In Situ Imaging of an Anisotropic Layer-by-Layer Phase Transition in Few-Layer MoTe 2 .

Understanding the phase transition mechanisms in two-dimensional (2D) materials is a key to precisely tailor their properties at the nanoscale. Molybdenum ditelluride (MoTe 2 ) exhibits multiple phases at room temperature, making it a promising candidate for phase-change applications. Here, we fabricate lateral 2 H - T d interfaces with laser irradiation and probe their phase transitions from micro- to atomic scales with in situ heating in the transmission electron microscope (TEM). By encapsulating the MoTe 2 with graphene protection layers, we create an in situ reaction cell compatible with atomic resolution imaging. We find that the T d -to- 2H phase transition initiates at phase boundaries at low temperatures (200-225 °C) and propagates anisotropically along the b -axis in a layer-by-layer fashion. We also demonstrate a fully reversible 2H - T d - 2H phase transition cycle, which generates a coherent 2H lattice containing inversion domain boundaries. Our results provide insights on fabricating 2D heterophase devices with atomically sharp and coherent interfaces.