Donor Quantum Dots Research Articles

Improving performance of CdSe/ZnS QDs via enhancing förster resonance energy transfer and suppressing auger recombination

In quantum dots (QDs) systems, Förster resonance energy transfer (FRET) and Auger recombination (AR) processes are crucial mechanisms that significantly impact their photoluminescence (PL) performance. Developing a strategy to simultaneously enhance FRET process and suppress the AR process is crucial. In this study, we reported that FRET process was enhanced, and the AR process was suppressed in the carbon-CdSe/ZnS QDs system via increasing carbon QDs donor ratios. The enhanced FRET rate is assigned to the increased adsorbed numbers and shortened donor-acceptor distance. The reduced AR rate is assigned to the decoration of the surface for CdSe/ZnS QDs acceptor in increased adsorbed numbers. As a result, the PL intensity of CdSe/ZnS QDs acceptor increased by over 3-fold. Our findings will stimulate the development of highly defined cascaded energy-transfer structures, aiming to achieve high-performance light-emitting devices.

Atomistic first-principles modeling of single donor spin-qubit

Using an impurity atom in crystal silicon as a spin-1/2 qubit has been made experimentally possible recently where the impurity atom acts as a quantum dot (QD). Quantum transport in and out of such a donor QD occurs in the sequential tunneling regime where a physical quantity of importance is the charging (addition) energy, which measures the energy necessary for adding an electron into the donor QD. In this work, we present a first-principles method to quantitatively predict the addition energy of the donor QD. Using density functional theory (DFT), we determine the impurity states that serve as the basis set for subsequent exact diagonalization calculation of the many-body states and energies of the donor QD. Due to the large effective Bohr radius of the conduction electrons in Si, very large supercells containing more than 10 000 atoms must be used to obtain accurate results. For the donor QD of a phosphorus impurity in bulk Si, the combined DFT and exact diagonalization predicts the first addition energy to be 53 meV, in good agreement with the corresponding experimental value. For the donor QD of an arsenic impurity in Si, the first addition energy is predicted to be 44.2 meV. The calculated many-body wave functions provide a vivid electronic picture of the donor QD.

(Invited) Photon Upconversion: Getting Molecules and Nanocrystals to Talk Triplets

Photon upconversion offers the prospect of combining low energy near-infrared radiation to make high energy photons in the visible wavelengths to improve photovoltaics and introduce new bioimaging techniques. In order to harness the intrinsic ability of colloidal semiconductor nanocrystals to couple strongly with light, it is important to efficiently outcouple energy from photoexcited quantum dots (QDs), much like how nature uses molecular antennas to direct light during photosynthesis. This talk focuses on the organic-inorganic interface for triplet-fusion based photon upconversion, where orbital overlap between the QD donor and molecular acceptor is critical for efficient energy transduction. In the past 8 years, we have learnt a great deal about how the electronic interactions between chalcogenide nanocrystals and acene conjugated molecules affect the photon upconversion quantum yield. We apply the lessons learnt to extending photon upconversion to silicon nanocrystal light absorbers, addressing synthetic challenges with invaluable insight from time-resolved spectroscopy.

Micellular fluorescence resonance energy transfer based fluorescent ratiometric response to hydrocarbon analytes.

Fluorescence resonance energy transfer (FRET) has been utilised to develop numerous selective and sensitive fluorescent ratiometric sensors. Typically, FRET-based fluorescent ratiometric sensors rely on chemical interactions between the sensor and analyte to illicit a response, thus unreactive hydrocarbons are a neglected analyte and a source for new sensors. By containing an unbound donor-acceptor system within micelles, energy transfer is enabled by spatial confinement. This offers the potential of a ratiometric response as a hydrocarbon analyte is added. Introducing a hydrocarbon analyte to this system causes micelles to swell, increasing the donor-acceptor distance and thus reducing the amount of observed energy transfer. We present InP/ZnS quantum dot donors interacting with a Nile Red acceptor, confined by cetyltrimethylammonium bromide (CTAB)-based micelles. We alleviated spatial confinement of the pair within micelles using common laboratory solvents to represent hydrocarbons, (toluene, hexane and octadecene). We constructed calibration curves for each solvent and found effective sensing ranges of 0.009-0.21, 0.008-0.27 and 0.003-0.06 M for toluene, hexane and octadecene, respectively. This study contributes towards the development of new hydrocarbon sensors utilising this new mechanism.

Ultrafast spectroscopy of size-dependent hole-transfer-mediated triplet energy transfer in a quantum dot-molecule complex

Triplet energy transfer (TET) from quantum dots to molecules has attracted tremendous attention in the last decade because the long-lived triplet state of molecules benefits various photophysical or photochemical processes and quantum dots (QDs), as the triplet sensitizer, provide prominent optical absorption and emission tunable through flexible size and composition control. Although several dynamical models for some certain QD-molecule complexes have been proposed, the TET mechanism in such complexes is still incomplete due to intricate energy level configuration. More specifics of TET being revealed is advantageous for its further applications. In this paper, a series of QD-molecule complexes composed of CdSe QDs with different diameters and 5-tetracene carboxylic acid (TCA) are assembled. The precise size control of CdSe QDs is realized by combining the transmission electron microscope and steady-state spectra. All CdSe-TCA complexes display expected Type-II like band alignment, which favors the hole-transfer-mediated TET confirmed by a series of spectral measurements. On the surface, ultrafast spectroscopy shows the charge dynamics, specifically the stepwise hole and electron transfer during TET, slow down with increasing size of the CdSe QDs. However, comprehensive analysis of the experimental results reveals that the hole transfer and the following electron transfer essentially depend on the energy level difference and wavefunction overlap between the QD donor and molecule acceptor rather than simply the QD size. The conclusions can facilitate our understanding of the TET mechanism in a QD-molecule complex and help optimize the performance of TET-related photoelectronic devices designed based on size control.

(Invited) Minimizing Thermal Losses at Quantum Dot-Metal Oxide Interfaces for Solar Energy Conversion

The sensitization of mesoporous oxides by semiconductor quantum dots (QDs) represents an appealing route for the development of low-cost solutions for solar energy production and storage (e.g. solar cells and fuel architectures). In order to boost photoconversion efficiency at these technologically relevant interfaces is important to minimize energy losses upon photon absorption; Energy losses that are intimately linked to kinetic completion at the relevant donor-acceptor interface [1]. Two main thermal energy losses can be identified following the absorption of a photon at the QD-oxide interface: firstly, any excess energy of the impinging photon vs the QD absorption onset is generally wasted as heat within the QD donor; secondly, the excess energy between QD electron donating state and the oxide accepting state (bottom of the conduction band) constitutes as well a thermal loss (now within the oxide) which reduces the available energy in the system to produce work. Reducing thermal losses associated with the latter mechanism is mandatory for allowing these architectures to reach efficiencies up to ~33% (the so called Shockley-Queisser limit). Preventing charge carrier cooling within the QD absorber after photon absorption, e.g. by enabling hot electron transfer toward the oxide, could – in principle - pave the way for developing QD dot based hot carrier solar cells with efficiencies beyond the Shockley-Queisser limit.Here I will present, from a charge carrier dynamics perspective, our studies aiming bypassing thermal losses at QD-oxide interfaces. First I will show how attempts to reduced QD-oxide donor-acceptor interfacial energetics by exploiting QD dipolar molecular capping can be in vain in strongly coupled QD-oxide sensitized systems [2]. This is linked to Fermi level pinning at the QD-oxide interface. I will show how this effect can be bypassed by inserting an insulating layer between QD donor and oxide acceptor. In the second part of my talk, I will discuss results demonstrating room-temperature hot electron transfer (HET) in strongly coupled QD-sensitized mesoporous oxides [3]. We quantify HET rates and show that the HET efficiency is determined by interfacial kinetic competition that can be optimized by both, maximizing QD-oxide coupling and/or suppressing electron cooling within the QD. Implications of these results for the challenging development of ultra-high efficiency solar converters will be discussed.

One-step self-assembly of quantum dot-based spherical nucleic acid nanostructure for accurate monitoring of long noncoding RNA MALAT1 in living cells and tissues

Simple and sensitive measurement of long non-coding RNA (lncRNA) is critical for early detection of malignancies. Herein, we demonstrate one-step self-assembly of quantum dot (QD)-based spherical nucleic acid (SNA) nanostructure for accurate monitoring of lncRNAs in living cells and tissues. When target lncRNA is present, it binds with a dumbbell probe to expose the complementary domain of Cy5-labeled primer, which subsequently induces cascade primer exchange reaction to produce abundant Cy5-labeled initiators. The Cy5-labeled initiators subsequently hybridize with hairpin probes on the QD surface to activate isothermal circular strand-displacement polymerization reaction, generating the QD-DNA-Cy5 nanostructures and inducing efficient Förster resonance energy transfer (FRET) between donor QD and acceptor Cy5. The obtained FRET signals are accurately quantified by single-molecule imaging. Notably, the single QD-based SNA nanostructure functions not only as a signal transmitter but also as a protector against non-specific amplification. Moreover, this assay utilizes only one DNA polymerase to achieve two-stage amplification, avoiding careful modulation of multiple enzymes. The self-assembly of QD nanosensor can be accomplished in single-step and single-tube manners at room temperature, eliminating precise temperature control and labor-intensive reaction protocols. This QD nanosensor achieves high sensitivity with a limit of detection (LOD) of 65.25 aM, and it is capable of quantifying lncRNA expression at single-cell level, differentiate tumor cells from normal cells, and distinguish breast cancer patients from healthy individuals, providing a versatile paradigm for biomedical research and early clinic diagnostics.

Nanoscale imaging of quantum dot dimers using time-resolved super-resolution microscopy combined with scanning electron microscopy

Time-resolved super-resolution microscopy was used in conjunction with scanning electron microscopy to image individual colloidal CdSe/CdS semiconductor quantum dots (QD) and QD dimers. The photoluminescence (PL) lifetimes, intensities, and structural parameters were acquired with nanometer scale spatial resolution and sub-nanosecond time resolution. The combination of these two techniques was more powerful than either alone, enabling us to resolve the PL properties of individual QDs within QD dimers as they blinked on and off, measure interparticle distances, and identify QDs that may be participating in energy transfer. The localization precision of our optical imaging technique was ∼3 nm, low enough that the emission from individual QDs within the dimers could be spatially resolved. While the majority of QDs within dimers acted as independent emitters, at least one pair of QDs in our study exhibited lifetime and intensity behaviors consistent with resonance energy transfer from a shorter lifetime and lower intensity donor QD to a longer lifetime and higher intensity acceptor QD. For this case, we demonstrate how the combined super-resolution optical imaging and scanning electron microscopy data can be used to characterize the energy transfer rate.

Dispersive cavity-mediated quantum gate between driven dot-donor nuclear spins

Nuclear spins show exceptionally long coherence times but the underlying good isolation from their environment is a challenge when it comes to controlling nuclear spin qubits. A particular difficulty, not only for nuclear spin qubits, is the realization of two-qubit gates between distant qubits. Recently, strong coupling between an electron spin and microwave resonator photons as well as a microwave resonator mediated coupling between two electron spins both in the resonant and the dispersive regime have been reported and, thus, a microwave resonator mediated electron spin two-qubit gate seems to be in reach. Inspired by these findings, we theoretically investigate the interaction of a microwave resonator with a hybrid quantum dot-donor (QDD) system consisting of a gate-defined Si QD and a laterally displaced $^{31}\mathrm{P}$ phosphorous donor atom implanted in the Si host material. We find that driving the QDD system allows to compensate the frequency mismatch between the donor nuclear spin splitting in the MHz regime and typical superconducting resonator frequencies in the GHz regime, and also enables an effective nuclear spin-photon coupling. While we expect this coupling to be weak, we predict that coupling the nuclear spins of two distant QDD systems dispersively to the microwave resonator allows the implementation of a resonator-mediated nuclear spin two-qubit $\sqrt{i\mathrm{SWAP}}$ gate with a gate fidelity approaching $90%$.

Quantifying the Variation in the Number of Donors in Quantum Dots Created Using Atomic Precision Advanced Manufacturing

Atomic-precision advanced manufacturing enables unique silicon quantum electronics built on quantum dots fabricated from small numbers of phosphorus dopants. The number of dopant atoms comprising a dot plays a central role in determining the behavior of charge and spin confined to the dots and thus overall device performance. In this work, we use both theoretical and experimental techniques to explore the combined impact of lithographic variation and stochastic kinetics on the number of P incorporations in quantum dots made using these techniques and how this variation changes as a function of the size of the dot. Using a kinetic model of PH3 dissociation augmented with novel reaction barriers, we demonstrate that for a 2 × 3 silicon dimer window the probability that no donor incorporates goes to zero, allowing for certainty in the placement of at least one donor. However, this still comes with some uncertainty in the precise number of incorporated donors (either one or two), and this variability may still impact certain applications. We also examine the impact of the size of the initial lithographic window, finding that the incorporation fraction saturates to δ-layer-like coverage as the circumference-to-area ratio decreases. We predict that this incorporation fraction depends strongly on the dosage of the precursor and that the standard deviation of the number of incorporations scales as ∼√n, as would be expected for a sequence of largely independent incorporation events. Finally, we characterize an array of 36 experimentally prepared multidonor 3 × 3 nm lithographic windows with scanning tunneling microscopy, measuring the fidelity of the lithography to the desired array and the final location of PHx fragments within these lithographic windows. We use our kinetic model to examine the expected variability due to the observed lithographic error, predicting a negligible impact on incorporation statistics. We find good agreement between our model and the inferred incorporation locations in these windows from scanning tunneling microscope measurements.

Energy-Transfer-Induced Enhanced Valley Splitting of Excitonic Emission of Inorganic CdTe@ZnS QDs in the Presence of Organic J-Aggregates: A Spectroscopic Insight into the Efficient Exciton (Inorganic)–Exciton (Organic) Coupling

An inorganic–organic nanohybrid material comprising an inorganic exciton (CdTe@ZnS) and an organic exciton (J-aggregates) has been synthesized, and subsequently investigations on exciton–exciton coupling between two components of the materials are carried out by employing various spectroscopic techniques. Analysis of the data has revealed very high Förster-type resonance energy transfer (FRET) from the donor quantum dots (QDs) to the acceptor J-aggregates. Interestingly, a clear splitting of the QD emission band during the FRET event is observed, indicating efficient coupling between inorganic and organic excitons. The observation of valley splitting of QD emission by organic J-aggregates, even in the absence of an external field, is new and exciting. More interestingly, the investigations have also shown that the valley splitting of the QD emission can be controlled by regulating the FRET process. In fact, a linear relationship between the energy separation value of the valley splitting and the FRET efficiency between QDs and J-aggregates is obtained. Moreover, in the present hybrid system, the presence of an organic moiety becomes advantageous in the sense that the valley splitting can also be controlled easily through chemical modification of the organic moiety. The outcome of the present investigation essentially demonstrates that the present nanohybrid system can be considered as a potential candidate for the fabrication of valleytronic materials.

Single-Shot Readout of Multiple Donor Electron Spins with a Gate-Based Sensor

Proposals for large-scale semiconductor spin-based quantum computers require high-fidelity single-shot qubit readout to perform error correction and read out qubit registers at the end of a computation. However, as devices scale to larger qubit numbers integrating readout sensors into densely packed qubit chips is a critical challenge. Two promising approaches are minimizing the footprint of the sensors, and extending the range of each sensor to read more qubits. Here we show high-fidelity single-shot electron spin readout using a nanoscale single-lead quantum dot (SLQD) sensor that is both compact and capable of reading multiple qubits. Our gate-based SLQD sensor is deployed in an all-epitaxial silicon donor spin-qubit device, and we demonstrate single-shot readout of three 31P donor quantum dot electron spins with a maximum fidelity of 95%. Importantly, in our device the quantum dot confinement potentials are provided inherently by the donors, removing the need for additional metallic confinement gates and resulting in strong capacitive interactions between sensor and donor quantum dots. Our results are consistent with a 1/d1.4 scaling of the capacitive coupling between sensor and 31P dots (where d is the sensor-dot distance), compared to 1/d2.5−3.0 in gate-defined quantum dot devices. Due to the small qubit size and strong capacitive interactions in all-epitaxial donor devices, we estimate a single sensor can achieve single-shot readout of approximately 15 qubits in a linear array, compared to 3–4 qubits for a similar sensor in a gate-defined quantum dot device. Our results highlight the potential for spin-qubit devices with significantly reduced sensor densities.Received 19 October 2021Revised 19 August 2022Accepted 4 January 2023DOI:https://doi.org/10.1103/PRXQuantum.4.010319Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasQuantum computationQuantum information with solid state qubitsQuantum metrologyQuantum Information

Water-soluble graphene quantum dot‐based polymer nanoparticles with internal donor/acceptor heterojunctions for efficient and selective detection of cancer cells

We present a new, insightful donor–acceptor (D-A) energy transfer-based strategy for the preparation and development of water-soluble multifunctional pH-responsive heterojunction nanoparticles. Hydrophilic tertiary amine-grafted polythiophene (WPT) as a donor and blue fluorescent graphene quantum dots (GQD) as an acceptor spontaneously form co-assembled nanoparticles that function as a highly pH-sensitive and efficient biosensor appropriate for the detection of cancer cells. These WPT/GQD nanoparticles exhibit a number of unique physical characteristics—such as broad-range, tunable GQD-loading contents and particle sizes, extremely low cytotoxicity in normal and cancer cells, and highly sensitive pH-responsiveness and rapid acid-triggered fluorescent behavior under aqueous acidic conditions. We show these features are conferred by self-aggregation of the GQD within the nanoparticles and subsequent aggregation-induced fluorescence of GQD after disassembly of the nanoparticles and dissociation of the D-A interactions under acidic conditions. Importantly, in vitro fluorescence imaging experiments clearly demonstrated the WPT/GQD nanoparticles were gradually taken up into normal and cancer cells in vitro. Selective formation of GQD aggregates subsequently occurred in the acidic microenvironment of the cancer cells and the interior of the cancer cells exhibited strong blue fluorescence; these phenomena did not occur in normal cells. In contrast, pristine WPT and GQD did not exhibit cellular microenvironment-triggered fluorescence transitions in cancer or normal cell lines. Therefore, this newly discovered water-soluble heterojunction system may represent a strongly fluorescent highly pH-sensitive bioprobe for rapid detection of cancer cells.

Fӧrster resonance energy transfer (FRET) between CdSe quantum dots and ABA phosphorus(V) corroles

In this article, highly fluorescent phosphorus(V) corrole was synthesised which was then combined with CdSe quantum dots (QDs) in order to study Förster resonance energy transfer (FRET) mechanism between CdSe QDs (donor) and phosphorus corrole (acceptor). Spectral overlap between QD’s emission profile and corrole’s absorption profile was found to be significant enough to result into Förster resonance energy transfer (FRET). The UV–vis spectrum experienced increase in the absorption bands on addition of phosphorus corrole to CdSe QDs suggesting QD-corrole conjugation. In the steady state fluorescence measurements, emission spectrum observed quenching in the fluorescence intensity of prepared CdSe QDs on addition of phosphorus corrole. Likewise, in case of time-resolved fluorescence measurements it was noticed that the CdSe QD’s lifetime was greatly quenched by the presence of a corrole acceptor. Stern-Volmer plot was made to show quenching in this case was dynamic in nature. Based on the results of UV–vis, steady state and time-resolved fluorescence measurements the plausible mechanism behind such observations is considered to be FRET.

Variable-Barrier Quantum Coulomb Blockade Effect in Nanoscale Transistors.

Current-voltage characteristics of a quantum dot in double-barrier configuration, as formed in the nanoscale channel of silicon transistors, were analyzed both experimentally and theoretically. Single electron transistors (SET) made in a SOI-FET configuration using silicon quantum dot as well as phosphorus donor quantum dots were experimentally investigated. These devices exhibited a quantum Coulomb blockade phenomenon along with a detectable effect of variable tunnel barriers. To replicate the experimental results, we developed a generalized formalism for the tunnel-barrier dependent quantum Coulomb blockade by modifying the rate-equation approach. We qualitatively replicate the experimental results with numerical calculation using this formalism for two and three energy levels participated in the tunneling transport. The new formalism supports the features of most of the small-scaled SET devices.

Effect of the Donor/Acceptor Size on the Rate of Photo-Induced Electron Transfer

The photo-induced electron transfer has been under intensive investigation for a few decades already, and a good understanding of the reaction was developed based on thorough study of the molecular donor–acceptor (DA) system. The recent shift to hybrid DA systems opens the question of transferring the knowledge to analyze and design these new materials. One of the apparent differences is the size increase of the donor or acceptor entities. The electronic wave functions of larger entities occupy a larger volume, but since these are still one-electron wave functions, their amplitudes are lower. A simple analysis proposed here demonstrates that this leads to roughly inverse third power dependence of the electron transfer rate constant on the donor or acceptor size, kET∝R−3. This dependence can be observed upon switching from molecular to quantum dot donor in DA systems with a fullerene acceptor.

Hybrid Nucleic Acid-Quantum Dot Assemblies as Multiplexed Reporter Platforms for Cell-Free Transcription Translation-Based Biosensors.

Cell-free synthetic biology has emerged as a valuable tool for the development of rapid, portable biosensors that can be readily transported in the freeze-dried form to the point of need eliminating cold chain requirements. One of the challenges associated with cell-free sensors is the ability to simultaneously detect multiple analytes within a single reaction due to the availability of a limited set of fluorescent and colorimetric reporters. To potentially provide multiplexing capabilities to cell-free biosensors, we designed a modular semiconductor quantum dot (QD)-based reporter platform that is plugged in downstream of the transcription-translation functionality in the cell-free reaction and which converts enzymatic activity in the reaction into distinct optical signals. We demonstrate proof of concept by converting restriction enzyme activity, utilized as our prototypical sensing output, into optical changes across several distinct spectral output channels that all use a common excitation wavelength. These hybrid Förster resonance energy transfer (FRET)-based QD peptide PNA-DNA-Dye reporters (QD-PDDs) are completely self-assembled and consist of differentially emissive QD donors paired to a dye-acceptor displayed on a unique DNA encoding a given enzyme's cleavage site. Three QD-based PDDs, independently activated by the enzymes BamHI, EcoRI, and NcoI, were prototyped in mixed enzyme assays where all three demonstrated the ability to convert enzymatic activity into fluorescent output. Simultaneous monitoring of each of the three paired QD-donor dye-acceptor spectral channels in cell-free biosensing reactions supplemented with added linear genes encoding each enzyme confirmed robust multiplexing capabilities for at least two enzymes when co-expressed. The modular QD-PDDs are easily adapted to respond to other restriction enzymes or even proteases if desired.

DNA sequencing by Förster resonant energy transfer.

We propose a new DNA sequencing concept based on nonradiative Förster resonant energy transfer (FRET) from a donor quantum dot (QD) to an acceptor molecule. The FRET mechanism combined with the nanopore-based DNA translocation is suggested as a novel concept for sequencing DNA molecules. A recently-developed hybrid quantum/classical method is employed, which uses time-dependent density functional theory and quasistatic finite difference time domain calculations. Due to the significant absorbance of DNA bases for photon energies higher than 4 eV, biocompatibility, and stability, we use Zinc-Oxide (ZnO) QD as a donor in the FRET mechanism. The most sensitivity for the proposed method to DNA is achieved for the Hoechst fluorescent-dye acceptor and 1 nm ZnO-QD. Results show that the insertion of each type of DNA nucleobases between the donor and acceptor changes the frequency of the emitted light from the acceptor molecule between 0.25 to 1.6 eV. The noise analysis shows that the method can determine any unknown DNA nucleobases if the signal-to-noise ratio is larger than 5 dB. The proposed concept and excellent results shed light on a new promising class of DNA sequencers.

Minimizing the Reorganization Energy of Cobalt Redox Mediators Maximizes Charge Transfer Rates from Quantum Dots

AbstractLight‐induced charge separation is at the very heart of many solar harvesting technologies. The reduction of energetic barriers to charge separation and transfer increases the rate of separation and the overall efficiency of these technologies. Here we report that the internal reorganization energy of the redox acceptor, the movement of the atoms with changing charge, has a profound effect on the charge transfer rates from donor quantum dots. We experimentally studied and modelled with Marcus Theory charge transfer to cobalt complexes that have similar redox potentials covering 350 mV, but vastly different reorganization energies spanning 2 eV. While the driving force does influence the electron transfer rates, the reorganization energies had a far more profound effect, increasing charge transfer rates by several orders of magnitude. Our studies suggest that careful design of redox mediators to minimize reorganization energy is an untapped route to drastically increase the efficiency of quantum dot applications that feature charge transfer.

Minimizing the Reorganization Energy of Cobalt Redox Mediators Maximizes Charge Transfer Rates from Quantum Dots.

Light-induced charge separation is at the very heart of many solar harvesting technologies. The reduction of energetic barriers to charge separation and transfer increases the rate of separation and the overall efficiency of these technologies. Here we report that the internal reorganization energy of the redox acceptor, the movement of the atoms with changing charge, has a profound effect on the charge transfer rates from donor quantum dots. We experimentally studied and modelled with Marcus Theory charge transfer to cobalt complexes that have similar redox potentials covering 350 mV, but vastly different reorganization energies spanning 2 eV. While the driving force does influence the electron transfer rates, the reorganization energies had a far more profound effect, increasing charge transfer rates by several orders of magnitude. Our studies suggest that careful design of redox mediators to minimize reorganization energy is an untapped route to drastically increase the efficiency of quantum dot applications that feature charge transfer.