Temperature-Dependent Permeability of the Ligand Shell of PbS Quantum Dots Probed by Electron Transfer to Benzoquinone

This paper describes an increase in the yield of collisionally gated photoinduced electron transfer (electron transfer events per collision) from oleate-capped PbS quantum dots (QDs) to benzoquinone (BQ) with increasing temperature (from 0 to 50 °C), due to increased permeability of the oleate adlayer of the QDs to BQ. The same changes in intermolecular structure of the adlayer that increase its permeability to BQ also increase its permeability to the solvent, toluene, resulting in a decrease in viscous drag and an apparent increase in the diffusion coefficient of the QDs, as measured by diffusion-ordered spectroscopy (DOSY) NMR. Comparison of NMR and transient absorption spectra of QDs capped with flexible oleate with those capped with rigid methylthiolate provides evidence that the temperature dependence of the permeability of the oleate ligand shell is due to formation of transient gaps in the adlayer through conformational fluctuations of the ligands

Description of the Adsorption and Exciton Delocalizing Properties of <i>p</i>‑Substituted Thiophenols on CdSe Quantum Dots

This work describes the quantitative characterization of the interfacial chemical and electronic structure of CdSe quantum dots (QDs) coated in one of five <i>p</i>-substituted thiophenolates (X-TP, X = NH₂, CH₃O, CH₃, Cl, or NO₂), and the dependence of this structure on the <i>p</i>-substituent X. ¹H NMR spectra of mixtures of CdSe QDs and X-TPs yield the number of X-TPs bound to the surface of each QD. The binding data, in combination with the shift in the energy of the first excitonic peak of the QDs as a function of the surface coverage of X-TP and Raman and NMR analysis of the mixtures, indicate that X-TP binds to CdSe QDs in at least three modes, two modes that are responsible for exciton delocalization and a third mode that does not affect the excitonic energy. The first two modes involve displacement of OPA from the QD core, whereas the third mode forms cadmium–thiophenolate complexes that are not electronically coupled to the QD core. Fits to the data using the dual-mode binding model also yield the values of Δ<i>r</i>₁, the average radius of exciton delocalization due to binding of the X-TP in modes 1 and 2. A 3D parametrized particle-in-a-sphere model enables the conversion of the measured value of Δ<i>r</i>₁ for each X-TP to the height of the potential barrier that the ligand presents for tunneling of excitonic hole into the interfacial region. The height of this barrier increases from 0.3 to 0.9 eV as the substituent, X, becomes more electron-withdrawing

Dependence of the Band Gap of CdSe Quantum Dots on the Surface Coverage and Binding Mode of an Exciton-Delocalizing Ligand, Methylthiophenolate

Displacement of native octylphosphonate (OPA) ligands for methylthiophenolate (CH₃-TP) on the surfaces of CdSe quantum dots (QDs) causes a moderate (up to 50 meV) decrease in the band gap (<i>E</i>_g) of the QD. Plots of the corresponding increase in apparent excitonic radius, Δ<i>R</i>, of the QDs versus the surface coverage of CH₃-TP, measured by ¹H NMR, for several sizes of QDs reveal that this ligand adsorbs in two distinct binding modes, (1) a tightly bound mode (<i>K</i>_a = 1.0 ± 0.3 × 10⁴ M^–1) capable of exciton delocalization, and (2) a more weakly bound mode (<i>K</i>_a = 8.3 ± 9.9 × 10² M^–1) that has no discernible effect on exciton confinement. For tightly bound CH₃-TP, the degree of delocalization induced in the QD is approximately linearly related to the fractional surface area occupied by the ligand for all sizes of QDs. Comparison of the dependence of Δ<i>R</i> on surface coverage of CH₃-TP over a range of physical radii of the QDs, <i>R</i> = 1.1–2.4 nm, to analogous plots simulated using a 3D spherical potential well model yield a value for the confinement barrier presented to the excitonic hole by tightly bound CH₃-TP of ∼1 eV

Enhanced Rate of Radiative Decay in CdSe Quantum Dots upon Adsorption of an Exciton-Delocalizing Ligand

This paper describes the enhancement of the quantum yield of photoluminescence (PL) of CdSe quantum dots (QDs) upon the adsorption of an exciton-delocalizing ligand, phenyldithiocarbamate. Increasing the apparent excitonic radius by only 10% increases the value of the radiative rate constant by a factor of 1.8 and the PL quantum yield by a factor of 2.4. Ligand exchange therefore simultaneously perturbs the confinement energy of charge carriers and enhances the probability of band-edge transitions