Evolution of Raman spectra in Mo\(_{1-x}\)W\(_x\)Te\(_2\) alloys

The structural polymorphism in transition metal dichalcogenides (TMDs) provides exciting opportunities for developing advanced electronics. For example, MoTe\(_2\) crystallizes in the 2H semiconducting phase at ambient temperature and pressure, but transitions into the 1T\(^\prime\) semimetallic phase at high temperatures. Alloying MoTe\(_2\) with WTe\(_2\) reduces the energy barrier between these two phases, while also allowing access to the T\(_d\) Weyl semimetal phase. The MoWTe\(_2\) alloy system is therefore promising for developing phase change memory technology. However, achieving this goal necessitates a detailed understanding of the phase composition in the MoTe\(_2\)-WTe\(_2\) system. We combine polarization-resolved Raman spectroscopy with X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) to study MoWTe\(_2\) alloys over the full compositional range x from 0 to 1. We identify Raman and XRD signatures characteristic of the 2H, 1T\(^\prime\), and T\(_d\) structural phases that agree with density-functional theory (DFT) calculations, and use them to identify phase fields in the MoTe\(_2\)-WTe\(_2\) system, including single-phase 2H, 1T\(^\prime\), and T\(_d\) regions, as well as a two-phase 1T\(^\prime\) + T\(_d\) region. Disorder arising from compositional fluctuations in MoWTe\(_2\) alloys breaks inversion and translational symmetry, leading to the activation of an infrared 1T\(^\prime\)-MoTe\(_2\) mode and the enhancement of a double-resonance Raman process in 2H-MoWTe\(_2\) alloys. Compositional fluctuations limit the phonon correlation length, which we estimate by fitting the observed asymmetric Raman lineshapes with a phonon confinement model. These observations reveal the important role of disorder in MoWTe\(_2\) alloys, clarify the structural phase boundaries, and provide a foundation for future explorations of phase transitions and electronic phenomena in this system.