Atomically precise synthesis and simultaneous heterostructure integration of 2D transition metal dichalcogenides through nano-confinement.

Two-dimensional (2D) materials, such as graphene, transition metal dichalcogenides (TMDs) and hexagonal boron nitride, exhibit intriguing properties that are sensitive to their atomic-scale structures and can be further enriched through van der Waals (vdW) integration. However, the precise synthesis and clean integration of 2D materials remain challenging. Here, using graphene or hexagonal boron nitride as a vdW capping layer, we create a nano-confined environment that directs the growth kinetics of 2D TMDs (such as NbSe2 and MoS2), enabling precise formation of TMD monolayers with tailored morphologies, from isolated monolayer domains to large-scale continuous films and intrinsically patterned rings. Moreover, Janus S-Mo-Se monolayers are synthesized with atomic precision via vdW-protected bottom-plane chalcogen substitution. Importantly, our approach simultaneously produces ultraclean vdW interfaces. This in situ encapsulation reliably preserves air-sensitive materials, as evidenced by the enhanced superconductivity of nano-confined NbSe2 monolayers. Altogether, our study establishes a versatile platform for the controlled synthesis and integration of 2D TMDs for advanced applications.