Role of Interligand Coupling in Determining the Interfacial Electronic Structure of Colloidal CdS Quantum Dots

Displacement of cadmium oleate (Cd­(oleate)₂) ligands for the exciton-delocalizing ligand 4-hexylphenyldithiocarbamate (C6-PTC) on the surfaces of CdS quantum dots (QDs) causes a decrease in the band gap (<i>E</i>_g) of the QD of ∼100 meV for QDs with a radius of 1.9 nm and ∼50 meV for QDs with a radius of 2.5 nm. The primary mechanism of this decrease in band gap, deduced in previous work, is a decrease in the confinement barrier for the excitonic hole. The increase in apparent excitonic radius of the QD that corresponds to this decrease in <i>E</i>_g is denoted Δ<i>R</i>. The dependence of Δ<i>R</i> on the surface coverage of C6-PTC, measured by ¹H NMR spectroscopy, appears to be nonlinear. Calculations of the excitonic energy of a CdS QD upon displacement of native insulating ligands with exciton-delocalizing ligands using a 3D spherical potential well model show that this response includes the contributions to Δ<i>R</i> from both isolated, bound C6-PTC ligands and groups of adjacent C6-PTC ligands. Fits to the experimental plots of Δ<i>R</i> <i>vs</i> surface coverage of C6-PTC with a statistical model that includes the probability of formation of clusters of bound C6-PTC on the QD surface allow for the extraction of the height of the confinement barrier presented by a single, isolated C6-PTC molecule to the excitonic hole. This barrier height is less than 0.6 eV for QDs with a radius of 1.9 nm and between 0.6 and 1.2 eV for QDs with a radius of 2.5 nm

Control of Exciton Confinement in Quantum Dot–Organic Complexes through Energetic Alignment of Interfacial Orbitals

This paper describes a method to control the quantum confinement, and therefore the energy, of excitonic holes in CdSe QDs through adsorption of the hole-delocalizing ligand phenyldithiocarbamate, PTC, and para substitutions of the phenyl ring of this ligand with electron-donating or -withdrawing groups. These substitutions control hole delocalization in the QDs through the energetic alignment of the highest occupied orbitals of PTC with the highest density-of-states region of the CdSe valence band, to which PTC couples selectively

Model for Adsorption of Ligands to Colloidal Quantum Dots with Concentration-Dependent Surface Structure

A study of the adsorption equilibrium of solution-phase CdS quantum dots (QDs) and acid-derivatized viologen ligands (<i>N</i>-[1-heptyl],<i>N</i>′-[3-carboxypropyl]-4,4′-bipyridinium dihexafluorophosphate, V²⁺) reveals that the structure of the surfaces of the QDs depends on their concentration. This adsorption equilibrium is monitored through quenching of the photoluminescence of the QDs by V²⁺ upon photoinduced electron transfer. When modeled with a simple Langmuir isotherm, the equilibrium constant for QD–V²⁺ adsorption, <i>K</i>_a, increases from 6.7 × 10⁵ to 8.6 × 10⁶ M^–1 upon decreasing the absolute concentration of the QDs from 1.4 × 10^–6 to 5.1 × 10^–8 M. The apparent increase in <i>K</i>_a upon dilution results from an increase in the mean number of available adsorption sites per QD from 1.1 (for [QD] = 1.4 × 10^–6 M) to 37 (for [QD] = 5.1 × 10^–8 M) through desorption of native ligands from the surfaces of the QDs and through disaggregation of soluble QD clusters. A new model based on the Langmuir isotherm that treats both the number of adsorbed ligands per QD and the number of available binding sites per QD as binomially distributed quantities is described. This model yields a concentration-independent value for <i>K</i>_a of 8.7 × 10⁵ M^–1 for the QD–V²⁺ system and provides a convenient means for quantitative analysis of QD–ligand adsorption in the presence of competing surface processes

Control of Exciton Confinement in Quantum Dot–Organic Complexes through Energetic Alignment of Interfacial Orbitals

This paper describes a method to control the quantum confinement, and therefore the energy, of excitonic holes in CdSe QDs through adsorption of the hole-delocalizing ligand phenyldithiocarbamate, PTC, and para substitutions of the phenyl ring of this ligand with electron-donating or -withdrawing groups. These substitutions control hole delocalization in the QDs through the energetic alignment of the highest occupied orbitals of PTC with the highest density-of-states region of the CdSe valence band, to which PTC couples selectively

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

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