Electron-phonon and phonon-phonon anharmonic interactions in 2H-MoX2 (X = S, Te): A comprehensive resonant Raman study

Transition metal dichalcogenides (TMDs) are layered materials which show excellent potential for nanoelectronic and optoelectronic applications. However, as many of the exciting features of these materials are controlled by anharmonic effects, a proper understanding of the phonon properties and anharmonicity associated with these materials is essential for the proposed applications to be realized. We present here a comprehensive study on the phonon properties of two different TMDs, viz., MoS2 and MoTe2, as a function of temperature, laser excitations, and polarization, as well as flake thickness. This paper includes the measurement of anharmonicity in the first-order and higher order Raman scattering processes. Variations in anharmonicity with the thickness of MoS2 and MoTe2 have been explained in terms of their phonon symmetries, electron-phonon coupling, and phonon-phonon interactions. Further, the effect of the underlying substrate on the anharmonic properties of in-plane and out-of-plane phonons has also been estimated from Raman measurements, thus elucidating the intrinsic phonon properties of two-dimensional layered materials.