Sub-Nanosecond Resonance Energy Transfer in the Near-Infrared within Self-Assembled Conjugates of PbS Quantum Dots and Cyanine Dye J‑Aggregates

Energy transfer (EnT) of near-infrared (NIR) excitons enables applications in harvesting of solar energy and biological imaging. Fast exciton extraction from NIR-absorbing Pb-chalcogenide quantum dots (QDs) may allow utilization of the photon downconversion (multiple exciton generation) process that occurs in those QDs to amplify signal in QD-based sensors or photocurrent in QD-based photovoltaics. This paper describes subnanosecond extraction of NIR excitons from PbS QDs by adsorbed J-aggregates of cyanine dye in aqueous dispersions. The QD/J-aggregate complexes form through electrostatic self-assembly, and the rate and yield of EnT within the complexes can be optimized by adjusting spectral overlap between QD emission and the J-aggregate absorption, which are controlled by density of charged ligands on the QD surface and the pH. The primary EnT pathways have rate constants ranging from (800 ps)⁻¹ to (2.2 ns)⁻¹, which are 1–2 orders of magnitude faster than previously reported examples with PbS QDs as exciton donors. The fastest EnT process occurs in 90 ps and is potentially competitive with Auger recombination of biexcitonic states in PbS QDs

Accelerating FRET between Near-Infrared Emitting Quantum Dots Using a Molecular J‑Aggregate as an Exciton Bridge

Fast energy transfer (EnT) among quantum dots (QDs) with near-infrared (NIR) emission is essential for fully exploiting their light harvesting and photon downconversion (multiexciton generation) abilities. This paper demonstrates a relayed EnT mechanism that accelerates the migration of NIR excitons between PbS QDs by a factor of 20 from that of one-step EnT through a polyelectrolyte and even a factor of ∼2 from that of one-step EnT between QDs in direct contact, by employing a J-aggregate (J-agg) of a cyanine dye as an exciton bridge. The donor QDs, acceptor QDs, and J-agg are electrostatically assembled into a sandwich structure with layer-by-layer deposition. Estimates of EnT rate and yield from transient and steady-state absorption and photoluminescence spectroscopies show that the rate-limiting step in the relay is EnT from the donor QD to the J-agg, while EnT from the J-agg to the acceptor QD occurs in <10 ps. A comparison of this system to the analogous solution-phase system suggests that the overall donor-to-acceptor EnT yield in the relay (18%) can be improved by depositing the J-agg with more intermolecular order. This work demonstrates the viability of relayed EnT through a molecular bridge as a strategy for accelerating long-distance exciton migration in assemblies of QDs, in particular in the near-infrared

Influence of Interparticle Structure on the Steady-State and Transient Current within Arrays of Thiocyanate-Treated PbS Nanocubes

This paper describes the dependence of the DC conductivity, film charging dynamics, and transient photocurrent dynamics of quasi-two-dimensional arrays of thiocyanate-capped PbS nanocubes (NCs) on the edge length of the NC. Arrays were prepared monolayer-by-monolayer using self-assembly at a liquid–air interface. Across-film conductivity increases with NC size with a dependence consistent with a simple diffusional hopping model. Upon application of a constant source-drain bias, the measured dark current decays exponentially to a nonzero steady-state value as immobile hole traps fill. Illumination with 532-nm light produces a repeatable photoresponse, which also fits to an exponential function. The lifetimes associated with decay of the dark current and growth of the photocurrent both increase with increasing NC size. Comparison of the electrical data with electron microscopy images reveals that this trend is related to the connectivity of the percolation networks within the film, which depends on the interparticle order and, in turn, on the edge length of the NCs. Correlations between interparticle order and electrical properties are made possible by the highly ordered films that result from the liquid–air interface deposition method

Role of Organosulfur Compounds in the Growth and Final Surface Chemistry of PbS Quantum Dots

This paper describes the mechanism by which reaction of sulfur with 1-octadecene (ODE) induces a change in the shape of PbS quantum dots (QDs), synthesized from the S/ODE precursor and lead­(II) oleate, from cubic to hexapodal by altering the ligand chemistry of the growing QDs. ¹H NMR and optical spectroscopies indicate that extended heating of sulfur and ODE at 180 °C produces a series of organosulfur compounds with optical transitions in the visible region and that the binding of organosulfur ligands to the growing QD induces a preferential growth at the ⟨100⟩ faces (over the ⟨111⟩ faces) and, therefore, a hexapodal geometry for the particles. The study shows that S/ODE can be made a more reliable precursor by reducing the temperature and duration of the sulfur dissolution step and that any metal sulfide QD synthesis using elemental sulfur heated to high temperatures should take steps to reduce the in situ yield of organosulfur byproducts by avoiding olefinic solvents

Mechanisms of Symmetry Breaking in a Multidimensional Flashing Particle Ratchet

Ratcheting is a mechanism that produces directional transport of particles by rectifying nondirectional energy using local asymmetries rather than a net bias in the direction of transport. In a flashing ratchet, an oscillating force (here, an AC field) is applied perpendicular to the direction of transport. In an effort to explore the properties of current experimentally realizable ratchet systems, and to design new ones, this paper describes classical simulations of a damped flashing ratchet that transports charged nanoparticles within a transport layer of finite, non-zero thickness. The thickness of the layer, and the decay of the applied field in the <i>z</i>-direction throughout that thickness, provide a mechanism of symmetry breaking in the system that allows the ratchet to produce directional transport using a temporally <i>unbiased</i> oscillation of the AC driving field, a sine wave. Sine waves are conveniently produced experimentally or harvested from natural sources but cannot produce transport in a 1D or pseudo-1D system. The sine wave drive produces transport velocities an order of magnitude higher than those produced by the common on/off drive, but lower than those produced by a temporally biased square wave drive (unequal durations of the positive and negative states). The dependence of the particle velocity on the thickness of the transport layer, and on the homogeneity of the oscillating field within the layer, is presented for all three driving schemes

Mechanisms of Symmetry Breaking in a Multidimensional Flashing Particle Ratchet

Ratcheting is a mechanism that produces directional transport of particles by rectifying nondirectional energy using local asymmetries rather than a net bias in the direction of transport. In a flashing ratchet, an oscillating force (here, an AC field) is applied perpendicular to the direction of transport. In an effort to explore the properties of current experimentally realizable ratchet systems, and to design new ones, this paper describes classical simulations of a damped flashing ratchet that transports charged nanoparticles within a transport layer of finite, non-zero thickness. The thickness of the layer, and the decay of the applied field in the <i>z</i>-direction throughout that thickness, provide a mechanism of symmetry breaking in the system that allows the ratchet to produce directional transport using a temporally <i>unbiased</i> oscillation of the AC driving field, a sine wave. Sine waves are conveniently produced experimentally or harvested from natural sources but cannot produce transport in a 1D or pseudo-1D system. The sine wave drive produces transport velocities an order of magnitude higher than those produced by the common on/off drive, but lower than those produced by a temporally biased square wave drive (unequal durations of the positive and negative states). The dependence of the particle velocity on the thickness of the transport layer, and on the homogeneity of the oscillating field within the layer, is presented for all three driving schemes

Control of the Redox Activity of Quantum Dots through Introduction of Fluoroalkanethiolates into Their Ligand Shells

Increasing the fraction of 1<i>H</i>,1<i>H</i>,2<i>H</i>,2<i>H</i>-perfluorodecanethiol (PFDT) in the mixed-PFDT/oleate ligand shell of a PbS quantum dot (QD) dramatically reduces the permeability of the ligand shell to alkyl-substituted benzoquinones (s-BQs), as measured by a decrease in the efficiency of collisional photoinduced electron transfer. Replacing only 21% of the oleates on the QD surface with PFDT reduces the yield of photo-oxidation by tetramethyl BQ by 68%. Experiments with s-BQ quenchers of two different sizes reveal that the degree of protection provided by the PFDT-doped monolayer, relative to a decanethiolate (DT)-doped monolayer at similar coverage, is due to both size exclusion (PFDT is larger and more rigid than DT), and the oleophobicity of PFDT. This work demonstrates the usefulness of fluorinated ligands in designing molecule-selective and potentially corrosion-inhibiting surface coatings for QDs for applications as robust emitters or high fidelity sensing platforms

The Chemical Environments of Oleate Species within Samples of Oleate-Coated PbS Quantum Dots

A combination of FT-IR, ¹H NMR, nuclear Overhauser effect (NOESY), and diffusion-ordered (DOSY) NMR spectroscopies shows that samples of oleate-coated PbS quantum dots (QDs) with core radii ranging from 1.6 to 2.4 nm, and purified by washing with acetone, contain two species of oleate characterized by the stretching frequencies of their carboxylate groups, the chemical shifts of their protons, and their diffusion coefficients. One of these oleate species exists primarily on the surfaces of the QDs and either chelates a Pb²⁺ ion or bridges two Pb²⁺ ions. The ratio of bridging oleates to chelating oleates on the surfaces of the QDs is approximately 1:1 for all sizes of the QDs we studied. The second oleate species in these samples bridges two Pb²⁺ ions within clusters or oligomers of lead oleate (with a hydrodynamic radius of ∼1.4 nm), which are byproducts of the QD synthesis. The concentration of these clusters increases with increasing size of the QDs because larger QDs are produced by increasing the concentration of the oleic acid ligand in the reaction mixture. The oleate molecules on the surfaces of the QDs and within the lead oleate clusters are in rapid exchange with each other. Additional washes with methanol progressively eliminate the contaminating clusters from the PbS QD samples. This work quantitatively characterizes the distribution of binding geometries at the inorganic/organic interface of the nanocrystals and demonstrates the utility of using organic ligands as probes for the composition of a colloidal QD sample as a function of the preparation procedure

Dual-Time Scale Photoinduced Electron Transfer from PbS Quantum Dots to a Molecular Acceptor

A combination of picosecond and microsecond transient absorption dynamics reveals the involvement of two mechanisms by which 1,4-benzoquinone (BQ) induces the decay of the excited state of PbS quantum dots (QDs): (i) electron transfer to BQ molecules adsorbed to the surfaces of PbS QDs and (ii) collisionally gated electron transfer to freely diffusing BQ. Together, these two mechanisms quantitatively describe the quenching of photoluminescence upon addition of BQ to PbS QDs in dichloromethane solution. This work represents the first quantitative study of a QD–ligand system that undergoes both adsorbed and collisionally gated photoinduced charge transfer within the same sample. The availability of a collisionally gated pathway improves the yield of electron transfer from PbS QDs to BQ by an average factor of 2.5 over that for static electron transfer alone

Computational Study of the Influence of the Binding Geometries of Organic Ligands on the Photoluminescence Quantum Yield of CdSe Clusters

This article describes density-functional-theory- (DFT-) based calculations of the rate constants for radiative (<i>k</i>_R) and nonradiative (<i>k</i>_NR) decay from the lowest singlet excited state (S₁) to the ground state (S₀) of a Cd₁₆Se₁₃ cluster ligated with various molecules in various binding geometries. The value of <i>k</i>_R is suppressed by ligands whose localized orbitals, as a result of their binding geometry, become the cluster’s frontier orbitals and, thereby, decrease the overlap of the electron densities of the HOMO and LUMO necessary for efficient dipole coupling. Thiolate ligands in a monodentate geometry and dithioate ligands in a bridging geometry decrease <i>k</i>_R in this manner. The value of <i>k</i>_NR is also sensitive to the binding geometries of the ligands: binding geometries that are less rigid yield a greater change in nuclear coordinates between the S₁ and S₀ electronic states, which, in turn, increases the rate of nonradiative decay by maximizing the vibronic coupling between the band-edge exciton and the vibrational modes of the ligands. This work suggests that the photoluminescence quantum yield of CdSe nanoparticles can be maximized by ensuring that the bridging binding mode is the dominant binding mode of the ligands; bridging modes decrease the nonradiative decay rate and eliminate mid-band-gap trap states in all cases studied, except for dithioate ligands