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The optical response of semiconducting monolayer transition-metal dichalcogenides (TMDCs) is dominated by strongly bound excitons that are stable even at room temperature. However, substrate-related effects such as screening and disorder in currently available specimens mask many anticipated physical phenomena and limit device applications of TMDCs. Here, we demonstrate that that these undesirable effects are strongly suppressed in suspended devices. Extremely robust (photogain > 1,000) and fast (response time < 1 ms) photoresponse allow us to study, for the first time, the formation, binding energies, and dissociation mechanisms of excitons in TMDCs through photocurrent spectroscopy. By analyzing the spectral positions of peaks in the photocurrent and by comparing them with first-principles calculations, we obtain binding energies, band gaps and spin-orbit splitting in monolayer TMDCs. For monolayer MoS2, in particular, we obtain an extremely large binding energy for band-edge excitons, Ebind ≥ 570 meV. Along with band-edge excitons, we observe excitons associated with a van Hove singularity of rather unique nature. The analysis of the source-drain voltage dependence of photocurrent spectra reveals exciton dissociation and photoconversion mechanisms in TMDCs.

Luminescent solar concentrators serving as semitransparent photovoltaic windows could become an important element in net zero energy consumption buildings of the future. Colloidal quantum dots are promising materials for luminescent solar concentrators as they can be engineered to provide the large Stokes shift necessary for suppressing reabsorption losses in large-area devices. Existing Stokes-shift-engineered quantum dots allow for only partial coverage of the solar spectrum, which limits their light-harvesting ability and leads to colouring of the luminescent solar concentrators, complicating their use in architecture. Here, we use quantum dots of ternary I-III-VI2 semiconductors to realize the first large-area quantum dot-luminescent solar concentrators free of toxic elements, with reduced reabsorption and extended coverage of the solar spectrum. By incorporating CuInSexS2-x quantum dots into photo-polymerized poly(lauryl methacrylate), we obtain freestanding, colourless slabs that introduce no distortion to perceived colours and are thus well suited for the realization of photovoltaic windows. Thanks to the suppressed reabsorption and high emission efficiencies of the quantum dots, we achieve an optical power efficiency of 3.2%. Ultrafast spectroscopy studies suggest that the Stokes-shifted emission involves a conduction-band electron and a hole residing in an intragap state associated with a native defect.

Quantum dots (QDs) of ternary I-III-VI compounds such as CuInS and CuInSe have been actively investigated as heavy-metal-free alternatives to cadmium- and lead-containing semiconductor nanomaterials. One serious limitation of these nanostructures, however, is a large photoluminescence (PL) line width (typically >300 meV), the origin of which is still not fully understood. It remains even unclear whether the observed broadening results from considerable sample heterogeneities (due, e.g., to size polydispersity) or is an unavoidable intrinsic property of individual QDs. Here, we answer this question by conducting single-particle measurements on a new type of CuInS (CIS) QDs with an especially thick ZnS shell. These QDs show a greatly enhanced photostability compared to core-only or thin-shell samples and, importantly, exhibit a strongly suppressed PL blinking at the single-dot level. Spectrally resolved measurements reveal that the single-dot, room-temperature PL line width is much narrower (down to ∼60 meV) than that of the ensemble samples. To explain this distinction, we invoke a model wherein PL from CIS QDs arises from radiative recombination of a delocalized band-edge electron and a localized hole residing on a Cu-related defect and also account for the effects of electron-hole Coulomb coupling. We show that random positioning of the emitting center in the QD can lead to more than 300 meV variation in the PL energy, which represents at least one of the reasons for large PL broadening of the ensemble samples. These results suggest that in addition to narrowing size dispersion, future efforts on tightening the emission spectra of these QDs might also attempt decreasing the "positional" heterogeneity of the emitting centers.

We report 65 tesla magneto-absorption spectroscopy of exciton Rydberg states in the archetypal monolayer semiconductor WSe2. The strongly field-dependent and distinct energy shifts of the 2s, 3s, and 4s excited neutral excitons permits their unambiguous identification and allows for quantitative comparison with leading theoretical models. Both the sizes (via low-field diamagnetic shifts) and the energies of the ns exciton states agree remarkably well with detailed numerical simulations using the non-hydrogenic screened Keldysh potential for 2D semiconductors. Moreover, at the highest magnetic fields the nearly-linear diamagnetic shifts of the weakly-bound 3s and 4s excitons provide a direct experimental measure of the exciton's reduced mass, mr = 0.20 ± 0.01 m0.The burgeoning interest in atomically-thin transitionmetal dichalcogenide (TMD) semiconductors such as monolayer MoS 2 and WSe 2 derives in part from their direct optical bandgap and very strong light-matter coupling [1, 2]. In a pristine TMD monolayer, the fundamental optical excitation -the ground-state neutral "A" exciton (X 0 )-can, remarkably, absorb >10% of incoming light [3]. Moreover, in doped or highly excited monolayers distinct resonances due to charged excitons or multiexciton states can develop in optical spectra [4][5][6][7][8][9]. The ability to spectrally resolve these and other features depends critically on material quality, which has markedly improved in recent years as techniques for synthesis, exfoliation, and surface passivation have steadily progressed.The optical quality of exfoliated WS 2 and WSe 2 monolayers has recently improved to the point where signatures of the much weaker excited Rydberg states of X 0 (2s, 2p, 3s, etc.) have been reported based on various linear and nonlinear optical spectroscopies [10][11][12][13][14][15][16]. Correct identification and quantitative measurements of excited excitons are of critical importance in this field, because they provide direct insight into the non-hydrogenic attractive potential between electrons and holes that is believed to exist in 2D materials due to dielectric confinement and nonlocal screening [17][18][19][20][21]. This potential leads, for example, to an unconventionally-spaced Rydberg series of excited excitons and can generate an anomalous ordering of (s, p, d ) levels [10]. Crucially, these excited states allow one to directly estimate the free-particle bandgap and binding energy of the X 0 ground state [10][11][12][13][14][15], both key material parameters that are otherwise difficult to measure in monolayer TMDs, and which are necessarily very sensitive to the surrounding dielectric environment [14,21,22,24]. Greatly desired, therefore, are incisive experimental tools for detailed studies of excited excitons in 2D semiconductors.Historically, optical spectroscopy in high magnetic fields B has provided an especially powerful way to identify and quantify excited excitons [25][26][27][28][29], because each excited state shifts very differently with B. Crucially, these shift...