Double resonance Raman modes in mono- and few-layer MoTe$_2$

TL;DR

This study combines theoretical calculations and experiments to analyze second-order Raman modes in mono- and few-layer MoTe$_2$, revealing the role of double resonance processes at the M points in the Brillouin zone.

Contribution

It provides a detailed understanding of the second-order Raman scattering mechanism in MoTe$_2$, linking theoretical predictions with experimental observations.

Findings

Second-order Raman peaks match theoretical predictions.

Double resonance involves inter-valley electron-phonon coupling.

Resonance occurs at the M point with strong optical absorption.

Abstract

We study the second-order Raman process of mono- and few-layer MoTe$_2$, by combining {\em ab initio} density functional perturbation calculations with experimental Raman spectroscopy using 532, 633 and 785 nm excitation lasers. The calculated electronic band structure and the density of states show that the electron-photon resonance process occurs at the high-symmetry M point in the Brillouin zone, where a strong optical absorption occurs by a logarithmic Van-Hove singularity. Double resonance Raman scattering with inter-valley electron-phonon coupling connects two of the three inequivalent M points in the Brillouin zone, giving rise to second-order Raman peaks due to the M point phonons. The predicted frequencies of the second-order Raman peaks agree with the observed peak positions that cannot be assigned in terms of a first-order process. Our study attempts to supply a basic understanding of the second-order Raman process occurring in transition metal di-chalcogenides (TMDs) and may provide additional information both on the lattice dynamics and optical processes especially for TMDs with small energy band gaps such as MoTe$_2$ or at high laser excitation energy.