Raman Scattering with infrared excitation resonant with MoSe$_2$ indirect band gap

TL;DR

This study explores resonance Raman scattering in MoSe₂ across various laser energies, revealing how resonance effects enhance high-order phonon modes at specific energies and how temperature influences these resonances.

Contribution

It provides a detailed analysis of two-phonon modes and their coupling with electrons at different excitation energies, highlighting the role of the indirect band gap in resonance phenomena.

Findings

High-order Raman modes are strongly enhanced at 1.16 eV excitation energy.

Two-phonon modes originate mainly from opposite momenta phonons.

Lowering temperature suppresses resonance-enhanced modes due to band gap increase.

Abstract

Resonance Raman scattering, which probes electrons, phonons and their interplay in crystals, is extensively used in two-dimensional materials. Here we investigate Raman modes in MoSe$_2$ at different laser excitation energies from 2.33 eV down to the near infrared 1.16 eV. The Raman spectrum at 1.16 eV excitation energy shows that the intensity of high-order modes is strongly enhanced if compared to the first-order phonon modes intensity due to resonance effects with the MoSe$_2$ indirect band gap. By comparing the experimental results with the two-phonon density of states calculated with density functional theory, we show that the high-order modes originate mostly from two-phonon modes with opposite momenta. In particular, we identify the momenta of the phonon modes that couple strongly with the electrons to produce the resonance process at 1.16 eV, while we verify that at 2.33 eV the two-phonon modes lineshape compares well with the two-phonon density of state calculated over the entire Brillouin Zone. We also show that, by lowering the crystal temperature, we actively suppress the intensity of the resonant two-phonon modes and we interpret this as the result of the increase of the indirect band gap at low temperature that moves our excitation energy out of the resonance condition.