Tunneling Of Conduction Electrons Research Articles

Impact of 100 MeV high-energy proton irradiation on β-Ga2O3 solar-blind photodetector: Oxygen vacancies formation and resistance switching effect

β-Ga2O3 based solar-blind photodetectors have strong radiation hardness and great potential applications in Earth's space environment due to the large bandgap and high bond energy. In this work, we investigated the photoelectric properties influence of β-Ga2O3 photodetector irradiated by 100 MeV high-energy protons which are the primary components in the inner belt of the Van Allen radiation belts where solar-blind photodetectors mainly worked. After proton irradiation, due to the formation of more oxygen vacancies and their migration driven by bias at the metal/semiconductor interface, transportation of carriers transforms with electron tunneling conduction for low-resistance state and thermionic emission for high resistance state. As a result, the current–voltage curves of β-Ga2O3 solar-blind photodetectors exhibit apparent hysteresis loops. The photoresponsivity of β-Ga2O3 photodetectors slightly increases from 1.2 × 103 to 1.4 × 103 A/W after irradiation, and the photoresponse speed becomes faster at a negative voltage while slower at positive voltage. The results reveal the effects of high-energy proton irradiation on β-Ga2O3 solar-blind photodetectors and provide a basis for the study of their use in a radiation harsh environment.

Electron transport in discontinuous metal thin films

The structure and basic experimental electrical properties of vacuum evaporated discontinuous (island) metal thin films of discrete metal nanoparticles on insulating substrates are briefly reviewed. Then the widely accepted Neugebauer and Webb (N&W) electrostatically activated electron tunneling conduction model is covered (with enhancements) before the numerous discrepancies between this model and experimental observations are identified, e.g. minimal substrate bias effect, non-linear field distribution, anomalous AC effects, asymmetrical contact effects, and switching. A modified model, based on contact electron injection and extraction, and computer simulations are introduced which explain these discrepancies at a qualitative level. However, quantitative experimental verification of the model is not possible without stable, reproducible films of known structures. The paper concludes with a review of possible preparation techniques which could yield satisfactory samples, especially self-assembly of organically protected metal nanoparticles. One of these has already demonstrated electrostatically activated conduction.

Spectroelectrochemical Behavior of Polycrystalline Gold Electrode Modified by Reverse Micelles.

The increasing demand for raising the reliability of electronic contacts has led to the development of methods that protect metal surfaces against atmospheric corrosion agents. This severe problem implies an important economic cost annually but small amounts of corrosion inhibitors can control, decrease or avoid reactions between a metal and its environment. In this regard, surfactant inhibitors have displayed many advantages such as low price, easy fabrication, low toxicity and high inhibition efficiency. For this reason, in this article, the spectroelectrochemical behavior of polycrystalline gold electrode modified by reverse micelles (water/polyethyleneglycol-dodecylether (BRIJ 30)/n-heptane) is investigated by atomic force microscopy (AFM), potentiodynamic methods and electrochemical impedance spectroscopy (EIS). Main results indicate a strong adsorption of a monolayer of micelles on the gold substrate in which electron tunneling conduction is still possible. Therefore, this method of increasing the corrosion resistance of gold contacts is usable only in conditions of long-term storage but not in the operation of devices with such contacts. In this regard, the micelle coating must be removed from the surface of the gold contacts before use. Finally, the aim of the present work is to understand the reactions occurring at the surfactant/metal interface, which may help to improve the fabrication of novel electrodes.

Interface driven electrical and magneto-transport properties of (100 – x)% La0.7Sr0.3MnO3– x% Paraffin wax (0 ≤ x ≤ 1) hybrid nanocomposites

We report electrical and magneto-transport properties of La0.7Sr0.3MnO3(LSMO)/Paraffin wax nanocomposites prepared through low temperature chemical pyrophoric reaction process. A core shell structure is expected to be formed having a ferromagnetic LSMO core and a spin disordered shell of paraffin wax. Enhanced charging energy due to coulomb blockade contribution with increasing paraffin wax is observed for the composites attributing enhanced intergranular distance between LSMO grains. Effective spin-polarized tunneling of conduction electrons resulting in sharp low field magnetoresistance (MR) of the composites is due to reordering of spin disorder at paraffin wax shell with magnetic field. Moreover, observation of enhanced magnetoresistive hysteresis with decrease in temperature has been attributed to increased blocked states at LSMO grain surface. Calculated surface spin susceptibility (χb) has been found to follow similar behavior of MR with temperature indicating surface magnetization dependent MR. Fitting of magnetoconductance versus field curves reveal increase of Para to ferro transition temperature, TC and χb indicate higher antiferromagnetic coupling of shell spins for composite samples than pristine LSMO above 50 K. χb also indicate systematic decrease in surface spin disorder of the composite samples with respect to pure LSMO which is expected to be due to the probable covalent bond formation between Paraffin wax molecules and surface atoms of LSMO nanoparticles.

The effect of thermal magnonic excitations on the electronic conductance of a magnetic nanowire

Abstract In this paper, we study the effect of thermal magnonic excitations on the electronic conductance of an anti-ferromagnetic nanowire which is coupled to two electrodes in the framework of equilibrium statistical mechanics. The electron-magnon scattering is assumed to be present only in the center part, not in the electrodes. For each magnonic mode of the chain, we calculate a Green’s function and the corresponding mode-dependent transmission coefficient via the linear response theory. Then, the total electronic transmission through the system is obtained by the summation of the mode-dependent ones. We examine the influence of magnonic waves on three different configurations for the center wire including uniform, alternative on-site (AB) and polyacetylene like (PA-like) chain. Two types of nonmagnetic and ferromagnetic structures are chosen for the left and right leads. The results show that the effect of magnonic excitations on the electronic conductance of the considered systems is important at high temperatures. Especially, the electron tunneling conductance is improved in the presence of magnonic excitations in the PA-like and AB nanowires in contrast to the uniform one. While for all configurations, the conductance decreases in the resonance region by increasing the magnonic effects. Finally, we discuss the spin polarization and we show that the effect of thermal magnonic excitations is more observable in AB with respect to PA-like and in PA-like with respect to the uniform configuration of the center wire.

Defect-Assisted Tunneling Electroresistance in Ferroelectric Tunnel Junctions.

Recent experimental results have demonstrated ferroelectricity in thin films of SrTiO_{3} induced by antisite Ti_{Sr} defects. This opens a possibility to use SrTiO_{3} as a barrier layer in ferroelectric tunnel junctions (FTJs)-emerging electronic devices promising for applications in nanoelectronics. Here using density functional theory combined with quantum-transport calculations applied to a prototypical Pt/SrTiO_{3}/Pt FTJ, we demonstrate that the localized in-gap energy states produced by the antisite Ti_{Sr} defects are responsible for the enhanced electron tunneling conductance which can be controlled by ferroelectric polarization. Our tight-binding modeling, which takes into account multiple defects, shows that the predicted defect-assisted tunneling electroresistance effect is greatly amplified when the defect energy levels are brought to the Fermi energy by one of the polarization states. Our results have implications for FTJs based on conventional ferroelectric barriers with defects and can be employed for the design of new types of FTJs with enhanced performance.

A shear-enhanced CNT-assembly nanosensor platform for ultra-sensitive and selective protein detection

Detection and quantification of low-concentration proteins in heterogeneous media are generally plagued by two distinct obstacles: lack of sensitivity due to high dissociation equilibrium constant KD and non-specificity due to an abundance of non-targets with similar KD. Herein, we report a nanoscale protein-sensing platform with a non-equilibrium on-off switch that employs dielectrophoretic and hydrodynamic shear forces to overcome these thermodynamic limitations with irreversible kinetics. The detection sensitivity is achieved with complete association of the antibody-antigen-antibody (Ab-Ag-Ab) complex by precisely and rapidly assembling carbon nanotubes (CNT) across two parallel electrodes via sequential DC electrophoresis and AC dielectrophoresis (DEP), and with single-CNT electron tunneling conductance. The high selectivity is achieved with a critical hydrodynamic shear rate between the activated dissociation shear rates of target and non-target linkers of the aligned CNTs. We are able to reach detection limits of 100 attomolar (aM) and 10 femtomolar (fM) in pure samples for two ELISA assays with low and high dissociation constant: biotin/streptavidin (10 fM) and HER2/HER2 antibody (0.44 ± 0.07nM), respectively. For both models, irreversible capture and shearing allow us to tune the dynamic range up to 5 decades by increasing the CNT numbers. We also demonstrate in spiked serum sample high selectivity towards target HER2 proteins against non-target HER2 isoform of a similar KD. The detection limit for HER2 in serum is lower than 100fM.

Large low-field magnetoresistance of Fe3O4 nanocrystal at room temperature

Superparamagnetic magnetite (Fe3O4) nanoparticles with an average size of 6.5nm and good monodispersion were synthesized and investigated by X-ray diffraction, Raman spectrometer, transmission electron microscopy and vibrating sample magnetometer. Corresponding low-field magnetoresistance (LFMR) was tested by physical property measurement system. A quite high LFMR has been observed at room temperature. For examples, at a field of 3000Oe, the LFMR is −3.5%, and when the field increases to 6000Oe, the LFMR is up to −5.1%. The electron spin polarization was estimated at 25%. This result is superior to the previous reports showing the LFMR of no more than 2% at room temperature. The conduction mechanism is proposed to be the tunneling of conduction electrons between adjacent grains considering that the monodisperse nanocrystals may supply more grain boundaries increasing the tunneling probability, and consequently enhancing the overall magnetoresistance.

Conductance hysteresis and inelastic excitations at hydrogen decorated cerium atoms and clusters in a tunnel junction

Voltage-controlled conductance and switching induced by single molecules or atoms are ideally studied in scanning tunneling microscope (STM) tunnel junctions. While the objects under consideration are mostly used in their original form, little is known of the possibilities of in situ adjustments of their properties. Here, we evidence properties of a tunnel junction made of a Ce atom/cluster built by atomic manipulation on Au(111) at a temperature of 4.6 K in the presence of H2. The conductance through the object is characterized by a switching voltage corresponding to an opening or closing of an inelastic electron tunneling conductance channel at 50 mV for a Ce atom and 140 mV for a Ce cluster and by charging. We demonstrate that the electronic properties of an STM junction can be engineered in a simple way by in situ guiding of the H2 pinning at an atomic cluster.

Quantum transport through a multi-quantum-dot-pair chain side-coupled with Majorana bound states**Project supported by the National Natural Science Foundation of China (Grant Nos. 11274040 and 10974015) and the Program for New Century Excellent Talents in University of China (Grant No. NCET-08-0044).

We investigate the quantum transport properties through a special kind of quantum dot (QD) system composed of a serially coupled multi-QD-pair (multi-QDP) chain and side-coupled Majorana bound states (MBSs) by using the Green functions method, where the conductance can be classified into two kinds: the electron tunneling (ET) conductance and the Andreev reflection (AR) one. First we find that for the nonzero MBS-QDP coupling a sharp AR-induced zero-bias conductance peak with the height of e2/h is present (or absent) when the MBS is coupled to the far left (or the other) QDP. Moreover, the MBS-QDP coupling can suppress the ET conductance and strengthen the AR one, and further split into two sub-peaks each of the total conductance peaks of the isolated multi-QDPs, indicating that the MBS will make obvious influences on the competition between the ET and AR processes. Then we find that the tunneling rate ΓL is able to affect the conductances of leads L and R in different ways, demonstrating that there exists a ΓL-related competition between the AR and ET processes. Finally we consider the effect of the inter-MBS coupling on the conductances of the multi-QDP chains and it is shown that the inter-MBS coupling will split the zero-bias conductance peak with the height of e2/h into two sub-peaks. As the inter-MBS coupling becomes stronger, the two sub-peaks are pushed away from each other and simultaneously become lower, which is opposite to that of the single QDP chain where the two sub-peaks with the height of about e2/2h become higher. Also, the decay of the conductance sub-peaks with the increase of the MBS-QDP coupling becomes slower as the number of the QDPs becomes larger. This research should be an important extension in studying the transport properties in the kind of QD systems coupled with the side MBSs, which is helpful for understanding the nature of the MBSs, as well as the MBS-related QD transport properties.

Electronic structure of a gold nanotube

The electronic structure of a gold nanotube has been studied by quantum-chemical methods. The energy dependences of total and partial densities of states of a nanotube with 16 atoms in a translational unit cell have been calculated by the linearized augmented-cylindrical-wave method. It has been demonstrated that the nanotube has a metal-like band structure. The s(Au) states are located completely in the valence band and are not involved in electron transport. The Fermi level is located at the peak of the total and partial d(Au) densities of states, which should contribute to the high electron tunneling conductance of the system. The valence band width is 11 eV.

Spin injection via (110)-grown semiconductor barriers

We study the tunneling of conduction electrons through a (110)-oriented single-barrier heterostructure grown from III-V semiconductor compounds. It is shown that, due to low spatial symmetry of such a barrier, the tunneling current through the barrier leads to an electron spin polarization. The inverse effect, generation of a direct tunneling current by spin polarized electrons, is also predicted. We develop the microscopic theory of the effects and show that the spin polarization emerges due to the combined action of the Dresselhaus spin-orbit coupling within the barrier and the Rashba spin-orbit coupling at the barrier interfaces.

Enhancement of Inelastic Electron Tunneling Conductance Caused by Electronic Decoupling in Iron Phthalocyanine Bilayer on Ag(111)

The effect of electronic decoupling on the inelastic electron tunneling process of iron phthalocyanine (FePc) molecules on Ag(111) was investigated using scanning tunneling microscopy (STM). A drastic difference in the inelastic electron tunneling to individual FePc molecules was found for the first and the second layer molecules grown on Ag(111). The spectrum of the first layer molecule is essentially structureless, whereas the second layer molecules provide giant conductance changes reaching several tens % due to the vibrational excitations. This is the first clear example to demonstrate, by using inelastic tunneling spectroscopy with STM, the enhancement of vibrational inelastic tunneling driven through the electronic decoupling of the molecules from the substrate.

Negative differential resistance in electron tunneling in ultrathin films near the two-dimensional limit

We report on our observation of negative differential resistance (NDR) in electron tunneling conductance in atomic-scale ultrathin Ag films on Si(111) substrates. NDR was observed by scanning tunneling spectroscopy measurements. The tunneling conductance depends on the electronic local density of states (LDOS) of the sample. We show that the sample bias voltage, at which negative differential resistance and peak negative conductance occur, depends on the film thickness. This can be understood from the variation in the LDOS of the Ag films as a function of film thickness down to the two-dimensional limit of one atomic layer. First principles density functional theory calculations have been used to explain the results.

Electric field assisted dissolution of Au rods in gold-doped silicate glass

Dissolution of Au rods in gold-doped silicate glass is observed experimentally during the dc electric field thermal poling. Scanning electron microscopy characterizations show that some Au rods with a high aspect ratio are dissolved to spherelike particles and others still keep an elongated structure, which is well accorded with the absorption spectroscopy results. The mechanism for dissolution of Au particles is attributed to electron tunneling conduction and Au cationic conduction, based on electrical measurements during the electric field assisted dissolution process. Electric field thermal poling provides a promising method for the controlling of structural and optical properties of noble metal-doped silicate glass.

Electrical conductance properties for magnetic tunnel junctions with MgO barriers

We measured inelastic electron tunneling (IET) spectra and conductance for MgO tunneling magnetoresistance (TMR) films to obtain information on the ferromagnetic/barrier layer interface. The IET spectra showed the difference between amorphous and crystalline structures in the barrier. In the magnetic tunnel junction (MTJ) with a crystalline barrier the IET spectra indicated an Mg-O phonon peak at a low bias voltage by measurement with a parallel magnetization configuration. On the other hand, no peak was observed in the MTJ with an amorphous barrier.

Transport properties and magnetoresistance of nano-polycrystalline La0.7Sr0.3MnO3－δ

The resistivity and magnetoresistance of bulk polycrystalline La0.7Sr0.3MnO3-δ, fabricated to produce nanoscale grains sintered at different temperatures, have been investigated as functions of temperature. With decreasing temperature below room temperature the resistivity (ρ) exhibits a maximum near 250K, below which it shows “metallic" behaviour, i.e. ρ∝T2 in the temperature range from 50K to 170K. However, ρ subsequentlyexhibits a minimum near 50K, below which it is well fit by the predictions of the tunneling of conduction electrons through insulating interfacial layers, viz lnρ ∝T1/2, a result interpreted as conduction-electron tunneling between adjacent LSM granules. The minimum of ρ occuring in these samples may result from the competition of the tunneling effect and the intrinsic metallic transport mechanism of LSM. The temperature dependence of the associated tunneling magnetoresistance is also reported.

Nano-interfacial space charge and single electron tunneling conduction in metal/polyimide/metal junctions

The surface potential of metal/polyimide (PI) ultra-thin film, prepared by spin-coat technique, was measured by Kelvin probe method at a temperature of 16 K. The photoirradiation effect on the surface potential was examined as the functions of the wavelength and irradiation time. It was found that the wavelength of 400 nm photoirradiation caused the charge injection from the metal electrode to PI films, rather than that of 600 nm. The behaviors of the metal/PI interfacial space charge estimated by the surface potential measurement well explained the photocurrent observed in the current–voltage characteristics of Au/PI/Al junctions, and the threshold voltage shift appeared in the Coulomb staircase of Au/PI:C 60/Al junction.

Ultrafast nonlinear optical response in silica-capped gold nanoparticle films

The imaginary part of the third-order nonlinear optical susceptibility (Im χ (3)) and the response time of Au nanoparticle-insulator composites with various concentrations of nanoparticles (NPs) have been investigated by femtosecond pump and probe spectroscopy. Au-NP composite films have been fabricated by depositing NP coated by insulating materials, and the volume fraction of NPs p was varied between 0.02 and 0.66. Im χ (3) from the surface plasmon resonance increases with p up to 0.34, and decreases for p>0.39. We have observed an enhancement of Im χ (3) from the interaction between NPs for p<0.39 which is in addition to the enhancement from the local-field effect of the Au NP-insulator composite system. The decrease of Im χ (3) for p>0.39 is attributed to the suppression of the local-field effect associated with the tunneling of conduction electrons between adjacent NPs. The response time is independent of p over the range 0.05–0.66, because photoexcited electrons in an NP relax by interaction with phonons within the NP.

Transport properties and magnetoresistance effect of polycrystalline La0.7Sr0.3MnO3 at low temperatures

The resistivity and magnetoresistance of bulk polycrystalline La0.7Sr0.3MnO3(LSM), deliberately fabricated to produce nanoscale grains, has been investigated as functions of temperature. With decreasing temperature below room temperature the resistivity (ρ) exhibits a maximum near 250K， below which it displays a “metallic" behaviour. However, ρ exhibits subsequently a minimum near 50K, below which it is well fit by the predictions for the tunneling of conduction electrons through insulating interfacial layers viz. lnρ∝T-1/2, a result interpreted as conduction-electron tunneling between adjacent LSM granules. The minimum of ρ occurs in this particular sample may result from the competition of the tunneling effect and the intrinsic transport mechanism of LSM. The temperature dependence of the associated tunneling magnetoresistance is also reported in this low temperature regime.