George Mason University, USA
Title: Structural and optical properties of the MoTe2-WTe2 alloy system
Biography: Patrick M Vora
The structural polymorphism intrinsic to transition metal dichalcogenides provides exciting opportunities for engineering novel devices. Of special interest are memory technologies that rely upon controlled changes in crystal phase, collectively known as phase change memories (PCMs). MoTe2 is ideal for PCMs as the ground state energy difference between the hexagonal (2H, semiconducting) and monoclinic (1T’, metallic) phases is minimal. This energy difference can be further reduced by substituting W for Mo on the metal sublattice, thus improving PCM performance. Therefore, understanding the properties of Mo1-xWxTe2 alloys across the entire compositional range is vital for the technological application of these versatile materials. We combine Raman spectroscopy with aberration-corrected scanning transmission electron microscopy and x-ray diffraction to explore the MoTe2-WTe2 alloy system. The results of these studies enable the construction of the complete alloy phase diagram, while polarization-resolved Raman measurements provide phonon mode and symmetry assignments for all compositions. Temperature-dependent Raman measurements indicate a transition from 1T’-MoTe2 to a distorted orthorhombic phase (Td) below 250 K and facilitate identification of the harmonic contributions to the optical phonon modes in bulk MoTe2 and Mo1-xWxTe2 alloys. We also identify a Ramanforbidden MoTe2 mode that is activated by compositional disorder and find that the main WTe2 Raman peak is asymmetric for x<1. This asymmetry is well-fit by a phonon confinement model, which allows the determination of the phonon correlation length. Our work is foundational for future studies of MoxW1-xTe2 alloys and provides new insights into the impact of disorders in transition metal dichalcogenides.