Electron Tunneling Model Research Articles

Octafluorobiphenyl-4,4’-diyl 9-oxothioxanthene-1,4-diyl polyether – a promising material for organic film based memristors: synthesis, memristive effect and charge transport mechanism

Polycondensation of perfluorobiphenyl with 1,4-dihydroxy- thioxanthen-9-one resulted in the title highly thermally stable polyether containing the thioxanthenone-type electron acceptor in-chain blocks in its structure. Model resistive memory devices based on it as the active medium exhibited a bipolar switching memristive effect with a 4-order difference in resistance between the low and high resistance states. The charge transport in this polyether is quantitatively described by the Nasyrov–Gritsenko model of phonon-assisted electron tunneling between traps.

Alkali-metal-based molecular-ion secondary ion mass spectrometry for precise quantitative analysis of low-dimensional materials without standards

Advancement in the understanding of fundamental aspects of Secondary Ion Mass Spectrometry (SIMS) has made this technique to be extremely powerful for quantitative materials analysis. The secondary ion emission process is a complex phenomenon involving various mechanisms to understand the mechanism. Among the existing mechanisms, the “electron-tunneling model” is the most accepted concept in the understanding of ionization probability for positive and negative ions. This model essentially concerns the survival probability of an ion emitted from the surface. As the intensity of secondary ions of a certain element depends strongly on the efficiency of ionization of a sputtered atom or molecule, the “local surface chemistry” plays a significant role in the secondary ion emission. This is purported as the “Matrix Effect” in SIMS, making the technique extremely challenging for quantification of materials in spite of the fact that it has the highest detection sensitivity (<ppb) and exceptional depth-resolution (<1[Formula: see text]nm). Therefore, it is required to correct the “matrix effect” for quantitative analysis. If the probing element (M) in its close proximity sees an alkali element (such as Li, Rb, K, Na and Cs, referred to as “A”) on the sample surface, the sputtered neutral atom (M0) of the probing element can attach an alkali-ion forming a quasi-molecular (MA)[Formula: see text] ion. Such a phenomenon can also occur if an alkali ion beam is chosen as the primary beam for sputtering. The MA[Formula: see text] molecular ion formation is directly related to the atomic polarizability of the element M. Since the sputtering of neutral atom (M0) is unconnected with the formation of MA[Formula: see text] ion, the “matrix effect” decreases dramatically in the analysis using MA[Formula: see text] ions. The phenomenon is very similar to the mechanism of “secondary neutral mass spectrometry” (SNMS). Although these (MA)[Formula: see text] molecular ions are potentially applicable in the direct quantification of materials without the need for “calibration standards”, it generally has a weak intensity. In such a case, the (MA)[Formula: see text] molecular ions are preferred in the analysis because of their higher detection sensitivity (by a few orders of magnitude in some cases), compared to that of (MA)[Formula: see text] molecular ions. Monitoring of the molecular ions is routinely preferred in conventional SIMS experiments to discriminate the sputtered ion species which are affected by mass interference or have poor dynamic ranges. For example, while making the SIMS analysis of GaAs, carbon as an impurity element is detected by monitoring (AsC)[Formula: see text] molecular ions instead of C− ions, because the C− ions have high background arising from the carbon-containing residual-gas species in the analysis chamber. Caesium is decidedly chosen for MCs[Formula: see text] or MCs[Formula: see text] molecular ions in SIMS because of the strongest reactivity and electropositive nature of caesium. This paper deals with a comprehensive discussion on the ion-emission phenomena in sputtering, the “matrix effect” and its compensation, and the potential application of the “MCs[Formula: see text]-SIMS” ([Formula: see text]) in the effective quantitative chemical analysis of materials. A special prominence has been given to the quantification of low-dimensional materials, superlattices and quantum-confined structures using this state-of-the-art “MCs[Formula: see text]-SIMS” approach in all complexities.

Structural Determinants of Redox Conduction Favor Robustness over Tunability in Microbial Cytochrome Nanowires.

Structural determinants of a 103-fold variation in electrical conductivity for helical homopolymers of tetra-, hexa-, and octa-heme cytochromes (named Omc- E, S, and Z, respectively) from Geobacter sulfurreducens are investigated with the Pathways model for electron tunneling, classical molecular dynamics, and hybrid quantum/classical molecular mechanics. Thermally averaged electronic couplings for through-space heme-to-heme electron transfer in the "nanowires" computed with density functional theory are ≤0.015 eV. Pathways analyses also indicate that couplings match within a factor of 5 for all "nanowires", but some alternative tunneling routes are found involving covalent protein backbone bonds (Omc- S and Z) or propionic acid-ligating His H-bonds on adjacent hemes (OmcZ). Reorganization energies computed from electrostatic vertical energy gaps or a version of the Marcus continuum expression parameterized on the total (donor + acceptor) solvent-accessible surface area typically agree within 20% and fall within the range 0.48-0.98 eV. Reaction free energies in all three "nanowires" are ≤|0.28| eV, even though Coulombic interactions primarily tune the site redox energies by 0.7-1.2 eV. Given the conserved energetic parameters, redox conductivity differs by < 103-fold among the cytochrome "nanowires". Redox currents do not exceed 3.0 × 10-3 pA at a physiologically relevant 0.1 V bias, with the slowest electron transfers being on a (μs) timescale much faster than typical (ms) enzymatic turnovers. Thus, the "nanowires" are proposed to be functionally robust to variations in structure that provide a habitat-customized protein interface. The 30 pA to 30 nA variation in conductivity previously reported from atomic force microscopy experiments is not intrinsic to the structures and/or does not result from the physiologically relevant redox conduction mechanism.

Novel Dielectric Nanogranular Materials with an Electrically Tunable Frequency Response

AbstractThe electrical modulation of the functionality of matter is of significant interest in physics and multifunctional tunable electronic device applications. Here, new dielectric materials with nanogranular structures comprised of nano‐sized Co granular metals dispersed in a Mg‐fluoride‐based dielectric matrix are explored. The dielectric relaxation frequency (fr), which represents a sharp decrease in dielectric permittivity in dielectrics, can be tuned by a DC electric field (E). As E increases, the position of fr first shifts to the low‐frequency side and then to the high‐frequency side, achieving a tunable fr in a certain frequency range. The ability to electrically modulate the relaxation frequency may help construct novel tunable frequency filters. The dielectric properties are theoretically examined based on the asymmetric electron tunnelling model that considers the size difference of granular pairs, offering an insightful understanding of the structure‐property relationship in disordered granular solids.

Deciphering I-V characteristics in molecular electronics with the benefit of an analytical model.

We share our perspective that a simple analytical model for electron tunneling in molecular junctions can greatly aid quantitative analysis of experimental data in molecular electronics. In particular, the single-level model (SLM), derived from first principles, provides a precise prediction for the current-voltage (I-V) characteristics in terms of key electronic structure parameters, which in turn depend on the molecular and contact architecture. SLM analysis thus facilitates understanding of structure-property relationships and provides metrics that can be compared across different types of tunnel junctions, as we illustrate with several examples.

Oxygen vacancy levels mediated photophysical pathways of NIR-II responsive broadband upconversion

The photon upconversion (UC) is the process that has been known for a long time. Especially, lanthanide-based UC materials recently have attracted huge interest of scientific community due to their narrow visible emission bands upon near-infrared (NIR) excitation. However, the excitation in the high excitation power of NIR can cause highly bright and broad emission (B-UC). Such explosive B-UC was distinct at high excitation condition with UC wavelength encompassing the entire visible range. Previously, the proposed model of B-UC origination includes heat, multi-photon absorption, and accumulated thermal energy transfer. In this article, we will show various experimental and theoretical evidence supporting the idea that B-UC arises from photophysical pathways. Here, the key experimental evidence for photophysical pathways model of B-UC is to measure B-UC in the low-temperature environment (22 K). In addition, the density functional theory was used to regenerate energy levels which would be compared to the absorption spectral bands in Er2O3. As a result, we are confident of an electron tunneling model from 2P3/2 of Er3+ to oxygen vacancy level (VO) of Er2O3. Then, VO electrons are sequentially excited to the conduction band of Er2O3 by energy transfer from 4F9/2 of Er3+. Finally, the electrons return to the VO of Er2O3 with concomitant B-UC.

An analytical model for electron tunneling in triangular quantum wells

An analytical expression for the decay rate of quasi-bound states (QBS) in a triangular quantum well is derived by considering the resonance scattering of particles from the triangular potential profile. Asymptotic properties of the Airy functions that are solutions to the Schrödinger equation for a linear or triangular potential and a perturbative expansion for a small broadening of the QBS are utilized to derive the expression for its location E 0 as well as the decay rate Γ that further gives the net tunneling current from the well. The triangular well model is commonly used to represent the band-bending near a heterojunction due to the electrostatic field. This compact expression shows excellent agreement with a full numerical solver and improvement over Wentzel–Kramers–Brillouin based calculations is demonstrated.

3D-printed graphene/polymer structures for electron-tunneling based devices

Designing 3D printed micro-architectures using electronic materials with well-understood electronic transport within such structures will potentially lead to accessible device fabrication for ‘on-demand’ applications. Here we show controlled nozzle-extrusion based 3D printing of a commercially available nano-composite of graphene/polylactic acid, enabling the fabrication of a tensile gauge functioning via the readjustment of the electron-tunneling barrier width between conductive graphene-centers. The electronic transport in the graphene/polymer 3D printed structure exhibited the Fowler Nordheim mechanism with a tunneling width of 0.79–0.95 nm and graphene centers having a carrier concentration of 2.66 × 1012/cm2. Furthermore, a mechanical strain that increases the electron-tunneling width between graphene nanostructures (~ 38 nm) by only 0.19 Ǻ reduces the electron flux by 1e/s/nm2 (from 18.51 to 19.51 e/s/nm2) through the polylactic acid junctions in the 3D-printed heterostructure. This corresponds to a sensitivity of 2.59 Ω/Ω%, which compares well with other tensile gauges. We envision that the proposed electron-tunneling model for conductive 3D-printed structures with thermal expansion and external strain will lead to an evolution in the design of next-generation of ‘on-demand’ printed electronic and electromechanical devices.

Phonon-assisted electron tunneling between traps in silicon oxide films treated in hydrogen plasma

The charge transport in thin thermal silicon oxide films treated in electron cyclotron resonance hydrogen plasma at different exposure times was investigated. X-ray photoelectron studies show that such treatment leads to the oxygen deficiency of the films. It was established that the treatment of the films in plasma leads to an increase of their conductivity by a factor of about 102. The film charge transport properties were studied at different temperatures and analyzed within four theoretical dielectric conductivity models. It was found that the charge transport mechanism is described by Fowler-Nordheim model in the initial silicon oxide and by the model of phonon-assisted electron tunneling between neutral traps after the treatment in hydrogen plasma. The thermal trap ionization energy value (Wt = 1.6 eV) measured from transport experiments is in agreement with that obtained from ab initio calculations for the oxygen vacancy (Si-Si bond) in SiO2.

Critical properties and charge transport in ethylene bridged organosilica low-κ dielectrics

Organosilicate-glass-based low-κ films containing both terminal methyl groups and an ethylene bridge between the silicon atoms are spin-on deposited by using 1,2-bis(trimethoxysilyl)ethane and methyltrimethoxysilane, Brij30 template, and thermal curing. The chemical composition, porosity, and internal defects are studied using Fourier-transform infrared spectroscopy, x-ray photoelectron spectroscopy, electron energy loss spectroscopy, UV induced luminescence, and ellipsometric porosimetry. It was found that the studied films contain oxygen-deficient centers (Si—Si bonds). The high defect density of the states near the valence-band edge of the studied low-κ films leads to a relatively small bandgap value of about 6.3 eV. The current–voltage characteristics at different temperatures were analyzed using six theoretical charge transport models where the transport is limited by the traps ionization. It was found that the best qualitative and quantitative agreement between the calculations and experimental data is achieved by using the model of phonon-assisted electron tunneling between the neutral traps and is supplemented by considering the space charge and charge carrier kinetics. Since the thermal and optical energies of the traps in the studied films are 1.6 eV and 3.2 eV, respectively, it is concluded that the traps are responsible for the charge transport in the Si—Si bonds.

Tunneling spectroscopy of an indium adatom precisely manipulated on the cross-sectional surface of InAs/GaSb quantum structures

Indium atom manipulation was achieved on the (110) cross-sectional surface of a quantum structure consisting of InAs/GaSb multilayer using a scanning tunneling microscope (STM) at low temperature. Positioning, removing, and lateral displacement by means of tip-induced hopping was performed on both InAs and GaSb layers. Electron tunneling via the electron orbital of the manipulated atom (adatom) was observed not only on the InAs layer but also on the GaSb layer, even when the energy of the orbital was in the GaSb band gap. The tunneling process from the STM tip to the InAs conduction band via the adatom and the GaSb band gap is explained well by the single electron tunneling model through the double-barrier tunneling junction. In the case of the adatom being very close to the InAs/GaSb heterointerface, interference with the electron tunneling to the InAs conduction electronic states through the GaSb barrier with/without passing the adatom was observed. The peak energy in the tunneling spectrum corresponding to the orbital of the adatom depends on the adatom position on the GaSb, demonstrating the fundamental interactions between the surface atomic-scale structures and semiconductor heterostructures.

A Model of Electron Tunneling to the Surface States in TiO2 with Application to Photocatalysis

Tunneling transmission of electrons excited by the sun light in the bulk TiO2 to its surface states is simulated. The model is built for the TiO2/Si nanostructure. The tunneling transmission coefficient was calculated by the phase functions method. The surface states are considered to be formed due to adsorption of impurities and organic compounds on TiO2. Energy dependence of the transmission coefficient is influenced by an interference of transmitted and reflected waves under conditions of the complex potential relief.

Multiphonon trap ionization transport in nonstoichiometric SiNx

It is generally accepted that the charge transport in dielectrics is governed by coulombic trap ionization due to a barrier lowering in high electric fields (Frenkel effect). In this paper, the charge transport mechanism in Si3N4 and nonstoichiometric silicon rich SiNx is experimentally studied and quantitatively analyzed with five theoretical models: Frenkel model of Coulomb traps ionization, Hill-Adachi model of overlapping Coulomb traps, Shklovskii-Efros percolation model, Makram-Ebeid and Lannoo model of multiphonon isolated traps ionization and Nasyrov-Gritsenko model of phonon-assisted electron tunneling between nearby traps. It is shown that the charge transport in Si3N4 and SiNx is qualitatively described by Frenkel effect, but Frenkel effect predicts an enormously low attempt to escape factor value. The charge transport at traps energies Wt = 1.6 eV and Wopt = 3.2 eV in Si3N4 and SiNx can be described by an increase in traps concentration in the framework of Makram-Ebeid and Lannoo model and Nasyrov-Gritsenko model. The Makram-Ebeid and Lannoo model quantitatively describes the charge transport in Si3N4 and SiNx with low silicon enrichment. The charge transport in nonstoichiometric SiNx with high silicon enrichment is well explained by Nasyrov-Gritsenko model.

Photoconductance of gold nano-island film induced by plasmonic effect

In this work, we experimentally investigate the photoconductance behavior of gold nano-island film due to the plasmonic effect. The current-voltage measurements are performed under continuous wave (CW) low laser irradiance at the wavelength properly matching the localized surface plasmon resonance (LSPR), λ = 532 nm. We measure the photoconductance of the nanostructured film as a function of laser irradiance. An enhancement of the film´s current is observed by increasing the applied voltage and the laser irradiance. Furthermore, our results show that the photoconductance is attributed to an additional voltage induced by the strong confined plasmonic electric field on the nano-islands’ surface. The experimental results are in good agreement with the Simmons’ model of electron tunneling.

Mathematical Modeling of Electron and Hole Tunneling through the Deposit Layers in Li-Oxygen and Li-Sulfur Batteries

The main discharge products in Li-oxygen and Li-sulfur batteries, as evidenced by x-ray microscopy, are predominantly crystalline lithium peroxide, lithium superoxide, and lithium sulfide [1, 2]. These products have a high electric resistivity and, practically, the only conduction mechanism in these materials is electron and hole tunneling. In this presentation we develop a mathematical model to describe the tunneling current density in Li-oxygen and Li-sulfur batteries. The model is based on the WKB approximation, which is used to model the electron and hole tunneling probability through the insulating layer. The derivation of the tunneling current is relatively similar to the derivation by Simmons [3], which analyzed metal-insulator-metal systems (MIM) but, in our case, the mathematical derivation is carefully modified to describe metal-insulating-electrolyte systems (instead of MIM systems). More importantly, we derive closed-form equations for the tunneling current density that can be readily used in finite element simulations or in the mathematical modeling of the ohmic losses across the discharge layer in Li-oxygen and Li-sulfur batteries. As we expect the tunneling current density depends on the shape of the conduction and valence bands, applied voltage, and permittivity (because of the image charges created in the carbon). At normal applied voltages (below 0.5 V) the tunneling currents are negligibly small, however, they can become large enough to affect the discharge characteristics of the batteries for low discharge rates (e.g. below 0.1 C) if the barrier heights of the valance band in the case of holes and conduction band in the case of electrons are small. More details about the mathematical derivation, the approximations used in the derivation, the closed-form equations, and a comparison with current models in the literature will be presented at the meeting. [1] N. Cañas, S. wolff, N. Wagner, K. Andreas Friedrich, In-situ X-ray diffraction studies of lithium-sulfur batteries, 2012. [2] K.R. Ryan, L. Trahey, J.S. Okasinski, A.K. Burrell, B.J. Ingram, In situ synchrotron X-ray diffraction studies of lithium oxygen batteries, Journal of Materials Chemistry A, 1 (2013) 6915-6919. [3] J.G. Simmons, Generalized Formula for the Electric Tunnel Effect between Similar Electrodes Separated by a Thin Insulating Film, Journal of Applied Physics, 34 (1963) 1793-1803. Figure 1

Electromigrated electrical optical antennas for transducing electrons and photons at the nanoscale.

Background: Electrically controlled optical metal antennas are an emerging class of nanodevices enabling a bilateral transduction between electrons and photons. At the heart of the device is a tunnel junction that may either emit light upon injection of electrons or generate an electrical current when excited by a light wave. The current study explores a technological route for producing these functional units based upon the electromigration of metal constrictions.Results: We combine multiple nanofabrication steps to realize in-plane tunneling junctions made of two gold electrodes, separated by a sub-nanometer gap acting as the feedgap of an optical antenna. We electrically characterize the transport properties of the junctions in the light of the Fowler–Nordheim representation and the Simmons model for electron tunneling. We demonstrate light emission from the feedgap upon electron injection and show examples of how this nanoscale light source can be coupled to waveguiding structures.Conclusion: Electromigrated in-plane tunneling optical antennas feature interesting properties with their unique functionality enabling interfacing electrons and photons at the atomic scale and with the same device. This technology may open new routes for device-to-device communication and for interconnecting an electronic control layer to a photonic architecture.

Multifunctional polyaniline/chloroplatinic acid composite material: Characterization and potential applications

Polyaniline/chloroplatinic acid (PANI/H2PtCl6.H2O) composite is synthesized viain situoxidative polymerization method. Fourier transform infrared, ultraviolet‐visible, energy‐dispersive X‐ray, and Thermogravimetric techniques are used for the characterization studies. The surface morphology of the samples is investigated by scanning electron microscopy and atomic force microscopy. Dielectric measurements are carried out depending on the temperature. The dispersion process increases the thermal degradation temperature of PANI by about 60°C, while reducing the optical energy gap of PANI from 1.99 to 1.76 eV. Chloroplatinic acid significantly increases the agglomeration and grain size (up to 10 μm) on the surface of the PANI. Gas sensor measurements show that Polyaniline/chloroplatinic acid composite can be a candidate for ethanol detection. With the dispersion process, the electrical conductivity of PANI increases almost 2.5 times. The charge transport mechanism of the samples is consistent with electron tunneling model up to 97°C and with small polaron tunneling model at higher temperatures. The hopping distance of the charge carriers reduced to 4.82 from 5.30 nm with dispersion process. Experimental results reveal that PANI/chloroplatinic acid composite can be a rather suitable multifunctional material for various applications such as gas sensors, optoelectronic and semiconductor devices. POLYM. ENG. SCI., 59:66–73, 2019. © 2018 Society of Plastics Engineers

Resistance switching of La doped SrTiO3 single crystals

To date, there has not been a consensus about the resistance switching mechanism of donor-doped SrTiO3. The La doped STO (LaSTO) single crystal is a donor-doped material and has an N-type conductivity since La3+ could easily substitute Sr2+. In this study, the Pt/LaSTO/In memory device is fabricated based on (100) LaSTO single crystal with 0.5 wt% La doping. Through a series of electrical tests, it is found that the Pt/LaSTO/In memory device has a stable multi-stage resistive switching property, and the maximum switching ratio is 104. The fitting I-V curve at the high resistance state (HRS) shows that there is an interface barrier in the memory device. However, the fitting I-V curve at low resistance state (LRS) is consistent with the characteristic of the electron tunneling model. The spectrum of electron paramagnetic resonance (EPR) indicates that LaSTO single crystal has only one EPR signal of g=2.012. Considering the fact that g=gobs-ge (where gobs is the g factor obtained from the sample, ge=2.0023 is the free electron value) is positive, the signal can be regarded as being due to hole center. The hole center is positively charged and can trap electrons. Comprehensive analysis indicates that the transition between the HRS and LRS of the device can be explained by the modulation of Pt/LaSTO interface barrier, which is caused by the electron trapping and detrapping of interfacial vacancy defects. In addition, it is found that illumination could reduce the low resistance of the Pt/LaSTO/In device. This is due to the photo-generated carriers causing a tunneling current because of the narrow Schottky barrier when the Pt/LaSTO/In device is in the LRS. However, the Schottky barrier plays a leading role in HRS, so the change in carrier concentration, caused by illumination, does not lead to a significant change in current for HRS. The experimental results provide theoretical and technical guidance for the applications of LaSTO single crystals in resistive memory devices.

Bias dependence of spin transfer torque in Co2MnSi Heusler alloy based magnetic tunnel junctions

Heusler compounds are of interest as electrode materials for use in magnetic tunnel junctions (MTJs) due to their half metallic character, which leads to 100% spin polarization and high tunneling magnetoresistance. Most work to date has focused on the improvements to tunneling magnetoresistance that can stem from the use of Heusler electrodes, while there is much less work investigating the influence of Heusler electrodes on the spin transfer torque properties of MTJs. Here, we investigate the bias dependence of the anti-damping like and field-like spin transfer torque components in both symmetric (Co2MnSi/MgO/Co2MnSi) and asymmetric (Co2MnSi/MgO/CoFe) structure Heusler based MTJs using spin transfer torque ferromagnetic resonance. We find that while the damping like torque is linear with respect to bias for both MTJ structures, the asymmetric MTJ structure has an additional linear component to the ordinarily quadratic field like torque bias dependence and that these results can be accounted for by a free electron tunneling model. Furthermore, our results suggest that the low damping and low saturation magnetization properties of Heusler alloys are more likely to lead significant improvements to spin torque switching efficiency rather than their half metallic character.

Modeling of electron tunneling through a tilted potential barrier

Tunnel junctions are interesting for both studying fundamental physical phenomena and providing new technological applications. Modeling of the tunneling current is important for understanding the tunneling processes and interpreting experimental data. In this work, the tunneling current is modeled using the Tsu-Esaki formulation with numerically calculated transmission. The feasibility of analytical formulae used for fitting experimental results is studied by comparing them with this model. The Tsu-Esaki method with numerically calculated transmission provides the possibility to calculate tunneling currents and fit experimental I–V curves for wide bias voltage and barrier width ranges as opposed to the more restricted analytical formulae. I–V curve features typical of tilted barrier structures are further analyzed to provide insight into the question, which of the phenomena can be explained with this simple barrier model. In particular, a small change in the effective barrier width is suggested as a possible explanation for experimental I–V curve features previously interpreted by a change in the tilt and height of the barrier.