A Study of the Phase Change Material Mo_1-xW_xTe₂

Structural polymorphism in the transition metal dichalcogenide (TMD) MoTe₂ is ideal for enabling ultrathin phase change memories. Under ambient conditions, MoTe₂ crystallizes in the 2H semiconducting phase, but can be grown in the 1T’ semimetallic phase that transitions into the T_d phase below ~250 K. Alloying MoTe₂ with WTe₂ is theorized to reduce the barrier between the 2H and 1T’ phases, which may improve the efficiency of TMD-based memories and provide on-demand topological phase transitions. Contradictory reports of the Mo_1-xW_xTe₂ phase diagram [1,2] demonstrate the need for new investigations of this alloy system. Here, we combine Raman spectroscopy, x-ray diffraction, scanning transmission electron microscopy, and density-functional theory to explore the phase boundaries of bulk Mo_1-xW_xTe₂ alloys. Our work confirms the existence of 2H, 1T’, and T_d regions, as well as a two-phase 1T’-T_d region in the MoTe₂-WTe₂ system [3]. We extract the phonon correlation length through the phonon confinement model, identify disorder-enhanced two-phonon scattering, and observe disorder-activation of infrared modes. These results provide a foundation for future applications of the Mo_1-xW_xTe₂ alloys.

[1] Rhodes et al. Nano Lett. 2017 [2] Lv et al. Sci. Rep. 2017 [3] Oliver et al. 2D Mat. 2017