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The Different Nature of Band Edge Absorption and Emission in Colloidal PbSe/CdSe Core/Shell Quantum |
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We present a quantitative analysis of the absorption and luminescence of colloidal PbSe/CdSe core/shell quantum dots (QDs). In absorption, both the energy and the oscillator strength of the first exciton transition coincide with that of plain PbSe QDs. In contrast, luminescence lifetime measurements indicate that the oscillator strength of the emitting transition is reduced by at least a factor of 4 compared to PbSe core QDs. Moreover, the addition of an electron scavenger quenches the PbSe/CdSe emission, while a hole scavenger does not. This implies that the electron wave function reaches the QD surface, while the hole is confined to the PbSe core. These observations are consistent with calculations based on the effective mass model, which show that PbSe/CdSe QDs are at the boundary between the type-I and quasi-type-II regime, where the electron spreads over the entire nanoparticle and the hole remains confined in the PbSe core. However, as this only leads to a minor reduction of the oscillator strength, it follows that the drastic reduction of the oscillator strength in emission cannot be explained in terms of electron delocalization. In combination with the increased Stokes shift for PbSe/CdSe QDs, this indicates that the emission results from lower energy states that are fundamentally different from the absorbing states.
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The absorption coefficient of PbSe/CdSe core/shell colloidal quantum dots |
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PbSe/CdSe core/shell colloidal quantum dots (QDs) are used as a model system to study the absorption coefficient of colloidal QD heterostructures, consisting of at least two semiconductor materials. We show that at energies far above the band gap (3.1 and 3.5 eV) the experimental intrinsic absorption coefficient is in excellent agreement with the Maxwell–Garnett effective medium theory for core/shell heterostructures and bulk values for the dielectric function. This allows for a straightforward measurement of the QD concentration from the absorbance spectrum. It also implies that basic optical measurements on core/shell heterostructures, such as measurements of the oscillator strength and photoluminescence lifetime, can be corrected for the local field reduction in QD heterostructures.
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Multiple Dot-in-Rod PbS/CdS Heterostructures with High Photoluminescence Quantum Yield |
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Pb cations in PbS quantum rods made from CdS quantum rods by successive complete cationic exchange reactions are partially re-exchanged for Cd cations. Using STEM-HAADF, we show that this leads to the formation of unique multiple dot-in-rod PbS/CdS heteronanostructures, with a photoluminescence quantum yield of 45–55%. We argue that the formation of multiple dot-in-rods is related to the initial polycrystallinity of the PbS quantum rods, where each PbS crystallite transforms in a separate PbS/CdS dot-in-dot. Effective mass modeling indicates that electronic coupling between the different PbS conduction band states is feasible for the multiple dot-in-rod geometries obtained, while the hole states remain largely uncoupled.
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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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