While the surface termination of quasi-spherical metal chalcogenide nanocrystals or quantum dots has been widely investigated, it remains unclear whether the ensuing surface chemistry models apply to similar nanocrystals with anisotropic shapes. In this work, we report on the surface-chemistry of 2D CdSe nanoplatelets, where we make use of an improved synthesis strategy that yields stable and aggregation free nanoplatelet suspensions with a photoluminescence quantum yield as high as 55%. We confirm that such nanoplatelets are enriched in Cd and, by means of 1 H nuclear magnetic resonance spectroscopy, we show that the Cd-rich surface is terminated by X-type carboxylate ligands. Not unlike CdSe quantum dots (QDs), entire cadmium carboxylate entities can be displaced by the addition of amines, and the desorption isotherm points toward a considerable binding site heterogeneity. Moreover, we find that even the slightest displacement of cadmium carboxylate ligands quenches the nanoplatelet photoluminescence. These experimental findings are further confirmed by density functional theory (DFT) calculations on a 5 monolayer model CdSe nanoplatelet. These simulations show that the most labile ligands are located in the vicinity of facet edges, and that the displacement of ligands from such edge sites creates midgap states that can account for the observed photoluminescence quenching. Next to extending surface chemistry insights from colloidal QDs to nanoplatelets, this work indicates that CdSe nanoplatelets constitute a unique nanocrystal model system to establish a comprehensive description of midgap trap states, which includes their structural, chemical, and electronic properties.

Ultrathin two-dimensional (2D) materials have received much attention in the past years for a wide variety of photonic applications because of their pronounced roomtemperature excitonic features, leading to unique properties in terms of light−matter interaction. However, only a few studies focus on light amplification and the complex photophysics at high excitation density. The beneficial nature of strong excitonic effects on optical gain remain hence unquantified, and despite the increased binding energies of the excitonic species, it remains unclear what the involvement of 2D excitons would be in optical gain. Here, we use colloidal CdSe nanoplatelets as a model system for colloidal 2D materials and show, using a quantitative and combinatory approach to ultrafast spectroscopy, that several excitation density-dependent optical gain regimes exist. At low density, optical gain originates from excitonic molecules delivering large material gains up to 20 000 cm −1 with an Auger limited lifetime of a few hundred picoseconds. At increasing pair density, we observe a persistence of this excitonic gain regime and the unexpected coexistence of blue-shifted and significantly enhanced optical gain up to 10 5 cm −1. We show that this peculiar situation originates from a carrier cooling bottleneck at high density that limits further exciton formation from unbound charge carriers. The insulating (multi-)exciton gas is found to coexist with the conductive phase, indicating the absence of a full Mott transition. Our results shed a new light on the photophysics of excitons in strongly excited 2D materials and pave the way for the development of more efficient (broadband) optical gain media and/or high exciton density applications.

We show that optical gain in 2D CdSe colloidal quantum wells (CQWs) shows little saturation and coexists with exciton absorption over a broad range of excitation densities, in stark contrast with 0D CdSe colloidal quantum dots (CQDs). In addition, we demonstrate that photoexcited CQWs can absorb or emit light through the thermodynamically driven formation or radiative recombination of singlet excitonic molecules. Invoking stimulated emission through the molecule−exciton transition, we can quantify all of the remarkable gain characteristics of CQWs using only experimentally determined parameters, an advance that highlights a fundamental difference between multiexcitons in CQWs and CQDs. While strong confinement prohibits the dissociation of multiexcitons into separate excitons in 0D CQDs, excitons and excitonic molecules coexist in a 2D CQW at room temperature, with densities governed by an association/ dissociation equilibrium, not by state-filling. Our finding points out future directions to optimize stimulated emission by excitonic 2D materials in general.

Inorganic nanostructures: Alloyed Cu2ZnSn(S1−xSex)4 wurtzite nanocrystals (10 nm in size) with a varying composition (x=0–1) were synthesized using a colloidal hot injection route. A photoluminescence (PL) emission study of these nanocrystals shows a compositionally tunable band‐gap ranging between 0.9–1.4 eV that directly correlates to the sulfur‐to‐selenium ratio (see picture).

The formation of Cu2SnSe3 tetrapod nanocrystals is reported using a hot injection colloidal synthesis. The ternary copper chalcogenide nanocrystals nucleate with a cubic core with four short wurzite arms.

Abstract2D 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 FStark 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 FStark is independent of the nanoplatelet area, correctly predicts the radiative lifetime, and lines up for strongly confined quantum dot systems.

Hydroxide anion conducting polymer membranes also termed as anion exchange membranes (AEMs) are recently becoming important materials for electrochemical technology, alkaline fuel cells, and electrolyzers. In this work, the preparation procedure for AEMs based on poly(vinyl alcohol) (PVA) and copolymer of poly(acrylonitrile (PAN)-dimethylamino ethylmethacrylate) (DMAEMA) with strongly basic quaternary ammonium in aqueous media has been reported. This simplified procedure avoids the use of chloromethyl methyl ether (CME), a carcinogen that is harmful to human health, generally used for chloromethylation during AEM preparation. Developed AEMs were extensively characterized by studying physicochemical and electrochemical properties, to assess their suitability for electrodialytic ion separation. These membranes were designed to possess all the required properties of a highly anion conductive membrane such as reasonable water uptake, good ion-exchange capacity (1.18 mequiv g(-1)), high permselectivity (0.90), along with reasonable conductivity (3.45 mS cm(-1)) due to quaternary ammonium group functionality. The membrane conductivity values in conjunction with solution conductivity have been used for the estimation of the isoconductivity point, considering the membrane as a combination of the gel phase and integral phase. Electroosmotic studies revealed quite low mass drag and equivalent pore radius (2.7-4.0 A) of the membrane, which are also desirable properties of an AEM. The excellent electrotransport property of AEM-70 for practical anion separation was concluded from i-v studies. Electrodialytic performance of the AEM-70 membrane revealed its suitability for applications in electromembrane processes.

CsPb(Cl 1−x Br x ) 3 perovskite nanocrystals (NCs) doped with Yb 3+ ions have recently attracted large attention for their applications in photovoltaics in view of the high quantum yield, exceeding 100% of Yb 3+ emission at ∼1 μm. In contrast, the particularly relevant Er 3+ emission at 1.5 μm in the third telecommunication window, of high interest in silicon integrated photonics, has been so far largely neglected in view of the weak emission performance displayed by Er 3+ -doped NCs. Comprehensive steady-state and time-resolved spectroscopic measurements provide insights into the underlying mechanisms of Yb 3+ and Er 3+ sensitization to rationalize the anomalous different behavior of these two emitters in singly doped NCs. We propose that single-photon excitation of two Yb 3+ ions possibly occurs through a transient internal redox mechanism in the perovskite host, while this pathway is unviable for Er 3+ . In turn, Yb 3+ -bridged Er 3+ sensitization, boosts the Er 3+ luminescence at ∼1.5 μm by 10 4 -fold compared to Er 3+ singly doped NCs, and a relative high quantum yield of ∼6% and extremely long lifetime (∼3 ms) are obtained. The resulting high Er 3+ excited state densities, combined with the large absorption cross-sections of the semiconducting CsPbCl 3 matrix make Er 3+ -doped perovskite promising innovative materials to realize photonic devices operating at telecommunication wavelengths.