  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.
