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Article Cite This: Acc. Chem. Res. 2018, 51, 2565−2573

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Directing Charge Transfer in Quantum Dot Assemblies Brian P. Bloom,† Ruibin Liu,‡ Peng Zhang,‡ Supriya Ghosh,† Ron Naaman,§ David N. Beratan,‡,∥,⊥ and David H. Waldeck*,† †

Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States Department of Chemistry, Duke University, Durham, North Carolina 27708, United States § Department of Chemical and Biological Physics, Weizmann Institute, Rehovot 76100, Israel ∥ Department of Physics, Duke University, Durham, North Carolina 27708, United States ⊥ Department of Biochemistry, Duke University, Durham, North Carolina 27710, United States

Acc. Chem. Res. 2018.51:2565-2573. Downloaded from pubs.acs.org by REGIS UNIV on 10/16/18. For personal use only.

‡

CONSPECTUS: The optical and electronic properties of semiconductor quantum dots (QDs) make them attractive candidates for applications in photovoltaics, spintronics, photocatalysis, and optoelectronics. Understanding how to control the flow of charge in QD assemblies is essential for realizing novel applications. This Account explores some unique characteristics of charge transport in QD dyads, triads, and their assemblies. The emerging features of these assemblies that provide new opportunities to manipulate charge flow at the nanoscale are (1) cascading energy landscapes and band offsets to inhibit charge recombination, (2) electrostatic fields that direct charge flow through QD− QD and QD-conjugated polymer junctions, and (3) QD chirality and chiral imprinting that promotes vectorial electron and spin selective transport. Charge flow kinetics is determined by a combination of familiar electron transfer parameters (reaction free energy, reorganization energy, and electronic coupling), donor and acceptor electronic densities of states, and internal electric fields. Electron transfer and electronic structure theory, combined with kinetic modeling, place the measured kinetics of QD electron transfer donor−acceptor assemblies into a unified conceptual context. The experimental transfer rates measured in these systems depend upon structure and the internal electric fields that are present in the assemblies. A negatively charged donor and positively charged acceptor, for example, facilitates (inhibits) electron (hole) transfer, while an electric field of opposite orientation (reversal of charges) inhibits (promotes) electron (hole) transfer. These and other emerging rules that govern charge flow in NP assemblies provide a strategy to design the directionality and yield of interfacial charge transport. Chirality at the nanoscale can induce spin selective charge transport, providing new ways to direct charge (and spin) flow in QD assemblies. Magnetoresistance and magnetic conductive probe atomic force microscopy experiments show spin selective electron transport for chirally imprinted QD assemblies. Photoinduced electron transfer from achiral donor-QDs to chiral acceptor-QDs depends on the electron spin and chiroptical properties of the acceptor-QDs. These assemblies show transport characteristics that correlate with features of the QDs’ circular dichroism spectra, presenting intriguing challenges to theory, and indicating that spectroscopic signatures may assist in the design and diagnosis of functional molecular assemblies. Theoretical and experimental studies of charge transport in well-defined QD assemblies are establishing design principles for vectorial charge transport and are also refining questions surrounding the mechanism and control of these processes. These intensified efforts are forging links between fundamental discoveries regarding mechanism and practical applications for these novel assembled nanostructures.

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INTRODUCTION Future energy capture and conversion schemes based upon assembled functional nanomaterials are likely to rely on an ability to achieve precise control of competing electronic processes. ‘“Photovoltaic paint”, based on organic−inorganic bulk heterojunction photovoltaic composites, offers the promise to reduce the cost of solar-to-electric energy transduction. For example, quantum dot dyads, composed of quantum dot (QD) pairs that form a staggered band edge energy alignment and possess an amphiphilic surface chemistry © 2018 American Chemical Society

(e.g., one hydrophilic-QD and one hydrophobic-QD), can function as a photoactivated charge transfer element that binds at the interface between the cathodic and anodic phases of a conjugated polymer (CP) composite and drives charge separation. The performance of hybrid organic−inorganic bulk-heterojunction photovoltaics lags behind that of QD solar cells and organic bulk heterojunction cells, largely because of Received: July 16, 2018 Published: October 5, 2018 2565

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Figure 1. Schematic of a molecular dyad assembly on a colloidal template (A) and the staggered type-II heterojunction inset (top right; green is CdSe and red is CdTe band edges). (B) STEM micrograph of an example assembly. Scale bar is 50 nm, and inset shows a 2-fold magnification of the original micrograph (left) and enhanced with an FFT filter (right). (C) Dependence of kET on the reaction Gibbs energy, −ΔrG. Black squares are the experimental data, and error bars are representative of the full width at half-maximum of the distribution fitting. The data set was fit to the semiclassical nonadiabatic equation (red curve) by summing over the first two electronic levels on the acceptor-QD. Reproduced with permission from ref 24. Copyright 2016 American Chemical Society.

rectifying interface;16 tandem cells based on state-of-the-art perovskite and colloidal quantum dot designs are proposed to provide even greater efficiencies because of their increased light harvesting capabilities.17 Electric fields can be used to tune charge transport in heterojunction structures by changing tunneling barrier heights and transport thermodynamics, thus altering charge-carrier mobilities.18 While the effects of external electric fields on solar cell devices are well-known,19 the influence of internal electrostatic fields (inside of the active material) is less well explored. Yaacobi-Gross et al. studied solar cells with QDs passivated with amine and thiol ligands.20 The ligand induced electric field imparted charge carrier directionality and increased the exciton mobility in the device, leading to a 37% enhancement in external quantum efficiencies compared to single ligand passivated QD devices. Studies of electrochemistry, photoemission, single molecule conductance, and magnetoresistance show that molecular chirality makes electron transport spin-dependent.21−23 The chirality induced spin selectivity (CISS) effect,22,23 arising from the coupling between the linear momentum of the electron and its spin orientation, can break the degeneracy of the electron spin states, exhibiting differences in charge separation and charge recombination rates for transport in chiral systems. This asymmetry appears to be dynamical in origin and offers promise for directing spin selective electron transfer in QD assemblies; an important first step for spintronic type applications. This Account describes our recent progress in enhancing the charge transfer and charge separation efficiency in QD assemblies by employing (1) energy cascades, (2) built-in electrostatic fields, and (3) chiral symmetry.

challenges in facilitating efficient charge separation at the nanoscale interfaces. The photoinduced charge separation efficiency depends on the photogeneration of electron−hole pairs and their subsequent spatial separation while avoiding charge recombination. By understanding the fundamental principles of charge transfer in organic−inorganic materials (QD−QD and QD−CP interfaces), we are elucidating new design principles that promise to boost the efficiency of low cost emergent technologies which may arise from these functional nanostructures. A fundamental understanding of charge transfer at the molecular scale has emerged from extensive studies of covalent and noncovalent donor−bridge−acceptor assemblies.1−3 Those studies validated the predictions of Marcus electron transfer theory,1 while exposing the critical role of fluctuations and solvent polarization dynamics in these reactions.4 This Account extends our understanding of bridge-mediated charge transport to nanoscale assemblies that incorporate semiconducting QD donors and acceptors. We discuss some of the new emergent design characteristics of these assemblies, including their electrostatic fields and chiral symmetries, that can be used to enhance their photoinduced charge separation efficiencies. The tunable electronic, optical, and surface chemistry properties of semiconducting QDs make them appealing candidates for applications in photovoltaics, catalysis, and spintronics. Energy cascades, resulting from staggered energyband alignment in QD films, are known to improve charge transport,5−7 and the properties of QD assemblies can be engineered by the choice of QD composition,8 sizes,9,10 surface ligands,10,11 and Fermi level pinning.12−15 Unidirectional charge transfer in photovoltaic devices was demonstrated by using energy band alignment of CdSe-CdTe QD layers.5 Chuang et al. used ligand induced energy-band shifts for PbS QDs to create a type II heterojunction in photovoltaic cells that exhibit >8% efficiency,6 and an 11.28% efficient cell was constructed by manipulating the energy alignment at a

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MARCUS DESCRIPTION OF CHARGE TRANSFER IN QD ASSEMBLIES The design motif shown in Figure 1A was used to establish a quantitative understanding of QD-to-QD charge transfer.24 2566

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Figure 2. (A) Geometry for a QD dyad. The z-axis joins the QD centers. One effective bridge orbital appears at the origin. (B) Plot showing contour lines at fixed values of log10(kTS/kTB) as a function of CdTe (donor) and CdSe (acceptor) radii. (C) Plot showing contour lines at fixed values of log10(kET/kHT) as a function of CdTe and CdSe radii. (D) Energy scheme of a QD triad based device sandwiched between ϒ and Λ electrodes. Images are adapted with permission from refs 27 and ref 30. Copyright 2017 and 2018 American Chemical Society.

kTS, respectively.27 Figure 2A shows the coordinates used to describe the positions for QDs of radii r1 and r2, separated by a center-to-center distance of 2a.27 A contour plot of the log10(kTS /kTB) is shown as a function of the donor-QD and acceptor-QD sizes in Figure 2B. kTB is larger than kTS for small QD radii. As the QD radii increase, kTS dominates because the number of interatomic interactions that contribute significantly to the through-space coupling increases, while the number of through-bond interactions is fixed. In addition, the wave function amplitudes on the QDs in the linker group attachment regions shrink with increasing QD radius. Both the through-space and through-bond electronic couplings between QDs decrease approximately exponentially, each with a similar distance decay constant (measuring edge-toedge), and the QD distance does not significantly influence the relative magnitudes of the through-space and through-bond contributions to the overall electron transfer rate. Because electronic coupling, reorganization energy, and ΔrG depend on QD sizes, they produce size-dependent electron transfer rates in QD−QD assemblies. The dependence of the reorganization energy and −ΔrG on the QD radii is well understood in the context of classical electrostatics. However, the through-space and through-bond couplings depend differently on the QD radii. This difference produces a switch in the dominant coupling mechanism as the QD radii change. The acceptor density-of-states plays a crucial role in determining kET as a function of the QD radii, making it necessary to include a sum over final states of the acceptorQD, unlike the case in small molecules. Importantly, the densities-of-states of the CdTe hole acceptor-QD and of the CdSe electron acceptor-QD depend differently on their radii, which can lead to an imbalance between kET and kHT (hole transfer rate constant) in CdTe−CdSe dyads.

CdSe QDs (red circles) were assembled electrostatically onto a SiO2 colloidal template to which a second CdTe QD (green circles) of opposite charge was covalently linked using carbodiimide chemistry. Figure 1B shows a STEM image of the structures. The QD sizes, and their capping ligands, enable the formation of type-II heterojunctions (Figure 1A, top right) in a donor−bridge−acceptor QD−QD dyad. The CdTe QD serves as the donor, and upon photoexcitation an electron is transferred to the CdSe acceptor-QD. The electron transfer rate, kET (calculated from time-resolved fluorescence), decays exponentially with the QD-to-QD distance (β = 0.68 per methylene unit), and the dependence of kET on the energy band offset is consistent with Marcus theory (Figure 1C).24 As ΔrG becomes more exergonic, both the Se and Pe electronic states of the CdSe QD can accept electrons, and a sum over final states is required to compute the overall electron transfer rate; this feature masks the inverted free energy effects that are found in molecular charge transfer systems. Note that other models have also been used to describe charge transfer in which the inverted Marcus regime is masked, but they are not required for a good fit to these data.25 These findings indicate that our understanding of electron transfer in molecular donorbridge-acceptor assemblies can be generalized to describe charge flow in QD−QD assemblies. The tunneling decay exponent (β) of 0.68 per methylene, is smaller than ∼1 per methylene found for molecular systems.26 Quantum chemical modeling of QD clusters predicts β = 0.46 per methylene, in reasonable agreement with the experiment (see ref 27 for more details on kET calculations).27 A tightbinding model (parametrized via DFT calculations of QD clusters) finds that both through-bond (TB) and throughspace (TS) interactions contribute to the QD-donor QDacceptor electronic coupling, with corresponding rates kTB and 2567

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Figure 3. (A) Photoluminescence decay of MPA-CdTe QDs in solution (black) and upon addition of TMA-CdSe (red) and DEA-CdSe (blue) QDs. (B) Photoluminescence decay of DEA-CdTe QDs in solution (red) and upon addition of MPA-CdSe (blue) QDs. (C) Structure of ligands used to cap the QDs; mercaptopropionic acid (MPA), N,N,N-trimethyl(11-mercaptoundecyl)ammonium (TMA) and N,N-dimethyl-2aminoethanethiol (DEA). (D) Diagram illustrating how an interfacial electric field affects the electron transfer kinetics of QD aggregates. Reproduced with permission from ref 31. Copyright 2010 American Chemical Society.

Figure 4. Energy diagram of the PDPPPV and CdTe heterojunction and a diagram illustrating the interfacial electric field resulting from their assembly (A). Ultrafast time-resolved transient absorption measurements of the bleach recovery at the absorption maximum of positively (B) and negatively (C) charged QDs before (red trace) and after (blue trace) the addition of negative (B) and positive (C) PDPPV. Adapted with permission from ref 32. Copyright 2014 Royal Society of Chemistry.

Optimized solar cells based on QDs should have approximately equal kET and kHT constants.28 Imbalance in kET and kHT slows the charge separation kinetics, enhancing charge recombination and diminishing device efficiency. Figure 2C shows how the model may be used to identify size regimes where kET ≈ kHT. The zero line for log10(kET/kHT) represents a RCdTe:RCdSe radius ratio of ∼1.3 and is determined by the QDs’ density-of-states and QD−QD electronic couplings.

Combining nonadiabatic electron transfer theory in the high temperature (Marcus) limit with a kinetic master equation analysis,29 we explored the benefits of using multi-QD assemblies as solar energy harvesting and conversion engines. We found that the external and internal power conversion efficiencies of QD dyads (CdTe−CdSe) can be significantly improved by introducing a third QD between the donor and acceptor dots, creating a QD triad assembly, Figure 2D.30 The improved efficiency depends on the band-edge energy offset of 2568

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assemblies should inhibit kET (see Figure 3D), the electron transfer is facile, with a rate of 1010 s−1. Why does the electrostatic field effect differ for covalent assemblies compared to electrostatic driven aggregates? The reduction in the electric field effect may arise from a difference between the electronic couplings and their associated effective tunneling barriers in the different structures. For the aggregates, the electronic coupling between donor and acceptor is weak and must proceed through an electrostatic junction, whereas the covalent dyad can use through-bond coupling pathways that “bypass” the unfavorable electrostatic junctions. These differences in coupling underscore the rich characteristics of charge transfer that can be accessed in nanoparticle assemblies.

the middle QD (M-QD) from those of its two neighbors. In general, the triads exhibited open-circuit voltages and shortcircuit currents up to 17% and 400% larger than those of the dyads, respectively. Greater internal and external conversion efficiencies arise when the M-QD band edge energies facilitate charge separation and inhibit charge recombination. The valence band edge energy of the M-QD should be tuned to enhance hole transfer and to slow the charge recombination from the conduction band of the acceptor-QD (R-QD in Figure 2D) to the valence band of the M-QD. The conduction band edge energy of the M-QD should be tuned to slow the back electron transfer from the R-QD to the L-QD when the charge recombination rates are slow, and should kinetically inhibit charge recombination between the M-QD and L-QD when the charge recombination rates are fast. Positioning the conduction band of the M-QD at an optimum value is the most important consideration in maximizing device efficiency.

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CHIRALITY AND SPIN TRANSPORT Electron transport through a chiral structure breaks the charge transport degeneracy of spin 1/2 and −1/2 electrons, and provides a new strategy to direct current flow with chiral QDs. Recent studies show that chiral ligands can induce circular dichroism (CD) and optical rotatory dispersion (ORD) on QDs.33−37 Upon attachment of cysteine ligands to the surface of CdSe QDs, a bisignate peak is induced on the excitonic transitions of the QD. Figure 5A shows mirror image CD

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ELECTRIC FIELD EFFECTS ON CHARGE FLOW How does an internal electric field influence the charge transfer characteristics of QDs? To answer this question, we monitored kET between QD aggregates in solution involving CdTe donorQDs attached electrostatically to CdSe acceptor-QDs; see Figure 3D.31 The charge on a colloidal QD is controlled by the surface ligands used to stabilize it, and by the solution pH and ionic strength. Because the ligand shell’s electrostatic charge did not shift the measured redox potentials of the QDs, the pH of the solution was used to control the surface charge of the QDs and to produce aggregates of similar sizes. Photoexcitation of the CdTe QDs with a negative surface charge causes facile electron transfer to CdSe QDs that have a positive surface charge (Figure 3A). For DEA-coated CdSe QDs (blue trace) the excited state lifetime was quenched more strongly than for TMA-coated CdSe QDs (red trace), because of the change in electron transfer distance of the donor−acceptor aggregates. When the charge on the donor-QDs is positive (DEA-CdTe), the fluorescence (Figure 3B, red trace) decays are essentially the same for DEA-CdTe and MPA-CdSe/DEACdTe. When the donor-QD is negatively charged and the acceptor-QD is positively charged, the internal electric field facilitates efficient electron transfer. Conversely, when the charges are reversed, electron transfer is inhibited; see Figure 3D. The electric field effect on charge transfer between QDs was also demonstrated for hole transfer in QD-conjugated polymer (QD-CP) heterojunctions (Figure 4).32 Modification of the polymer PDPPPV with anionic or cationic side chains allows electrostatic attachment to positive or negative CdTe QDs, similar to the electrostatic QD-QD aggregates in the previous study. The charges of the QDs and PDPPPV did not shift the redox potentials significantly in the dark, and the average sizes of the assembly and polymer molecular weight were similar in the studies. The photoluminescence data show efficient quenching (large kET) when the QD is positively charged and the PDPPPV is negatively charged, but minimal quenching when the charges are reversed. These results were confirmed in time-resolved transient absorption studies, which followed the bleach recovery of the QD’s exciton and the CP’s radical cation. The data in Figure 4 show a higher kET value for the + CdTe/−PDPPPV (Figure 4B) than for the −CdTe/+PDPPPV systems (Figure 4C). The covalently linked QD dyads in Figure 1 also possess an electrostatic field. Even though the electric field in the dyad

Figure 5. CD spectrum of L- (black) and D-cysteine (red) capped CdSe QDs in the first excitonic absorption region of the QD (A) and the absorption spectra of the L- (black, solid) and D-cysteine (red, dashed) coated CdSe QDs (B).

spectra of the two CdSe-QD enantiomers, and 5B shows their corresponding absorption spectra. The intensities of the CD peaks depend strongly on the size of the QD and the ligand surface coverage.38 How does the chirality of QDs influence spin-mediated transport properties? To answer this question, the spin-filtering properties of chiral QDs were investigated by magnetic conductive-probe atomic force microscopy (mCP-AFM) and magnetoresistance (MR) measurements.37 Figure 6 shows findings from the magnetoresistance measurements on chiral QD devices (the device architecture is indicated at the bottom 2569

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more populated than under zero field. Therefore, the spin “up” electrons, preferentially transmitted by the D-cysteine capped QDs, find fewer unfilled accepting levels in the Ni and a higher resistance is observed. The opposite asymmetric MR response for L- and D-cysteine QDs indicates that the chirality of the QDs is the underlying cause of the spin-filtering. When the QD is capped with achiral mercaptopropionic acid (MPA) ligands, the data become symmetric (panel B) indicating no dependence of spin transport on magnetic field orientation. Note that the magnitude of the MR response is relatively unchanged in the range of 20−300 K. The role of chirality in photoinduced electron transfer was studied for QD donor-bridge-acceptor dyad assemblies, see Figure 1A. Circularly polarized light was used to photoexcite an achiral CdTe donor-QD which generates a spin-polarized excited state population.39 The subsequent electron transfer to the chiral CdSe acceptor-QDs was monitored by time-resolved fluorescence spectroscopy. Figure 7 shows the photoluminescence decays for QD dyads containing the same donor, but different acceptor-QDs: L-cysteine CdSe (panel A), mercaptopropionic acid CdSe (panel B), and D-cysteine CdSe (panel C). The CdTe donor-QD was photoexcited with clockwise (cw) circularly polarized light (blue), linearly polarized light (black), and counterclockwise (ccw) circularly polarized light (green).40 Each decay curve was fit to a distribution of lifetime components and the short lifetime component, associated with electron transfer to CdSe, is shown in the inset. The short lifetime components are on the order of a few hundred picoseconds,39 and the electron transfer occurs before decoherence of the donor-QDs’ spin polarized excited state population. When the acceptor-QDs are L- or D-cysteine capped CdSe, the lifetime decay of the CdTe changes with excitation polarization because the acceptor promotes electron transfer for one spin type, while it impedes the electron transfer for the other spin type. For L-cysteine CdSe acceptor-QDs, the lifetime decay was fastest for ccw polarized excitation and slowest for cw polarized excitation of the donor. As was found for transmission in the magnetoresistance studies described above, the spin preference for electron transfer to the CdSe acceptor-QD flips with chirality. When the acceptor-QD is achiral, the excited state lifetime decay of the CdTe QDs does not change with light polarization.

Figure 6. Magnetoresistance data of L-cysteine (A), mercaptopropionic acid (B), and D-cysteine (C) passivated CdSe QDs. The bottom of each panel shows the architecture of the device and the arrows indicate the scan direction with the green arrow being the origin of the scan. Measurements were performed at 20 K and at a fixed current of 1 mA. Reproduced with permission from ref 37. Copyright 2016 American Chemical Society.

of each panel). The nickel layer acts as an analyzer for the electron spin; application of a magnetic field splits its spin states into sub-bands. Figure 6 shows the magnetoresistance data as a function of the QD capping ligand’s chirality, %MR =

R(H ) − R(0) × 100% R(0)

where R(H) is the resistance at a particular magnetic field strength and R(0) is the resistance at zero magnetic field. For L- and D-cysteine capped QDs (panels A and C, respectively), the asymmetry of the magnetoresistance response with magnetic field can be understood from their spin-filtering properties. For D-cysteine capped QDs, the MR is negative when a negative magnetic field is applied and is positive when a positive magnetic field is applied. With a negative magnetic field, the spin sub-bands of the Ni are split, leading to spin “up” states that are less populated than they are with zero field. The spin “up” electrons, which are preferentially transmitted by the D-cysteine capped QDs (electron spin aligned parallel to the propagation direction), find more accepting levels in the Ni, and thus experience a lower resistance. A positive magnetic field splits the spin sub-bands of the Ni in the opposite orientation to the negative magnetic field; spin “up” states are

Figure 7. Photoluminescence decays of the CdTe donor-QDs covalently attached to different acceptor-QDs; L-cysteine (A), mercaptopropionic acid (MPA, B), and D-cysteine (C) coated CdSe QDs. The excitation light polarization is indicated by different colors; clockwise circularly polarized (cw, blue), linearly polarized (black) and counterclockwise circularly polarized (ccw, green) light. The insets show the short component to a distribution fit of the decay. Reproduced with permission from ref 40. Copyright 2017 American Chemical Society. 2570

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Figure 8. Relationship between the asymmetry in electron transfer rate, Pet, and the circular dichroism signal intensity of the acceptor-QD determined using the short time constant (A) and by integrating all parts of the decay that involve polarization (B). Each data point represents a different assembly and the symbol type indicates the capping ligand on the acceptor-QD: magenta squares (D-cysteine), green triangles (N-acetyl-Lcysteine), and blue circles (L-cysteine). The gray curve is a sigmoidal best fit to the data. The error bars associated with Pet were determined using assemblies where the acceptor-QDs were capped with achiral mercaptopropionic acid instead of cysteine. Adapted with permission from ref 40. Copyright 2017 American Chemical Society.

Figure 9. (A) Schematic structure of a GaN-based magnetless Hall device. A two-dimensional electron gas is formed between the AlGaN layer and the GaN layer underneath. Nanoparticles are attached to the substrate through chiral oligopeptides adsorbed on top of the conductive channel (zoomed inset). (B) Hall potential measured on the device when the nanoparticles are illuminated by either cw (gray, right) or ccw (red, left) circularly polarized light. Adapted with permission from ref 41. Copyright 2016 Nature.

which can influence the fitting of the short time constant. To address these issues, the data were also analyzed by subtracting the long time components, in which no polarization is observed, from the decay and then integrating. The asymmetry in electron transport is thus defined as

To quantify differences in electron transfer rates we define an asymmetry parameter Pet as Pet =

kcw − kccw kcw + kccw

Here, kcw and kccw are the electron transfer rates found for cw and ccw polarized excitation. The asymmetry in electron transfer rates was plotted versus the acceptor-QD’s circular dichroism intensity; calculated as the difference between a change in the molar extinction coefficients at the peak and the trough of the lowest energy bisignate band (Figure 8A).40 Each symbol type represents a different kind of capping ligand on the acceptor-QD: L-cysteine (blue circles), N-acetyl-L-cysteine (green triangles), and D-cysteine (magenta squares). The data fall approximately on a sigmoidal curve (gray line), indicating the correlation between Pet and the chiroptical properties of the acceptor-QDs. While the short time constant is a good representation of the asymmetry in electron transfer rate, other features also contribute to the observed differences in the cw and ccw lifetime decays. For instance, when an assembly is excited with a light polarization that is unfavorable for electron transfer, a rise time in the decay is present (see ref 40 for more details)

Pet =

Icw − Iccw Icw + Iccw

where Icw and Iccw are the integrated areas of the subtracted fluorescence intensity decays for cw and ccw excitation, respectively. The correlation between the asymmetry in electron transfer rates and chiroptical properties of the QDs is plotted in Figure 8B. The magnitude of the asymmetry in Figure 8B is smaller than that of Figure 8A because it includes parts of the decay in which depolarization has begun to occur; however, the chirality effect on electron transport is still observed. How does the CISS effect fit into the general framework of electron transfer theory? To explore this question, the dependence of Pet on the reaction Gibbs energy, −ΔrG, was evaluated. While the overall charge transfer rates increase with more negative ΔrG values, no correlation of Pet with ΔrG is evident.40 This finding suggests that the spin selectivity arises 2571

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Accounts of Chemical Research from electronic coupling rather than from the thermodynamic driving force of the reaction. In the event that a change in electronic coupling does not influence Pet, a new parameter may be required to account for the electron spin influence in a Marcus framework for chiral systems. While the linkage between the circular dichroism response and Pet is yet to be fully elucidated, a first step will be to understand the nature of the nanoparticle’s chiral imprint and spin-filtering. The role of CISS in electron transfer was also studied with a newly developed Hall-effect device, shown in Figure 9.41 The Hall effect42 arises when current is flowing in a substrate between two electrodes, and a magnetic field applied perpendicular to the current flow induces an electric potential perpendicular both to the current and the magnetic fields. As Figure 9A shows, chiral oligopeptides, NH2-{Ala-Aib}8COCHNH 2 CH 2 PO 4 H 2 and SHCH 2 CH 2 CO-{Ala-Aib} 5 COOH, were self-assembled as a monolayer onto a GaN surface via the acid group and CdSe QDs were attached to the top of the oligopeptides through the amine or thiol groups (Figure 9A, inset). Figure 9B shows the Hall potential measured when the nanoparticles are illuminated by cw (right, red) or ccw (left, gray) circularly polarized light. These studies were performed without an external magnetic field, and demonstrate that spin-filtering in chiral assemblies displays a Hall-effect. The Hall voltage arises from the photoaccumulation of spin-polarized electrons. Based on calibrations with an external magnetic field, the signal corresponds to a magnetization of approximately 100 G.

David N. Beratan: 0000-0003-4758-8676 David H. Waldeck: 0000-0003-2982-0929 Notes

The authors declare no competing financial interest. Biographies Brian P. Bloom completed his B.Sc. in Chemistry and B.A. in Physics at Duquesne University, Pittsburgh, PA, and his Ph.D. at the University of Pittsburgh. He is currently working as a postdoctoral associate at the University of Pittsburgh. Ruibin Liu studied Chemical Physics at the University of Technology and Science of China (B.S.), Anhui, China and received his Ph.D. in Chemistry at Duke University. Peng Zhang is a Research Associate Professor of Chemistry at Duke. Supriya Ghosh completed his B.Sc. in Chemistry at Calcutta University, Kolkata, India, M. Sc. in Chemistry at IIT Bombay, Mumbai India, and M. Sc, in Chemistry at University of Alberta, Edmonton Canada. He is currently a Ph.D. candidate in Chemistry at the University of Pittsburgh. Ron Naaman completed his B.Sc. in Chemistry at Ben Gurion University, and his Ph.D. at the Weizmann Institute, Israel. He was a postdoctoral fellow at Stanford University and Harvard University before returning to the Weizmann Institute where he is now a Full Professor in Chemical Physics. David N. Beratan studied chemistry at Duke (B.S.) and Caltech (Ph.D.). After an N.R.C. fellowship and technical staff appointment at NASA’s Jet Propulsion Laboratory, he moved to the University of Pittsburgh and later to Duke University, where he is the R. J. Reynolds Professor of Chemistry, Professor of Biochemistry, and Professor of Physics.

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CONCLUDING REMARKS This Account describes emerging design principles for charge transfer in QD assemblies. Many of the experimental findings can be placed into a mechanistic context through electronic structure analysis and the tenets of nonadiabatic electron transfer theory. While familiar roles are played by key charge transfer parameters (ΔrG, electronic coupling, and reorganization energy), complexities arise from the need to include the electronic densities-of-states and account for changes in the electronic coupling pathways with QD size. It is exciting to find that new phenomena appear in these systems, including the effects of internal electrostatic fields and of chiral symmetry. These effects await the development of new theories that may provide a more complete understanding of the electron transfer dynamics. For instance, it is not fully understood why covalent and electrostatic assemblies show marked differences in their charge transport characteristics in the presence of an electric field, and the physical origins of the observed asymmetries in electron transfer rates for chiral QDs are yet to be understood. Nonetheless, the recent experimental and theoretical findings described here establish a comprehensive framework to understand and explore charge transfer and transport among nanoparticles. Importantly, the electrostatic field and chiral symmetry effects provide new approaches to direct charge flow in dyad and higher-order QD assemblies as well as to inhibit charge recombination at device layer interfaces.

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David H. Waldeck completed his B.Sc. in Chemistry at the University of Cincinnati and his Ph.D. at the University of Chicago. He was an IBM postdoctoral fellow, at the University of California, Berkeley, before joining the chemistry faculty at the University of Pittsburgh, where he is now a Professor of Chemistry.

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ACKNOWLEDGMENTS This work was supported by the Department of Energy; Grant No. ER46430 to R.N. and D.H.W. and DE-SC0010662ER46952 to D.N.B. R.N. also acknowledges the partial support of the European Research Council under the European Union’s Seventh Framework Program (FP7/2007-2013)/ ERC Grant Agreement No. 338720 CISS.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ron Naaman: 0000-0003-1910-366X 2572

DOI: 10.1021/acs.accounts.8b00355 Acc. Chem. Res. 2018, 51, 2565−2573

Article

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DOI: 10.1021/acs.accounts.8b00355 Acc. Chem. Res. 2018, 51, 2565−2573