhanced interlayer coupling.