Control Of Electron Transport Research Articles

Electrochemical impedance spectroscopy analysis of dye-sensitized solar cells composed of electrospun composite photoanodes: A comparative study of natural and synthetic sensitizers

In this study, a transmission line model is applied to the electrochemical impedance spectroscopy (EIS) data of the fabricated dye-sensitized solar cells (DSSCs) to evaluate the charge transfer mechanism through the cells. Natural dye from black plum (Syzygium cumini) fruit was used as a cell sensitizer (SC-DSSC) and compared its photovoltaic and electron transport capabilities to those of a cell using a synthetic sensitizer (N719-DSSC). TiO2/ZnO electrospun composite nanofibers were used as the semiconductor layer of the photoanode to enhance electron transfer. The EIS analysis revealed the role of electron resistances through shant, interfaces, and electrolyte solution by measuring the electron transfer kinetic parameters of each element. Based on the results, the SC-DSSC and N719-DSSC are appropriate photovoltaic cells because their ratios of effective electron diffusion length to photoanode thickness are 12.5 and 2.8, respectively. The EIS analysis showed that the electrospun composite nanofiber coated on the photoanode reduces the semiconductor layer's electrical resistance to the cell's total resistance. The extracted natural dye also boosted electron lifetime to 3.68 ms and diffusion coefficient to 54.3×10−6 m2/s while minimizing back-electron recombination at the semiconductor-electrolyte interface. Moreover, the semiconductor-electrolyte interface resistance is over 85% of the overall resistance for both DSSCs and controls electron transport through the cells, which is due to the dye-semiconductor binding intensity. Based on the photovoltaic data, the SC-DSSC cell efficiency was lower than N719-DSSC which is attributed to its higher electron transfer rate-controlling element. Thus, enhancing dye-semiconductor interactions will decrease the rate-controlling impedance and enhance cell performance.

Thylakoid membrane stacking controls electron transport mode during the dark-to-light transition by adjusting the distances between PSI and PSII.

The balance between linear electron transport (LET) and cyclic electron transport (CET) plays an essential role in plant adaptation and protection against photo-induced damage. This balance is largely maintained by phosphorylation-driven alterations in the PSII-LHCII assembly and thylakoid membrane stacking. During the dark-to-light transition, plants shift this balance from CET, which prevails to prevent overreduction of the electron transport chain and consequent photo-induced damage, towards LET, which enables efficient CO2 assimilation and biomass production. Using freeze-fracture cryo-scanning electron microscopy and transmission electron microscopy of Arabidopsis leaves, we reveal unique membrane regions possessing characteristics of both stacked and unstacked regions of the thylakoid network that form during this transition. A notable consequence of the morphological attributes of these regions, which we refer to as 'stacked thylakoid doublets', is an overall increase in the proximity and connectivity of the two photosystems (PSI and PSII) that drive LET. This, in turn, reduces diffusion distances and barriers for the mobile carriers that transfer electrons between the two PSs, thereby maximizing LET and optimizing the plant's ability to utilize light energy. The mechanics described here for the shift between CET and LET during the dark-to-light transition are probably also used during chromatic adaptation mediated by state transitions.

Cytochrome c lysine acetylation regulates cellular respiration and cell death in ischemic skeletal muscle

Skeletal muscle is more resilient to ischemia-reperfusion injury than other organs. Tissue specific post-translational modifications of cytochrome c (Cytc) are involved in ischemia-reperfusion injury by regulating mitochondrial respiration and apoptosis. Here, we describe an acetylation site of Cytc, lysine 39 (K39), which was mapped in ischemic porcine skeletal muscle and removed by sirtuin5 in vitro. Using purified protein and cellular double knockout models, we show that K39 acetylation and acetylmimetic K39Q replacement increases cytochrome c oxidase (COX) activity and ROS scavenging while inhibiting apoptosis via decreased binding to Apaf-1, caspase cleavage and activity, and cardiolipin peroxidase activity. These results are discussed with X-ray crystallography structures of K39 acetylated (1.50 Å) and acetylmimetic K39Q Cytc (1.36 Å) and NMR dynamics. We propose that K39 acetylation is an adaptive response that controls electron transport chain flux, allowing skeletal muscle to meet heightened energy demand while simultaneously providing the tissue with robust resilience to ischemia-reperfusion injury.

Regulating the orientation of a single coordinate bond by the synergistic action of mechanical forces and electric field.

The molecular binding orientation with respect to the electrode plays a pivotal role in determining the performance of molecular devices. However, accomplishing in situ modulation of single-molecule binding orientation remains a great challenge due to the lack of suitable testing systems and characterization approaches. To this end, by employing a developed STM-BJ technique, we demonstrate that the conductance of pyridine-anchored molecular junctions decreases as the applied voltage increases, which is determined by the repeated formation of thousands of gold-molecule-gold dynamic break junctions. In contrast, the static fixed molecular junctions (the distance between two electrodes is fixed) with identical molecules exhibit a reverse tendency as the bias voltage increases. Supported by flicker noise measurements and theoretical calculations, we provide compelling evidence that the orientation of nitrogen-gold bonds (a universal coordinate bond) in the pyridine-anchored molecular junctions can be manipulated to align with the electric field by the synergistic action of the mechanical stretching force and the electric fields, whereas either stimulus alone cannot achieve the same effect. Our study provides a framework for characterizing and regulating the orientation of a single coordinate bond, offering an approach to control electron transport through single molecular junctions.

Quantum interference of electrons through electric-field-induced edge states in stacked graphene nanoribbons

In this work, we investigate, by means of numerical simulations, the quantum interference of electrons in stacked graphene structures consisting of two unequal width, armchair-edged graphene nanoribbons. Electronic states residing near the edges of the system are induced when an external electric field is applied normal to the ribbons. By reversing the direction of electric field in the central region, one can create an electronic analogue of the optical Fabry-Pérot (FP) interferometer. Electronic junctions formed at the boundaries between the central region and the left and right ones in the former play the role of the partially reflected mirrors in the latter. The observed conductance oscillations demonstrate that electrons in the edge states transporting through the system experience quantum interference similar to that of light waves passing through an optical FP interferometer. Moreover, electronic states formed at the junctions enhance inter-edge scattering which affects electron transmission significantly. The possibility to control electron transport via electric gates is also considered.

Photon and magnetic field controlled electron transport of a multiply-resonant photon-cavity double quantum dot system

We study electron transport through double quantum dots (DQD) coupled to a cavity with a single photon mode. The DQD is connected to two electron reservoirs, and the total system is under an external perpendicular magnetic field. The DQD system exhibits a complex multi-level energy spectrum. By varying the photon energy, several anti-crossings between photon dressed electron states of the DQD-cavity system are found at low strength of the magnetic field. The anti-crossings are identified as multiple Rabi resonances arising from the photon exchange between these states. As the results, a dip in the current is seen caused by the multiple Rabi resonances. By increasing the strength of the external magnetic field, a dislocation of the current dip to a lower photon energy is found and the current dip can be diminished. The interplay of the strength of the magnetic field and the geometry of the states the DQD system can weaken the multiple Rabi resonances in which the exchange of photon between the anti-crossings is decreased. We can therefore confirm that the electron transport behavior in the DQD-cavity system can be controlled by manipulating the external magnetic field and the photon cavity parameters.

(Nanocarbons Division Richard E. Smalley Research Award) Evolution of Nanocarbons in Energy Conversion and Storage

The discovery of fullerenes led to the exploration of their excited state and electron storage properties in 1990s. This wave of research quickly led to the growth of carbon nanotube research, followed by graphene based nanocarbon materials. After a span of more than three decades, we now have a better understanding of properties of these nanocarbons and how to utilize them in various applications. Of particular interest are graphene–semiconductor or −metal nanoparticle composites with the capability to function as efficient, multifunctional materials for energy conversion and storage. In particular, reduced graphene oxide (RGO) is a suitable composite substrate because of its two-dimensional structure, outstanding surface area, and electrical conductivity. The presentation will focus on common employed methods to design graphene oxide-semiconductor composite materials and examine key studies that characterize their excited state interactions. The favorable energetics of RGO for intercepting excited states and inducing charge separation, along with the ease of developing electrode materials with tailored properties make it an exceptionally important mediator for improving the performance of solar cells and photocatalyst arrays. Strategies to develop graphene composites and control electron capture and transport through the 2D carbon network will be discussed.

Flavodiiron proteins enhance the rate of CO2 assimilation in Arabidopsis under fluctuating light intensity.

The proton concentration gradient (ΔpH) and membrane potential (Δψ) formed across the thylakoid membrane contribute to ATP synthesis in chloroplasts. Additionally, ΔpH downregulates photosynthetic electron transport via the acidification of the thylakoid lumen. K+ exchange antiporter 3 (KEA3) relaxes this downregulation by substituting ΔpH with Δψ in response to fluctuation of light intensity. In the Arabidopsis (Arabidopsis thaliana) line overexpressing KEA3 (KEA3ox), the rate of electron transport is elevated by accelerating the relaxation of ΔpH after a shift from high light (HL) to low light. However, the plant cannot control electron transport toward photosystem I (PSI), resulting in PSI photodamage. In this study, we crossed the KEA3ox line with the line (Flavodiiron [Flv]) expressing the Flv proteins of Physcomitrium patens. In the double transgenic line (Flv-KEA3ox), electrons overloading toward PSI were pumped out by Flv proteins. Consequently, photodamage of PSI was alleviated to the wild-type level. The rate of CO2 fixation was enhanced in Flv and Flv-KEA3ox lines during HL periods of fluctuating light, although CO2 fixation was unaffected in any transgenic lines in constant HL. Upregulation of CO2 fixation was accompanied by elevated stomatal conductance in fluctuating light. Consistent with the results of gas exchange experiments, the growth of Flv and Flv-KEA3ox plants was better than that of WT and KEA3ox plants under fluctuating light.

Controlling electron transport towards efficient all-solution-processed quantum dot light emitting diodes

A high maximum external quantum efficiency of an all-solution-processed quantum dot light-emitting diode is demonstrated to be 28.6% by tailoring the electron injection for balancing the charge injection in a QLED.

Electrostatic Fermi level tuning in large-scale self-assembled monolayers of oligo(phenylene-ethynylene) derivatives.

Understanding and controlling the orbital alignment of molecules placed between electrodes is essential in the design of practically-applicable molecular and nanoscale electronic devices. The orbital alignment is highly determined by the molecule-electrode interface. Dependence of orbital alignment on the molecular anchor group for single molecular junctions has been intensively studied; however, when scaling-up single molecules to large parallel molecular arrays (like self-assembled monolayers (SAMs)), two challenges need to be addressed: 1. Most desired anchor groups do not form high quality SAMs. 2. It is much harder to tune the frontier molecular orbitals via a gate voltage in SAM junctions than in single molecular junctions. In this work, we studied the effect of the molecule-electrode interface in SAMs with a micro-pore device, using a recently developed tetrapodal anchor to overcome challenge 1, and the combination of a single layered graphene top electrode with an ionic liquid gate to solve challenge 2. The zero-bias orbital alignment of different molecules was signalled by a shift in conductance minimum vs. gate voltage for molecules with different anchoring groups. Molecules with the same backbone, but a different molecule-electrode interface, were shown experimentally to have conductances that differ by a factor of 5 near zero bias. Theoretical calculations using density functional theory support the trends observed in the experimental data. This work sheds light on how to control electron transport within the HOMO-LUMO energy gap in molecular junctions and will be applicable in scaling up molecular electronic systems for future device applications.

Piezoelectric tunability and topological insulator transition in a GaN/InN/GaN quantum-well device

Using an 8-band model it is demonstrated through the combination of strain and piezoelectricity that increasing the InN quantum-well thickness of a GaN-InN-GaN device changes the InN material from a positive bandgap semiconductor to a topological insulator (negative bandgap). Moderate strain tuning of a four monolayer InN layer for a GaN-InN-GaN device reveals a giant (one order of magnitude) tuning of current–voltage characteristics. It is verified that piezoelectricity plays an important role in controlling electron transport through the InN layer.

Abstract 331: Impact of Acetylated Cyclophilin D on the Mitochondrial Permeability Transition Pore

Background: The mitochondrial matrix protein cyclophilin D (CypD) is a key regulator of mitochondrial function. CypD controls electron transport chain activity and ATP synthesis by regulating the permeability transition pore (PTP). The activity of CypD is regulated by several post-translational modifications including acetylation of lysine 166 in the mouse. Objective: To investigate how acetylation at lysine 166 of CypD specifically in the heart modifies its ability to regulate the PTP and the ATP synthase. Results: We generated a conditional cardiac knock-in mouse model where lysine 166 has been mutated into glutamine (CypD K166Q ) to mimic permanent acetylation of CypD. The mice were either +/+, +/- or -/- for the expression of native CypD. Results show that mitochondrial oxygen consumption was not affected by the expression of CypD K166Q . The calcium retention capacity (CRC) was measured with Arsenazo III and decreased significantly when CypD K166Q was expressed. The CypD inhibitor cyclosporine A significantly increased the CRC in WT mice. However, cyclosporine A was did not inhibit CypD in the hearts of mice expressing only CypD K166Q or in addition to wild-type CypD. The ability of the ATP synthase to create dimers or oligomers was assessed by western blotting and the hydrolysis of ATP in in-gel assays and shows that expression of CypD K166Q decreased the assembly of the ATP synthase into dimers or oligomers. Conclusions: Our data show that the expression of CypD K166Q increases the sensitivity of PTP opening to calcium and limits the assembly of ATP synthase into oligomers.

Acetylation of Cyclophilin D Increases Calcium Sensitivity of the Permeability Transition Pore

The mitochondrial matrix protein cyclophilin D (CypD) is key regulator of mitochondrial function. It controls electron transport chain activity and ATP synthesis by regulating the permeability transition pore (PTP). The activity of CypD is tuned by several post-translational modifications including acetylation of lysine 166 in the mouse. To investigate how acetylation at lysine 166 modifies the ability of CypD to regulate the PTP and the ATP synthesis. We generated a knock-in mouse model, were specifically in the heart, lysine 166 has been mutated into glutamine (CypDK166Q) to mimic permanent acetylation of CypD. The mice were either +/+, +/- or -/- for the expression of CypD. Calcium retention capacity (CRC) was measured with Arsenazo III in the absence or presence of cyclosporine A (CsA) or ADP. The ability of the ATP synthase to create dimers or oligomers was accessed western blotting and by the hydrolysis of ATP in an in-gel assay. The CRC decreased significantly in mice when CypDK166Q mutation was expressed, while cyclosporine A or NIM811 increased the CRC. ADP increased the CRC in heart mitochondria from all mice in all experimental groups significantly (p ≤ 0.001). In-gel assays and western blotting after clear native electrophoresis show that in mitochondria from mice expressing CypD K166Qthe ATP synthase is less likely to form dimers or oligomers. Our data show that the expression of CypD mutant of CypD increases the sensitivity to calcium and limits the generation of ATP synthase oligomers.

Spin-Dependent Electron Transport through Bacterial Cell Surface Multiheme Electron Conduits.

Multiheme cytochromes, located on the bacterial cell surface, function as long-distance (>10 nm) electron conduits linking intracellular reactions to external surfaces. This extracellular electron transfer process, which allows microorganisms to gain energy by respiring solid redox-active minerals, also facilitates the wiring of cells to electrodes. While recent studies have suggested that a chiral induced spin selectivity effect is linked to efficient electron transmission through biomolecules, this phenomenon has not been investigated in extracellular electron conduits. Using magnetic conductive probe atomic force microscopy, Hall voltage measurements, and spin-dependent electrochemistry of the decaheme cytochromes MtrF and OmcA from the metal-reducing bacterium Shewanella oneidensis MR-1, we show that electron transport through these extracellular conduits is spin-selective. Our study has implications for understanding how spin-dependent interactions and magnetic fields may control electron transport across biotic-abiotic interfaces in both natural and biotechnological systems.

Controlling polaron formation at hematite surfaces by molecular functionalization probed by XUV reflection-absorption spectroscopy.

Small polaron formation is known to limit the photocatalytic charge transport efficiency of hematite via ultrafast carrier self-trapping. While small polaron formation is known to occur in bulk hematite, a complete description of surface polaron formation in this material is not fully understood. Theoretical predictions indicate that the kinetics and thermodynamics of surface polaron formation are different than those in bulk. However, to test these predictions requires the ability to experimentally differentiate polaron formation dynamics at the surface. Near grazing angle extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy is surface sensitive and provides element and oxidation state specific information on a femtosecond time scale. Using XUV-RA, we provide a systematic comparison between surface and bulk polaron formation kinetics and energetics in photoexcited hematite. We find that the rate of surface polaron formation (250 ± 40 fs) is about three times slower than bulk polaron formation (90 ± 5 fs) in photoexcited hematite. Additionally, we show that the surface polaron formation rate can be systematically tuned by surface molecular functionalization. Within the framework of a Marcus type model, the kinetics and energetics of polaron formation are discussed. The slower polaron formation rate observed at the surface is found to result from a greater lattice reorganization relative to bulk hematite, while surface functionalization is shown to tune both the lattice reorganization as well as the polaron stabilization energies. The ability to tune the kinetics and energetics of polaron formation and hopping by molecular functionalization provides the opportunity to synthetically control electron transport in hematite.

Hidden anisotropy in the Drude conductivity of charge carriers with Dirac-Schrödinger dynamics

We show that the conductivity of a two-dimensional electron gas can be intrinsically anisotropic despite isotropic Fermi surface, energy dispersion, and disorder configuration. In the model we study, the anisotropy stems from the interplay between Dirac and Schr\"odinger features combined in a special two-band Hamiltonian describing the quasiparticles similar to the low-energy excitations in phosphorene. As a result, even scalar isotropic disorder scattering alters the nature of the carriers and results in anisotropic transport. Solving the Boltzmann equation exactly for such carriers with point-like random impurities we find a hidden knob to control the anisotropy just by tuning either the Fermi energy or temperature. Our results are expected to be generally applicable beyond the model studied here, and should stimulate further search for the alternative ways to control electron transport in advanced materials.

Spatial Scales and Plasma Properties of the Distant Magnetopause: Evidence for Selective Ion and Electron Transport

AbstractOne of the most important sources of magnetospheric plasma is particle entry through the distant magnetotail boundary, the nightside magnetopause. This entry mechanism depends on the magnetopause configuration. Off the equator, the strong lobe magnetic field renders the magnetopause a tangential or a rotational discontinuity, and thus the magnetosheath field orientation predominantly controls particle entry through magnetic reconnection. At the equatorial, distant tail magnetopause, however, the magnetic field's control of particle entry is diminished because the plasma beta there is large on both sides of the boundary. Thus, transport there can be significantly different from that at the dayside and off‐equatorial magnetopauses. Using observations from two Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun probes, we investigate plasma transport mechanisms around the distant equatorial magnetopause. We find that transport occurs as a series of abrupt transitions in density, ion and electron temperatures, and ion kinetic energy of spatial scales as small as a typical plasma sheet ion gyroradius. Analysis of the particle phase space density reveals that an energy‐selection mechanism controls electron transport across the magnetopause, whereas ion transport is likely controlled by spatial diffusion driven by low‐frequency magnetic field fluctuations. We discuss the importance of these fluctuations for the magnetopause structure (e.g., the thickness of the transitions in plasma density, ion and electron temperatures, and ion kinetic energy).

Protein Electronics: Chemical Modulation of Contacts Control Energy Level Alignment in Gold-Azurin-Gold Junctions.

Making biomolecular electronics a reality will require control over charge transport across biomolecules. Here we show that chemical modulation of the coupling between one of the electronic contacts and the biomolecules in a solid-state junction allows controlling electron transport (ETp) across the junction. Employing the protein azurin (Az), we achieve such modulation as follows: Az is covalently bound by Au-S bonding to a lithographically prepared Au electrode (Au-Az). Au nanowires (AuNW) onto which linker molecules, with free carboxylic group, are bound via Au-S bonds serve as top electrode. Current-voltage plots of AuNW-linkerCOOH//Az-Au junctions have beenshownearlier to exhibitstep-like features, due to resonant tunneling through discrete Az energy levels. Forming an amide bond between the free carboxylic group of the AuNW-bound linker and Az yields AuNW-linkerCO-NH-Az-Au junctions. This Az-linker bond switches the ETp mechanism from resonant to off-resonant tunneling. By varying the extent of this amide bonding, the current-voltage dependence can be controlled between these two mechanisms, thus providing a platform for altering and controlling the ETp mechanism purely by chemical modification in a two-terminal device, i.e., without a gate electrode. Using results from conductance, including the energy barrier and electrode-molecule coupling parameters extracted from current-voltage fitting and normalized differential conductance analysis and from inelastic-electron-tunneling and photoelectron spectroscopies, we determine the Az frontier orbital energies, with respect to the Au Fermi level, for four junction configurations, differing only in electrode-protein coupling. Our approach and findings open the way to both qualitative and quantitative control of biomolecular electronic junctions.

OCIAD1 Controls Electron Transport Chain Complex I Activity to Regulate Energy Metabolism in Human Pluripotent Stem Cells.

SummaryPluripotent stem cells (PSCs) derive energy predominantly from glycolysis and not the energy-efficient oxidative phosphorylation (OXPHOS). Differentiation is initiated with energy metabolic shift from glycolysis to OXPHOS. We investigated the role of mitochondrial energy metabolism in human PSCs using molecular, biochemical, genetic, and pharmacological approaches. We show that the carcinoma protein OCIAD1 interacts with and regulates mitochondrial complex I activity. Energy metabolic assays on live pluripotent cells showed that OCIAD1-depleted cells have increased OXPHOS and may be poised for differentiation. OCIAD1 maintains human embryonic stem cells, and its depletion by CRISPR/Cas9-mediated knockout leads to rapid and increased differentiation upon induction, whereas OCIAD1 overexpression has the opposite effect. Pharmacological alteration of complex I activity was able to rescue the defects of OCIAD1 modulation. Thus, hPSCs can exist in energy metabolic substates. OCIAD1 provides a target to screen for additional modulators of mitochondrial activity to promote transient multipotent precursor expansion or enhance differentiation.

Graphene-Complex-Oxide Nanoscale Device Concepts.

The integration of graphene with complex-oxide heterostructures such as LaAlO3/SrTiO3 offers the opportunity to combine the multifunctional properties of an oxide interface with the exceptional electronic properties of graphene. The ability to control interface conduction through graphene and understanding how it affects the intrinsic properties of an oxide interface are critical to the technological development of multifunctional devices. Here we demonstrate several device archetypes in which electron transport at an oxide interface is modulated using a patterned graphene top-gate. Nanoscale devices are fabricated at the oxide interface by conductive atomic force microscope (c-AFM) lithography, and transport measurements are performed as a function of the graphene gate voltage. Experiments are performed with devices written adjacent to or directly underneath the graphene gate. Distinct capabilities of this approach include the ability to create highly flexible device configurations, the ability to modulate carrier density at the oxide interface, and the ability to control electron transport up to the single-electron tunneling regime, while maintaining intrinsic transport properties of the oxide interface. Our results facilitate the design of a variety of nanoscale devices that combine excellent transport properties of these two proximal two-dimensional electron systems.