Multistep Tunneling Research Articles

Photokatalytische CO2 Reduktion mit CO2‐bindenden Enzymen

AbstractNeuartige Konzepte zur Nutzung von Kohlendioxid sind erforderlich, um eine kreislauforientierte Kohlenstoffwirtschaft zu erreichen und Umweltprobleme zu minimieren. Um diese Ziele zu erreichen, sind Photo‐, Elektro‐, thermische und Biokatalyse die Schlüsselwerkzeuge, die vorzugsweise in wässrigen Lösungen eingesetzt werden. Dennoch gibt es nur wenige katalytische Systeme, die in Wasser effizient arbeiten. Hier stellen wir eine allgemeine Strategie zur Identifizierung von Enzymen vor, die für die CO2‐Reduktion geeignet sind und zwar auf der Grundlage von Strukturanalysen für potenzielle Kohlendioxid‐Bindungsstellen und anschließenden Mutationen. Wir entdeckten, dass die Phenolsäure‐Decarboxylase aus Bacillus subtilis (BsPAD) die wässrige photokatalytische CO2‐Reduktion in Gegenwart eines Ruthenium‐Photosensibilisators und Natriumascorbat selektiv zu Kohlenmonoxid‐Bildung ermöglicht. Mit modifizierten Varianten von BsPAD wurden TONs von bis zu 978 und Selektivitäten von bis zu 93 % (wobei die gewünschte CO‐ gegenüber der H2‐Erzeugung bevorzugt wurde) erreicht. Durch Mutationen im Bereich des aktiven Zentrums von BsPAD wurden die Umsatzzahlen für die CO‐Erzeugung weiter verbessert. Dabei zeigte sich, dass der Elektronentransfer geschwindigkeitsbegrenzend ist und durch mehrstufiges Tunneln erfolgt. Die Allgemeingültigkeit dieses Ansatzes wurde durch die Verwendung von acht anderen Enzymen bewiesen, die alle die gewünschte Aktivität zeigten, was unterstreicht, dass eine Reihe von Proteinen zur photokatalytischen CO2‐Reduktion in der Lage ist.

Photocatalytic CO2 Reduction Using CO2-Binding Enzymes.

Novel concepts to utilize carbon dioxide are required to reach a circular carbon economy and minimize environmental issues. To achieve these goals, photo-, electro-, thermal-, and biocatalysis are key tools to realize this, preferentially in aqueous solutions. Nevertheless, catalytic systems that operate efficiently in water are scarce. Here, we present a general strategy for the identification of enzymes suitable for CO2 reduction based on structural analysis for potential carbon dioxide binding sites and subsequent mutations. We discovered that the phenolic acid decarboxylase from Bacillus subtilis (BsPAD) promotes the aqueous photocatalytic CO2 reduction selectively to carbon monoxide in the presence of a ruthenium photosensitizer and sodium ascorbate. With engineered variants of BsPAD, TONs of up to 978 and selectivities of up to 93 % (favoring the desired CO over H2 generation) were achieved. Mutating the active site region of BsPAD further improved turnover numbers for CO generation. This also revealed that electron transfer is rate-limiting and occurs via multistep tunneling. The generality of this approach was proven by using eight other enzymes, all showing the desired activity underlining that a range of proteins is capable of photocatalytic CO2 reduction.

Temperature dependent transition of conduction mechanism from carrier injection to multistep tunneling in Fe3O4 (111)/Alq3/Co organic spin valve

We have presented experimental results addressing the origin of spin valve (SV) magneto-resistance (MR), in both injection and tunnel conduction regimes, in our fabricated Fe 3 O 4 (111)/Alq 3 /Co SV device. Experimental evidences have shown that any alternative MR process, such as ‘tunneling anisotropic MR’ is not at the origin of this SV MR. Spin resolved density of states of electrodes indicate that both the conduction mechanisms induce different spin dependent scattering which inturn modify the MR signal of the device. This modification helped in maintaining a non-monotonous quenching of MR signal with increase in temperature. We have also proposed a phenomenological model for device operation where the concept of charge gap modification at Fermi level across Verwey transition is envisaged to offer this unique scenario of tuning the conduction mode and hence MR in this ferrite based organic SV. The model is also supported by an established theoretical study which considers high temperature phonon assisted tunneling through defect states at electrode–organic interface of the device. • Conduction mechanism transition from carrier injection to multistep tunneling. • Transition due to Verwey transition of Fe 3 O 4 . • Spin flip scattering at Alq 3 spacer above Verwey transition temperature. • Non-monotonous MR quenching with temperature. • Room temperature MR signal of hybrid spin valve.

Effects of current transportation and deep traps on leakage current and capacitance hysteresis of AlGaN/GaN HEMT

In the temperature range of 40–320 K, we have studied the current-voltage I–V and capacitance-voltage C–V characteristics of a (Pt/Au)–Al0.2Ga0.8N/GaN/Al2O3 HEMT structure. Based on the I–V characteristics, we found that the leakage current and the three main parameter values of diodes (ideality factor n, series resistance Rs, and barrier height ΦB) were very sensitive to temperature changes. In addition, by considering the different current-transport mechanisms, the experimental data showed that the tunnelling current is the dominant element at all temperatures, which seems to be owing to a multistep tunnelling along dislocations. The C–V analysis showed an increase in the pinch-off voltage, a hysteresis phenomenon, and residual capacitance variation. This unusual behaviour, observed in the I–V and C–V characteristics, has been related to trapping and de-trapping processes of charge carriers happening in the structure. A one hole trap, with an activation energy of 0.316 eV and capture cross sections of 1.3 × 10−21 cm2, was observed in the deep-level transient spectroscopy (DLTS) spectra. DLTS measurements with different gate-voltages (Vgs) showed that this trap can be associated to an extended defect in the (Pt/Au)–Al0.2Ga0.8N/GaN/Al2O3 HEMT structure, such as threading dislocations. The relationship between the anomalies shown in the C–V-T and, I–V-T characteristics, and DLTS measurements, has been established.

Transport and storage dynamics of 30% In-rich InGaN/GaN MQW LED p–i–n structure

DC and AC electrical characteristics of an InGaN/GaN multi-quantum well light emitting diode based on PIN structures were investigated through temperature-dependent current–voltage (I--T) and admittance-temperature-frequency (Y-T-) measurements within 80–375 K temperature interval. Multi-step tunneling was discerned as a carrier conduction mechanism for whole reverse and small forward biases at the high-temperature side in the studied temperature interval while hopping conduction at the low side. Electron and hole as the tunneling carriers were identified through activation energy, 110 meV and 60 meV respectively. The values were determined from I--T/C-T- measurements in conjunction with conductance of quantum well itself and consisted with the reported activation energy; 120–220 meV for electrons, 50–80 meV for holes.

Regimes of current transport mechanisms in CdS/CdTe solar cells

Forward bias recombination (current transport) mechanisms have been evaluated for thin film solar cells and correlated to the in-gap trap levels present. Here CdTe/CdS devices were chosen as an archetypal example of a modern thin film solar cell, and a set of devices with a range of design variables was used in order to reveal the full range of behaviours that may operate to limit current transport. Experimental current–voltage–temperature datasets were compared to mathematical models of transport, and the in-gap traps were evaluated by thermal admittance spectroscopy. The current transport mechanisms operating are presented on a temperature–voltage diagram. Three regimes were identified: at ‘intermediate’ voltages, the behaviour was temperature dependent. From 300 K down to 240 K, thermally activated Shockley Read Hall recombination mediated by a 0.38 eV trap (VCd) dominated the transport. Between 200 and 240 K the transport was thermally activated but below 200 K the mechanism became dominated by tunnel assisted interface recombination. At ‘low’ voltages (and for all devices at all voltages when measured at T < 200 K) band to band recombination is via multi-step tunnelling through in-gap states. At high voltage, the forward current is dominated by the well-known limiting effect of the back Schottky contact to the CdTe which is in reverse bias. The current transport behaviour is also correlated with the n-CdS thickness and CdCl2 processing conditions, both of which are critical to device performance.

Two Tryptophans Are Better Than One in Accelerating Electron Flow through a Protein.

We have constructed and structurally characterized a Pseudomonas aeruginosa azurin mutant Re126WWCuI, where two adjacent tryptophan residues (W124 and W122, indole separation 3.6–4.1 Å) are inserted between the CuI center and a Re photosensitizer coordinated to the imidazole of H126 (ReI(H126)(CO)3(4,7-dimethyl-1,10-phenanthroline)+). CuI oxidation by the photoexcited Re label (*Re) 22.9 Å away proceeds with a ∼70 ns time constant, similar to that of a single-tryptophan mutant (∼40 ns) with a 19.4 Å Re–Cu distance. Time-resolved spectroscopy (luminescence, visible and IR absorption) revealed two rapid reversible electron transfer steps, W124 → *Re (400–475 ps, K1 ≅ 3.5–4) and W122 → W124•+ (7–9 ns, K2 ≅ 0.55–0.75), followed by a rate-determining (70–90 ns) CuI oxidation by W122•+ ca. 11 Å away. The photocycle is completed by 120 μs recombination. No photochemical CuI oxidation was observed in Re126FWCuI, whereas in Re126WFCuI, the photocycle is restricted to the ReH126W124 unit and CuI remains isolated. QM/MM/MD simulations of Re126WWCuI indicate that indole solvation changes through the hopping process and W124 → *Re electron transfer is accompanied by water fluctuations that tighten W124 solvation. Our finding that multistep tunneling (hopping) confers a ∼9000-fold advantage over single-step tunneling in the double-tryptophan protein supports the proposal that hole-hopping through tryptophan/tyrosine chains protects enzymes from oxidative damage.

Living with Oxygen.

Work on the electronic structures of metal–oxo complexes began in Copenhagen over 50 years ago. This work led to the prediction that tetragonal multiply bonded transition metal–oxos would not be stable beyond the iron–ruthenium–osmium oxo wall in the periodic table and that triply bonded metal–oxos could not be protonated, even in the strongest Brønsted acids. In this theory, only double bonded metal–oxos could attract protons, with basicities being a function of the electron donating ability of ancillary ligands. Such correlations of electronic structure with reactivity have gained importance in recent years, most notably owing to the widespread recognition that high-valent iron–oxos are intermediates in biological reactions critical to life on Earth.In this Account, we focus attention on the oxygenations of inert organic substrates by cytochromes P450, as these reactions involve multiply bonded iron–oxos. We emphasize that P450 iron–oxos are strong oxidants, so strong that they would destroy nearby amino acids if substrates are not oxygenated rapidly; it is our view that these high-valent iron–oxos are such dangerous reactive oxygen species that Nature surely found ways to disable them. Looking more deeply into this matter, mainly by examining many thousands of structures in the Protein Data Bank, we have found that P450s and other enzymes that require oxygen for function have chains of tyrosines and tryptophans that extend from active-site regions to protein surfaces. Tyrosines are near the heme active sites in bacterial P450s, whereas tryptophan is closest in most human enzymes. High-valent iron–oxo survival times taken from hole hopping maps range from a few nanoseconds to milliseconds, depending on the distance of the closest Trp or Tyr residue to the heme. In our proposed mechanism, multistep hole tunneling (hopping) through Tyr/Trp chains guides the damaging oxidizing hole to the protein surface, where it can be quenched by soluble protein or small molecule reductants. As the Earth’s oxygenic atmosphere is believed to have developed about 2.5 billion years ago, the increase in occurrence frequency of tyrosine and tryptophan since the last universal evolutionary ancestor may be in part a consequence of enzyme protective functions that developed to cope with the environmental toxin, O2.

Graphite/p-SiC Schottky Diodes Prepared by Transferring Drawn Graphite Films onto SiC

Graphite/p-SiC Schottky diodes are fabricated using the recently suggested technique of transferring drawn graphite films onto p-SiC single-crystal substrates. The current–voltage and capacitance–voltage characteristics are measured at different temperatures and at different frequencies of a small-signal AC signal, respectively. The temperature dependences of the potential-barrier height and of the series resistance of the graphite/p-SiC junctions are measured and analyzed. The dominant mechanisms of the charge–carrier transport through the diodes are determined. It is shown that the dominant mechanisms of the transport of charge carriers through the graphite/p-Si Schottky diodes at a forward bias are multi-step tunneling recombination and tunneling described by the Newman formula (at high bias voltages). At reverse biases, the dominant mechanisms of charge transport are the Frenkel–Poole emission and tunneling. It is shown that the graphite/p-SiC Schottky diodes can be used as detectors of ultraviolet radiation since they have the open-circuit voltage Voc = 1.84 V and the short-circuit current density Isc = 2.9 mA/cm2 under illumination from a DRL 250-3 mercury–quartz lamp located 3 cm from the sample.

Диоды Шоттки графит/p-SiC, полученные методом переноса нарисованной пленки графита на SiC

AbstractGraphite/ p -SiC Schottky diodes are fabricated using the recently suggested technique of transferring drawn graphite films onto p -SiC single-crystal substrates. The current–voltage and capacitance–voltage characteristics are measured at different temperatures and at different frequencies of a small-signal AC signal, respectively. The temperature dependences of the potential-barrier height and of the series resistance of the graphite/ p -SiC junctions are measured and analyzed. The dominant mechanisms of the charge–carrier transport through the diodes are determined. It is shown that the dominant mechanisms of the transport of charge carriers through the graphite/ p -Si Schottky diodes at a forward bias are multi-step tunneling recombination and tunneling described by the Newman formula (at high bias voltages). At reverse biases, the dominant mechanisms of charge transport are the Frenkel–Poole emission and tunneling. It is shown that the graphite/ p -SiC Schottky diodes can be used as detectors of ultraviolet radiation since they have the open-circuit voltage V _oc = 1.84 V and the short-circuit current density I _sc = 2.9 mA/cm^2 under illumination from a DRL 250-3 mercury–quartz lamp located 3 cm from the sample.

Electric field activated nonlinear and disorder-induced charge transport in doped polymer devices

Abstract In the present study, temperature dependent conductivity and field dependent nonlinear transport are adopted to provide insight into the charge transport in poly(3-methylthiophene) polymer thin film devices that are electropolymerized at different temperatures. The samples obtained at different temperature show relative disorder quantified by the conductivity ratio, σr = σ (84 K)/σ (300 K) which is further correlated with the morphology of the samples. Charge transport in each sample is governed by variable range hopping mechanism and the parameters like density of states at the Fermi level [N(EF)], average hopping distance (R) and average hopping energy (W) have been estimated. Electric field activated nonlinear transport is described by multistep tunneling model of Glazman-Matveev (GM model) over four separate channels of conduction. It is concluded that the field dependent conductivity has a ‘field-scale’ in all the samples where the conductivity converges to a single master curve in the temperature range of observation. The results obtained from the conductivity ratio and temperature dependent conductivity are strongly supported by electric field dependent conductivity and nonlinear exponents.

Structure Controlled Long-Range Sequential Tunneling in Carbon-Based Molecular Junctions

Carbon-based molecular junctions consisting of aromatic oligomers between conducting sp2 hybridized carbon electrodes exhibit structure-dependent current densities (J) when the molecular layer thickness (d) exceeds ∼5 nm. All four of the molecular structures examined exhibit an unusual, nonlinear ln J vs bias voltage (V) dependence which is not expected for conventional coherent tunneling or activated hopping mechanisms. All molecules exhibit a weak temperature dependence, with J increasing typically by a factor of 2 over the range of 200-440 K. Fluorene and anthraquinone show linear plots of ln J vs d with nearly identical J values for the range d = 3-10 nm, despite significant differences in their free-molecule orbital energy levels. The observed current densities for anthraquinone, fluorene, nitroazobenzene, and bis-thienyl benzene for d = 7-10 nm show no correlation with occupied (HOMO) or unoccupied (LUMO) molecular orbital energies, contrary to expectations for transport mechanisms based on the offset between orbital energies and the electrode Fermi level. UV-vis absorption spectroscopy of molecular layers bonded to carbon electrodes revealed internal energy levels of the chemisorbed films and also indicated limited delocalization in the film interior. The observed current densities correlate well with the observed UV-vis absorption maxima for the molecular layers, implying a transport mechanism determined by the HOMO-LUMO energy gap. We conclude that transport in carbon-based aromatic molecular junctions is consistent with multistep tunneling through a barrier defined by the HOMO-LUMO gap, and not by charge transport at the electrode interfaces. In effect, interfacial "injection" at the molecule/electrode interfaces is not rate limiting due to relatively strong electronic coupling, and transport is controlled by the "bulk" properties of the molecular layer interior.

Characterization of nonOhmic electrical transport in double perovskite compounds through bias scale and nonlinearity exponent

Scaling analysis of nonOhmic electrical transport in double perovskite (DP) compounds like La2NiMnO6 and Sr2Fe0.3Mn0.7MoO6 is presented over a wide range of electric bias and temperatures. It is shown that the voltage V0(T) at which conductance deviates from its Ohmic value Σ0(T) scales with Σ0(T) as V0(T)∼Σ0(T)xT, xT being the onset exponent characterizing the onset of nonOhmic conduction. Interestingly, it was found that xT is negative and insensitive to the nature of conduction mechanism in DPs but is related to the characteristic temperature T0 and the mean hopping length Hm. We provide a scaling formalism in terms of the parameters V0(T) and xT in DPs for deeper understanding of the spintronic application and the electrode functioning in solid oxide fuel cells (SOFC). Inelastic multi-step tunneling is found to be the suitable mechanism of electronic transport characterized completely by these two parameters.

Investigation of electrical transport properties in heterojunctions comprised of silicon substrate and nanocrystalline iron disilicide films

In this study, n-type nanocrystalline FeSi2/p-type Si heterojunctions were fabricated using facing-target direct-current sputtering (FTDCS). The possible transportation mechanisms of carriers were investigated by analysing the dark J-V characteristics at temperatures ranging between 60 and 300 K. The ideality factor (n) was estimated from the slope of the linear region for the forward lnJ-V characteristics. The value of n was 1.87 at 300 K and nearly constant at temperatures ranging from 140 to 300 K, suggesting that a recombination process at the junction interface was dominant in the transportation mechanism of carriers. At temperatures below 140 K, the value of n increased by more than two and the value of A was virtually constant. Owing to the consistency of A, combined with the temperature dependent value of n, the implication is that a trap-assisted multi-step tunnelling mechanism governed the carrier transport. From the analysis of J-V characteristics, the value of barrier height was 0.58 eV at 300 K and decreased to 0.19 eV at 60 K.

Abnormal bias dependence of magnetoresistance in CoFeB/MgO/Si spin-injection tunnel contacts

We report a strong bias voltage dependence of magnetoresistance (MR) in CoFeB/MgO/Si spin-injection tunnel contacts using the three-terminal Hanle geometry. When a bias voltage is relatively large, the MR is composed of two characteristic signals: a conventional Hanle signal observed at a low magnetic field, which is due to the precession of injected spins, and another signal originating from the rotation of the magnetization at a larger magnetic field. In contrast, for a small bias voltage, additional signals appear at a wide range of magnetic fields, which occasionally overwhelms the conventional Hanle signals. Because the additional signals are pronounced at a low bias and are significantly reduced by annealing at moderate temperatures, they can be attributed to multi-step tunneling via defect states at the interfaces or tunnel barrier. Our results demonstrate that the spin injection signal caused by the defect states can be evaluated by its bias voltage dependence.

Transport characteristics of n-ZnO/p-Si heterojunction as determined from temperature dependent current–voltage measurements

Zinc oxide (ZnO) nanorods have been synthesized by a two-step chemical bath deposition process on silicon substrates having different dopant densities and orientations. Scanning electron microscopy and X-ray diffraction analysis reveal that the orientation of the Si substrate does not affect the orientation, distribution or crystallinity of the nanostructures. The electrical properties of the ZnO/Si heterojunction are also investigated by current–voltage (I–V) measurements. The ideality factor is found to be 2.6 at 295K, indicating that complex current transport mechanisms are at play. Temperature dependent I–V characteristics have been used to determine the dominant transport mechanism. The experimental results suggest that in the low bias region the current is dominated by a trap assisted multi-step tunneling process.

Effects of CuO film thickness on electrical properties of CuO/ZnO and CuO/ZnS hetero-junctions

It is known that the quality of the diode and the transport of the charge carriers across the junction may be greatly influenced by the quality of the interface, and depends on the crystallinity of thin layers. In this work, we have prepared the p-CuO/n-ZnO and p-CuO/n-ZnS hetero-junctions by ultrasonic spray pyrolysis at 300°C under atmospheric pressure. We have investigated the influence of the thickness of CuO film on the electrical properties of the hetero-junctions. The X-ray diffraction indicates that both the CuO and ZnO thin films grow according to polycrystalline nature while ZnS films show a quasi-amorphous nature. The current voltage temperature (I–V–T) and capacitance voltage (C–V) characteristics reveal that CuO/ZnS is an abrupt junction, whereas the data of CuO/ZnO hetero-junctions suggest that the potential barrier heights are spatially non-uniform (junction not abrupt). The electrical behavior of the junction is strongly affected by the defect state distribution at the hetero-interface. The recombination process involving the interface states predominates at low forward bias (V<0.5V), whereas trap assisted multi-step tunneling capture emission (MTCE) prevails in the higher bias region (V>0.5V) on the thin CuO side. The junction parameters (ideality factor, rectification ratio, built in potential etc…) are of the same order of magnitude for both heterojunction diodes i.e. ZnO/CuO and ZnS/CuO.

Mechanism of Carrier Transport in n-Type β-FeSi&lt;sub&gt;2&lt;/sub&gt;/Intrinsic Si/p-Type Si Heterojunctions

Preparation of n-type β-FeSi2/intrinsic Si/p-type Si heterojunctions was accomplished by facing-target direct-current sputtering (FTDCS) and measuring their current-voltage characteristic curves at low temperatures ranging from 300 K down to 50 K. A mechanism of carrier transport in the fabricated heterojunctions was investigated based on thermionic emission theory. According to this theory, the ideality factor was calculated from the slope of the linear part of the forward lnJ-V plot. The ideality factor was 1.12 at 300 K and increased to 1.99 at 225 K. The estimated ideality factor implied that a recombination process was the predominant mechanism of carrier transport. When the temperatures decreased below 225 K, the ideality factor was estimated to be higher than two and parameter A was estimated to be constant. The obtained results implied that the mechanism of carrier transport was governed by a trap-assisted multi-step tunneling process. At high forward bias voltage, the predominant mechanism of carrier transport was changed into a space charge limit current process.

Electron flow through biological molecules: does hole hopping protect proteins from oxidative damage?

Biological electron transfers often occur between metal-containing cofactors that are separated by very large molecular distances. Employing photosensitizer-modified iron and copper proteins, we have shown that single-step electron tunneling can occur on nanosecond to microsecond timescales at distances between 15 and 20 Å. We also have shown that charge transport can occur over even longer distances by hole hopping (multistep tunneling) through intervening tyrosines and tryptophans. In this perspective, we advance the hypothesis that such hole hopping through Tyr/Trp chains could protect oxygenase, dioxygenase, and peroxidase enzymes from oxidative damage. In support of this view, by examining the structures of P450 (CYP102A) and 2OG-Fe (TauD) enzymes, we have identified candidate Tyr/Trp chains that could transfer holes from uncoupled high-potential intermediates to reductants in contact with protein surface sites.

Could tyrosine and tryptophan serve multiple roles in biological redox processes?

Single-step electron tunnelling reactions can transport charges over distances of 15-20 Åin proteins. Longer-range transfer requires multi-step tunnelling processes along redox chains, often referred to as hopping. Long-range hopping via oxidized radicals of tryptophan and tyrosine, which has been identified in several natural enzymes, has been demonstrated in artificial constructs of the blue copper protein azurin. Tryptophan and tyrosine serve as hopping way stations in high-potential charge transport processes. It may be no coincidence that these two residues occur with greater-than-average frequency in O(2)- and H(2)O(2)-reactive enzymes. We suggest that appropriately placed tyrosine and/or tryptophan residues prevent damage from high-potential reactive intermediates by reduction followed by transfer of the oxidizing equivalent to less harmful sites or out of the protein altogether.