Optical Signatures of Structural Polymorphism in MoTe2 and Mo1-xWxTe2 Alloys

P.M. Vora, R. Beams, S.M. Oliver, S. Krylyuk, A.K. Singh, I. Kalish, A. Bruma, F. Tavazza, J. Joshi, I.R. Stone, S.J. Stranick, A.V. Davydov
George Mason University,
United States

Keywords: transition metal dichalcogenides, phase change, Raman, spectroscopy, MoTe2, alloys

Summary:

Transition metal dichalcogenides (TMDs) are two-dimensional materials that follow 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. Additionally, TMDs can exist in multiple structural phases that allow for further control over electronic properties and the development of dynamic memory technology relying upon changes in structural phase. Recent theoretical studies have suggested that MoTe2 exhibits a minimal energy difference between the hexagonal (2H, semiconducting) and monoclinic (1T’, metallic) structural phases, making it ideal for phase change memory applications [1,2]. In this presentation, I will discuss our collaboration’s efforts to explore the vibrational properties and structural phases of 1T’-MoTe2 and Mo1-xWxTe2. Temperature-dependent and polarization-resolved Raman spectroscopy are the primary tools in these studies and are complimented by aberration-corrected transmission electron microscopy (TEM) as well as x-ray diffraction (XRD) measurements. Temperature-dependent Raman measurements provide evidence for a transition from 1T’-MoTe2 to a distorted orthorhombic phase (Td) below 250 K and facilitate identification of the anharmonic contributions to the optical phonon modes in bulk MoTe2 and 1T’/Td Mo1-xWxTe2 [3,4]. 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 demonstrate that this anharmonic behavior is consistent with the decay of an optical phonon into multiple acoustic phonons. We also explore the composition-dependent optical properties of Mo1-xWxTe2. alloys and identify clear signatures of the 2H, 1T’, and Td structural phases [5]. Polarization-resolved Raman measurements enable the assignment of all vibrational modes as well as the evolution of mode symmetry and frequency with x. We identify a previously unobserved, Raman-forbidden MoTe2 mode that is activated by compositional disorder and find that the 212 cm-1 WTe2 Raman peak is asymmetric for x < 1. This asymmetry is well-fit by the phonon confinement model and allows the determination of the phonon correlation length [6] which serves as a measure of WTe2 domain size. Combining these results with TEM and XRD measurements allows for the construction of the alloy phase diagram. Our work is foundational for future studies of Mo1-xWxTe2 and provides new insights into the impact of disorder in transition metal dichalcogenides.