The progress of modern technology is increasingly driven by the development and evaluation of novel materials. Layered two-dimensional (2D) materials, collectively referred to as van der Waals solids, are receiving intense interest in the community due to their unconventional and diverse electronic behaviors. Transition metal dichalcogenides (TMDs) are a class of 2D material following the basic chemical formula MX2, where M = (Mo, W, Nb, Re, …) and X = (S, Se, Te, …). Varying the chemical composition allows access to semiconducting, semi-metallic, superconducting, or magnetic behaviors in the 2D limit. Of recent interest are telluride based TMDs such as MoTe2 and WTe2 which are predicted exhibit controllable structural phase transitions appropriate for phase change memory applications as well as topologically protected and spin polarized electronic states.
In this colloquium, I will present our efforts to understand the temperature-dependent optical properties of MoTe2 and MoxW1-xTe2 using temperature-dependent and polarization-resolved Raman spectroscopy. We have used this technique to identify the anharmonic contributions to the optical phonon modes in bulk MoTe2 occupying the distorted orthorhombic (Td) lattice structure. At temperatures ranging from 100 K to 200 K, we find that all modes redshift linearly with temperature however, below 100 K we observe nonlinear frequency shifts in some modes. We show that this anharmonic behavior is consistent with the decay of an optical phonon into multiple acoustic phonons. Furthermore, the highest frequency Raman modes show large changes in intensity and linewidth near 250 K that correlate well with a structural phase transition.
We also explore the composition-dependent optical properties of MoxW1-xTe2 alloys. Our observations identify signatures of the hexagonal (H), monoclinc (1T’) and Td structural phases. Polarization-resolved Raman measurements allow for the assignment of all vibrational modes as well as the evolution of mode symmetry and frequency with composition. We discover a previously unobserved WTe2 mode as well as a Raman-forbidden MoTe2 mode that is activated by compositional disorder. The primary WTe2 Raman peak is asymmetric for x <= 0.1, and is well fit by the spatial correlation model. From these fits, we extract the spatial phonon correlation length which serves as an indirect measure of the WTe2 domain size. Our study is foundational for future studies of MoxW1-xTe2 and provides new insights into the impact of disorder in transition metal dichalcogenides.