Additional excitonic features and momentum-dark states in ReS2

TL;DR

This study uncovers new excitonic resonances and dark states in ReS2, revealing complex electronic and optical properties crucial for polarization-sensitive optoelectronic applications.

Contribution

It reports the discovery of additional excitonic features and momentum-dark states in ReS2, enhancing understanding of its electronic band structure and optical anisotropy.

Findings

Identification of two new excitonic resonances (X3 and X4).

Observation of low-lying momentum-dark states affecting PL intensity.

Determination of anisotropic dielectric function for ReS2.

Abstract

Unidirectional in-plane structural anisotropy in Rhenium-based transition metal dichalcogenides (TMDs) introduces a new class of 2-D materials, exhibiting anisotropic optical properties. In this work, we perform temperature dependent, polarization-resolved photoluminescence and reflectance measurements on several-layer ReS$_{2}$. We discover two additional excitonic resonances (X$_{3}$ and X$_{4}$), which can be attributed to splitting of spin degenerate states. Strong in-plane oscillator strength of exciton species X$_{1}$ and X$_{2}$ are accompanied by weaker counterparts X$_{3}$ and X$_{4}$ with similar polarization orientations. The in-plane anisotropic dielectric function has been obtained for ReS$_{2}$ which is essential for engineering light matter coupling for polarization sensitive optoelectronic devices. Furthermore, our temperature dependent study revealed the existence of low-lying momentum-forbidden dark states causing an anomalous PL intensity variation at 30 K, which has been elucidated using a rate equation model involving phonon scattering from these states. Our findings of the additional excitonic features and the momentum-dark states can shed light on the true nature of the electronic band structure of ReS$_{2}$.