Additional excitonic features and momentum-dark states in 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${}_2$.

9 pages, 4 figures (main text) 16 pages, 10 figures (supplemental material). To be published in Phys. Rev. B (Rapid Comm.)