Large-Size Superlattices Synthesized by Sequential Sulfur Substitution-Induced Transformation of Metastable MoTe2

We demonstrate a general protocol that uses a metastable phase as a template, followed by chalcogen substitution and phase transformation to obtain superlattices, or single crystals, of layered transition metal dichalcogenides (TMDs). In particular, the single-crystalline 2H-MoTe2 thin film, with the available wafer-scale synthesis, is selected as the template to study the chalcogen substitution mechanism. Analogous to the initiation polymerization process, a Te vacancy-initiated and S diffusion-mediated mechanism is proposed to describe the sequential substitution: the complete sulfurization of the top and bottom MoTe2 layers at the first stage, followed by the alloying process in the middle layers and finally the full conversion of the flake into MoS2. The substitution in each layer starts from the vacancy and expands to the nearby region catalyzed by the strain field, whereas the sulfurization sequence in the multilayer system is mediated by the cross-layer S diffusion process. This is confirmed by the cross-sectional observation of the intermediate state by scanning transmission electron microscopy and density functional theory studies. This unique mechanism enables us to fabricate the sub-centimeter scale, composition-tunable, and symmetric MoS2/MoTe2(1-x)S2x/MoS2 superlattices. Our work presents a new tool for the large-scale synthesis of TMD-based heterostructures toward industrial applications.