Efficiency Of Electron Tunneling Research Articles

Computer-Aided Rational Construction of Mediated Bioelectrocatalysis with π-Conjugated (Hetero)cyclic Molecules: Toward Promoted Distant Electron Tunneling and Improved Biosensing.

Highly π-conjugated (hetero)cyclic molecules having delocalized orbitals and tunable charge mobilities are attractive redox relays for mediated bioelectrocatalysis in manifold applications. As rigid molecules, their dynamics within the soft but confined intraprotein space becomes the crucial determinant of the enzyme-mediator electron-tunneling efficiency. However, it is rarely investigated in designing the mediated interface with a particular biocatalyst (e.g., oxidoreductase), which remains an empirical but try-and-error process. Herein, we present the computer-aided exploration of interactions between a flavin-containing reductive synthase and structurally diverse π-extended (hetero)cyclic mediators to realize reversed bioelectrocatalytic oxidation at low overpotentials. Compared to ring-fused systems with unbroken molecular planarity, heteroatom-bridged cyclic, and rotatable conjugated structures (e.g., indophenols) can experience unusually large dynamic torsion under biased forces of hydrogen bonding with enzyme residues. This behavior led to fast intraprotein reorientation (<50 ps) that shortened the electron-tunneling distance from 12 to 9 Å. Meanwhile, the lowest unoccupied molecular orbital level upon molecular torsion was decreased by 0.5 eV to further promote electron abstraction from the reduced flavin cofactor. An efficient distant electron tunneling also obviated mediator transport through the substrate channel, thus avoiding competitive inhibition on enzyme kinetics to broaden the operating concentration range. The resulting bioelectrocatalytic interface enables low-potential biosensing of glutamate with improved selectivity. Our finding provides new structural insights into constructing efficient long-range heterogeneous charge transport with biomacromolecular catalysts.

Nano-Fe0 embedded in N-doped carbon architectures for enhanced oxidation of aqueous contaminants

Nano-Fe0 embedded in N-doped carbon architectures (Fe0@N-C) were used as a peroxymonosulfate (PMS) activator for the mineralization of organic pollutants. Fe0@N-C was synthesized through a MOF-mediated recrystallization approach. Fe0@N-C exhibited high activity and reusability, better than that of most conventional metal catalysts. Further, the influences of some representative factors for the PMS activation were comprehensively analyzed and optimized. The enhanced performance of Fe0@N-C was primarily ascribed to abundant reactive oxygen species, especially 1O2, released in the oxidation process at the solid/aqueous interface. Nano-Fe0 wrapped with N-doped graphitic layers could preferentially generate effective Fe0 and Fe-Nx active sites with the electronic structure reconstruction, endowing the outstanding catalytic activity. The unique porous carbon structure not only protected the embedded nano-Fe0 from poisoning or leaching, but also provided suitable mass transfer channels and also promoted electron tunneling efficiency. These outstanding characteristics manifested Fe0@N-C had promising potential for organic pollutant degradation.

Nonvolatile Electrical Bistability Behaviors Observed in Au/Ag Nanoparticle-Embedded MOFs and Switching Mechanisms.

Electrically bistable devices play an important role in the next generation of information materials. Plasmonic noble metal nanoparticles (Au and Ag NPs) with diameters <6 nm were embedded into 3-D Cd-based metal-organic framework (MOF) matrixes via the photoreduction method to generate Au (Ag) NPs@MOF composites. Electrical bistability measurements on the sandwiched ITO/NPs@MOF/silver devices indicate that two switchable conductivity states with nonvolatile memory behaviors can be observed. The ITO/Au NP@2/Ag device with neutral matrix possesses the highest ON/OFF current ratio of 104, which can be attributed to its higher electron tunneling efficiency because of the better dispersity of Au NPs in the MOF matrix. A mechanism regarding the electric-field-induced charge-transfer process assisted by conformational change in the active layer was proposed.

Tunable Resistive Switching Enabled by Malleable Redox Reaction in the Nano-Vacuum Gap.

Neuromorphic computing has emerged as a highly promising alternative to conventional computing. The key to constructing a large-scale neural network in hardware for neuromorphic computing is to develop artificial neurons with leaky integrate-and-fire behavior and artificial synapses with synaptic plasticity using nanodevices. So far, these two basic computing elements have been built in separate devices using different materials and technologies, which poses a significant challenge to system design and manufacturing. In this work, we designed a resistive device embedded with an innovative nano-vacuum gap between a bottom electrode and a mixed-ionic-electronic-conductor (MIEC) layer. Through redox reaction on the MIEC surface, metallic filaments dynamically grew within the nano-vacuum gap. The nano-vacuum gap provided an additional control factor for controlling the evolution dynamics of metallic filaments by tuning the electron tunneling efficiency, in analogy to a pseudo-three-terminal device, resulting in tunable switching behavior in various forms from volatile to nonvolatile switching in a single device. Our device demonstrated cross-functions, in particular, tunable neuronal firing and synaptic plasticity on demand, providing seamless integration for building large-scale artificial neural networks for neuromorphic computing.

The reduction of 4-nitrobenzene diazonium electrografted layer: An electrochemical study coupled to in situ sum-frequency generation spectroscopy

This work describes an electrochemical study of 4-nitrobenzene diazonium (4-NBD) reduction onto glassy carbon (GC) electrode coupled to in situ sum-frequency generation (SFG) spectroscopy. After 4-NBD grafting at 0.3 V vs. saturated calomel electrode (SCE) onto GC, SFG allowed a clear signal assigned to the symmetrical vibration mode of the nitro (NO2) groups to be observed at 1349 cm−1 or 1353 cm−1 depending on whether the spectrum was recorded in air or inside the solution. This result proved that 4-NBD grafting actually occurs at a potential as high as 0.3 V vs. SCE. The combination of SFG data and cyclic voltammetry (CV) also indicated that at such a potential, NO2 groups did not experience reduction process into hydroxylamine (NHOH) or amine (NH2) groups. The electrolysis of grafted NO2 moieties at −0.1 V was followed by CV and in situ by SFG. The exponential decay of the NO2 signal located at 1353 cm−1 vs. electrolysis time was in accordance with a charge transfer-limited reaction rate for a species immobilized at the electrode surface, and allowed a first order kinetic rate constant for NO2 reduction to be estimated k = 0.006 s−1. The integration of the peaks observed on the corresponding cyclic voltammograms (CVs) which were attributed to the NO/NHOH reversible system showed that the NO2 reduction produced both hydroxylamine and amine groups and was not quantitative. The fact that SFG spectroscopy was silent for long electrolysis time values suggested the remaining nitro groups to be located far from the electrode surface, as a consequence of an electron tunneling efficiency which decreased throughout the film thickness. Further electrolysis at −0.8 V allowed the remaining nitro groups to be reduced into NH2 with almost quantitative yields. All these results suggest the existence of a stratified layer during the electrolysis process, in which there is no limitation due to H+ diffusion in the organic film.

Flexible Infrared Responsive Multi-Walled Carbon Nanotube/Form-Stable Phase Change Material Nanocomposites.

Flexible infrared (IR)-responsive materials, such as polymer nanocomposites, that exhibit high levels of IR responses and short response times are highly desirable for various IR sensing applications. However, the IR-induced photoresponses of carbon nanotube (CNT)/polymer nanocomposites are typically limited to 25%. Herein, we report on a family of unique nanocomposite films consisting of multi-walled carbon nanotubes (MWCNTs) uniformly distributed in a form-stable phase change material (PCM) that exhibited rapid, dramatic, reversible, and cyclic IR-regulated responses in air. The 3 wt % MWCNT/PCM nanocomposite films demonstrated cyclic, IR-regulated on/off electrical conductivity ratios of 11.6 ± 0.6 and 570.0 ± 70.5 times at IR powers of 7.3 and 23.6 mW/mm(2), respectively. The excellent performances exhibited by the MWCNT/PCM nanocomposite films were largely attributed to the IR-regulated cyclic and reversible form-stable phase transitions occurring in the PCM matrix due to MWCNT's excellent photoabsorption and thermal conversion capabilities, which subsequently affected the thickness of the interfacial PCM between adjacent conductive MWCNTs and thus the electron tunneling efficiency between the MWCNTs. Our findings suggest that these unique MWCNT/PCM nanocomposites offer promising new options for high-performance and flexible optoelectronic devices, including thermal imaging, IR sensing, and optical communication.

Computational Investigation into Photoswitching Efficiency of Diarylethene Derivatives: An Insight Based on the Decay Constant of Electron Tunneling

The switching efficiency (SE) of the intramolecular exchange interaction J between the open- and the closed-ring isomers of diarylethenes (DAEs) was investigated using DFT calculations of DAE biradicals with different core structures: DAEs with 3-thienyl, thiophene-S,S-dioxide-3-yl, 2-thienyl, or thiophene-S,S-dioxide-2-yl rings. The SE of DAE with a 3-thienyl ring is calculated to be around 400-fold, which is the largest among the four calculated DAEs. The decay constant β of the exchange interaction J for the DAE molecular wires was evaluated by calculating J for biradicals with different lengths of wires. For the wires of the closed-ring isomers of DAE with 3-thienyl- and thiophene-S,S-dioxide-3-yl rings, which are supposed to take a quinoid structure, allyl nitronyl nitroxide radical was successfully employed. The calculated β values showed a significant difference between the open- and the closed-ring isomers, and this difference of β is considered to be the origin of photoswitching of J. The difference of β upon isomerization, Δβ, is in good agreement with SE, and the largest Δβ was obtained for the DAE with a 3-thienyl ring. We can understand the switching of J as the switching of electron tunneling efficiency β between the open- and the closed-ring isomers.

Theoretical Investigation of the β Value of the π-Conjugated Molecular Wires by Evaluating Exchange Interaction between Organic Radicals

The decay constant β of π-conjugated molecular wire corresponds to the efficiency of electron tunneling through the molecule. We have calculated β values of intramolecular magnetic exchange interactions by density functional theory for organic bis(nitronyl nitroxide)s that have several molecular wire units, namely, oligo-p-phenylene (OPP), oligophenylene ethynylene (OPE), oligophenylene vinylene (OPV), oligophenylene imine (OPI), bicyclo[1.1.1]pentane oligomer (BCP), β,meso,β-triply fused porphyrin oligomer (TFP), and meso,meso-linked porphyrin oligomer (MLP). For OPP, OPE, OPV, OPI, BCP, TFP, and MLP, β was calculated to be 0.41, 0.25, 0.22, 0.32, 0.77, 0.03, and 0.65 Å–1, respectively. The obtained β values are in good agreement with the experimental value of the molecular tunneling conductance, suggesting similarity of the mechanism between exchange interaction and molecular conductance. This method can be used to estimate the efficiency of electron tunneling through the molecular wire.

Use of U-shaped donor-bridge-acceptor molecules to study electron tunneling through nonbonded contacts.

A systematic determination of electronic coupling matrix elements in U-shaped molecules is demonstrated. The unique architecture of these systems allows for the determination of the electronic coupling through a pendant molecular moiety that resides between the donor and acceptor groups; this moiety quantifies the efficiency of electron tunneling through nonbonded contacts. Experimental electron-transfer rate constants and reaction free energies are used to calibrate a molecular-based model that describes the solvation energy. This approach makes it possible to experimentally determine electronic couplings and compare them with computational values.

Electron Transport Through Composite Monolayers

Well-organized thiol monolayers on electrode surfaces are prepared using the Langmuir–Blodgett and self-assembly methods. Planned modification of the molecules building the monolayer allow the electron tunneling efficiency across the monolayer to be controlled. The barrier properties of the monolayers are probed by electrochemical methods. The extent of blocking for all systems under study indicates that contribution of the electroactive molecules that find direct access to the electrode surface can be neglected. These observations permit us to use the monolayers for the determination of the kinetic parameters of Fe(CN)3– 6 and IrCl2– 6 ion reduction. Such monolayers are employed for the studies of long-range electron transport. We show that insertion of amide bonds in appropriate positions of the alkyl chains of all molecules building the monolayer makes it possible to create a lateral hydrogen-bond network linking the internal amide groups in the monolayer and contributing to the electronic coupling between the redox probe and the electrode. The relation between the location of the amide moiety in the molecule and its importance for the electron tunneling efficiency through the intervening organic medium is discussed.

Kinetics of long-range electron transfer through alkanethiolate monolayers containing amide bonds

Non-electroactive alkanethiolate monolayers containing internal amide bonds were used as model systems for the studies of the effect of structure of the intervening medium on long-range electron transfer. The blocking properties and the kinetics of electron transfer across the monolayers immobilized on gold were studied by voltammetry with the hexachloroiridate(IV) ion as the redox probe in the solution. The electron transfer efficiency was measured over a large potential window. The three types of monolayers studied were simple octadecanethiol and two amide-containing systems with one or two amide moieties in place of selected methylene groups in the main alkyl chain. Enhanced electronic coupling between the redox probe and the metal of the electrode was found for the monolayers with internal amide bonds. We ascribed it to the contribution of a hydrogen bonded network to electron tunneling through the monolayer. In the case of monolayers formed by molecules containing two secondary amide groups, the location of amide moieties inside the monolayer was shown to play an important role in the electron transfer efficiency. The second amide moiety placed in the alkyl chain in the odd position relative to the first amide did not increase electronic coupling in the monolayer. This behavior can be explained as due to larger distances between the amide groups in the external plane of the monolayer leading to difficulty in the formation of the hydrogen bond network. The position of the amide group relative to the electrode surface may be also considered as an important factor determining the efficiency of electron tunneling through the monolayer.

Mercury–mercury tunneling junctions

Abstract We have investigated electrical properties of symmetric alkanethiolate bilayer junctions formed by contacting two hanging mercury drop electrodes each coated initially with a single alkanethiolate monolayer. Using potentiodynamic, current transient and ac impedance methods, we have shown that the initial, largely all-trans structure of the bilayers becomes disordered with time as a result of the van der Waals and coulombic forces squeezing the junction. Chain disorder involves both gauche defects and chain intercalation at the mid plane of the bilayer. We showed that the latter progress more rapidly for junctions formed with longer alkanethiols. To characterize the strength of the electronic coupling across these junctions, we measured the decay constant β for the native bilayers featuring largely all-trans conformation and for the disordered junctions. The former was obtained from fast scan (50–500 V s−1) j–V experiments and yielded β=1.29 per CH2 group (corresponding to β=1.0 A−1). Impedance measurements were used to investigate properties of the disordered junctions. Electron tunneling resistance and junction capacitance were measured simultaneously. The plot of the logarithm of junction resistance measured at zero voltage bias versus thickness gave β=1.6 A−1. These measurements demonstrate that the efficiency of electron tunneling decreases substantially when alkanethiolate chains become disordered.

The effect of image force on the efficiency of electron tunneling through macromolecular structures

In electron tunneling (ET) along the boundary of two media with different high-frequency dielectric permittivities, the energy gap for the tunneling electron is affected by image forces. Macromolecules and macromolecular structures contain segments possessing different permittivities. Therefore, in any description of ET one must take image forces into account. Here, the spatial form of donor-acceptor ET through bridged molecular groups situated near macromolecular segments is investigated. It is shown that image forces can significantly reduce the energy levels of an electron on the bridge groups. As a result, the ET rate in the donor-bridge-acceptor system can increase by many orders of magnitude. the effect is more favorable for extended bridged chains. This fact may be used in explaining donor-acceptor conductivity of electron pathways in real macromolecular biological structures. Bridged groups in electron pathways may be provided by molecular groups whose affinity for the tunneling electron is higher than that of other groups. Furthermore, the electron overlap integral between the bridged groups should be large enough. That is, the role of bridge can be played by the polypeptide chain backbone, by side-chains of some amino acids (particularly those with aromatic rings), and by conjugated systems of the carotinoid type situated near macromolecular segments or near the surfaces of membranes or channels.

Orientation-dependent tunneling of electrons between planar cyclic molecules

We have investigated the dependence of the efficiency of electron tunneling between planar cyclic molecules on their mutual orientation. An expression has been obtained for the dependence on orientation of the tunneling probability in terms of the “cyclic polyene” model. The magnitude of the orientation effect has been estimated for a change in the orientation of the molecules from the face-to-face to the edge-to-edge position.

Solvent effects on the kinetics of simple electrochemical reactions: Part I. Comparison of the behavior of Co(III)/(II) trisethylenediamine and ammine couples with the predictions of dielectric conitnuum theory

The electroreduction kinetics of Co(en) 3 3+ (en-ethylenediamine), Co(NH 3) 6 3+, and Co(NH 3) 5F 2+ have been investigated atmercury electrodes in aqueous solution and six nonaqueous solvents in order to explore the influence of the solvent upon the electrode kinetics of such simple one-electron outer-sphere reactions where the composition of the reactant's coordination sphere remains fixed. Substantial variations in the experimental rate parameters were observed as the solvent was altered; these reflect the influence of the outer-shell solvent upon the reorganization barrier to electron transfer. A simple phenomenological treatment of solvent effects upon the electrode kinetics of such simple redox reactions is given. It is pointed out that double-layer corrected rate constants k corr can be evaluated at the same Galvani potential in different solvents with useful accuracy; these quantities can provide particular insight into the chemical influence of the solvent. The utility of separating such solvent effects into “intrinsic” and “thermodynamic” contributions is noted. The substantial decreases in k corr , seen when substituting several non-aqueous solvents for water, were traced to increases in the outer-shell component of the intrinsic free-energy barrier (Δ G i # ) os. In contrast, small decreases in (Δ G i # ) os are predicted by the dielectric continuum model under these circumstances. These discrepancies between theory and experiment are ascribed to contributions to (Δ G i # ) os from extensive short-range reorientation of solvent molecules. Variations in the efficiency of electron tunneling within the transition state may also provide a contribution to the observed solvent effects.

Cathodes for HeNe Lasers

Vapor deposited aluminum cathodes for parallel plate HeNe lasers have been used with considerable success. It is shown that the aluminum cold cathode is improved by the addition of copper and that other alloying elements of bulk Al 2024 do not contribute to cathode performance. The plasma oxidation process of Hochuli improves cathode performance by cleaning the surface and optimizing the oxide film for maximum efficiency. There appears to be a trade-off between life and electron tunneling efficiency for optimum oxide thickness. Auger electron spectroscopy, ellipsometry, and scanning electron microscopy are the techniques used for surface characterization.