Unraveling the Photophysics of Liquid-Phase Exfoliated Two-Dimensional ReS2 Nanoflakes

TL;DR

This study investigates the charge carrier dynamics in liquid-phase exfoliated ReS2 nanoflakes using advanced spectroscopy, revealing unbound charge carriers dominate despite high exciton binding energy, with implications for optoelectronic applications.

Contribution

It provides a detailed analysis of charge carrier behavior in ReS2, highlighting the presence of free carriers and their dynamics, which was not previously well-understood.

Findings

Unbound charge carriers dominate in ReS2 despite high exciton binding energy.

Two distinct temporal regimes show different effects on exciton absorption.

ReS2 exhibits potential for free charge carrier generation relevant to photovoltaics.

Abstract

Few-layered transition metal dichalcogenides (TMDs) are increasingly popular materials for optoelectronics and catalysis. Amongst the various types of TMDs available today, rhenium-chalcogenides (ReX2) stand out due to their remarkable electronic structure, such as the occurrence of anisotropic excitons and potential direct bandgap behavior throughout multi-layered stacks. In this letter, we have analyzed the nature and dynamics of charge carriers in highly crystalline liquid-phase exfoliated ReS2, using a unique combination of optical pump-THz probe and broadband transient absorption spectroscopy. Two distinct time regimes are identified, both of which are dominated by unbound charge carriers despite the high exciton binding energy. In the first time regime, the unbound charge carriers cause an increase and a broadening of the exciton absorption band. In the second time regime, a peculiar narrowing of the excitonic absorption profile is observed, which we assign to the presence of built-in fields and/or charged defects. Our results pave the way to analyze spectrally complex transient absorption measurements on layered TMD materials and indicate the potential for ReS2 to produce mobile free charge carriers, a feat relevant for photovoltaic applications.