In Situ Imaging of an Anisotropic Layer-by-Layer Phase Transition in Few-Layer MoTe₂

Understanding the phase transition mechanisms in two-dimensional (2D) materials is a key to precisely tailor their properties at the nanoscale. Molybdenum ditelluride (MoTe₂) exhibits multiple phases at room temperature, making it a promising candidate for phase-change applications. Here, we fabricate lateral 2<i>H</i>-<i>T</i>_d interfaces with laser irradiation and probe their phase transitions from micro- to atomic scales with <i>in situ</i> heating in the transmission electron microscope (TEM). By encapsulating the MoTe₂ with graphene protection layers, we create an <i>in situ</i> reaction cell compatible with atomic resolution imaging. We find that the <i>T</i>_d-to-<i>2H</i> phase transition initiates at phase boundaries at low temperatures (200-225 °C) and propagates anisotropically along the <i>b</i>-axis in a layer-by-layer fashion. We also demonstrate a fully reversible <i>2H</i>-<i>T</i>_d-<i>2H</i> phase transition cycle, which generates a coherent <i>2H</i> lattice containing inversion domain boundaries. Our results provide insights on fabricating 2D heterophase devices with atomically sharp and coherent interfaces.