Dark excitons in transition metal dichalcogenides

TL;DR

This paper explores the nature of dark excitons in monolayer transition metal dichalcogenides, revealing their spectral positions and impact on emission efficiency, which is crucial for technological applications.

Contribution

It provides a detailed excitonic landscape of TMDs, including dark states, using Wannier equation solutions, and clarifies the relation between dark excitons and photoluminescence.

Findings

Dark excitons significantly influence emission efficiency.

Spectral positions of dark states vary among TMDs.

Reduced photoluminescence does not necessarily indicate a transition to a non-direct band gap.

Abstract

Monolayer transition metal dichalcogenides (TMDs) exhibit a remarkably strong Coulomb interaction that manifests in tightly bound excitons. Due to the complex electronic band structure exhibiting several spin-split valleys in the conduction and valence band, dark excitonic states can be formed. They are inaccessibly by light due to the required spin-flip and/or momentum transfer. The relative position of these dark states with respect to the optically accessible bright excitons has a crucial impact on the emission efficiency of these materials and thus on their technological potential. Based on the solution of the Wannier equation, we present the excitonic landscape of the most studied TMD materials including the spectral position of momentum- and spin-forbidden excitonic states. We show that the knowledge of the electronic dispersion does not allow to conclude about the nature of the material's band gap, since excitonic effects can give rise to significant changes. Furthermore, we reveal that an exponentially reduced photoluminescence yield does not necessarily reflect a transition from a direct to a non-direct gap material, but can be ascribed in most cases to a change of the relative spectral distance between bright and dark excitonic states.