Davydov splitting and excitonic resonance effects in Raman spectra of few-Layer MoSe2

TL;DR

This study investigates how excitonic resonances influence the Raman spectra of few-layer MoSe2, revealing new resonance-dependent peaks, Davydov splitting, and quantifying inter-layer interactions.

Contribution

It provides a detailed analysis of resonance effects in Raman spectra of MoSe2, including the first observation of Davydov splitting and quantification of inter-layer coupling strengths.

Findings

Resonance profiles reflect the joint density of states for optical transitions.

Selective enhancement of specific Raman modes depends on exciton energy.

Inter-layer interactions are about 30% of nearest-neighbor interactions.

Abstract

Raman spectra of few-layer MoSe2 were measured with 8 excitation energies. New peaks that appear only near resonance with various exciton states are analyzed, and the modes are assigned. The resonance profiles of the Raman peaks reflect the joint density of states for optical transitions, but the symmetry of the exciton wave functions leads to selective enhancement of the A1g mode at the A exciton energy and the shear mode at the C exciton energy. We also find Davydov splitting of intra-layer A1g, E1g, and A2u modes due to inter-layer interaction for some excitation energies near resonances. Furthermore, by fitting the spectral positions of inter-layer shear and breathing modes and Davydov splitting of intra-layer modes to a linear chain model, we extract the strength of the inter-layer interaction. We find that the second-nearest-neighbor inter-layer interaction amounts to about 30% of the nearest-neighbor interaction for both in-plane and out-of-plane vibrations.