Two-step Tunneling Research Articles

Identification and two-step tunnel engineering of a carbonyl reductase for biosynthesis of an (R)-α-lipoic acid intermediate

As a “universal antioxidant”, α-lipoic acid (LA) is beneficial for health and therapy, combatting Alzheimer's disease, diabetes. The main active form of LA has the R-configuration. Biocatalytic synthesis of (R)-ethyl 8‑chloro-6-hydroxycaprylate ((R)-ECHO) from ethyl 8‑chloro-6-oxocrylate (ECOO) coupled with subsequent chemical synthesis is a competitive route to (R)-LA. However, very few enzymes have been confirmed to prepare the key intermediate (R)-ECHO with high efficiency and absolute stereoselectivity. In this work, carbonyl reductase CoCR13 from Candida orthopsilosis Co 90–125 was mined, and then engineered by a stepwise strategy with the aim of expanding the substrate entry tunnel. This led to the optimal variant M3 (R129L/D210T/S126A), which exhibited a 93.42-fold higher catalytic efficiency (kcat/Km = 85.95 mm−1·s−1) towards ECOO than the wild type, an ee value greater than 99.5 % (R) from initial 86 % (R), and a space time yield of 99.9 g·L−1·h−1. This study expands the scarce enzyme inventory for the biosynthesis of (R)-ECHO and offers a new choice for the industrial preparation of (R)-LA through a synergistic enzymatic-chemical approach.

Scaling with MgO thickness of the proximity exchange field in a Fe/MgO/Si contact

The scaling of the proximity exchange field, which ferromagnetic Fe exerts on the spin accumulation at the MgO/Si interface of Fe/MgO/Si tunnel contacts, was investigated using the inverted Hanle effect. Data as a function of the applied bias voltage shows that the exchange field varies with bias in the same way as the square of the tunnel spin polarization (TSP). The decay of the exchange field with increasing MgO thickness ${t}_{\mathrm{MgO}}$ is rather weak and nonexponential, which we argue to be related to the improved epitaxial quality of the MgO, and the, consequently, improved spin-filtering properties at larger MgO thickness. Using the TSP data to correct for this yields an exponential decay of the exchange field with ${t}_{\mathrm{MgO}}$ with a decay length of $\ensuremath{\lambda}=\ensuremath{-}0.42$ nm. Comparing this to the decay length for the tunnel conductance ($\ensuremath{\lambda}=\ensuremath{-}0.21$ nm) suggests that the exchange interaction is dominated by two-step tunneling processes.

Hydrogen atom quantum diffusion in solid parahydrogen: The H + N2O → cis-HNNO → trans-HNNO reaction.

The diffusion and reactivity of hydrogen atoms in solid parahydrogen at temperatures between 1.5 K and 4.3 K are investigated by high-resolution infrared spectroscopy. Hydrogen atoms are produced within solid parahydrogen as the by-products of the 193 nm in situ photolysis of N2O, which induces a two-step tunneling reaction, H + N2O → cis-HNNO → trans-HNNO. The second-order rate constant for the first step to form cis-HNNO is found to be inversely proportional to the N2O concentration after photolysis, indicating that the hydrogen atoms move through solid parahydrogen via quantum diffusion. This reaction only readily occurs at temperatures below 2.8 K, not due to an increased rate constant for the first reaction step at low temperatures but rather due to an increased selectivity to the reaction. The rate constant for the second step of the reaction mechanism involving unimolecular isomerization is shown to be independent of the N2O concentration as expected. The inverse concentration dependence of the rate constant for the reaction step that involves the hydrogen atom demonstrates clearly that quantum diffusion influences the reactivity of the hydrogen atoms in solid parahydrogen, which does not have an analogy in classical reaction kinetics.

Targeting the Rich Conformational Landscape of N-Allylmethylamine Using Rotational Spectroscopy and Quantum Mechanical Calculations.

The highly variable conformational landscape of N-allylmethylamine (AMA) was investigated using Fourier transform microwave spectroscopy aided by high-level theoretical calculations to understand the energy relationship governing the interconversion between nine stable conformers. Spectroscopically, transitions belonging to four low energy conformers were identified and their hyperfine patterns owing to the 14 N quadrupolar nucleus were unambiguously resolved. The rotational spectrum of the global minimum geometry, conformer I, shows an additional splitting associated with a tunneling motion through an energy barrier interconnecting its enantiomeric forms. A two-step tunneling trajectory is proposed by finding transition state structures corresponding to the allyl torsion and NH inversion. Natural bond orbital and non-covalent interaction analyses reveal that an interplay between steric and hyperconjugative effects rules the conformational preferences of AMA.

Electrical power generation from moderate-temperature radiative thermal sources.

Moderate-temperature thermal sources (100° to 400°C) that radiate waste heat are often the by-product of mechanical work, chemical or nuclear reactions, or information processing. We demonstrate conversion of thermal radiation into electrical power using a bipolar grating-coupled complementary metal-oxide-silicon (CMOS) tunnel diode. A two-step photon-assisted tunneling charge pumping mechanism results in separation of charge carriers in pn-junction wells leading to a large open-circuit voltage developed across a load. Electrical power generation from a broadband blackbody thermal source has been experimentally demonstrated with converted power densities of 27 to 61 microwatts per square centimeter for thermal sources between 250° and 400°C. Scalable, efficient conversion of radiated waste heat into electrical power can be used to reduce energy consumption or to power electronics and sensors.

Effect of OFF-State Stress on Gate-Induced Drain Leakage by Interface Traps in Buried-Gate FETs

The gate-induced drain leakage (GIDL) current is one of the major leakage sources in a dynamic random-access memory (DRAM) cell transistor. In addition to band-to-band tunneling (BTBT), which causes GIDL in the gate-to-drain overlap region, the generation of interface traps in the gate dielectric increases the GIDL current by trap-assisted two-step tunneling (TATT). In this article, the influence of OFF-state stress on the generation of the interface traps, which deteriorates GIDL, was quantitatively analyzed with charge pumping (CP) characterization method in buried-channel array transistors (BCATs) for DRAM cells. The applied stress increased the GIDL current while simultaneously degrading device performance including such as transconductance (g <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> ) and ON-state current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> ), due to the generation of the interface traps. By using the CP characterization, the interface traps were spatially profiled along the gate-to-drain overlap region in the junction. Trap distribution inside the energy bandgap was also characterized.

DTER: Optimal Two-Step Dual Tunnel Energy Requesting for RF-Based Energy Harvesting System

We propose a new energy harvesting strategy that uses a dedicated energy source (ES) to optimally replenish energy for radio frequency (RF) energy harvesting powered Internet of Things. Specifically, we develop a two-step dual tunnel energy requesting (DTER) strategy that minimizes the energy consumption on both the energy harvesting device and the ES. Besides the causality and capacity constraints that are investigated in the existing approaches, DTER also takes into account the overhead issue and the nonlinear charge characteristics of an energy storage component to make the proposed strategy practical. Both offline and online scenarios are considered in the second step of DTER. To solve the nonlinear optimization problem of the offline scenario, we convert the design of offline optimal energy requesting problem into a classic shortest path problem and thus a global optimal solution can be obtained through dynamic programming (DP) algorithms. The online suboptimal transmission strategy is developed as well. Simulation study verifies that the online strategy can achieve almost the same energy efficiency as the global optimal solution in the long term.

Hole Hopping through Tryptophan in Cytochrome P450.

Electron-transfer kinetics have been measured in four conjugates of cytochrome P450 with surface-bound Ru-photosensitizers. The conjugates are constructed with enzymes from Bacillus megaterium (CYP102A1) and Sulfolobus acidocaldarius (CYP119). A W96 residue lies in the path between Ru and the heme in CYP102A1, whereas H76 is present at the analogous location in CYP119. Two additional conjugates have been prepared with (CYP102A1)W96H and (CYP119)H76W mutant enzymes. Heme oxidation by photochemically generated Ru3+ leads to P450 compound II formation when a tryptophan residue is in the path between Ru and the heme; no heme oxidation is observed when histidine occupies this position. The data indicate that heme oxidation proceeds via two-step tunneling through a tryptophan radical intermediate. In contrast, heme reduction by photochemically generated Ru+ proceeds in a single electron tunneling step with closely similar rate constants for all four conjugates.

Spin-dependent transport mechanisms in CoFe/MgO/n+-Si junctions investigated by frequency response of signals

We investigated spin-dependent transport properties in CoFe/MgO/n+-Si junctions by measuring Hanle signals and their dependence on the measurement frequency. The CoFe/MgO/n+-Si junctions exhibited two types of Hanle curves with different half-widths. Hanle signals with a broad half-width were observed mainly in the low-bias region, and these signals exhibited apparent frequency dependence and disappeared in the high-frequency region though Hanle signals with narrow half-widths were almost independent of the measurement frequency used in this study. This frequency dependence is explained by the mechanism of two-step tunneling. These results show that investigating the frequency response signals gives clear information on spin-dependent transport mechanisms.

Role of defects in the thermal droop of InGaN-based light emitting diodes

This paper reports an investigation of the physical origin of the thermal droop (the drop of the optical power at high temperatures) in InGaN-based light-emitting diodes. We critically investigate the role of various mechanisms including Shockley-Read-Hall recombination, thermionic escape from the quantum well, phonon-assisted tunneling, and thermionic trap-assisted tunneling; in addition, to explain the thermal droop, we propose a closed-form model which is able to accurately fit the experimental data by using values extracted from measurements and simulations and a limited set of fitting parameters. The model is based on a two-step phonon-assisted tunneling over an intermediate defective state, corrected in order to take into account the pure thermionic component at zero bias and the field-assisted term.

Different charging behaviors between electrons and holes in Si nanocrystals embedded in SiNx matrix by the influence of near-interface oxide traps**Project supported by the National Basic Research Program of China (Grant No. 2010CB934402) and the National Natural Science Foundation of China (Grant No. 11374153).

Si-rich silicon nitride films are prepared by plasma-enhanced chemical vapor deposition method, followed by thermal annealing to form the Si nanocrystals (Si-NCs) embedded in SiNx floating gate MOS structures. The capacitance–voltage (C–V), current–voltage (I–V), and admittance–voltage (G–V) measurements are used to investigate the charging characteristics. It is found that the maximum flat band voltage shift (ΔVFB) due to full charged holes (∼ 6.2 V) is much larger than that due to full charged electrons (∼ 1 V). The charging displacement current peaks of electrons and holes can be also observed by the I–V measurements, respectively. From the G–V measurements we find that the hole injection is influenced by the oxide hole traps which are located near the SiO2/Si-substrate interface. Combining the results of C–V and G–V measurements, we find that the hole charging of the Si-NCs occurs via a two-step tunneling mechanism. The evolution of G–V peak originated from oxide traps exhibits the process of hole injection into these defects and transferring to the Si-NCs.

Spin-dependent Transport Properties of CrO2 Micro Rod.

The CrO2 micro rod powder was synthesized by decomposing the CrO3 flakes at a specific temperature to yield precursor and annealing such a precursor in a sealed glass tube. The magneto-transport properties have been measured by a direct current four-probe method using a Cu/CrO2 rods/colloidal silver liquid electrode sandwich device. The largest magnetoresistance (MR) around ~72 % was observed at 77 K with applied current of 0.05 μA. The non-linear I–V curve indicates a tunneling type transport properties and the tunneling barrier height is around 2.2 ± 0.04 eV at 77 K, which is obtained with fitting the non-linear I–V curves using Simmons’ equation. A mixing of Cr oxides on the surface of CrO2 rod observed by X-ray photoemission spectroscopy provides a tunneling barrier rather than a single phase of Cr2O3 insulating barrier. The MR shows strong bias voltage dependence and is ascribed to the two-step tunneling process.

Tunneling anisotropic magnetoresistance inC60-based organic spintronic systems

${\mathrm{C}}_{60}$ fullerenes are interesting molecular semiconductors for spintronics since they exhibit weak spin-orbit and hyperfine interactions, which is a prerequisite for long spin lifetimes. We report spin-polarized transport in spin-valve-like structures containing ultrathin (10 nm) ${\mathrm{C}}_{60}$ layers, ferromagnetic (FM) epitaxial face-centered-cubic (fcc) Co (111) contacts, ${\mathrm{AlO}}_{x}$ tunnel barriers, and nonmagnetic Al counter electrodes. Even though genuine spin-valve behavior cannot occur for only one FM contact, we find significant tunneling anisotropic magnetoresistance (TAMR) upon rotating the in-plane magnetization, originating from spin-orbit interaction (SOI) induced anisotropy of the fcc (111) Co bands. The uniaxial magnetocrystalline anisotropy of the Co electrodes results in a predominantly twofold symmetric in-plane TAMR effect. We investigated the TAMR effect in the direct tunneling regime (2 nm ${\mathrm{C}}_{60}$), at the transition point to two-step tunneling (4 nm ${\mathrm{C}}_{60}$), and in the multistep regime (8 nm ${\mathrm{C}}_{60}$). A sizable TAMR of 4.5% is found at 5 K under application of a 500-mT in-plane magnetic field for ${\mathrm{C}}_{60}$ layers of 2 nm, which is strongly suppressed at 8 nm thickness, indicating that TAMR may strongly contribute to the ``spin-valve'' signal for direct tunneling, but not for multistep tunneling. The TAMR effect is proposed to be due to a combination of SOI induced modulation of the tunneling DOS upon rotating the in-plane magnetization of the fcc epitaxial Co thin film, resonant tunneling processes involving interfacial states, and different Bychkov-Rashba SOI at the different interfaces.

Giant enhancement of spin detection sensitivity in (Ga,Mn)As/GaAs Esaki diodes

We investigate the correlation between spin signals measured in three-terminal (3T) geometry by the Hanle effect and the spin accumulation generated in a semiconductor channel in a lateral (Ga,Mn)As/GaAs Esaki diode device. We systematically compare measurements using a 3T configuration, probing spin accumulation directly beneath the injecting contact, with results from nonlocal measurements, where solely spin accumulation in the GaAs channel is probed. We find that the spin signal detected in the 3T configuration is dominated by a bias-dependent spin detection sensitivity, which in turn is strongly correlated with charge-transport properties of the junction. This results in a particularly strong enhancement of the detected spin signal in a region of increased differential resistance. We find additionally that two-step tunneling via localized states (LS) in the gap of (Ga,Mn)As does not compromise spin injection into the semiconductor conduction band.

Anomalous scaling of spin accumulation in ferromagnetic tunnel devices with silicon and germanium

The magnitude of spin accumulation created in semiconductors by electrical injection of spin-polarized electrons from a ferromagnetic tunnel contact is investigated, focusing on how the spin signal detected in a Hanle measurement varies with the thickness of the tunnel oxide. An extensive set of spin-transport data for Si and Ge magnetic tunnel devices reveals a scaling with the tunnel resistance that violates the core feature of available theories, namely, the linear proportionality of the spin voltage to the injected spin current density. Instead, an anomalous scaling of the spin signal with the tunnel resistance is observed, following a power law with an exponent between 0.75 and 1 over 6 decades. The scaling extends to tunnel resistance values larger than 10 9 �μ m 2 , far beyond the regime where the classical impedance mismatch or back flow into the ferromagnet play a role.Thisscalingisincompatiblewithexistingtheoryfordirecttunnelinjectionofspinsintothesemiconductor.It also demonstrates conclusively that the large spin signal does not originate from two-step tunneling via localized states near the oxide/semiconductor interface. Control experiments show that spin accumulation in localized states within the tunnel barrier or artifacts are also not responsible. Altogether, the scaling results suggest that, contrary to all existing descriptions, the spin signal is proportional to the applied bias voltage, rather than the (spin) current.

Two-step tunneling technique of deep brain stimulation extension wires—a description

While a significant body of literature exists on the intracranial part of deep brain stimulation surgery, the equally important second part of the intervention related to the subcutaneous tunneling of deep brain stimulation extension wires is rarely described. The tunneling strategy can consist of a single passage of the extension wires from the frontal incision site to the subclavicular area, or of a two-step approach that adds a retro-auricular counter-incision. Each technique harbors the risk of intraoperative and postoperative complications. At our center, we perform a two-step tunneling procedure that we developed based on a cadaveric study. In 125 consecutive patients operated since 2002, we did not encounter any complication related to our tunneling method. Insufficient data exist to fully evaluate the advantages and disadvantages of each tunneling technique. It is of critical importance that authors detail their tunneling modus operandi and report the presence or absence of complications. This gathered data pool may help to formulate a definitive conclusions on the safest method for subcutaneous tunneling of extension wires in deep brain stimulation.

Planar tunneling measurements of the energy gap in biased bilayer graphene

We present an analysis on the determination of the energy gap in biased bilayer graphene using tunneling measurements, report our experimental results obtained from planar tunneling spectroscopy, and compare them with those from electrical transport measurements. Bilayer graphene flakes were prepared by exfoliating from bulk graphite onto SiO2 thermally grown on a doped Si substrate. Due to the low carrier density of bilayer graphene, the Fermi level and electronic structure are expected to be highly sensitive to tunnel bias-induced charging, which is neglected in traditional tunnel junctions. We found that the tunneling signal generally exhibited a “V”-shaped tunneling conductance background that did not shift with back gate voltage, possibly due to a two-step tunneling process. We observed a tunable suppression in the tunneling conductance that follows theoretical predictions for a band gap in biased bilayer graphene. We explore the evolution of the band gap by tuning the electric field and charge carrier density produced by the tunneling bias and back gate, and compare experimental results with numerical simulations. Finally, we compare these findings with transport measurements of top- and bottom-gated bilayer graphene field effect transistors featuring similar gate dielectrics.

Enhanced tunneling in the GaAs p+–n+ junction by embedding InAs quantum dots

GaAs p+–n+ junctions with and without a layer of InAs quantum dots (QDs) embedded at the interface are discussed in this article. The current density versus voltage (I–V) characteristics show that the junctions without QDs are weak degenerate due to the Beryllium(Be) atoms diffusion of nominal p++-GaAs; the junctions with QDs generate enhanced tunneling current at forward bias, because the QDs layer reduces the Be diffusion and enables a two-step tunneling process. At room temperature, the current density of the sample with QDs is enhanced to 122 A cm−2 at a forward bias of +0.32 V, which is about 2 orders of magnitude higher than the reference sample without QDs.

Tunneling behavior of bismuth telluride nanoplates in electrical transport

We study the electrical transport properties of ensembles of bismuth telluride (Bi2Te3) nanoplates grown by solution based chemical synthesis. Devices consisting of Bi2Te3 nanoplates are fabricated by surface treatment after dropping the solution on the structured gold plates and the temperature dependence of resistance shows a nonmetallic behavior. Symmetric tunneling behavior in I–V was observed in both our experimental results and theoretical calculation of surface conductance based on a simple Hamiltonian, which excludes carrier–carrier interactions. Here, we present two devices: one showing symmetric, the other showing a two-step tunneling behavior. The latter can be understood in terms of disorder.

Injection and detection of spin in a semiconductor by tunneling via interface states

Injection and detection of spin accumulation in a semiconductor having localized states at the interface is evaluated. Spin transport from a ferromagnetic contact by sequential, two-step tunneling via interface states is treated not in itself, but in parallel with direct tunneling. The spin accumulation induced in the semiconductor channel is not suppressed, as previously argued, but genuinely enhanced by the additional spin current via interface states. Spin detection with a ferromagnetic contact yields a weighted average of the spin accumulation in the channel and in the localized states. In the regime where the spin accumulation in the localized states is much larger than that in the channel, the detected spin signal is insensitive to the spin accumulation in the localized states and the ferromagnet probes the spin accumulation in the semiconductor channel.