13 research outputs found

    Quantum Yield Regeneration: Influence of Neutral Ligand Binding on Photophysical Properties in Colloidal Core/Shell Quantum Dots

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    This article describes an experiment designed to identify the role of specific molecular ligands in maintaining the high photoluminescence (PL) quantum yield (QY) observed in as-synthesized CdSe/CdZnS and CdSe/CdS quantum dots (QDs). Although it has been possible for many years to prepare core/shell quantum dots with near-unity quantum yield through high-temperature colloidal synthesis, purification of such colloidal particles is frequently accompanied by a reduction in quantum yield. Here, a recently established gel permeation chromatography (GPC) technique is used to remove weakly associated ligands without a change in solvent: a decrease in ensemble QY and average PL lifetime is observed. Minor components of the initial mixture that were removed by GPC are then added separately to purified QD samples to determine whether reintroduction of these components can restore the photophysical properties of the initial sample. We show that among these putative ligands trioctylphosphine and cadmium oleate can regenerate the initial high QY of all samples, but only the “L-type” ligands (trioctyphosphine and oleylamine) can restore the QY without changing the shapes of the optical spectra. On the basis of the PL decay analysis, we confirm that quenching in GPC-purified samples and regeneration in ligand-introduced samples are associated chiefly with changes in the relative population fraction of QDs with different decay rates. The reversibility of the QY regeneration process has also been studied; the introduction and removal of trioctylphosphine and oleylamine tend to be reversible, while cadmium oleate is not. Finally, isothermal titration calorimetry has been used to study the relationship between the binding strength of the neutral ligands to the surface and photophysical property changes in QD samples to which they are added

    Probing Surface Saturation Conditions in Alternating Layer Growth of CdSe/CdS Core/Shell Quantum Dots

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    We monitor effective band gap energy shifts and free reagent concentration during the formation of CdS shells on CdSe nanocrystals to test the hypothesis that alternating addition of stoichiometric doses of precursors can effectively saturate surface sites and thereby enforce conformal shell growth. The selective ionic layer addition and reaction (SILAR) mechanism has been proposed to describe growth under such conditions, and the method is attractive because of the opportunity to (1) avoid cross-reaction of precursors in growing binary films in solution and (2) enforce conformal growth, rather than regioselective growth, by saturating all available surface sites in a self-limiting manner in each half-cycle. The strong red shift that takes place when CdS shells are grown on CdSe cores provides a convenient process monitoring tool that complements scanning transmission electron microscopy imaging and analytical measurements of free reagent concentration. We find that, under commonly used conditions, a cadmium oleate precursor reacts incompletely at chalcogenide-saturated nanocrystal surfaces. Although approximately spherical particles are obtained, the growth does not proceed via saturating cycles, as described in the SILAR mechanism. This has implications for the rational control of conformal and regioselective growth of epilayers on nanocrystal quantum dots and higher-dimensional chalcogenide semiconductor nanostructures via solution processes

    Purification of Quantum Dots by Gel Permeation Chromatography and the Effect of Excess Ligands on Shell Growth and Ligand Exchange

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    This article describes the use of gel permeation chromatography (GPC) as a means to separate natively capped colloidal CdSe and CdSe/Cd<sub><i>x</i></sub>Zn<sub>1–<i>x</i></sub>S quantum dots (QDs) from small-molecule impurities in hydrophobic solvents. A range of analysis techniques, including <sup>1</sup>H NMR, diffusion-ordered NMR analysis (DOSY), and thermogravimetric analysis (TGA) have been used to compare the nature and quantities of ligands adsorbed on the QDs after GPC and after alternative purification methods. We show that the GPC purified samples display lower ligand-to-QD ratio (135 oleate substituents per nanocrystal for CdSe QDs with lowest-energy absorption peak at 534 nm) than what we can achieve by the multiple precipitation/redissolution method, and the GPC purified samples are stable at both room temperature and high temperature (180–200 °C for CdSe QDs). The achievement of an efficient and highly reproducible method for the preparation of clean QD samples allowed us to test whether impurities that reside in samples prepared by standard purification methods have a significant effect on further surface modification reactions. We found that the reactivity of CdSe QDs toward precursors for CdS shell growth was profoundly affected by the presence of excess ligands in impure QD samples prepared by multiple precipitations and that the removal of excess ligands and impurities significantly improved the speed and reliability by which water-soluble CdSe/Cd<sub><i>x</i></sub>Zn<sub>1–<i>x</i></sub>S QDs could be prepared by ligand exchange with cysteine. GPC purification provides a preparative-scale, consistent, size-based purification of QDs without perturbing the solvent environment and as such could serve as the basis for advanced syntheses and enable detailed measurements of QD surface chemical properties

    Purification and In Situ Ligand Exchange of Metal-Carboxylate-Treated Fluorescent InP Quantum Dots via Gel Permeation Chromatography

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    Recently the addition of M<sup>2+</sup> Lewis acids (M = Cd, Zn) to InP quantum dots (QDs) has been shown to enhance the photoluminescence quantum yield (PL QY). Here we investigate the stability of this Lewis acid layer to postsynthetic processing such as purification and ligand exchange. We utilize gel permeation chromatography to purify the quantum-dot samples as well as to aid in the ligand-exchange reactions. The Lewis-acid-capped particles are stable to purification and maintain the enhanced luminescence properties. We demonstrate successful ligand exchange on the quantum dots by switching the native carboxylate ligands to phosphonate ligands. Changes in the optical spectra after exposure to ambient environment indicate that both carboxylate- and phosphonate-capped QDs remain air-sensitive

    Copolymerization and Synthesis of Multiply Binding Histamine Ligands for the Robust Functionalization of Quantum Dots

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    The polymeric functionalization of quantum dots via ligand exchange is a robust method for the preparation of stable fluorescent particles with high quantum yields. For most biological applications of quantum dots, water solubility is a key requirement; to achieve biocompatibility, polymeric ligand systems that can provide water solubility as well as effective anchoring groups are advantageous. In this work, histamine functional polymers bearing poly­(ethylene glycol) (PEG) side chains were prepared using RAFT polymerization. A versatile postmodification strategy using activated ester units of <i>N</i>-methacryloxysuccinimide (NMS) and poly­(ethylene glycol) methacrylate in the polymer chain afforded copolymers ranging from 6K to 50K with low polydispersities, along with tailored composition of each monomer along the copolymer chain. By controlling the monomer ratio, PEGMA molecular weight, time, and temperature, the composition could be tuned to study its effect on quantum dot functionalization. Representative oleate-capped CdSe/Cd<sub><i>x</i></sub>Zn<sub>1–<i>x</i></sub>S QDs purified by a recently established gel permeation chromatography (GPC) method were used to test the effectiveness of the histamine-bearing polymers for preparation of water-soluble QDs. Successful ligand exchange of the QDs was characterized by good dispersions in water, lack of aggregation between QDs, and good quantum yields in water. Overall, the synthetic method demonstrates a facile and robust postmodification strategy for the formation of multiply binding, histamine-bearing copolymers, which can be applied to nanomaterials for fundamental investigations and bioimaging/biodistribution studies

    Fluorescence Polarization Measurements to Probe Alignment of a Bithiophene Dye in One-Dimensional Channels of Self-Assembled Phenylethynylene Bis-Urea Macrocycle Crystals

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    This paper describes the use of polarized fluorescence microscopy to directly probe guest molecule orientation in bis-urea macrocycle crystals. These macrocycles assemble to afford one-dimensional (1D) microchannels ∼9 Å in diameter that have previously been shown to exhibit normal Fickian diffusion and induce selective reactivity among the confined guest molecules. In the present work, we take advantage of the quasi-1D morphology of fiber-like microcrystals with the extended dimension corresponding to the channel axis to measure excitation and emission polarization values for a bithiophene guest. Guest fluorescence is shown to be polarized along the fiber axis with emission polarization values up to 0.729, indicating a high degree of orientational order within the 1D channels. The observed behavior is consistent with calculated results for the guest orientation and electronic transition dipole moment. The results indicate the value of functional fluorescent probes as a measure of guest confinement in low-dimensional environments

    Single Crystal to Single Crystal Polymerization of a Self-Assembled Diacetylene Macrocycle Affords Columnar Polydiacetylenes

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    This manuscript describes a single-crystal-to-single-crystal polymerization of the dihydrate of diacetylene <b>1</b> (<b>1</b>·2H<sub>2</sub>O) to give an unusual polydiacetylene (PDA) structure that consists of aligned nanotubes, with each covalently bonded nanotube having two parallel PDA chains that run parallel down opposite sides of a channel defined by the macrocycle. Each PDA nanotube is connected with four other columns via amide hydrogen bonds with an N-(H)···O distance of 2.888(4) Å. Such well-ordered polymers should display quasi-one-dimensional electronic structures and may be of interest for the formation of highly conductive organic materials. We obtained the <b>1</b>·2H<sub>2</sub>O form in bulk, which was polymerized by heating. Powder X-ray diffraction suggests that bulk PDA powder is single phase and displays a similar structure as the PDA single crystals. Furthermore, we showed that PDA crystals absorbed I<sub>2</sub> vapor. We believe that this unique PDA structure, which is amenable to property control via adsorbed guests, will be an attractive one for investigating charge-transfer doping in PDA-based organic electronic materials. We also observed two additional crystal forms, including <b>1</b>·MeOH, which also shows a columnar assembly, and a tetrahydrate <b>1</b>·4H<sub>2</sub>O that shows water tetramers that link four cycles into a 2D sheet structure

    Single Crystal to Single Crystal Polymerization of a Self-Assembled Diacetylene Macrocycle Affords Columnar Polydiacetylenes

    No full text
    This manuscript describes a single-crystal-to-single-crystal polymerization of the dihydrate of diacetylene <b>1</b> (<b>1</b>·2H<sub>2</sub>O) to give an unusual polydiacetylene (PDA) structure that consists of aligned nanotubes, with each covalently bonded nanotube having two parallel PDA chains that run parallel down opposite sides of a channel defined by the macrocycle. Each PDA nanotube is connected with four other columns via amide hydrogen bonds with an N-(H)···O distance of 2.888(4) Å. Such well-ordered polymers should display quasi-one-dimensional electronic structures and may be of interest for the formation of highly conductive organic materials. We obtained the <b>1</b>·2H<sub>2</sub>O form in bulk, which was polymerized by heating. Powder X-ray diffraction suggests that bulk PDA powder is single phase and displays a similar structure as the PDA single crystals. Furthermore, we showed that PDA crystals absorbed I<sub>2</sub> vapor. We believe that this unique PDA structure, which is amenable to property control via adsorbed guests, will be an attractive one for investigating charge-transfer doping in PDA-based organic electronic materials. We also observed two additional crystal forms, including <b>1</b>·MeOH, which also shows a columnar assembly, and a tetrahydrate <b>1</b>·4H<sub>2</sub>O that shows water tetramers that link four cycles into a 2D sheet structure

    Single Crystal to Single Crystal Polymerization of a Self-Assembled Diacetylene Macrocycle Affords Columnar Polydiacetylenes

    No full text
    This manuscript describes a single-crystal-to-single-crystal polymerization of the dihydrate of diacetylene <b>1</b> (<b>1</b>·2H<sub>2</sub>O) to give an unusual polydiacetylene (PDA) structure that consists of aligned nanotubes, with each covalently bonded nanotube having two parallel PDA chains that run parallel down opposite sides of a channel defined by the macrocycle. Each PDA nanotube is connected with four other columns via amide hydrogen bonds with an N-(H)···O distance of 2.888(4) Å. Such well-ordered polymers should display quasi-one-dimensional electronic structures and may be of interest for the formation of highly conductive organic materials. We obtained the <b>1</b>·2H<sub>2</sub>O form in bulk, which was polymerized by heating. Powder X-ray diffraction suggests that bulk PDA powder is single phase and displays a similar structure as the PDA single crystals. Furthermore, we showed that PDA crystals absorbed I<sub>2</sub> vapor. We believe that this unique PDA structure, which is amenable to property control via adsorbed guests, will be an attractive one for investigating charge-transfer doping in PDA-based organic electronic materials. We also observed two additional crystal forms, including <b>1</b>·MeOH, which also shows a columnar assembly, and a tetrahydrate <b>1</b>·4H<sub>2</sub>O that shows water tetramers that link four cycles into a 2D sheet structure

    Single Crystal to Single Crystal Polymerization of a Self-Assembled Diacetylene Macrocycle Affords Columnar Polydiacetylenes

    No full text
    This manuscript describes a single-crystal-to-single-crystal polymerization of the dihydrate of diacetylene <b>1</b> (<b>1</b>·2H<sub>2</sub>O) to give an unusual polydiacetylene (PDA) structure that consists of aligned nanotubes, with each covalently bonded nanotube having two parallel PDA chains that run parallel down opposite sides of a channel defined by the macrocycle. Each PDA nanotube is connected with four other columns via amide hydrogen bonds with an N-(H)···O distance of 2.888(4) Å. Such well-ordered polymers should display quasi-one-dimensional electronic structures and may be of interest for the formation of highly conductive organic materials. We obtained the <b>1</b>·2H<sub>2</sub>O form in bulk, which was polymerized by heating. Powder X-ray diffraction suggests that bulk PDA powder is single phase and displays a similar structure as the PDA single crystals. Furthermore, we showed that PDA crystals absorbed I<sub>2</sub> vapor. We believe that this unique PDA structure, which is amenable to property control via adsorbed guests, will be an attractive one for investigating charge-transfer doping in PDA-based organic electronic materials. We also observed two additional crystal forms, including <b>1</b>·MeOH, which also shows a columnar assembly, and a tetrahydrate <b>1</b>·4H<sub>2</sub>O that shows water tetramers that link four cycles into a 2D sheet structure
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