Millimeter-Scale Single-Crystalline Semiconducting MoTe ₂ via Solid-to-Solid Phase Transformation

Among the Mo- and W-based two-dimensional (2D) transition metal dichalcogenides, MoTe ₂ is particularly interesting for phase-engineering applications, because it has the smallest free energy difference between the semiconducting 2H phase and metallic 1T′ phase. In this work, we reveal that, under the proper circumstance, Mo and Te atoms can rearrange themselves to transform from a polycrystalline 1T′ phase into a single-crystalline 2H phase in a large scale. We manifest the mechanisms of the solid-to-solid transformation by conducting density functional theory calculations, transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. The phase transformation is well described by the time-temperature-transformation diagram. By optimizing the kinetic rates of nucleation and crystal growth, we have synthesized a single-crystalline 2H-MoTe ₂ domain with a diameter of 2.34 mm, a centimeter-scale 2H-MoTe ₂ thin film with a domain size up to several hundred micrometers, and a seamless 1T′-2H MoTe ₂ coplanar homojunction. The 1T′-2H MoTe ₂ homojunction provides an elegant solution for ohmic contact of 2D semiconductors. The controlled solid-to-solid phase transformation in 2D limit provides a new route to realize wafer-scale single-crystalline 2D semiconductor and coplanar heterostructure for 2D circuitry.