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The dielectric function of colloidal lead chalcogenide quantum dots |
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By combining the Maxwell-Garnett effective medium theory with the Kramers-Krönig relations, the dielectric function of colloidal quantum dots is derived from their absorbance spectrum. This way, the real and imaginary part, or equivalently the refractive index and exinction coefficient, are obtained over the entire spectral range, from energies below the band gap to transitions deep in the bands. The experimental static dielectric constant ε0 is compared to ab initio calculations, where we show that ε0 is comparable to bulk. Importantly, this allows to exclude a recently proposed model, which states that ε0 is size-dependent due to an opening of the band gap. Rather, our calculations show that ε0is bulk-like in the inner Qdot volume, and can be enhanced or suppressed at the Qdot surface due to surface polarization. However, in lead salt Qdots this surface term has only a minor influence on ε0, leading to an essentially size-independent and bulk-like value. Download the paper |
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Binding of Phosphonic Acids to CdSe Quantum Dots: A Solution NMR Study |
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We have used 1H and 31P solution nuclear magnetic resonance spectroscopy to analyze the binding of phosphonic acid ligands to wurtzite CdSe quantum dots (CdSe QDs). CdSe QDs synthesized with phosphonic acids as a surfactant have a ligand shell composed of phosphonic acid and phosphonic acid anhydride moieties. Titrations of as-synthesized QDs with excess oleic acid do not induce desorption of phosphonic species, whereas titration of oleic-acid-exchanged QDs with excess phosphonic acid shows that the latter quantitatively replaces the oleic acid with a 1:1 stoichiometry. Both the stoichiometry of the oleic acid/phosphonic acid exchange interaction and the ratio between the Cd surface excess and the ligand density indicate that phosponic acids bind as hydrogen phosphonates to the CdSe surface.
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Dynamic Stabilization of CdSe Quantum Dots by Alkylamines |
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Using nuclear magnetic resonance spectroscopy, we show that dodecylamine ligands adsorbed to CdSe Qdots are in a dynamic equilibrium between free and adsorbed states. This contrasts with previous results on oleic acid capped PbSe Qdots , where we observed only adsorbed ligands, but is in line with the ligand dynamics observed for dodecylamine capped CdTe and octylamine capped ZnO Qdots . Diffusion ordered spectroscopy allows to extract a lower limit of 50 s-1 for the ligand desorption rate, which is substantially faster than what has been calculated in literature using luminescence spectroscopy.
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Langmuir–Blodgett monolayers of colloidal lead chalcogenide quantum dots |
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Monolayers of PbSe, PbSeCdSe and PbS quantum dots are fabricated using the Langmuir-Blodgett technique (see also our previous manuscripts on LB deposition of InP and PbSe Qdots). This study focuses on the morphological and optical properties of the resulting monolayers. We show that homogenenous monolayers are obtained for a three materials, yet on a local scale PbSe Qdots form small clusters via oriented attachment. From an optical perspective, PbS Qdots are the most promising material, as they still emit brightly after deposition, while the PbSe and PbSeCdSe Qdot luminescence is strongly quenched.
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PbSe quantum dots: Finite, off-stoichiometric, and structurally distorted |
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We recently published in Physical Review B our study on the structural properties of colloidal PbSe quantum dots. Using synchrotron X-ray diffraction and pair-distribution function analysis, the atomic ordering in these Qdots was modeled. Our data reveal that small colloidal Qdots are off-stoichiometric, which confirms our previous results using mass spectrometry and nuclear magnetic resonance spectroscopy . More importantly, we show that the crystal lattice constant increases with decreasing size; deviations of up to 0.05Å (1% of the bulk lattice constant) are observed. Also, substantial structural disorder is present in the Qdots: atoms are randomly displaced from their equilibrium positions by as much as 0.2Å.
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