Distance-Dependence of Interparticle Energy Transfer in the Near-Infrared within Electrostatic Assemblies of PbS Quantum Dots

Abstract

This paper describes control of the rate constant for near-infrared excitonic energy transfer (EnT) within soluble aqueous assemblies of PbS quantum dots, cross-linked by Zn, by changing the length of the mercapto-alkanoic acid (MAA) that serves as the cross-linking ligand. Sequestration of Zn by a chelating agent or zinc hydroxide species results in deaggregation of the assemblies with EnT turned "off". Upon decreasing the number of methylene groups in MAAs from 16 to 3, the interparticle separation decreases from 5.8 nm to 3.7 nm and the average observed EnT rate increases from ∼(150 ns) to ∼(2 ns). A master equation translates intrinsic (single-donor-single-acceptor) EnT rate constants predicted for each ligand length using Förster theory to observed average rate constants. For interparticle distances greater than ∼4 nm, the point dipole approximation (PDA) implementation of Förster theory agrees with experimentally measured rates. At shorter interparticle distances, the PDA drastically underestimates the observed EnT rate. The prediction of the rates of these short-distance EnT processes is improved by ∼20% by replacing the PDA with a transition density cube calculation of the interparticle Coulombic coupling.

Energy transfer-enhanced photocatalytic reduction of protons within quantum dot light-harvesting–catalyst assemblies

“…Our conclusion that the enhancement of catalytic activity of the system is due to energy funneling is also supported by the agreement of our experimental data with the values of IQE and IQE cQD extracted from simulations of exciton dynamics in the QD assemblies using a master equation model (36,37,41) (Fig. 4 B and C, red), where EnT is the only interaction among QDs.…”

“…Here, electrostatic assemblies of QDs form spontaneously (35) upon mixing sQDs with cQDs in 1 M ascorbic acid (AA) at pH 4.7, and shaking for 30 s. Ascorbic acid serves as both a proton source and a sacrificial reductant for H 2 production (13). As opposed to, for instance, QD assemblies formed through metal ion coordination to the QDs' ligands (36,37), electrostatic self-assembly allows for the preferential formation of sQD−cQD contacts over cQD−cQD or sQD−sQD contacts. This more specific assembly leads to higher yields of EnT to the cQDs than does nonspecific QD assembly; see SI Appendix, Fig.…”

Influence of a Capping Ligand on the Band Gap and Excitonic Levels in Colloidal Solutions and Films of ZnSe Quantum Dots

“…In the former case, the gap reduction makes it possible to "pull out" an electron or a hole from the QD and to prevent their recombination. In the latter case, on the contrary, owing to the compression of the exciton wave function, it becomes possible to increase the energy of − ℎ pairs [1][2][3][4][5][6]. Along with the quantum confinement effect, which nowadays is the most studied property of semiconductor QDs, this circumstance allows their energy to be controlled in a wide interval of their sizes, and the wave function of charge carriers in QDs to be governed without changing the actual sizes of QDs.…”

“…Ligands possess different affinities with respect to superficial QD atoms, which depend on the type of the bond that they mutually form between them. For example, the highest affinities with respect to CdSe and CdS QDs belong to thiols (thiolates), which form a covalent bond with the superficial Cd atoms [4][5][6]. The treatment of the surface of CdS and CdSe QDs with dithiocarbamates reduces the forbidden gap in those QDs by about 970 and 220 meV, respectively, by means of the hole delocalization at a ligand molecular orbital.…”

“…If a QD-ligand complex belongs to type-II, then holes rapidly transit from the QD into the ligand shell, as it takes place, e.g., in the CdSe QDphenyldithiocarbamate (PTC) complex [7][8][9][10][11][12]. Nowadays, those processes are mainly studied on the basis of QDs of CdX and PbX (X = S, Se, Te) chalcogenides [1,[4][5][6]. At the same time, ZnX QDs remain little researched owing to a complicated procedure of their synthesis and their rather wide optical gap.…”