Description of the Adsorption and Exciton Delocalizing Properties of p-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 p-substituted thiophenolates (X-TP, X = NH2, CH3O, CH3, Cl, or NO2), and the dependence of this structure on the p-substituent X. (1)H 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 Δr1, 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 Δr1 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.

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.

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.

The role of interfacial charge transfer-type interactions in the decay of plasmon excitations in metal nanoparticles.

This perspective describes the influence of interfacial charge transfer-type interactions on the optical spectra and hot electron cooling processes in plasmonic nanoparticles (NPs), and ongoing work to optimize these interactions for charge extraction from the plasmon or hot electron state. The manuscript focuses on interfaces of metal NPs with organic molecules and with semiconductors. Charge extraction from multi-electron excited states has applications in photodetection, sensing, and conversion of solar energy to electricity and fuels.

Identification of parameters through which surface chemistry determines the lifetimes of hot electrons in small Au nanoparticles.

This paper describes measurements of the dynamics of hot electron cooling in photoexcited gold nanoparticles (Au NPs) with diameters of ∼3.5 nm, and passivated with either a hexadecylamine or hexadecanethiolate adlayer, using ultrafast transient absorption spectroscopy. Fits of these dynamics with temperature-dependent Mie theory reveal that both the electronic heat capacity and the electron-phonon coupling constant are larger for the thiolated NPs than for the aminated NPs, by 40% and 30%, respectively. Density functional theory calculations on ligand-functionalized Au slabs show that the increase in these quantities is due to an increased electronic density of states near the Fermi level upon ligand exchange from amines to thiolates. The lifetime of hot electrons, which have thermalized from the initial plasmon excitation, increases with increasing electronic heat capacity, but decreases with increasing electron-phonon coupling, so the effects of changing surface chemistry on these two quantities partially cancel to yield a hot electron lifetime of thiolated NPs that is only 20% longer than that of aminated NPs. This analysis also reveals that incorporation of a temperature-dependent electron-phonon coupling constant is necessary to adequately fit the dynamics of electron cooling.

Controlling the rate of electron transfer between a quantum dot and a tri-ruthenium molecular cluster by tuning the chemistry of the interface.

Ultrafast transient absorption measurements reveal that the rate of photoinduced electron transfer (PET) from colloidal CdSe quantum dots (QDs) to oxo-centered triruthenium clusters (Ru(3)O) depends on the structure of the chemical headgroup by which the Ru(3)O clusters adsorb to the QDs. Complexes comprising QDs and Ru(3)O clusters adsorbed through a pyridine-4-carboxylic acid ligand (nic-Ru(3)O) have an intrinsic PET rate constant of (4.9 ± 0.9) × 10(9) s(-1) whereas complexes comprising QDs and Ru(3)O clusters adsorbed through a 4-mercaptopyridine ligand (thiol-Ru(3)O) have an intrinsic PET rate constant of (36 ± 7) × 10(9) s(-1). Cyclic voltammetry measurements of nic-Ru(3)O and thiol-Ru(3)O yield reduction potentials vs. Ag/AgCl of -0.93 V for both clusters, and density functional theory calculations of the nic-Ru(3)O and thiol-Ru(3)O clusters yield internal reorganization energies for the cluster radical anion of -0.17 eV and -0.19 eV, respectively. The small differences in driving force and reorganization energy between the two complexes rule out these parameters as possible explanations for the factor-of-seven difference in the rate constants for PET. The difference in the observed rates of PET for the two complexes is therefore attributable to a difference in donor-acceptor electronic coupling, which, according to electronic structure calculations, is modulated by the torsional angle between the Ru(3)O core of the cluster and the functionalized pyridine ligand that bridges the cluster to the QD surface.