Electronic Structure and Excited State Dynamics of Cadmium Chalcogenide Nanorods.

The cylindrical quasi-one-dimensional shape of colloidal semiconductor nanorods (NRs) gives them unique electronic structure and optical properties. In addition to the band gap tunability common to nanocrystals, NRs have polarized light absorption and emission and high molar absorptivities. NR-shaped heterostructures feature control of electron and hole locations as well as light emission energy and efficiency. We comprehensively review the electronic structure and optical properties of Cd-chalcogenide NRs and NR heterostructures (e.g., CdSe/CdS dot-in-rods, CdSe/ZnS rod-in-rods), which have been widely investigated over the last two decades due in part to promising optoelectronic applications. We start by describing methods for synthesizing these colloidal NRs. We then detail the electronic structure of single-component and heterostructure NRs and follow with a discussion of light absorption and emission in these materials. Next, we describe the excited state dynamics of these NRs, including carrier cooling, carrier and exciton migration, radiative and nonradiative recombination, multiexciton generation and dynamics, and processes that involve trapped carriers. Finally, we describe charge transfer from photoexcited NRs and connect the dynamics of these processes with light-driven chemistry. We end with an outlook that highlights some of the outstanding questions about the excited state properties of Cd-chalcogenide NRs.

Exciton and biexciton transient absorption spectra of CdSe quantum dots with varying diameters.

Transient absorption (TA) spectroscopy of semiconductor nanocrystals (NCs) is often used for excited state population analysis, but recent results suggest that TA bleach signals associated with multiexcitons in NCs do not scale linearly with exciton multiplicity. In this manuscript, we probe the factors that determine the intensities and spectral positions of exciton and biexciton components in the TA spectra of CdSe quantum dots (QDs) of five diameters. We find that, in all cases, the peak intensity of the biexciton TA spectrum is less than 1.5 times that of the single exciton TA spectrum, in stark contrast to a commonly made assumption that this ratio is 2. The relative intensities of the biexciton and exciton TA signals at each wavelength are determined by at least two factors: the TA spectral intensity and the spectral offset between the two signals. We do not observe correlations between either of these factors and the particle diameter, but we find that both are strongly impacted by replacing the native organic surface-capping ligands with a hole-trapping ligand. These results suggest that surface trapping plays an important role in determining the absolute intensities of TA features for CdSe QDs and not just their decay kinetics. Our work highlights the role of spectral offsets and the importance of surface trapping in governing absolute TA intensities. It also conclusively demonstrates that the biexciton TA spectra of CdSe QDs at the band gap energy are less than twice as intense as those of the exciton.

High Affinity Electrostatic Interactions Support the Formation of CdS Quantum Dot:Nitrogenase MoFe Protein Complexes.

Nitrogenase MoFe protein can be coupled with CdS nanocrystals (NCs) to enable photocatalytic N2 reduction. The nature of interactions that support complex formation is of paramount importance in intermolecular electron transfer that supports catalysis. In this work we have employed microscale thermophoresis to examine binding interactions between 3-mercaptopropionate capped CdS quantum dots (QDs) and MoFe protein over a range of QD diameters (3.4-4.3 nm). The results indicate that the interactions are largely electrostatic, with the strength of interactions similar to that observed for the physiological electron donor. In addition, the strength of interactions is sensitive to the QD diameter, and the binding interactions are significantly stronger for QDs with smaller diameters. The ability to quantitatively assess NC protein interactions in biohybrid systems supports strategies for understanding properties and reaction parameters that are important for obtaining optimal rates of catalysis in biohybrid systems.

Cryo-annealing of Photoreduced CdS Quantum Dot-Nitrogenase MoFe Protein Complexes Reveals the Kinetic Stability of the E4(2N2H) Intermediate.

A critical step in the mechanism of N2 reduction to 2NH3 catalyzed by the enzyme nitrogenase is the reaction of the four-electron/four-proton reduced intermediate state of the active-site FeMo-cofactor (E4(4H)). This state is a junction in the catalytic mechanism, either relaxing by the reaction of a metal bound Fe-hydride with a proton forming H2 or going forward with N2 binding coupled to the reductive elimination (re) of two Fe-hydrides as H2 to form the E4(2N2H) state. E4(2N2H) can relax to E4(4H) by the oxidative addition (oa) of H2 and release of N2 or can be further reduced in a series of catalytic steps to release 2NH3. If the H2 re/oa mechanism is correct, it requires that oa of H2 be associative with E4(2N2H). In this report, we have taken advantage of CdS quantum dots in complex with MoFe protein to achieve photodriven electron delivery in the frozen state, with cryo-annealing in the dark, to reveal details of the E-state species and to test the stability of E4(2N2H). Illumination of frozen CdS:MoFe protein complexes led to formation of a population of reduced intermediates. Electron paramagnetic resonance spectroscopy identified E-state signals including E2 and E4(2N2H), as well as signals suggesting the formation of E6 or E8. It is shown that in the frozen state when pN2 is much greater than pH2, the E4(2N2H) state is kinetically stable, with very limited forward or reverse reaction rates. These results establish that the oa of H2 to the E4(2N2H) state follows an associative reaction mechanism.

Low-temperature trapping of N2 reduction reaction intermediates in nitrogenase MoFe protein-CdS quantum dot complexes.

The biological reduction of N2 to ammonia requires the ATP-dependent, sequential delivery of electrons from the Fe protein to the MoFe protein of nitrogenase. It has been demonstrated that CdS nanocrystals can replace the Fe protein to deliver photoexcited electrons to the MoFe protein. Herein, light-activated electron delivery within the CdS:MoFe protein complex was achieved in the frozen state, revealing that all the electron paramagnetic resonance (EPR) active E-state intermediates in the catalytic cycle can be trapped and characterized by EPR spectroscopy. Prior to illumination, the CdS:MoFe protein complex EPR spectrum was composed of a S = 3/2 rhombic signal (g = 4.33, 3.63, and 2.01) consistent with the FeMo-cofactor in the resting state, E0. Illumination for sequential 1-h periods at 233 K under 1 atm of N2 led to a cumulative attenuation of E0 by 75%. This coincided with the appearance of S = 3/2 and S = 1/2 signals assigned to two-electron (E2) and four-electron (E4) reduced states of the FeMo-cofactor, together with additional S = 1/2 signals consistent with the formation of E6 and E8 states. Simulations of EPR spectra allowed quantification of the different E-state populations, along with mapping of these populations onto the Lowe-Thorneley kinetic scheme. The outcome of this work demonstrates that the photochemical delivery of electrons to the MoFe protein can be used to populate all of the EPR active E-state intermediates of the nitrogenase MoFe protein cycle.

Light-driven carbon-carbon bond formation via CO2 reduction catalyzed by complexes of CdS nanorods and a 2-oxoacid oxidoreductase.

Redox enzymes are capable of catalyzing a vast array of useful reactions, but they require redox partners that donate or accept electrons. Semiconductor nanocrystals provide a mechanism to convert absorbed photon energy into redox equivalents for enzyme catalysis. Here, we describe a system for photochemical carbon-carbon bond formation to make 2-oxoglutarate by coupling CO2 with a succinyl group. Photoexcited electrons from cadmium sulfide nanorods (CdS NRs) transfer to 2-oxoglutarate:ferredoxin oxidoreductase from Magnetococcus marinus MC-1 (MmOGOR), which catalyzes a carbon-carbon bond formation reaction. We thereby decouple MmOGOR from its native role in the reductive tricarboxylic acid cycle and drive it directly with light. We examine the dependence of 2-oxoglutarate formation on a variety of factors and, using ultrafast transient absorption spectroscopy, elucidate the critical role of electron transfer (ET) from CdS NRs to MmOGOR. We find that the efficiency of this ET depends strongly on whether the succinyl CoA (SCoA) cosubstrate is bound at the MmOGOR active site. We hypothesize that the conformational changes due to SCoA binding impact the CdS NR-MmOGOR interaction in a manner that decreases ET efficiency compared to the enzyme with no cosubstrate bound. Our work reveals structural considerations for the nano-bio interfaces involved in light-driven enzyme catalysis and points to the competing factors of enzyme catalysis and ET efficiency that may arise when complex enzyme reactions are driven by artificial light absorbers.

Dissecting Electronic-Structural Transitions in the Nitrogenase MoFe Protein P-Cluster during Reduction.

The [8Fe-7S] P-cluster of nitrogenase MoFe protein mediates electron transfer from nitrogenase Fe protein during the catalytic production of ammonia. The P-cluster transitions between three oxidation states, PN, P+, P2+ of which PN↔P+ is critical to electron exchange in the nitrogenase complex during turnover. To dissect the steps in formation of P+ during electron transfer, photochemical reduction of MoFe protein at 231-263 K was used to trap formation of P+ intermediates for analysis by EPR. In complexes with CdS nanocrystals, illumination of MoFe protein led to reduction of the P-cluster P2+ that was coincident with formation of three distinct EPR signals: S = 1/2 axial and rhombic signals, and a high-spin S = 7/2 signal. Under dark annealing the axial and high-spin signal intensities declined, which coincided with an increase in the rhombic signal intensity. A fit of the time-dependent changes of the axial and high-spin signals to a reaction model demonstrates they are intermediates in the formation of the P-cluster P+ resting state and defines how spin-state transitions are coupled to changes in P-cluster oxidation state in MoFe protein during electron transfer.

Electron Transfer from Semiconductor Nanocrystals to Redox Enzymes.

This review summarizes progress in understanding electron transfer from photoexcited nanocrystals to redox enzymes. The combination of the light-harvesting properties of nanocrystals and the catalytic properties of redox enzymes has emerged as a versatile platform to drive a variety of enzyme-catalyzed reactions with light. Transfer of a photoexcited charge from a nanocrystal to an enzyme is a critical first step for these reactions. This process has been studied in depth in systems that combine Cd-chalcogenide nanocrystals with hydrogenases. The two components can be assembled in close proximity to enable direct interfacial electron transfer or integrated with redox mediators to transport charges. Time-resolved spectroscopy and kinetic modeling have been used to measure the rates and efficiencies of the electron transfer. Electron transfer has been described within the framework of Marcus theory, providing insights into the factors that can be used to control the photochemical activity of these biohybrid systems. The range of potential applications and reactions that can be achieved using nanocrystal-enzyme systems is expanding, and numerous fundamental and practical questions remain to be addressed.

Defining Intermediates of Nitrogenase MoFe Protein during N2 Reduction under Photochemical Electron Delivery from CdS Quantum Dots.

Coupling the nitrogenase MoFe protein to light-harvesting semiconductor nanomaterials replaces the natural electron transfer complex of Fe protein and ATP and provides low-potential photoexcited electrons for photocatalytic N2 reduction. A central question is how direct photochemical electron delivery from nanocrystals to MoFe protein is able to support the multielectron ammonia production reaction. In this study, low photon flux conditions were used to identify the initial reaction intermediates of CdS quantum dot (QD):MoFe protein nitrogenase complexes under photochemical activation using EPR. Illumination of CdS QD:MoFe protein complexes led to redox changes in the MoFe protein active site FeMo-co observed as the gradual decline in the E0 resting state intensity that was accompanied by an increase in the intensity of a new "geff = 4.5" EPR signal. The magnetic properties of the geff = 4.5 signal support assignment as a reduced S = 3/2 state, and reaction modeling was used to define it as a two-electron-reduced "E2" intermediate. Use of a MoFe protein variant, ß-188Cys, which poises the P cluster in the oxidized P+ state, demonstrated that the P cluster can function as a site of photoexcited electron delivery from CdS to MoFe protein. Overall, the results establish the initial steps for how photoexcited CdS delivers electrons into the MoFe protein during reduction of N2 to ammonia and the role of electron flux in the photochemical reaction cycle.

Comparison of Phonon Damping Behavior in Quantum Dots Capped with Organic and Inorganic Ligands.

Surface ligand modification of colloidal semiconductor nanocrystals has been widely used as a means of controlling photoexcited-state generation, relaxation, and coupling to the environment. While progress has been made in understanding how surface ligand modification affects the behavior of electronic states, less is known about the influence of surface ligand modification on phonon behavior, which impacts relaxation dynamics and transport phenomena. In this work, we compare the dynamics of optical and acoustic phonons in CdTe quantum dots (QDs), CdTe/CdSe core/shell QDs capped with octadecylphosphonic acid ligands, and CdTe QDs capped with Se2- to ascertain how ligand exchange from native aliphatic ligands to single-atom Se2- ligands affects phonon behavior. We use transient absorption spectroscopy and observe modulations in the kinetics of excited-state decay due to QD lattice vibrations from both optical and acoustic phonons, which we describe using the damped oscillator model. The longitudinal optical phonons have similar frequencies and damping behavior in all three samples. In contrast, the longitudinal acoustic phonon mode in the Se2--capped CdTe QDs is severely damped, much more so than in CdTe and CdTe/CdSe QDs capped with the native aliphatic ligands. We attribute these differences in the acoustic phonon behavior to the differences in how the QD dissipates vibrational energy to its surroundings as a function of ligand identity. Our results indicate that these inorganic surface-capping ligands enhance not only the electronic but also the mechanical coupling of nanocrystals with their environment.

Relationships between Exciton Dissociation and Slow Recombination within ZnSe/CdS and CdSe/CdS Dot-in-Rod Heterostructures.

Type-II and quasi type-II heterostructure nanocrystals are known to exhibit extended excited-state lifetimes compared to their single material counterparts because of reduced wave function overlap between the electron and hole. However, due to fast and efficient hole trapping and nonuniform morphologies, the photophysics of dot-in-rod heterostructures are more rich and complex than this simple picture. Using transient absorption spectroscopy, we observe that the behavior of electrons in the CdS "rod" or "bulb" regions of nonuniform ZnSe/CdS and CdSe/CdS dot-in-rods is similar regardless of the "dot" material, which supports previous work demonstrating that hole trapping and particle morphology drive electron dynamics. Furthermore, we show that the longest lived state in these dot-in-rods is not generated by the type-II or quasi type-II band alignment between the dot and the rod, but rather by electron-hole dissociation that occurs due to fast hole trapping in the CdS rod and electron localization to the bulb. We propose that specific variations in particle morphology and surface chemistry determine the mechanism and efficiency of charge separation and recombination in these nanostructures, and therefore impact their excited-state dynamics to a greater extent than the heterostructure energy level alignment alone.

Activation Thermodynamics and H/D Kinetic Isotope Effect of the Hox to HredH+ Transition in [FeFe] Hydrogenase.

Molecular complexes between CdSe nanocrystals and Clostridium acetobutylicum [FeFe] hydrogenase I (CaI) enabled light-driven control of electron transfer for spectroscopic detection of redox intermediates during catalytic proton reduction. Here we address the route of electron transfer from CdSe→CaI and activation thermodynamics of the initial step of proton reduction in CaI. The electron paramagnetic spectroscopy of illuminated CdSe:CaI showed how the CaI accessory FeS cluster chain (F-clusters) functions in electron transfer with CdSe. The Hox→HredH+ reduction step measured by Fourier-transform infrared spectroscopy showed an enthalpy of activation of 19 kJ mol-1 and a ∼2.5-fold kinetic isotope effect. Overall, these results support electron injection from CdSe into CaI involving F-clusters, and that the Hox→HredH+ step of catalytic proton reduction in CaI proceeds by a proton-dependent process.

Strong Visible Absorption and Broad Time Scale Excited-State Relaxation in (Ga(1-x)Zn(x))(N(1-x)O(x)) Nanocrystals.

(Ga(1-x)Zn(x))(N(1-x)O(x)) is a visible absorber of interest for solar fuel generation. We present a first report of soluble (Ga(1-x)Zn(x))(N(1-x)O(x)) nanocrystals (NCs) and their excited-state dynamics over the time window of 10(-13)-10(-4) s. Using transient absorption spectroscopy, we find that excited-state decay in (Ga0.27Zn0.73)(N0.27O0.73) NCs has both a short (<100 ps) and a long-lived component, with a long overall average lifetime of ∼30 µs. We also find that the strength of the visible absorption is comparable to that of direct band gap semiconductors such as GaAs. We discuss how these results may relate to the origin of visible absorption in (Ga(1-x)Zn(x))(N(1-x)O(x)) and its use in solar fuel generation.

Solvents effects on charge transfer from quantum dots.

To predict and understand the performance of nanodevices in different environments, the influence of the solvent must be explicitly understood. In this Communication, this important but largely unexplored question is addressed through a comparison of quantum dot charge transfer processes occurring in both liquid phase and in vacuum. By comparing solution phase transient absorption spectroscopy and gas-phase photoelectron spectroscopy, we show that hexane, a common nonpolar solvent for quantum dots, has negligible influence on charge transfer dynamics. Our experimental results, supported by insights from theory, indicate that the reorganization energy of nonpolar solvents plays a minimal role in the energy landscape of charge transfer in quantum dot devices. Thus, this study demonstrates that measurements conducted in nonpolar solvents can indeed provide insight into nanodevice performance in a wide variety of environments.

Competition between electron transfer, trapping, and recombination in CdS nanorod-hydrogenase complexes.

Electron transfer from photoexcited CdS nanorods to [FeFe]-hydrogenase is a critical step in photochemical H2 production by CdS-hydrogenase complexes. By accounting for the distributions in the numbers of electron traps and enzymes adsorbed, we determine rate constants and quantum efficiencies for electron transfer from transient absorption measurements.

Electron transfer kinetics in CdS nanorod-[FeFe]-hydrogenase complexes and implications for photochemical H2 generation.

This Article describes the electron transfer (ET) kinetics in complexes of CdS nanorods (CdS NRs) and [FeFe]-hydrogenase I from Clostridium acetobutylicum (CaI). In the presence of an electron donor, these complexes produce H2 photochemically with quantum yields of up to 20%. Kinetics of ET from CdS NRs to CaI play a critical role in the overall photochemical reactivity, as the quantum efficiency of ET defines the upper limit on the quantum yield of H2 generation. We investigated the competitiveness of ET with the electron relaxation pathways in CdS NRs by directly measuring the rate and quantum efficiency of ET from photoexcited CdS NRs to CaI using transient absorption spectroscopy. This technique is uniquely suited to decouple CdS→CaI ET from the processes occurring in the enzyme during H2 production. We found that the ET rate constant (k(ET)) and the electron relaxation rate constant in CdS NRs (k(CdS)) were comparable, with values of 10(7) s(-1), resulting in a quantum efficiency of ET of 42% for complexes with the average CaI:CdS NR molar ratio of 1:1. Given the direct competition between the two processes that occur with similar rates, we propose that gains in efficiencies of H2 production could be achieved by increasing k(ET) and/or decreasing k(CdS) through structural modifications of the nanocrystals. When catalytically inactive forms of CaI were used in CdS-CaI complexes, ET behavior was akin to that observed with active CaI, demonstrating that electron injection occurs at a distal iron-sulfur cluster and is followed by transport through a series of accessory iron-sulfur clusters to the active site of CaI. Using insights from this time-resolved spectroscopic study, we discuss the intricate kinetic pathways involved in photochemical H2 generation in CdS-CaI complexes, and we examine how the relationship between the electron injection rate and the other kinetic processes relates to the overall H2 production efficiency.

Photoelectron spectroscopy of CdSe nanocrystals in the gas phase: a direct measure of the evanescent electron wave function of quantum dots.

We present the first photoelectron spectroscopy measurements of quantum dots (semiconductor nanocrystals) in the gas phase. By coupling a nanoparticle aerosol source to a femtosecond velocity map imaging photoelectron spectrometer, we apply robust gas-phase photoelectron spectroscopy techniques to colloidal quantum dots, which typically must be studied in a liquid solvent or while bound to a surface. Working with a flowing aerosol of quantum dots offers the additional advantages of providing fresh nanoparticles for each laser shot and removing perturbations from bonding with a surface or interactions with the solvent. In this work, we perform a two-photon photoionization experiment to show that the photoelectron yield per exciton depends on the physical size of the quantum dot, increasing for smaller dots. Next, using effective mass modeling we show that the extent to which the electron wave function of the exciton extends from the quantum dot, the so-called "evanescent electron wavefunction", increases as the size of the quantum dot decreases. We show that the photoelectron yield is dominated by the evanescent electron density due to quantum confinement effects, the difference in the density of states inside and outside of the quantum dots, and the angle-dependent transmission probability of electrons through the surface of the quantum dot. Therefore, the photoelectron yield directly reflects the fraction of evanescent electron wave function that extends outside of the quantum dot. This work shows that gas-phase photoelectron spectroscopy is a robust and general probe of the electronic structure of quantum dots, enabling the first direct measurements of the evanescent exciton wave function.

Hole-scavenging in photo-driven N2 reduction catalyzed by a CdS-nitrogenase MoFe protein biohybrid system.

The light-driven reduction of dinitrogen (N2) to ammonia (NH3) catalyzed by a cadmium sulfide (CdS) nanocrystal­nitrogenase MoFe protein biohybrid is dependent on a range of different factors, including an appropriate hole-scavenging sacrificial electron donor (SED). Here, the impact of different SEDs on the overall rate of N2 reduction catalyzed by a CdS quantum dot (QD)-MoFe protein system was determined. The selection of SED was guided by several goals: (i) molecules with standard reduction potentials sufficient to reduce the oxidized CdS QD, (ii) molecules that do not absorb the excitation wavelength of the CdS QD, and (iii) molecules that could be readily reduced by sustainable processes. Earlier studies utilized buffer molecules or ascorbic acid as the SED. The effectiveness of ascorbic acid as SED was compared to dithionite (DT), triethanolamine (TEOA), and hydroquinone (HQ) across a range of concentrations in supporting N2 reduction to NH3 in a CdS QD-MoFe protein photocatalytic system. It was found that TEOA supported N2 reduction rates comparable to those observed for dithionite and ascorbic acid. HQ was found to support significantly higher rates of N2 reduction compared to the other SEDs at a concentration of 50 mM. A comparison of the rates of N2 reduction by the biohybrid complex to the standard reduction potential (Eo) of the SEDs reveals that Eo is not the only factor impacting the efficiency of hole-scavenging. These findings reveal the importance of the SED properties for improving the efficiency of hole-scavenging in the light-driven N2 reduction reaction catalyzed by a CdS QD-MoFe protein hybrid.

Ligand Equilibrium Influences Photoluminescence Blinking in CsPbBr3: A Change Point Analysis of Widefield Imaging Data.

Photoluminescence intermittency remains one of the biggest challenges in realizing perovskite quantum dots (QDs) as scalable single photon emitters. We compare CsPbBr3 QDs capped with different ligands, lecithin, and a combination of oleic acid and oleylamine, to elucidate the role of surface chemistry on photoluminescence intermittency. We employ widefield photoluminescence microscopy to sample the blinking behavior of hundreds of QDs. Using change point analysis, we achieve the robust classification of blinking trajectories, and we analyze representative distributions from large numbers of QDs (Nlecithin = 1308, Noleic acid/oleylamine = 1317). We find that lecithin suppresses blinking in CsPbBr3 QDs compared with oleic acid/oleylamine. Under common experimental conditions, lecithin-capped QDs are 7.5 times more likely to be nonblinking and spend 2.5 times longer in their most emissive state, despite both QDs having nearly identical solution photoluminescence quantum yields. We measure photoluminescence as a function of dilution and show that the differences between lecithin and oleic acid/oleylamine capping emerge at low concentrations during preparation for single particle experiments. From experiment and first-principles calculations, we attribute the differences in lecithin and oleic acid/oleylamine performance to differences in their ligand binding equilibria. Consistent with our experimental data, density functional theory calculations suggest a stronger binding affinity of lecithin to the QD surface compared to oleic acid/oleylamine, implying a reduced likelihood of ligand desorption during dilution. These results suggest that using more tightly binding ligands is a necessity for surface passivation and, consequently, blinking reduction in perovskite QDs used for single particle and quantum light experiments.

Charge transfer dynamics between photoexcited CdS nanorods and mononuclear Ru water-oxidation catalysts.

We describe the charge transfer interactions between photoexcited CdS nanorods and mononuclear water oxidation catalysts derived from the [Ru(bpy)(tpy)Cl](+) parent structure. Upon excitation, hole transfer from CdS oxidizes the catalyst (Ru(2+) → Ru(3+)) on a 100 ps to 1 ns timescale. This is followed by 10-100 ns electron transfer (ET) that reduces the Ru(3+) center. The relatively slow ET dynamics may provide opportunities for the accumulation of multiple holes at the catalyst, which is necessary for water oxidation.