Electron-Phonon Coupling, Thermal Expansion Coefficient, Resonance Effect and Phonon Dynamics in High Quality CVD Grown Mono and Bilayer MoSe2

TL;DR

This study provides a comprehensive analysis of phonon behaviors, electron-phonon interactions, and thermal expansion in high-quality monolayer and bilayer MoSe2 using temperature and polarization-dependent Raman spectroscopy, revealing new insights into their fundamental properties.

Contribution

It offers detailed experimental insights into phonon dynamics, electron-phonon coupling, and symmetry assignments in MoSe2, advancing understanding of 2D material properties.

Findings

Observation of phonon modes up to fourth-order including forbidden modes.

Identification of significant electron-phonon coupling effects, especially in bilayer MoSe2.

Extraction of thermal expansion coefficients for monolayer and bilayer MoSe2.

Abstract

Probing phonons, quasi-particle excitations and their coupling has enriched our understanding of these 2D materials and proved to be crucial for developing their potential applications. Here, we report comprehensive temperature, 4-330 K, and polarization-dependent Raman measurements on mono and bilayer MoSe2. Phonon's modes up to fourth-order are observed including forbidden Raman and IR modes, understood considering Frohlich mechanism of exciton-phonon coupling. Most notably, anomalous variations in the phonon linewidths with temperature pointed at the significant role of electron-phonon coupling in these systems, especially for the out-of-plane (A1g) and shear mode (E22g), which is found to be more prominent in the narrow-gaped bilayer than the large gapped monolayer. Via polarization-dependent measurements, we deciphered the ambiguity in symmetry assignments, especially to the peaks around ~ 170 cm-1 and ~ 350 cm-1. Temperature-dependent thermal expansion coefficient, an important parameter for the device performance, is carefully extracted for both mono and bilayer by monitoring the temperature-dependence of the real-part of the phonon self-energy parameter. Our temperature-dependent in-depth Raman studies provide a pave for uncovering the deeper role of phonons in these 2D layered materials from a fundamental as well as application point of view.