Few-layered transition-metal dichalcogenides (TMDs) are increasingly popular materials for optoelectronics and catalysis. Among 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 band gap behavior throughout multilayered stacks. In this paper, 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 broad-band 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.

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2D materials are considered for applications that require strong light-matter interaction because of the apparently giant oscillator strength of the exciton transitions in the absorbance spectrum. Nevertheless, the effective oscillator strengths of these transitions have been scarcely reported, nor is there a consistent interpretation of the obtained values. Here, we analyse the transition dipole moment and the ensuing oscillator strength of the exciton transition in 2D CdSe nanoplatelets by means of the optically induced Stark effect (OSE). Intriguingly, we find that the exciton absorption line reacts to a high intensity optical field as a transition with an oscillator strength F_Stark that is 50 times smaller than expected based on the linear absorption coefficient. We propose that the pronounced exciton absorption line should be seen as the sum of multiple, low oscillator strength transitions, rather than a single high oscillator strength one, a feat we assign to strong exciton center-of-mass localization. Within the quantum mechanical description of excitons, this 50-fold difference between both oscillator strengths corresponds to the ratio between the coherence area of the exciton’s center of mass and the total area, which yields a coherence area of a mere 6.1 nm². Since we find that the coherence area increases with reducing temperature, we conclude that thermal effects, related to lattice vibrations, contribute to exciton localization. In further support of this localization model, we show that F_Stark is independent of the nanoplatelet area, correctly predicts the radiative lifetime, and lines up for strongly confined quantum dot systems. [Figure not available: see fulltext.]

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Low-dimensional semiconductors have found numerous applications in optoelectronics. However, a quantitative comparison of the absorption strength of low-dimensional versus bulk semiconductors has remained elusive. Here, we report generality in the band-edge light absorptance of semiconductors, independent of their dimensions. First, we provide atomistic tight-binding calculations that show that the absorptance of semiconductor quantum wells equals mπα (m = 1 or 2 with α as the fine-structure constant), in agreement with reported experimental results. Then, we show experimentally that a monolayer (superlattice) of quantum dots has similar absorptance, suggesting an absorptance quantum of mπα per (confined) exciton diameter. Extending this idea to bulk semiconductors, we experimentally demonstrate that an absorptance quantum equal to mπα per exciton Bohr diameter explains their widely varying absorption coefficients. We thus provided compelling evidence that the absorptance quantum πα per exciton diameter rules the band-edge absorption of all direct semiconductors, regardless of their dimension.

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We studied the dynamics of transfer of photoexcited electronic states in a bilayer of the two-dimensional transition metal dichalcogenide ReS2 and tetracene, with the aim to produce triplets in the latter. This material combination was used as the band gap of ReS2 (1.5 eV) is slightly larger than the triplet energy of tetracene (1.25 eV). Using time-resolved optical absorption spectroscopy, transfer of photoexcited states from ReS2 to triplet states in tetracene was found to occur within 5 ps with an efficiency near 38%. This result opens up new possibilities for heterostructure design of two-dimensional materials with suitable organics to produce long-lived triplets. Triplets are of interest as sensitizers in a wide variety of applications including optoelectronics, photovoltaics, photocatalysis, and photon upconversion.

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Solution processed quantum dot (QD) lasers are one of the holy-grails of nanoscience. They are not yet commercialized because the lasing threshold is too high: one needs < 1 exciton per QD, which is hard to achieve due to fast non-radiative Auger recombination. The threshold can however be reduced by electronic doping of the QDs, which decreases the absorption near the band-edge, such that the stimulated emission (SE) can easily outcompete absorption. Here, we show that by electrochemically doping films of CdSe/CdS/ZnS QDs we achieve quantitative control over the gain threshold. We obtain stable and reversible doping more than two electrons per QD. We quantify the gain threshold and the charge carrier dynamics using ultrafast spectroelectrochemistry and achieve quantitative agreement between experiments and theory, including a vanishingly low gain threshold for doubly doped QDs. Over a range of wavelengths with appreciable gain coefficients, the gain thresholds reach record-low values of ∼10-5 excitons per QD. These results demonstrate an unprecedented level of control over the gain threshold in doped QD solids, paving the way for the creation of cheap, solution-processable low-threshold QD-lasers.

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Owing to their widely tuneable optical properties and strong light-matter interaction, colloidal quantum dots[1] (QDs) are considered for next-generation photonic devices such as solution processable lasers[2] and all-optical wavelength converters. Using QDs for this last application, i.e. wavelength conversion, is limited by either slow interband (106-109 s-1) or energy consuming multi-exciton dynamics (109-1012 s−1). Here we show, using white light pump-probe spectroscopy, that the interplay between two intrinsic material properties of PbS QDs, intraband absorption and interband bleach can lead to a very strong modulation of near-infrared light on an ultrafast, picosecond, timescale: A peculiar dynamic arises at the matching wavelength: a strong burst of absorption induced by the pump lasts for roughly 1 picosecond and vanishes, leaving the dots as they were before the pump pulse arrived.@en