This article reviews the mechanisms through which molecules adsorbed to the surfaces of semiconductor nanocrystals, quantum dots (QDs), influence the pathways for and dynamics of intra- and interband exciton relaxation in these nanostructures. In many cases, the surface chemistry of the QDs determines the competition between Auger relaxation and electronic-to-vibrational energy transfer in the intraband cooling of hot carriers, and between electron or hole-trapping processes and radiative recombination in relaxation of band-edge excitons. The latter competition determines the photoluminescence quantum yield of the nanocrystals, which is predictable through a set of mostly phenomenological models that link the surface coverage of ligands with specific chemical properties to the rate constants for nonradiative exciton decay.
This Communication describes the photoredox catalysis of a C-C coupling reaction between 1-phenylpyrrolidine (PhPyr) and phenyl trans-styryl sulfone by visible-light-absorbing colloidal CdS quantum dots (QDs), without a sacrificial oxidant or reductant, and without a co-catalyst. Simple kinetic analysis reveals that photo-oxidation of PhPyr by the QDs is the rate-limiting step. Disordering of the ligand shell of the QDs by creating mixed monolayers of oleate and octylphosphonate increases the initial rate of the reaction by a factor of 2.3, and the energy efficiency (mol product/joule of incident photons) of the reaction by a factor of 1.6, by facilitating the hole-transfer step.
Displacement of native octylphosphonate (OPA) ligands for methylthiophenolate (CH 3 -TP) on the surfaces of CdSe quantum dots (QDs) causes a moderate (up to 50 meV) decrease in the band gap (E g ) of the QD. Plots of the corresponding increase in apparent excitonic radius, ΔR, of the QDs versus the surface coverage of CH 3 -TP, measured by 1 H NMR, for several sizes of QDs reveal that this ligand adsorbs in two distinct binding modes, (1) a tightly bound mode (K a = 1.0 ± 0.3 × 10 4 M −1 ) capable of exciton delocalization, and (2) a more weakly bound mode (K a = 8.3 ± 9.9 × 10 2 M −1 ) that has no discernible effect on exciton confinement. For tightly bound CH 3 -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 ΔR on surface coverage of CH 3 -TP over a range of physical radii of the QDs, R = 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 CH 3 -TP of ∼1 eV. ■ INTRODUCTIONThis paper describes the dependence of the excitonic radius of CdSe quantum dots, QDs, on the surface coverage of an exciton-delocalizing ligand, methylthiophenolate, CH 3 -TP, and the use of this dependence to estimate both the number of binding geometries (with corresponding adsorption constants) for the ligand and the magnitude of the confinement potential that each binding mode presents for the exciton. Exciton delocalizing ligands, such as thiolates 1−4 and dithiocarbamates, 5−8 allow for increased electronic coupling of a quantum-confined exciton with the immediate surroundings of the QD, and therefore facilitate charge carrier or exciton extraction into proximate redox or energy acceptors, without changing the physical size or chemical composition of the QD core or broadening their optical spectra. 5−9 Exciton delocalization is also associated with an increase in the oscillator strength of band-edge transitions 3,4 and a resultant increase in the photoluminescence quantum yield of ensembles of QDs. 10 In addition to the benefits of delocalizing ligands for use of QDs as photovoltaic active materials, photocatalysts, and luminescent tags, the response of the excitonic energy of the QD to its surface chemistry is a sensitive probe of the degree of quantum confinement of its carriers, 6,8 and, as we show here, the chemical and electronic structure of the QD−ligand interface, which is often difficult to probe using traditional analytical techniques, especially in the solution phase.Displacement of native octylphosphonate ligands (OPA) by CH 3 -TP on the surfaces of the QDs, Figure 1A, causes a moderate (up to 50 meV) decrease in the band gap (E g ) of the QDs, which we measure by monitoring the position of the first excitonic peak of the QDs with UV−vis absorption spectroscopy, Figure 1B. We report the decrease in E g as an increase in the apparent radius of its quantum-confine...
This paper describes a procedure for transferring colloidal CdS and CdSe quantum dots (QDs) from organic solvents to water by exchanging their native hydrophobic ligands for phosphonopropionic acid (PPA) ligands, which bind to the QD surface through the phosphonate group. This method, which uses dimethylformamide as an intermediate transfer solvent, was developed in order to produce high-quality water soluble QDs with neither a sulfur-containing ligand nor a polymer encapsulation layer, both of which have disadvantages in applications of QDs to photocatalysis and biological imaging. CdS (CdSe) QDs were transferred to water with a 43% (48%) yield using PPA. The photoluminescence (PL) quantum yield for PPA-capped CdSe QDs is larger than that for QDs capped with the analogous sulfur-containing ligand, mercaptopropionic acid (MPA), by a factor of four at pH 7, and by up to a factor of 100 under basic conditions. The MPA ligands within MPA-capped QDs oxidize at Eox ~ +1.7 V vs. SCE, whereas cyclic voltammograms of PPA-capped QDs show no discerible oxidation peaks at applied potentials up to +2.5 V vs. SCE. The PPA-capped QDs are chemically and colloidally stable for at least five days in the dark, even in the presence of O2, and are stable when continuously illuminated for five days, when oxygen is excluded and a sacrificial reductant is present to capture photogenerated holes.
Time-resolved optical spectroscopies reveal multielectron transfer from the biexcitonic state of a CdS quantum dot to an adsorbed tetracationic compound cyclobis(4,4'-(1,4-phenylene) bipyridin-1-ium-1,4-phenylene-bis(methylene)) (ExBox(4+)) to form both the ExBox(3+•) and the doubly reduced ExBox(2(+•)) states from a single laser pulse. Electron transfer in the single-exciton regime occurs in 1 ps. At higher excitation powers the second electron transfer takes ∼5 ps, which leads to a mixture of redox states of the acceptor ligand. The doubly reduced ExBox(2(+•)) state has a lifetime of ∼10 ns, while CdS(+•):ExBox(3+•) recombines with multiple time constants, the longest of which is ∼300 μs. The long-lived charge separation and ability to accumulate multiple charges on ExBox(4+) demonstrate the potential of the CdS:ExBox(4+) complex to serve as a platform for two-electron photocatalysis.
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