Investigating the 1T' to T_dPhase Transition in Mo_1-xW_xTe₂ via Raman Spectroscopy

Molybdenum ditelluride (MoTe₂) is an exciting material for quantum applications due to its unique electronic properties and phase-dependent topological states. While the 2H phase of MoTe₂ is thermodynamically stable at room temperature, the 1T' phase, a monoclinic structure with higher-order topological properties, can also be stabilized. Upon cooling, the 1T' phase transitions into the orthorhombic T_d phase, which is a type-II Weyl semimetal. Alloying molybdenum with tungsten (Mo₁₋ₓWₓTe₂) has been identified as a promising route to engineer this transition. Although this alloy system has been previously studied using techniques such as X-ray diffraction (XRD) and neutron scattering, there remains disagreement regarding the width of the mixed-phase region and the mechanism driving the 1T' to T_d transition. Understanding this mixed-phase region is critical, as it can host topological interface states between 1T’ and T_d domains.

In this work, we demonstrate that Raman spectroscopy is a powerful tool for probing the 1T' to T_d phase transition in Mo₁₋ₓWₓTe₂, offering new insights into the transition mechanism. By tracking the low-frequency layer breathing modes, we show that the phase transition mechanism is closely linked to changes in the stacking order between layers. Using different laser wavelengths, we underscore the importance of excitation energy selection for precise characterization of phase transitions. Further, we correlate our Raman spectroscopy data with cross-sectional transmission electron microscopy (TEM) results to provide a comprehensive understanding of the structural changes occurring during the transition. These findings have significant implications for the design and optimization of quantum devices based on Mo₁₋ₓWₓTe₂ and similar materials.