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

TL;DR

This study visualizes the layer-by-layer anisotropic phase transition in MoTe2 at atomic resolution, revealing boundary-initiated, reversible phase changes crucial for 2D device engineering.

Contribution

It introduces in situ TEM imaging of phase transitions in MoTe2 with graphene encapsulation, uncovering anisotropic, layer-by-layer transition mechanisms at atomic scale.

Findings

Phase transition initiates at boundaries at 200-225°C.

Transition propagates anisotropically along the b-axis.

Reversible 2H-Td-2H phase cycle with coherent interfaces.

Abstract

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 2H-Td 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 MoTe2 with graphene protection layers, we create an in situ reaction cell compatible with atomic resolution imaging. We find that the Td-to-2H phase transition initiates at phase boundaries at low temperatures (200-225 degree C) and propagates anisotropically along the b-axis in a layer-by-layer fashion. We also demonstrate a fully reversible 2H-Td-2H phase transition cycle, which generates a coherent 2H lattice containing inversion domain boundaries. Our results provide insights on fabricating 2D hetero-phase devices with atomically sharp and coherent interfaces.