Anomalous Dimensionality-Driven Phase Transition of MoTe₂ in Van der Waals Heterostructure

Phase transition in nanomaterials is distinct from that in 3D bulk materials owing to the dominant contribution of surface energy. Among nanomaterials, 2D materials have shown unique phase transition behaviors due to their larger surface-to-volume ratio, high crystallinity, and lack of dangling bonds in atomically thin layers. Here, the anomalous dimensionality-driven phase transition of molybdenum ditelluride (MoTe₂) encapsulated by hexagonal boron nitride (hBN) is reported. After encapsulation annealing, single-crystal 2H-MoTe₂ transformed into polycrystalline T_d-MoTe₂ with tilt-angle grain boundaries of 60°-glide-reflection and 120°-twofold rotation. In contrast to conventional nanomaterials, the hBN-encapsulated MoTe₂ exhibit a deterministic dependence of the phase transition on the number of layers, in which the thinner MoTe₂ has a higher 2H-to-T_d phase transition temperature. In addition, the vertical and lateral phase transitions of the stacked MoTe₂ with different crystalline orientations can be controlled by inserted graphene layers and the thickness of the heterostructure. Finally, it is shown that seamless T_d contacts for 2H-MoTe₂ transistors can be fabricated by using the dimensionality-driven phase transition. The work provides insight into the phase transition of 2D materials and van der Waals heterostructures and illustrates a novel method for the fabrication of multi-phase 2D electronics.