Inductively coupled plasma mass spectrometry (ICP-MS) was combined with UV–vis absorption spectroscopy and transmission electron microscopy to determine the size, composition, and intrinsic ...absorption coefficient μi of 4 to 11 nm sized colloidal CsPbBr3 nanocrystals (NCs). The ICP-MS measurements demonstrate the nonstoichiometric nature of the NCs, with a systematic excess of lead for all samples studied. Rutherford backscattering measurements indicate that this enrichment in lead concurs with a relative increase in the bromide content. At high photon energies, μi is independent of the nanocrystal size. This allows the nanocrystal concentration in CsPbBr3 nanocolloids to be readily obtained by a combination of absorption spectroscopy and the CsPbBr3 sizing curve.
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Although solvent–ligand interactions play a major role in nanocrystal synthesis, dispersion formulation, and assembly, there is currently no direct method to study this. Here we examine the ...broadening of 1H NMR resonances associated with bound ligands and turn this poorly understood descriptor into a tool to assess solvent–ligand interactions. We show that the line broadening has both a homogeneous and a heterogeneous component. The former is nanocrystal-size dependent, and the latter results from solvent–ligand interactions. Our model is supported by experimental and theoretical evidence that correlates broad NMR lines with poor ligand solvation. This correlation is found across a wide range of solvents, extending from water to hexane, for both hydrophobic and hydrophilic ligand types, and for a multitude of oxide, sulfide, and selenide nanocrystals. Our findings thus put forward NMR line-shape analysis as an indispensable tool to form, investigate, and manipulate nanocolloids.
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Nanoscaled VO2 structures were made using a fast microwave-assisted synthesis method in acetophenone as solvent, starting from vanadium pentoxide V2O5. Insights in the reaction and purification ...process were elaborated by attenuated total reflection infrared and nuclear magnetic resonance spectroscopy, which validated the choice of oleyl amine as ligand and acetophenone as solvent. A straightforward heat treatment in inert atmosphere was used to convert the mixture of VOx (M) and VOOH phases to thermochromically active monoclinic VO2 (M) nanomaterials. This conversion process was monitored via in-situ X-ray diffraction and gave insight in the influence of ethanol and tert-butyl hydroperoxide on the crystal structure and morphology, monitored via bright-field transmission electron microscopy. The thermochromic switching was investigated with differential scanning calorimetry showing a latent heat up to 43.34 J/g.
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Nanocrystal–ligand interactions and ligand exchange processes are usually described by a uniform distribution of equal binding sites. Here, we analyze this assumption by a quantitative study of the ...displacement of Z-type cadmium oleate ligands from CdSe nanocrystals by addition of L-type ligands. First, we determined the stoichiometry of the displacement reaction by analyzing the equilibrium upon dilution using solution nuclear magnetic resonance spectroscopy. We found that 1 equiv of tetramethylethylene-1,2-diamine (TMEDA) or 2 equiv of n-butylamine or benzylamine bind to the displaced cadmium oleate. We only reached a comprehensive description of the displacement isotherm by including two types of binding sites with a different equilibrium constant. We corroborated this finding by density functional theory calculations on a CdSe model nanocrystal, which show that even single facets contain a broad variety of binding sites. Finally, we analyzed the thermodynamics of the displacement equilibrium for the weaker binding sites by constructing van’t Hoff plots for the different displacers. Whereas displacement with TMEDA appears to be enthalpically neutral, it is entropically favorable. In contrast, displacement with the primary amines is entropically unfavorable but is associated with a negative change in enthalpy. Since the distribution of binding energy emanates from the large fraction of edge and vertex sites on a nanocrystal facet, these findings are most likely inherent to nanocrystals in general and should be considered when analyzing surface reactions on such materials.
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Following the introduction of perovskites for photovoltaic solar energy conversion, the use of these materials as a general purpose optoelectronic material for displays, lighting, and lasing has been ...explored. However, while reports on stimulated emission and lasing by perovskites show great promise, a comprehensive quantification of their optical gain characteristics is lacking. Here, we measure gain coefficients, clarify the gain mechanism, and explore the gain dynamics of colloidal CsPbBr3 nanocrystals by deploying a unique combination of broadband transient absorption and ultrafast fluorescence spectroscopy. Opposite from current literature, we show that optical gain in such nanocrystals is supported by stimulated emission from free carriers, and not from excitons or biexcitons. Importantly, we demonstrate that the concomitant gain coefficients and thresholds agree with literature results reported for perovksite thin films. Finally, we show that, even in the case of fully inorganic lead halide perovskites, a cooling bottleneck hampers the development of net stimulated emission at high excitation density. Based on these results, we propose that bulk-like colloidal nanocrystals in general offer a unique testbed to quantify optical gain of novel photonic materials and in particular for lead halide perovskites.
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A multiplex lateral flow immunoassay (LFIA) for the determination of the mycotoxins deoxynivalenol, zearalenone, and T2/HT2-toxin in barley was developed with luminescent quantum dots (QDs) as label. ...The synthesized QDs were hydrophilized by two strategies, that is, coating with an amphiphilic polymer or silica. The water-soluble QDs were compared with regard to their bioconjugation with monoclonal antibody (mAb) and were tested on a LFIA. Silica-coated QDs that contained epoxy groups were most promising. Therefore, green, orange, and red epoxy-functionalized silica-coated QDs were conjugated with anti-ZEN, anti-DON, and anti-T2 mAb, respectively. The LFIA was developed in accordance with the European Commission legal limits with cutoff limits of 1000, 80, and 80 μg/kg for deoxynivalenol, zearalenone, and T2/HT2-toxin, respectively. The LFIA gave a fast result (15 min) with a low false-negative rate (<5%), and the results were easy to interpret without any sophisticated equipment.
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PEGylation, which has traditionally been the method of choice to enhance the colloidal stability of nanostructures designed for biological applications and to prevent nonspecific protein adsorption, ...is now being challenged by short zwitterionic ligands. Inspired by the zwitterionic nature of cell membranes, these ligands have the potential to push forward the field of nanoparticles for nanomedicine. In this work, we report a thorough analysis of the surface chemistry of silica-coated luminescent CdSe/CdS quantum dots functionalized with either PEG-silane or zwitterionic sulfobetaine-silane by quantitative nuclear magnetic resonance spectroscopy. We demonstrate the differences in the cellular uptake propensity between particles with these two ligands. Although both ligands offer good colloidal stability in a crowded cell culture medium, the zwitterionic-functionalized nanoparticles with an optimized ligand density showed to be more easily endocytosed by HeLa cells. This approach can readily be transferred to other nanoparticle systems offering a wealth of unique properties, with great potential for intracellular bioapplications.
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Two dimensional superlattices of epitaxially connected quantum dots enable size-quantization effects to be combined with high charge carrier mobilities, an essential prerequisite for highly ...performing QD devices based on charge transport. Here, we demonstrate that surface active additives known to restore nanocrystal stoichiometry can trigger the formation of epitaxial superlattices of PbSe and PbS quantum dots. More specifically, we show that both chalcogen-adding (sodium sulfide) and lead oleate displacing (amines) additives induce small area epitaxial superlattices of PbSe quantum dots. In the latter case, the amine basicity is a sensitive handle to tune the superlattice symmetry, with strong and weak bases yielding pseudohexagonal or quasi-square lattices, respectively. Through density functional theory calculations and in situ titrations monitored by nuclear magnetic resonance spectroscopy, we link this observation to the concomitantly different coordination enthalpy and ligand displacement potency of the amine. Next to that, an initial ∼10% reduction of the initial ligand density prior to monolayer formation and addition of a mild, lead oleate displacing chemical trigger such as aniline proved key to induce square superlattices with long-range, square micrometer order; an effect that is the more pronounced the larger the quantum dots. Because the approach applies to PbS quantum dots as well, we conclude that it offers a reproducible and rational method for the formation of highly ordered epitaxial quantum dot superlattices.
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Quantum dot core–shell structures are essential to superior optical properties. However, Cd-based quantum dots grown from wurtzite CdSe cores still suffer from irreproducibility due to gel formation ...during the core synthesis. Moreover, state-of-the-art shelling procedures often remain time-consuming. We report here (1) a new procedure of the wurtzite CdSe quantum dot synthesis which eliminates the phosphonic anhydride contamination and (2) the improvement and generalization of the flash synthesis for the growth of highly luminescent multishell heterostructures. The high versatility of the flash procedure enables the epitaxial shelling of wurtzite CdSe by CdS, ZnS and even CdZnS alloys, resulting in a significant enhancement of their photoluminescence efficiency. Thanks to a modification of the CdSe/CdS dot-in-rods synthesis to prevent tip-to-tip connections, the flash procedure could also be extended to these anisotropic structures for the growth of multishell nanorods as well. The high quality optical properties of these multishell flash heterostructures are of great interest for applications ranging from lighting and displays to long-term cell labeling.
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In this work, we present a combined experimental and theoretical analysis of two-component ligand shells passivating CdSe quantum dots. Using nuclear magnetic resonance spectroscopy, we first show ...that exposing oleate-capped quantum dots to primary carboxylic acids results in a one-for-one exchange that preserves the overall ligand surface concentration. Exposure to straight-chain acids leads to a binary ligand shell that behaves as an ideal mixture and that has a composition matching the overall acid composition of the dispersion. In the case of branched-chain acids, the exchange is restricted to about 25% of the original ligands. Based on molecular dynamics simulations, we argue that this behavior reflects the more favorable packing of oleates compared to branched carboxylates on the (100) facets of CdSe quantum dots.
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