Dark and bright exciton formation, thermalization, and photoluminescence in monolayer transition metal dichalcogenides

TL;DR

This study provides a microscopic understanding of how bright and dark excitons form, thermalize, and influence photoluminescence in monolayer transition metal dichalcogenides, highlighting the significant role of dark excitons in tungsten-based TMDs.

Contribution

It reveals the formation, dynamics, and impact of dark excitons, especially intervalley dark states, on exciton behavior and photoluminescence in TMDs, which was previously not well understood.

Findings

Dark excitons significantly affect exciton lifetimes and photoluminescence.

Intervalley dark excitons are crucial in tungsten-based TMDs.

Elevated temperatures enhance the role of dark excitons.

Abstract

The remarkably strong Coulomb interaction in atomically thin transition metal dichalcogenides (TMDs) results in an extraordinarily rich many-particle physics including the formation of tightly bound excitons. Besides optically accessible bright excitonic states, these materials also exhibit a variety of dark excitons. Since they can even lie below the bright states, they have a strong influence on the exciton dynamics, lifetimes, and photoluminescence. While very recently, the presence of dark excitonic states has been experimentally demonstrated, the origin of these states, their formation, and dynamics have not been revealed yet. Here, we present a microscopic study shedding light on time- and energy-resolved formation and thermalization of bright and dark intra- and intervalley excitons as well as their impact on the photoluminescence in different TMD materials. We demonstrate that intervalley dark excitons, so far widely overlooked in current literature, play a crucial role in tungsten-based TMDs giving rise to an enhanced photoluminescence and reduced exciton lifetimes at elevated temperatures.