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

Abstract

Few-layered transition metal dichalcogenides (TMDs) are increasingly popular materials for optoelectronics and catalysis. Amongst the various types of TMDs available today, rheniumchalcogenides (ReX2) stand out due to their remarkable electronic structure, such as the occurrence of anisotropic excitons and potential direct bandgap behavior throughout multilayered 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.

Band Edge Carrier-Induced Sign Reversal of an Ultrafast Nonlinear Optical Response in Few-Layer ReS2 Nanoflakes

“…However, the PIA is not decaying within our experimental time window. The PIA observed is in stark contrast to the result described by Schiettecatte et al, where absorption due to lattice heating appears at the exciton position itself, replacing the bleach at longer times . At this point of time, we assign the delayed growth, and slow decay of PIA can be associated with the excited-state absorption of the carriers (excitons) at the band edge and those trapped at the defects.…”

“…Recently, Schiettecatte et al prepared liquid-phase exfoliated ReS 2 with NMP as a solvent and described the photophysical properties. 40,41 In our case, 125 mg well-ground ReS 2 powder of single crystals was mixed with 25 mL NMP solution. The solution was sonicated using a probe sonicator of 125 mW power with 40 cycles consisting of a run time of 20 min and rest of 10 min to avoid conglomeration.…”

Generating Triplets in Organic Semiconductor Tetracene upon Photoexcitation of Transition Metal Dichalcogenide ReS2

“…We studied transfer photoexcited electronic states from ReS 2 to tetracene in a bilayer configuration. Please note that according to our recent study photoexcitation at 530 nm can lead to the formation of excitons and free charges in ReS 2 with an upper limit of the initial quantum yield of free charges near 20% and a lifetime before recombination to excitons or trapping of a few picoseconds . After photoexcitation of ReS 2 triplets can be produced in tetracene both due to transfer of excitons and to sequential transfer of electrons and holes from ReS 2 to tetracene.…”

“…Please note that according to our recent study photoexcitation at 530 nm can lead to the formation of excitons and free charges in ReS 2 with an upper limit of the initial quantum yield of free charges near 20% and a lifetime before recombination to excitons or trapping of a few picoseconds. 41 After photoexcitation of ReS 2 triplets can be produced in tetracene both due to transfer of excitons and to sequential transfer of electrons and holes from ReS 2 to tetracene. In this work, we do not distinguish these two processes and henceforth refer to them as transfer of photoexcited states to tetracene.…”

Ab initio investigations of a CoBiS monolayer with and without point defects

“…6 In addition, they exhibit diverse electrical, optical, thermal, and mechanical properties along the different in-plane crystal orientations due to their decreased crystal symmetry. 6,7 Low-symmetry TMDs, particularly those with the 1T 0 phase, have recently gained a lot of attention due to their intrinsic anisotropic properties of fundamental and technological importance. Despite these extraordinary properties, particularly their attractiveness for the fabrication of novel optoelectronic devices, few 2D TMDs, such as ReS(Se) 2 and W(Mo)Te 2 , have been synthesized in this phase.…”