Exciton Dissociation within Quantum Dot–Organic Complexes

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Exciton Dissociation within Quantum Dot−Organic Complexes: Mechanisms, Use as a Probe of Interfacial Structure, and Applications Kathryn E. Knowles, Mark D. Peterson, Martin R. McPhail, and Emily A. Weiss* Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States ABSTRACT: This article reviews the structural and electronic features of colloidal quantum dot (QD)−organic complexes that influence the rate of photoinduced charge separation (PCS) across the interface between the inorganic core of the QD and its organic surface ligands. While Marcus theory can be used to describe the rate of PCS in QD−organic complexes, uncertainties in the exact atomic configuration of the inorganic−organic interface and heterogeneities in this interfacial structure within an ensemble of QDs complicate the determination of the most fundamental Marcus parameterselectronic coupling, reorganization energy, and driving force. This article discusses strategies for accounting for uncertainties and heterogeneities when using Marcus theory to interpret rates of PCS in QD−organic complexes and highlights how measurement of PCS rates can provide information about the interfacial structure of the QD surface. Recent progress in the application of mechanistic knowledge of PCS to harvest multiple charge carriers from QDs containing multiple excitons and extend the lifetime of the charge-separated state is also discussed.

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INTRODUCTION This paper reviews the structural and electronic factors relevant to mechanistic studies of photoinduced charge separation (PCS) across the interface between the inorganic core of a colloidal semiconductor quantum dot (QD) and its organic ligands. Colloidal QDs have five properties that make them excellent candidates to be photoactive components in systems for conversion of solar energy to electricity and chemical fuel: (i) Their extinction coefficients at the first excitonic transition range from 104 to 106 M−1 cm−1,1,2 up to a factor of 100 larger than that for ruthenium(II) tris(bipyridine) dyes used in the most efficient dye-sensitized solar cells.3,4 (ii) They have bandgaps that are tunable with the size of the nanocrystalline core1,2 and with coverage of certain strongly coupled organic ligands.5,6 (iii) Their cores can accommodate multiple delocalized excitonic states, formed simultaneously either through carrier multiplication or multiphoton absorption. (iv) They are synthesized using wetchemistry methods and can be deposited from suspension under ambient conditions, which, in principle, enables QD films to be manufactured by high-throughput roll-to-roll printing techniques. In contrast, polycrystalline silicon films are typically deposited by chemical vapor deposition techniques that require high vacuum and high temperatures (100−600 °C).7 (v) They are more photostable than organic molecules with comparable extinction coefficients. For example, when used as fluorescent labels on cell membranes, CdSe/ZnS core/shell QDs maintained their photoluminescence intensity under continuous UV irradiation up to 4000 times longer than a substituted fluorescein dye.8 Solar energy conversion using QDs involves absorption of a photon to create a net-neutral electron−hole pair, called an © XXXX American Chemical Society

exciton, followed by dissociation of the exciton through extraction of one of the charge carriers, either the electron or the hole, from the QD. This charge separation process must happen faster than recombination of the electron and hole to regenerate the ground state of the QD. Knowledge of the chemical and electronic factors that control the rate of exciton dissociation in QDs is therefore essential for enabling rational design of efficient photovoltaic and photocatalytic systems that incorporate QDs. Marcus theory has successfully predicted the dependence of the rate constant of photoinduced charge separation in molecular systems on chemical and electronic factors such as molecular conformation, orbital energies, and dielectric environment.9−11 These predictions have informed the design of conductive polymers for organic photovoltaics,12,13 molecular photocatalysts,14,15 and molecular sensitizers for dye-sensitized solar cells.16,17 Unlike molecules, colloidal QDs do not have welldefined atomic configurations in the regions where their crystalline cores terminate. An ion on the surface of a QD is bound to core ions, other surface ions, and organic surface ligands, but the number of coordination sites occupied by each of these species varies among surface ions.18 The number and type of species to which a surface ion is coordinated determine whether the energies of its frontier orbitals lie within the conduction or valence band or within the bandgap of the QD, in which case the surface state is a thermodynamic trap for a charge carrier. In general, an undercoordinated surface cation forms an Received: January 21, 2013 Revised: April 23, 2013

A

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electron trap that lies energetically below the edge of the conduction band, and an undercoordinated surface anion forms a hole trap that lies above the edge of the valence band. Within an ensemble, the number of undercoordinated surface cations and anions, and thus the number of electron and hole trap states, varies from QD to QD. An ensemble of QDs is inherently heterogeneous with respect to the QD radius, the total number of surface ions per QD, the ratio of surface cations to surface anions, and the number of bound surface ligands per QD. Furthermore, a surface ligand may bind to the surface of a QD in more than one configurationfor example, an alkylphosphonate ligand may adsorb in a singly deprotonated or doubly deprotonated form, to either a cation or anion or both simultaneously.19,20 Current synthetic methods are not capable of eliminating any of these types of heterogeneities within a synthetic batch of QDs, although various analytical techniques can be applied to characterize their distributions within any given sample.21 This characterization is important in designing QD-based charge-transfer systems because all of these aspects of surface structure affect the ability of an excitonic charge carrier to move from a core state of the QD across the QD− ligand interface to an acceptor moiety. While Marcus theory is, in principle, just as applicable to charge-transfer processes in QD− molecule systems as it is to molecular systems, an attempt to use Marcus theory to predict the rate constant of photoinduced charge separation (PCS) from a QD is, in practice, complicated by these various heterogeneities and distributions. Ongoing developments in quantitative structural and chemical characterization of the QD−ligand interface19,22−25 will push forward both fundamental studies of photoinduced charge transfer and applied research in QD-based solar energy conversion materials. This article begins with a brief review of the various decay pathways available to excitonic charge carriers in QDs and of methods to spectroscopically distinguish photoinduced electron or hole transfer pathways from other nonradiative processes. We then describe how specific aspects of QD surface structure influence our ability to determine the magnitude of the most fundamental Marcus parameters for electron transfer reactionselectronic coupling, reorganization energy, and driving forceand discuss strategies for accounting for uncertainties when using Marcus theory to interpret rate constants of PCS in QDs. A summary of some of our recent work demonstrates how measurements of rate constants of PCS can provide information about the structure of the QD surface. Finally, we highlight perhaps the most exciting application of mechanistic knowledge of QD−ligand CS: some recent progress in harvesting multiple charge carriers from QDs containing multiple excitons.

Figure 1. Cartoon illustrating localization of an excitonic (delocalized) hole to (a) a lattice trap, (b) a surface trap, and (c) a hole-accepting ligand. The blue curves depict the wave function of the hole; the green ovals depict redox-active hole-accepting ligands; and the black curves depict surface ligands that couple to QD surface states to form surface traps.

Trapping events are critical to the understanding of mechanisms and measurement of PCS processes involving QDs for two reasons: (i) Initial localization of a carrier to a trap state can inhibit (or accelerate) the rate of subsequent carrier transfer to an acceptor molecule by decreasing (or increasing) the driving force or the orbital overlap between donor and acceptor states. (ii) Measuring photoluminescence (PL) quenching upon addition of an electron or hole acceptor molecule to a solution of QDs is one of the most highly utilized techniques for obtaining the rate and yield of electron and hole transfer, but trapping of either the electron or the hole to an orbital that is electronically uncoupled from the complementary delocalized carrier also quenches the band-edge PL of the QD. For instance, exchange of native trioctylphosphine oxide or phosphonic acid ligands for thiol ligands produces deep trap states that result in large decreases in band-edge PL and, in some cases, emergence of a broad low-energy emission feature.26 Controlled ligand exchange has been shown to have dramatic effects on the luminescent properties of QDs by altering these surface−ligand interactions.27,28 Measurement of PCS rate constants and yields by PL quenching is therefore complicated by the sensitivity of PL intensity to nonradiative processes other than PCS. Transient absorption (TA) spectroscopy allows several spectrally distinct features, which have amplitudes that are only sensitive to band-edge electrons, band-edge holes, or reduced/ oxidized forms of charge acceptors, to be monitored at once. For example, the amplitude of the ground-state (GS) bleach feature in CdSe and CdS QDs is proportional to the population of electrons in conduction band-edge states but is not affected by the presence of holes in valence band-edge states.29−31 The reason for the selective sensitivity of the CdX (X = S, Se) GS bleach feature is not conclusively known, but one proposed explanation is based on the higher degeneracy of the frontier orbitals of the valence band than the conduction band.29 Supporting this hypothesis, the bleach feature of PbX QDs, which have a more symmetric electronic structure across the bandgap, is sensitive to the presence of both holes and electrons at the band edges.32,33 Intraband absorption features in TA spectra can be used to separate electron dynamics from hole

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IDENTIFYING PHOTOINDUCED CHARGE SEPARATION PROCESSES Charge Trapping vs Charge Separation. Following photoexcitation, a variety of radiative and nonradiative relaxation pathways are available to both the electron and the hole. These pathways include (i) trapping of an electron or hole to a localized core state formed by a lattice defect (Figure 1a), (ii) trapping of an electron or hole to a surface trap state formed by an undercoordinated surface ion or by the mixing of ligand orbitals with surface atomic orbitals (Figure 1b), (iii) transfer of an electron or hole to a molecular acceptor that is spectroscopically distinct from states on the QD (Figure 1c), (iv) energy transfer from the photoexcited QD to an acceptor moiety (not shown), and (v) radiative recombination of the electron and hole (not shown). B

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QD to the lowest unoccupied molecular orbital (LUMO) of the acceptor,33,39 and the hole moves from the valence band of the QD to the highest occupied molecular orbital (HOMO) of the acceptor.49,50 Because the orbitals involved in electron and hole transfer have different energies, spatial overlaps, and symmetries, the electronic coupling for these processes is different. The degree to which the electron or hole couples to the QD surface can be selectively tuned by changing the height of the tunneling barrier presented to either charge carrier by the organic− inorganic interface or by changing the stoichiometry of the QD surface. Frederick et al. demonstrated selective delocalization of the hole into the ligand shell of CdS, CdSe, and PbS QDs by replacing the native ligand shell with phenyldithiocarbamate molecules whose HOMOs are energetically resonant with QD valence band-edge states and symmetric with the atomic orbitals of the anion that comprise the valence band of the QD.5 The extent to which the hole delocalizes into the ligand shell can be controlled by changing the position of the HOMO of the phenyldithiocarbamate ligand relative to the valence band-edge states through para-substitution with electron-donating or electron-withdrawing substitutents.6 The conduction and valence bands primarily comprise cation and anion atomic orbitals, respectively, so the stoichiometry of the QD surface should affect the relative rate constants of electron and hole transfer because the degree of cation or anion enrichment of the surface influences the degree to which each charge carrier electronically couples to the surface and, consequently, to a proximate acceptor. This relationship, in principle, would allow measurements of PCS rate constants to serve as probes of QD surface composition, although this type of experiment has not yet been reported. If this picture of interfacial charge transfer is correct, a cation-rich QD surface will be more likely to participate in electron transfer than hole transfer because it has more available surface sites through which a carrier can couple to an electron acceptor it has available for coupling to a hole acceptor.

dynamics. Intraband absorptions in the near-IR region corresponding to electrons in the conduction band of the QD generally occur at higher energies (∼900−1300 nm) than the intraband absorptions of valence band holes (∼1300−2500 nm), but electron intraband absorption also occurs in the mid-IR (∼2000−6500 nm).30,34−36 Depopulation of the conduction and/or valence band occurs in both charge trapping and charge-transfer processes. There are three ways to distinguish the two processes: (i) Some systems present a spectrally distinct feature from the reduced/oxidized molecular charge acceptor; this feature can be used to positively identify photoinduced electron transfer (PET) or photoinduced hole transfer (PHT) mechanisms.37,38 (ii) In the absence of a signal from the charge acceptor, shorter ground state recovery lifetimes or reduction of PL quantum yield upon addition of the charge acceptor implies that this species provides an additional nonradiative exciton decay process (namely, charge transfer) for the QDs. This test is sometimes complicated at large concentrations of the putative charge-accepting ligand because it can displace passivating ligands or cause surface reconstruction, which introduces new charge trapping pathways that also quench excited states and PL. (iii) Calculation of the driving force for electron or hole transfer determines whether those processes are thermodynamically feasible and, if the uncertainty associated with such a calculation is taken into account (see below), may eliminate them as sources of PL decay or bleach recovery. Electron Transfer vs Hole Transfer. Photoinduced electron transfer (PET) and photoinduced hole transfer (PHT) will generally have different rates, and one process can be dominant even when both are energetically favorable.39 PET from QDs is more often reported than PHT; however, to incorporate QDs into efficient, robust photovoltaic devices, we need to understand the structural and electronic factors that control hole transfer as well as electron transfer. For instance, the first step in the generation of current from a QD-sensitized solar cell is typically electron injection from a photoexcited QD into a semiconductor electrode. Rapid recombination of this electron with the hole left in the valence band of the QD is a primary factor limiting the quantum efficiency of QD-sensitized solar cells.40−42 Without extraction of this hole by a molecular reductant, the QD becomes more susceptible not only to recombination processes but also to oxidative damage. The reaction of photogenerated holes with oxygen and various redox couples can produce trap states, shifts in absorbance and photoluminescence features, and surface shelling and etching.42−45 These effects are particularly pronounced in liquid junction CdTe-sensitized solar cells where, upon illumination, a polysulfide redox couple rapidly forms a CdS shell on the CdTe dots that retards further hole extraction.46 The ligand shell of a quantum dot can help protect it from oxidative processes. Electrochemical measurements indicate that the presence of oleate ligands on the surfaces of PbS QDs produces a cathodic shift of the oxidation potential and increased robustness over multiple redox cycles compared to bulk PbS thin films.47 Removal of surface ligands through excessive washing leaves the QDs more susceptible to photooxidation.48 Addition of a larger bandgap semiconductor shelling layer to a QD can also slow down degradation by isolating the hole from reactive surface states and allow the use of the highly efficient iodide/triiodide redox couple in liquid junction QD-sensitized solar cells; this redox couple rapidly degrades unshelled dots.44 Different orbitals are involved in electron and hole transfer processes: the electron moves from the conduction band of the

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APPLICATIONS OF MARCUS THEORY TO PCS WITHIN QD−ORGANIC COMPLEXES Much of the work investigating PCS reactions between colloidal QDs and various redox partners has tested whether the rate constants of these processes are consistent with the predictions of Marcus theory.51−54 The majority of this work uses the Marcus equation for nonadiabatic charge transfer55 (eq 1) to interpret the observed rate constants (kCS) of PCS reactions involving QDs. Equation 1 k CS =

2π |Va , b|2 ℏ

⎛ (λ + ΔG )2 ⎞ 1 0 ⎟ exp⎜ − 4λkbT ⎠ 4πλkbT ⎝

(1)

contains three parameters that depend on the chemical and electronic structure of both the QD and its redox partner: (i) |Va,b|2 is the electronic coupling between the reactant (precharge transfer) state of the system and the product (postcharge transfer) state of the system at the nuclear coordinate at which their potential surfaces cross. For molecular PCS reactions, |Va,b|2 is usually estimated by calculating sums of orbital overlap integrals within the atomic configurations that provide pathways for the charge carrier to travel from the donor to the acceptor.10 (ii) The reorganization energy, λ, is the energy difference between the nuclear geometries of the reactant species and the charge-separated system formed upon charge transfer, and includes contributions from the donor, acceptor, and proximate solvent or other spectator molecules. (iii) ΔG0 is C

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Figure 2. (A) Cartoon illustrating how the donor−bridge−acceptor model maps onto the components of a typical QD−ligand complex, including the potential profile of the tunneling barrier presented by the bridge. (B) Diagram depicting the states that contribute to ΔG0 for electron transfer (ET) and hole transfer (HT) from a QD in the lowest energy excitonic state to an electron acceptor or hole acceptor, respectively.

includes, for example, the undercoordinated surface atom to which the ligand is bound, the binding headgroup of the ligand, and the alkyl chain separating the binding headgroup from the redox-active part of the ligand. This model is appropriate when there are weak couplings between the QD and the bridging group and between the bridging group and the redox-active group that is, when the system partitions easily into three relatively isolated parts. Figure 2A contains a qualitative sketch of the potential profile of such a system, where the linker between the donor and acceptor has orbital energies far off resonance with the donor and acceptor orbitals. In this case, PCS occurs via a direct or superexchange mechanism in which the bridge acts as a tunneling barrier, and the magnitude of |Va,b|2 and, consequently, the magnitude of kCS between the QD and redox partner decrease exponentially with increasing donor−acceptor distance.10 Electronic Coupling. In the D−B−A approximation, the parameters that determine the magnitude of the electronic coupling between the donor and acceptor are the distance between the centroids of the donor and acceptor orbitals and the energies of the intervening (bridging) orbitals relative to the donor and acceptor orbitals,10 where the energies of the orbitals of donor, bridge, and acceptor sites are estimated from their isolated components. A commonly used strategy for testing the dependence of kCS on |Va,b|2 (and thus evaluating the applicability of eq 1) is to vary the donor−acceptor distance, either by changing the length of the organic linker connecting a QD to its redox partner or by varying the thickness of an inorganic shell separating a QD core donor and a molecular acceptor. These strategies have repeatedly yielded decreases in PCS rate constants with increasing D−A distances, as expected. Some studies observe an exponential dependence of kCS on this distance, which is consistent with the D−B−A model, while others do not sample enough distances or perform precise enough measurements of kCS to establish the functional form of the dependence of kCS on D−A distance. Bakkers et al. observed that the rate constant of PET from a film of CdSe QDs to a Au electrode decreases exponentially as the length of the rigid bis-sulfide linker connecting the QDs to the electrode increases.64 Pernik et al. observed faster PET from CdSe QDs into films of mesoporous TiO2 for QDs directly adsorbed to the TiO2 than for QDs linked to TiO2 via the bifunctional linker mercaptopropionic acid.65 Similarly, Dibbell et al. observed decreasing yields of PET from CdS QDs to TiO2

the change in free energy associated with the PCS reaction (the energy difference between the minima of the reactant and product surfaces). Four features of QD−ligand systems make these Marcus parameters challenging to determine for PCS reactions involving QDs: (i) Many structural parameters, including QD size and surface chemistry, are heterogeneous within an ensemble of colloidal QDs. This problem is a synthetic one and a challenging one to solve because the final structure of a QD is determined by complex nucleation and growth processes that rely on hundreds of noncovalent interactions between unstable inorganic surfaces and surfactant molecules. (ii) There is only a small (although growing) set of analytical methods that offer precise knowledge of the chemical structure of the QD−ligand interface. This structure includes the conformation of ligands (both redox-active ligands and spectator ligands that passivate the surface of a QD), the coordination environment of the surface ion to which a redox-active ligand is bound, and the energies and symmetries of the orbitals involved in forming the bond between the QD surface and a ligand. There are several recent papers and reviews that discuss the quantitative analytical chemistry of QD surfaces.19,21−25 (iii) The QD comprises many hundreds of atoms, all of which contribute to the delocalized excited states of the QD that serve as donors in PCS reactions. A complete, highlevel electronic structure calculation, for the purposes of determining ΔG0, |Va,b|2, or reorganization energy, λ, is currently computationally unfeasible. Density functional theory (DFT) calculations on model systems comprising small clusters and a few ligands may provide some insight into possible geometries and electronic structures of surface-bound ligands.18,56−58 (iv) Electrochemical measurements on QDs are complicated by irreversible surface redox reactions that can be difficult to distinguish from redox processes involving delocalized core states of the QD and often result in degradation of the QD.59,60 A coarse-grained approach to analyzing PCS dynamics in QD−ligand systems is to approximate the QD and redox partner as single sites within a donor−bridge−acceptor (D−B−A) system, in which the bridge comprises all of the components that lie between the QD and the redox-active functional group of the ligand. This approximation is commonly used in molecular systems.61−63 Figure 2A contains a diagram illustrating how the D−B−A model maps onto the structure of a QD−ligand complex. The (photoexcited) QD core is the donor; the redoxactive component of the ligand is the acceptor; and the bridge D

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which spectral features, such as the ground-state bleach31,74 or intraband absorptions,30,34 whose amplitudes are proportional to the population of electrons or holes in conduction or valence band-edge states, decay concurrently with the appearance of a feature associated with the reduced or oxidized state of the molecular acceptor. If there is no clear spectral signal from the reduced or oxidized molecular acceptor and there is evidence that charge-trapping processes occur in the absence of the acceptor, then it is difficult to separate the dynamics of charge trapping from those of charge transfer, or even determine conclusively that charge transfer has occurred. Once the participating orbital in the QD has been identified, the second challenge in determining ΔG0 is to determine the energy of this orbital. Attempts to measure the energies of QD band-edge states by cyclic voltammetry often produce a series of irreversible oxidation and reduction waves because undercoordinated surface ions have redox potentials similar to the delocalized core states.59 Redox-active surfaces make it difficult to identify the exact redox processes that produce these signals, and irreversible surface reactions often cause QDs to aggregate and precipitate from solution during scans. Cross-linking QDs in a solid-state film75 or using an ionic liquid as electrolyte76 can produce reversible redox processes observable by cyclic voltammetry; however, the potentials measured by these methods may not precisely coincide with organic solutionphase potentials due to inter-QD coupling and large differences in solvent dielectric properties, respectively. An alternative to electrochemical measurements of QD redox potentials is measurement of ionization potentials of QD films by ultraviolet photoelectron spectroscopy (UPS). UPS provides a direct measurement of the energy of the valence band-edge or occupied surface midgap states, relative to vacuum; the energy of the conduction band-edge is then determined by adding the energy of the optical bandgap to the energy of the valence bandedge.77 The potential problem again is that UPS measurements are performed on films and under vacuum and thus do not simulate the environment of solution-phase PCS reactions. In lieu of experimentally measured values, many researchers use the effective mass approximation78 to estimate the energies of the QD band-edge states. This approach uses the value for the effective mass of the relevant charge carrier in the bulk semiconductor material, and the size of the QD, to calculate the confinement energy of the carrier as a function of the radius of the QD. Different methods for estimating the energies of frontier orbitals in QDs produce values that differ from each other by as much as, for example, 0.5 eV for CdSe. For instance, Tvrdy et al. estimate the energy of the conduction band-edge of a 2.8 nm CdSe QD to be −3.65 eV relative to vacuum using the effective mass approximation,53 while UPS measurements performed by Jacieniak et al. place this conduction band-edge energy level at −3.15 eV relative to vacuum.77 Figure 3 is a plot of the logarithm of the measured charge separation rate constant, log(kCS), versus the estimated driving force for the reaction, (−ΔG0), for a set of QD PCS reactions reported in the literature. For all of the PCS reactions summarized in Figure 3, the QD is photoexcited and donates an electron (solid shapes) or a hole (open shapes) to an acceptor. The type of QD and the corresponding charge carrier acceptor for each reaction are identified in the legend. To make the most physically meaningful comparison between PCS rate constants measured in different laboratories, we independently calculated the value of −ΔG0 for each donor−acceptor pair in Figure 3

nanoparticles as the length of mercaptoalkanoic acid linkers connecting the CdS QDs to the TiO2 nanoparticles increased.52 Zhu et al. reported that the rate constant of PET from CdSe/ZnS core/shell QDs to a carboxylated anthraquinone acceptor decreases exponentially with increasing thickness of the ZnS shell.66 These authors also observed a slower rate of PET from CdTe/CdSe core/shell QDs to the anthraquinone acceptor than for PET from the CdTe cores alone.67 In both cases, the authors attributed the decrease in PET rate constant to a decrease in | Va,b|2 due to a decrease in the degree of overlap between the QD electron wave function and the LUMO of the anthraquinone acceptor with increasing shell thickness. Reorganization Energy. The two most important considerations in calculating λ, the total nuclear reorganization energy, for a PCS reaction in a QD−molecular system are: (i) The QD lattice is rigid relative to organic components of the system, so its contribution to the reorganization energy is usually negligible, especially in solution-phase systems.68,69 (ii) The magnitude of the outer-sphere reorganization energy is often calculated for molecular systems by considering the solvent to be a dielectric continuum and therefore depends on the dielectric constant of the environment immediately surrounding the donor and acceptor.55,70 In QD−ligand systems, the dielectric environment local to the donor−acceptor pair comprises not only the solvent but also the inorganic surface of the QD and the organic passivating ligands. The relative contributions of these various components to the local dielectric constant is difficult to determine and may require a model more detailed than a multilayer dielectric continuum. There is some evidence that the dielectric constant of the solvent plays a significant role in determining the rate of photoinduced charge transfer (PCT) in QD−ligand systems. Jones et al. developed a model based on Marcus theory to interpret charge-trapping rate constants in CdSe/CdS/ZnS core/shell/shell QDs,71 as measured by time-resolved photoluminescence spectroscopy. This model included reorganization energy as an explicit fitting parameter; the authors found that the best-fit reorganization energy increased with increasing molecular polarizability of the solvent, as predicted by the continuum model.71 Hyun et al. found that the rate constant of PET from PbS QDs to a thiolated anthracene molecule increased with increasing static dielectric constant of the solvent and did not correlate with the optical dielectric constant of the solvent.51 Applying a Marcus equation in which only static dielectric effects determine reorganization energy yielded qualitatively good fits to the data when solvent contributions were allowed to dominate the reorganization energy.51 Driving Force. The driving force for a PCS reaction, ΔG0, depends primarily on the energy difference between the donor and acceptor orbitals, but also depends on the Coulombic penalty for separating charges, given the dielectric constant of the surrounding medium. In the case of molecules, energies of donor and acceptor orbitals can usually be determined experimentally from electrochemical measurements of the relevant redox potentials.72,73 For PCS reactions involving QDs, the first challenge in determining ΔG0 is identifying which orbital on the QD is involved in the charge-transfer reaction. In systems where the rate of intraband relaxation > the rate of charge transfer > the rate of charge trapping, a delocalized band-edge orbital acts as the charge donor. Figure 2B illustrates the energy levels that determine ΔG0 for typical PET and PHT from a QD to a molecular acceptor. Participation of band-edge states in PCS can be confirmed by transient absorption (TA) measurements in E

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two exceptions are the data sets from Tvrdy et al. and Robel et al. that examined PET from CdSe QDs linked to the surfaces of metal oxides. Tvrdy et al.53 measured rate constants of PET in systems comprising CdSe QDs of various sizes directly adsorbed to transparent nanostructured films of SnO2, TiO2, and ZnO (blue triangles).53 Robel et al. measured solution-phase rate constants of PET for CdSe QDs of various sizes connected to TiO2 nanoparticles with mercaptopropionic acid linkers (blue squares).54 The observed Marcus-normal dependence of kCS on −ΔG0 for the two data sets with nanostructured semiconductor acceptors indicates that electron transfer occurs from the LUMO of the photoexcited CdSe QDs to an energy level within the conduction band of the nanostructured semiconductor that has a low density of states. If a quasi-continuum of states within the conduction band of the semiconductor acted as the electron acceptor, then −ΔG0 would not change with the energy of the donor state, and the observed kCS would be constant for CdSe QDs of various sizes. Therefore, the nanostructured semiconductor acceptors shown in Figure 3 act as molecular-type acceptors, and PET from QDs to these acceptors is comparable to PET from QDs to the molecular acceptors shown in Figure 3. We note that the higher density of states within a semiconductor acceptor may, in principle, contribute to a larger kCS for a given −ΔG0 for a semiconductor acceptor compared to a molecular acceptor, but this effect does not appear in the data shown in Figure 3. Some of the studies summarized in Figure 351,53,54,79 measured the rate constants of PCS from various sizes of QDs to the same acceptor. Changing the size of the QD is the easiest way to systematically vary the driving force for a PCS reaction involving a QD, assuming that the donor state is a delocalized state of the QD core. As the size of a QD increases, the energy of the conduction band-edge decreases, and the energy of the valence band-edge increases; thus, all other factors being equal, the rate of PET or PHT from a QD to a particular acceptor decreases as the size of the QD increases. Inspection of the data sets in Figure 3 that contain more than one point reveals a Marcus-normal relationship between kCS and −ΔG0that is, kCS increases as −ΔG0 increases. Inclusion of all of the data sets in Figure 3, however, reveals a much weaker correlation between kCS and −ΔG0. For example, two similar PCS reactions in Figure 3 that each involve photoinduced hole transfer from CdSe QDs to molecular acceptors, p-phenylenediamine (open blue circle) and a carboxylated ruthenium bipyridine complex (open blue square), have the same driving force (−ΔG0 = 0.7 eV) but values of kCS that differ by more than 3 orders of magnitude. This range of 3 orders of magnitude indicates either that factors other than driving force, such as |Va,b|2 and λ, contribute to kCS and vary widely even among similar QD−acceptor systems or that the community is not consistent in their methodology for extracting a rate constant for CS from transient dynamics. For example, some reports include statistical distributions, as described in the next section, to account for the adsorption of multiple molecular acceptors per QD and extract an intrinsic charge separation rate constant, kCS,int, that corresponds to a single QD−molecule donor−acceptor pair, while other reports do not account for these statistical contributions and only report the observed charge separation rate constant, kCS,obs. Statistical Contributions to Observed CS Rate Constants. For systems that contain molecular acceptors, one important factor that determines the magnitude of the observed PCS rate constant, and is often overlooked, is the number of adsorbed redox-active molecules per QD. Equation 4

Figure 3. Plot of the logarithm of the reported charge separation rate constant (log(kCS)) versus the driving force for the charge separation reaction (−ΔG0) compiled for pairs of QDs and charge acceptors. The driving force for each pair is calculated as described in the text (eq 2). Closed shapes denote electron transfer data, and open shapes denote hole transfer data.

using the same method, eq 2. In eq 2, EDonor is the energy relative to vacuum of the orbital from −ΔG0 = −(EAcceptor − EDonor)

(2)

which the electron originates, and EAcceptor is the energy of the orbital to which the electron is transferred. For PET from a QD to an electron acceptor, EDonor is the energy of a conduction band-edge state of the QD and EAcceptor is the energy of the LUMO of the acceptor. For PHT from a QD to a hole acceptor, EDonor is the energy of the HOMO of the hole acceptor, and EAcceptor is the energy of a valence band-edge state of the QD. To estimate the energies of QD band-edge states, we used the UPS measurements of the size-dependent energies of valence bandedge states of CdSe and PbS QDs reported by Jacieniak et al.,77 estimates of valence band-edge energies of CdS QDs from the effective mass approximation,37,38 and bandgap energies and sizes of the QDs reported by the authors of the corresponding PCS study. We determined the energy relative to vacuum of the relevant frontier molecular orbital of the charge acceptor, EMO, using eq 3, where Eredox is the reported redox potential of the acceptor relative to the EMO = −Eredox − 5.1

(3)

ferrocenium/ferrocene (Fc /Fc) couple and −5.1 eV is the Fc+/ Fc potential relative to vacuum.72 We note that many of the values of −ΔG0 used as x-coordinates in Figure 3 differ from the values of −ΔG0 reported by the authors of the corresponding PCS study. Of the PCS rate constants shown in Figure 3, most correspond to solution-phase PCS from QDs to molecular acceptors. The +

F

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Figure 4. (A) Top: Chemical structure of a viologen molecule, HVR2+, functionalized at N and N′, respectively, with a (CH2)n−COOH group and a heptane group. Bottom: Plot of log(kCS,int) versus n (blue dots) for a series of HVR2+ molecules with n = 1−3 where kCS,int is the intrinsic rate constant of electron transfer from CdS QDs to HVR2+. The n = 0 case corresponds to methyl viologen. The dotted blue line is the best-fit dependence of log(kCS,int) on n assuming the electron transfer occurs via tunneling through the alkyl chain (“through-bond”) and kCS,int decreases exponentially with an attenuation factor, β, of 1.04 Å−1. (B) Illustration of the charge-transfer pathways available for electron transfer from CdS QDs to carboxylated viologen ligands with extended (Geometry 1) or bent (Geometry 2) alkylcarboxylate chains. The insensitivity of kCS,int to n demonstrated in (A) indicates that charge transfer occurs only through the pathway defined by the green arrow to viologen ligands bound to the surface in Geometry 2. Adapted with permission from ref 83. Copyright 2012 American Chemical Society.

k CS,obs = nk CS,int

constant and mischaracterize the influence of these parameters on the PCS rate constant.

(4)

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defines the relationship between the observed rate constant of charge separation, kCS,obs, and the intrinsic single donor−single acceptor charge separation rate constant, kCS,int, for a QD with n bound acceptors. Within an ensemble of QDs, not every QD has the same number of adsorbed acceptors or even the same number of available adsorption sites. Morris-Cohen et al.80 and Sadhu et al.81 developed models for determining kCS,int from transient absorption or photoluminescence measurements that use binomial and Poisson models, respectively, to describe the distribution of the number of potential charge acceptors adsorbed per QD within an ensemble of QDs. These models enable the simultaneous determination of kCS,int (the rate constant that should be correlated to ΔG0 in Figure 3) and the adsorption equilibrium constant, Keq, for a particular QD− acceptor system.37 Factors that affect the average number of acceptors adsorbed per QD in solution include the relative concentrations of QDs and acceptors, the surface composition of the QDs (i.e., the degree of cation or anion enrichment), the density of native ligands on the surfaces of the QDs, and the adsorption constant of the native ligands. The latter three factors determine the number of available adsorption sites per QD for added redox-active molecules and can be affected by the size andimportantlythe absolute concentration80 of the QDs. Like the number of adsorbed acceptors per QD, the number of available adsorption sites per QD is a binomially distributed variable; thus, a complete description of the adsorption of molecular acceptors within an ensemble of QDs requires the use of two nested binomial distributions.80 The necessity of including both binomial distributions in this analysis is apparent in the dependence of the efficiency with which a molecular charge acceptor quenches the PL of a QD sample on the absolute concentration of the QD sample.80,82 This dependence indicates that dilution of QDs changes their surface composition by changing the adsorption equilibrium of the active ligands. Analyses that aim to evaluate the influence of variables such as QD size, linker length, and solvent on the Marcus parameters ΔG0, |Va,b|2, and λ, but fail to control for the number of adsorbed acceptors per QDand therefore the number of available CS pathways per QDwill potentially overestimate the PCS rate

CHARGE TRANSFER AS A PROBE OF THE STRUCTURE OF THE QD−ORGANIC INTERFACE The intrinsic rate constant of PCS from QDs to molecular redox partners, kCS,int, obtained by accounting for the number of bound acceptors per QD in fits to transient dynamics, is the rate constant that is directly related to the Marcus parameters for the system and therefore can be used as a probe of the interfacial structure of a QD−organic complex. Morris-Cohen et al., using the binomial distribution model referenced above, measured kCS,int for PET from CdS QDs to a series of substituted viologen acceptors that differed in the number of methylene groups separating the carboxylate binding group from the redox-active viologen moiety (Figure 4A).83 While previous studies found that PCS rate constants generally decrease as the length of the linker between the binding headgroup and redox-active moiety increases,52,64 in this case, kCS,int did not change with increasing linker length (Figure 4A). The dotted line in Figure 4A depicts the dependence of kCS,int on the number of methylene groups in the linker predicted by treating the trans-extended alkyl linker as a tunneling barrier. The insensitivity of the rate constant of electron transfer from CdS QDs to substituted viologens to the length of the alkyl linker indicates that the PET occurs via a “through-space” rather than a “through-bond” pathway (Figure 4B), and suggests that the viologens adsorb to the QD in a geometry in which the linker does not separate the QD surface from the redox-active viologen moiety.83 Figure 4B illustrates a possible adsorption configuration in which the bipyridyl rings of the viologen are oriented parallel to the QD surface (the molecule is shown in a “side-on” orientation for clarity but is probably in a cofacial geometry). In addition to the adsorption geometry of molecular acceptors, measurement of intrinsic PCS rate constants can also provide insight into the conformation of the native organic ligand shell that passivates and solubilizes colloidal QDs. Tagliazucchi et al. measured rate constants of PET from CdSe QDs passivated with mercaptoalkanoate ligands of various lengths to polyviologen within multilayer films.84 The authors observed decreasing PET rate constants with increasing length of the mercaptoalkanoate surface ligand, which indicates that this surface ligand acts as a G

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Figure 5. Left: Normalized kinetic traces extracted at the peak of the GS bleach of the QDs from the transient absorption spectra of (a) oleate-coated PbS QDs (OL-PbS) and (b) decanethiolate-coated PbS QDs (DT-Pbs) in toluene solutions containing various concentrations of aminoferrocene (amFc). The GS bleach decays faster as [amFc] increases for OL-PbS QDs and is insensitive to the addition of amFc for DT-PbS QDs. Right: 1DNOESY (color line) and standard 1H NMR (black line) spectra of (c) OL-PbS and (d) DT-PbS in air-free toluene-d8 solutions containing amFc. The asterisks in (c) mark 1D-NOESY cross-peaks that indicate through-space coupling between protons in oleate ligands and the selectively excited Cp5 protons on amFc. The 1D-NOESY spectrum of DT-PbS and amFc does not contain such cross-peaks. Adapted from ref 86 with permission from the Royal Society of Chemistry.

packed (either in a liquid-like or crystalline motif) than OA chains on the surface of a PbS QD and thus do not present any gaps large enough to allow amFc to penetrate the ligand shell, and (ii) the thiolate headgroup of DT forms a much stronger bond to Pb2+ than does the carboxylate headgroup of OA, so amFc cannot effectively compete with DT for available binding sites on the QD. While it is currently unclear which of these mechanisms dominates, this work clearly demonstrates that the structure of the organic ligand shell plays an important role in the PCS behavior of QD−acceptor systems. Most PCS reactions in QD−ligand systems require a molecular acceptor to be adsorbed to the surface of the QD in order to extract a charge carrier before the exciton recombines; however, if the exciton lifetime is long enough, then collisional charge transfer between photoexcited QDs and freely diffusing solution-phase acceptors can also occur. The lifetime of an exciton in PbS QDs ranges from 1 to 3 μs;87,88 this time scale is long enough to enable both collisionally gated and static PET from solution-phase PbS QDs to freely diffusing and adsorbed benzoquinone (BQ) molecules, respectively.89 Less than 1% of collisions between PbS QDs and BQ molecules result in PET, which indicates that the diffusing BQ molecules have to permeate through the oleate ligand shell to a location close enough to the inorganic core of the QD to participate in PET before exciton recombination. This observation provides further evidence for the role of the organic ligand shell in determining the accessibility of the inorganic QD core to molecular redox partners. The lifetime of an exciton in CdSe or CdS QDs is 2 orders of magnitude shorter (∼10−50 ns)90 than in PbS QDs and thus

tunneling barrier for the PET reaction in these systems. Molecular models of the ligand shell structure suggest that, for mercaptoalkanoates containing up to seven carbons, a collapsed monolayer structure provides the best fit to the observed dependence of PET rate constants on mercaptoalkanoate chain length, while trans-extended structures provide the best fits for mercaptoalkanoates containing 10−15 carbons.84 This result suggests a “crystallization” of molecular adlayers of QDs with increasing chain length, as is observed for self-assembled monolayers on flat surfaces.85 Malicki et al. demonstrated that the structure of the native ligand shell influences the ability of a small molecular redox partner to access the surface of a QD.86 Addition of aminoferrocene (amFc) to a solution of PbS QDs passivated with oleate (OA) increases the rate of decay of the ground-state bleach feature in the TA spectrum (Figure 5A) due to PHT from the QD to amFc. Nuclear Overhauser effect spectroscopy (NOESY NMR) indicated that amFc molecules adsorb to the surfaces of PbS QDs passivated with OA (Figure 5C). In contrast, for QDs of the same size passivated with decanethiolate (DT), addition of amFc does not affect the TA spectrum (Figure 5B), and the 1-D NOESY NMR spectrum does not contain any evidence for surface-bound amFc (Figure 5D). PHT activity is thus correlated with adsorption of amFc to the surface of the QD. Quantitative 1H NMR measurements reveal similar numbers of bound OA and DT native ligands, so a difference in number density of the passivating ligands is not responsible for the difference in PHT behavior.86 Two hypotheses for the mechanism governing the change in PHT activity upon ligand exchange from OA to DT are: (i) DT chains are more tightly H

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Figure 6. (A) Transient absorption spectra of CdSe/CdS core/shell QDs with adsorbed methyl viologen (MV2+) electron acceptors 6−10 ps after photoexcitation of the QD with various excitation intensities. The absorption feature centered at ∼630 nm corresponds to reduced methyl viologen (MV+•). (B) Plots of the transient signal extracted at 630 nm for various excitation intensities. The amplitude of this signal corresponds to the average number of reduced MV+• species per QD. The traces are normalized such that the magnitudes of the GS bleach at long time scales, when only one exciton per QD remains, are equivalent across all excitation intensities. (C) Plots of the average number of photons encountered by each QD (black line), the average number of excitons generated per QD (red triangles), and the average number of reduced MV+• species produced per QD (green circles) versus the excitation intensity. Adapted with permission from ref 102. Copyright 2012 American Chemical Society.

in QDs. The decay of the GS bleach feature in TA spectra of QDs can be used to monitor the dependence of the exciton dynamics on excitation energy. At energies greater than the bandgap but less than twice the bandgap energy, the kinetics of the GS bleach feature do not depend on excitation energy because intraband relaxation of hot carriers is typically much faster than interband relaxation.26,116 At some threshold energy that is greater than twice the bandgap energy and that depends on the nature of the sample, a fast decay component appears in the GS bleach kinetics. This fast decay component corresponds to pairwise recombination of multiple excitons via Auger recombination, and its relative amplitude is proportional to the number of additional excitons generated per QD.97 Carrier multiplication has already enabled the fabrication of QD solar cells with internal117 and external118 quantum efficiencies greater than 100%. Sambur et al. used photocurrent spectroscopy to measure quantum yields greater than 100% in systems comprising PbS QDs of various diameters coupled to TiO2 single crystals.117 The absorbed photon to current efficiency exceeded 100% at ∼2.9 times the bandgap energy, but the incident photon to current efficiency was low due to the optically thin single layer of QDs on the anatase TiO2 electrode. Semonin et al. developed a PbSe-based solar cell with an internal quantum efficiency of ∼130% and an external quantum efficiency of ∼114%.118 The device consisted of indium tin oxide coated successively with 40−60 nm ZnO, sequential layers of PbSe QDs treated with either 1,2-ethanedithiol or hydrazine, and a thermally deposited thin gold anode. Solution-phase QDs containing multiple excitons generated from either carrier multiplication or multiphoton absorption can transfer electrons to several adsorbed acceptor molecules. Extraction of multiple carriers must occur faster than Auger recombination, which depends on both the carrier concentration and size of the QD; a typical lifetime for Auger recombination in CdSe QDs is ∼50 ps.104,119 Yang et al. used TA spectroscopy to measure carrier multiplication in PbS QDs followed by electron transfer to multiple adsorbed methylene blue molecules at an excitation energy ∼2.9 times the bandgap energy.120 They confirmed the mechanism of carrier multiplication by performing TA measurements with a combination of different excitation energies and intensities. Fast recovery dynamics are only observed in the GS bleach at low intensities for high energy photons, which is a signature of carrier multiplication. The number of electrons transferred to methylene blue was determined from the amplitude of the methylene blue radical absorption feature produced upon electron transfer. The authors

does not allow enough time for collisional charge transfer to occur.

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APPLICATION OF INTERFACIAL CHARGE TRANSFER TO SOLAR ENERGY CONVERSION: MULTIELECTRON CHARGE-TRANSFER REACTIONS A fundamental difference between QDs and organic chromophores is the ability of a QD to mediate multiple charge-transfer reactions due to the degeneracy of its lowest-energy excitonic state, which allows a QD to accommodate multiple excitons or injected charge carriers, and the availability of multiple sites for charge acceptors to adsorb to QD surfaces. Understanding and controlling the rates and mechanisms of multielectron transfer reactions in QD/organic complexes is essential for developing QD-based systems to address the most important problems in conversion of solar energy into electrical and chemical energy. For example, the theoretical maximum thermodynamic efficiency calculated for single junction solar cells increases from 31%91 to 45%92,93 for cells capable of producing multiple electrons from a single photon because when a photon of energy greater than the bandgap is absorbed in a conventional solar cell the excess energy is lost as heat, but carrier multiplication utilizes the excess photon energy to create a second charge carrier. The multiply degenerate band-edges of QDs are also potentially useful for storing multiple electrons or holes at similar redox potentials to be used as needed in multielectron photocatalytic processes, such as water splitting.94,95 There are two different processes by which multiple excitons can be generated in a QD: (i) Carrier multiplication, in which absorption of a single photon with energy greater than twice the bandgap energy produces multiple excitons. The three proposed mechanisms for carrier multiplication are impact ionization (the quantum mechanical inverse of Auger recombination),96,97 dephasing of a coherent superposition of single and multiexciton states,98,99 and direct excitation into a multiexciton state.100,101 (ii) Absorption of several photons with energy less than twice the bandgap energy can create multiple excitons per QD if all the photons are absorbed faster than the Auger recombination rate,102,103 which increases as the number of excitons per QD increases.104 Observations of carrier multiplication in QDs have been reported for PbSe,97,98,105 PbS,98 PbTe,106 CdSe,107,108 CdTe,109 InP,110 InAs,111 Si,112,113 and core/shell QDs.114,115 Transient absorption (TA) is the most common spectroscopic technique used to identify and characterize carrier multiplication processes I

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nanoparticle to solution-phase reductants that then diffuse away from the particle produce up to tens of electrons that live indefinitely in delocalized conduction band or surface-trapped states of the particle.123−125 These “photocharged” particles can then be used as reagents for multielectron reduction of O2 and H2O2 to water, reduction of nitrate ions to ammonia,123 and proton-coupled electron transfer reactions to phenoxyl and nitroxyl radicals.124,125 Spontaneous charge transfer to multiple adsorbed acceptors can also produce indefinitely stable QDs that contain multiple excess charges. For example, spontaneous electron transfer from sulfur atoms on the surfaces of PbS QDs in the ground electronic state to adsorbed tetracyanoquinodimethane (TCNQ) molecules produces an average of up to 4.5 stable charge-separated states per QD, where each chargeseparated state comprises an oxidized sulfur atom and a reduced TCNQ molecule.126 Core/shell nanoparticles, where the core is the charge donor and the shell is the charge acceptor, spatially separate excitonic charge carriers within the same nanoparticle. A so-called “type-II” band alignment in which the conduction and valence band-edge states of the shell are both lower in energy than the conduction and valence band-edge states of the core enables an internal PCS process that localizes the electron to the shell and the hole to the core.67 Zhu et al. observed that internal PCS in CdTe/CdSe core/shell QDs facilitates subsequent electron transfer from the CdSe shell to adsorbed anthraquinone acceptors, while the presence of the shell inhibits charge recombination with the hole localized in the CdTe core.67 In most semiconductor materials, the electron and hole have different effective masses and therefore different degrees of localization or “confinement” within the QD;5,127 these properties can also be used, in addition to band alignment, to spatially separate the electron and hole within core/shell nanoparticles. For example, Zhu et al. also observed a disproportionately large decrease in the charge recombination rate constant (compared to the decrease in charge separation rate constant) with shell thickness in type-I CdSe/ ZnS core/shell QDs with adsorbed anthraquinone acceptors.66 In this case, both the electron and hole delocalize into the shell, but the smaller effective mass of the electron in ZnS enables it to delocalize further into the shell than the hole. The ZnS shell thus presents a lower tunneling barrier to the electron than to the hole as measured by the exponential attenuation factor β, which is a factor of 3 smaller for charge separation (electron transfer from the CdSe/ZnS QD to anthraquinone) than for charge recombination (hole transfer from CdSe/ZnS to reduced anthraquinone).66

found that 1.1 electrons per QD were transferred to adsorbed methylene blue molecules with a charge-transfer lifetime of less than 2.3 ps.120 Matylitsky et al. reported the generation of four excitons per CdSe QD and the subsequent transfer of all four electrons to adsorbed methyl viologen molecules in ∼70 fs, when exposed to high fluence light with an energy equal to 1.4 times the bandgap energy.121 The simultaneous recovery of the GS bleach feature and appearance of a positive absorption feature corresponding to the viologen radical cation in TA spectra of CdSe QDs confirmed the occurrence of ultrafast charge transfer. Upon absorption of multiple low energy photons, Huang et al. observed the transfer of up to three electrons from a single CdSe QD to adsorbed methylene blue molecules in ∼2 ps.119 The authors observed a maximum number of transferred electrons at an absorption expectation value of about five photons per QD. At this saturation point, the number of reduced methylene blue molecules was roughly equivalent to the number of adsorbed methylene blue molecules, which indicates that the number of transferred electrons was limited by the number of adsorbed acceptors and not by the number of electrons in the QD available for transfer. Zhu et al. recently reported the generation of 19 excitons per QD in quasi type-II CdSe/CdS core/shell QDs by multiphoton absorption and the transfer of all 19 electrons to adsorbed methyl viologen (MV2+) molecules.102 The authors quantified the number of transferred electrons through comparison of the kinetics and amplitudes of the features of the methyl viologen radical cation (MV+•) and the QD ground state bleach. Figure 6 summarizes the TA data used to quantify the number of electrons transferred to adsorbed MV2+ molecules in this study. The same group also reported the transfer of 21 electrons to methyl viologen from CdSe quantum rods in ∼59 fs following multiphoton absorption.103 This ultrafast electron transfer is much shorter than even the intraband relaxation rate, which suggests that hot carriers are extracted before they relax to the band edge. The fast multielectron transfer processes described above represent a promising step toward using QDs in multielectron photocatalytic systems; however, all of these examples involved transfer of multiple electrons to multiple acceptors adsorbed to the surface of the QD. The next step toward effective QD photocatalytic systems is to demonstrate transfer of multiple charge carriers to the same acceptor in either a concerted or sequential charge-transfer reaction with controlled rate constants.

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EXTENDING THE LIFETIME OF THE CHARGE-SEPARATED STATE Achieving exciton dissociation in QD−ligand complexes is only useful if the separated charge carriers are extracted by an electrode in a photovoltaic system or used to reduce or oxidize a substrate in a photocatalytic system before charge recombination occurs. In addition to understanding how to control the rate of exciton dissociation, we also need to control the lifetime of the resulting charge-separated state. Charge separation lifetimes on the order of microseconds have been achieved in multichromophoric molecular systems by decreasing the electronic coupling between the electron and hole, either by increasing the donor−acceptor distance or manipulating the conformation of the bridging groups.9,61,122 One way to inhibit charge recombination in nanoparticle− ligand systems is to use a molecular acceptor that diffuses away from the QD after charge transfer. For example, successive photoinduced hole transfer reactions from a TiO2 or ZnO

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CONCLUSION Quantum dots and molecules each have their own advantages for conversion of light energy to electrical or chemical energy. Their combination is very powerful, not only because the composite material has characteristics of each but also because we can design architectures in which each enhances the other’s properties. Interfacial charge transfer is both a demonstration of control and a probe of the structure of the nanoscale inorganic/organic interface between a QD and molecules bound to its surface. Once we achieve this control, there are wellestablished theoretical models we can adapt to optimize chargetransfer processes in hybrid materials as we have in covalent organic systems. New synthetic and analytical approaches to QD−organic complexes and creative design of ligands for QDs are pushing this very interdisciplinary field toward truly functional QD-based energy conversion materials. J

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AUTHOR INFORMATION

Martin McPhail received his B.S. in chemistry from the Missouri University of Science and Technology in 2010 where he worked under the advisement of Charles Chusuei. He is currently a Ph.D. candidate in Emily Weiss’s group at Northwestern University. His research focuses on the influence of hierarchical organization on the charge carrier dynamics of self-assembled nanoparticle structures.

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies

Emily Weiss received her A.B. in Chemistry from Princeton University in 2000 and her Ph.D. in Chemistry from Northwestern University in 2005, upon completing her dissertation work on superexchangemediated electron transfer in organic donor−acceptor systems with Michael Wasielewski and Mark Ratner. She was a Petroleum Research Fund Postdoctoral Energy Fellow with George Whitesides at Harvard University from 2005 to 2008 and began her independent career at Northwestern in 2008. Her group (http://sites.weinberg.northwestern. edu/weiss-lab/) focuses on analytical and spectroscopic studies of the surface chemistry of semiconductor nanostructures and its influence on their photophysics.

Kathryn Knowles received a B.S. in chemistry and a B.A. in mathematics from the University of Rochester in 2008. She is currently a Ph.D. candidate in chemistry at Northwestern University where she studies the relationship between surface chemistry and exciton dynamics in colloidal semiconductor quantum dots in Emily Weiss’s group.

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ACKNOWLEDGMENTS This work was supported by the David and Lucile Packard Foundation through a Packard Foundation Fellowship for Science and Engineering to E.A.W. K.E.K. is supported by the Department of Energy Office of Science Graduate Research Fellowship Program (DOE SCGF), made possible in part by the American Recovery and Reinvestment Act of 2009, administered by ORISE-ORAU under contract no. DE-AC05-06OR23100.

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Mark Peterson graduated from the University of California, Irvine in 2009 after performing research on the hygroscopic growth and deliquescence of NaCl nanoparticles with Professor Sergey Nizkorodov. His research in the Weiss group at Northwestern University focuses on the charge-transfer properties of quantum dot−organic complexes, with applications in energy, medicine, and photocatalysis.

REFERENCES

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