Large External Electric Field Research Articles

Theoretical study on photoelectric emission of AlxGa1-xAs nanowire array photocathode with variable Al component under external electric field

The internal electric field formed by the AlxGa1-xAs structure with variable Al component can promote the carrier migration to the top surface of the nanowire to improve the photoelectric emission of the nanowire array. Similarly, under the action of the external electric field, the carrier inside the nanowire drift towards the top surface and change the trajectories of electrons between the nanowires. In this paper, the variable Al component AlxGa1-xAs nanowire model was established by using COMSOL Multipysics software, and the quantum efficiency of the nanowire was calculated based on the two-dimensional continuity equation with Matlab. The results show that the Al component in the second sublayer of AlxGa1-xAs nanowire can greatly affect the quantum efficiency of the array. Small external electric field can improve the quantum efficiency, while large external electric field and inclined light irradiation can reduce the quantum efficiency.

Electrical control of orbital and vibrational interlayer coupling in bi- and trilayer 2H−MoS2

Manipulating electronic interlayer coupling in layered van der Waals (vdW) materials is essential for designing opto-electronic devices. Here, we control vibrational and electronic interlayer coupling in bi- and trilayer 2H-MoS$_2$ using large external electric fields in a micro-capacitor device. The electric field lifts Raman selection rules and activates phonon modes in excellent agreement with ab-initio calculations. Through polarization resolved photoluminescence spectroscopy in the same device, we observe a strongly tunable valley dichroism with maximum circular polarization degree of $\sim 60\%$ in bilayer and $\sim 35\%$ in trilayer MoS$_2$ that are fully consistent with a rate equation model which includes input from electronic band structure calculations. We identify the highly delocalized electron wave function between the layers close to the high symmetry $Q$ points as the origin of the tunable circular dichroism. Our results demonstrate the possibility of electric field tunable interlayer coupling for controlling emergent spin-valley physics and hybridization driven effects in vdW materials and their heterostructures.

Contact barriers modulation of graphene/β-Ga2O3 interface for high-performance Ga2O3 devices

The ultra-wide-gap β-Ga2O3 has been regarded as a promising material for next-generation power electronic and deep-ultraviolet (UV) photodetectors. Exploring a suitable electrode is vital for realizing high performance β-Ga2O3 based nanodevices. Herein, the structural and contact properties of graphene/Ga2O3 interfaces are tuned and investigated by using the first-principles calculations. Results show that the small n-type Schottky barrier of about 0.07 eV for the graphene/Ga2O3 interface with weak interlayer interaction is irrespective of the interface stacking arrangement. Moreover, the intrinsic electronic property of Ga2O3 is well preserved in the interface. More interestingly, the n-type Schottky barrier to Ohmic contact transition can be obtained by shorting the interlayer distance, or increasing the graphene layers or applying a negative external electric field for the interface. Moreover, applying a large positive external electric field can realize the p-type Schottky barrier to Ohmic contact transition for graphene/Ga2O3 interface. These results are uncovered by analyzing the interfacial dipole and potential step of graphene/Ga2O3 interface, and expected to enhance the application potential of graphene electrode in the β-Ga2O3 based electronic and optoelectronic devices.

First-principle investigation of the elastic and plastic properties of the bismuthene: Effect of the external electric field

Abstract In this article, the density functional theory is employed to evaluate the elastic and plastic properties of the monolayer bismuthene. To investigate the influence of the external electric field on the bismuthene mechanical properties, the external electric fields with different magnitudes are applied to it. It is shown that Young's modulus of the bismuthene is not sensitive to small electric fields. However, a large external electric field can change it. Similarly, a sufficiently large external electric field can result in decreasing the yield strain of the monolayer bismuthene. Besides, it is shown that the Bi–Bi bond length is not sensitive to the presence of the external electric field.

Tuning Schottky barrier in graphene/InSe van der Waals heterostructures by electric field

The contacts between semiconductor and metal are vital in the fabrication of nano electronic and optoelectronic devices. The contact type has a great influence on the function realization and performance of the device. In order to prepare multifunctional devices with high performance, it is necessary to modulate the barrier height and contact type at the interface. First-principles calculations based on the density functional theory (DFT) are implemented in the VASP package. The generalized gradient approximation of Perdew, Burke, and Ernzerhof (GGA-PBE) with van der Waals (vdW) correction proposed by Grimme (DFT-D3) is chosen due to its good description of long-range vdW interactions. It is demonstrated that weak vdW interactions dominate between graphene and InSe with their intrinsic electronic properties preserved. We find that the n-type ohmic contact is formed at the graphene/InSe interface with the Fermi level through the conduction band of InSe (<i>Φ</i><sub>Bn</sub> < 0). The Fermi level of graphene/InSe heterostructure moves down to below the Dirac point of graphene layer, which results in p-type (hole) doping in graphene. Moreover, the external electric field is effective to tune the Schottky barrier, which can control not only the Schottky barrier height but also the type of contact. With the negative external electric field varying from 0 to –1 V/nm, the conduction band minimum of InSe below the Fermi level declines gradually but the n-type ohmic contact is still preserved. Nevertheless, with the positive external electric field varying from 0 to 0.8 V/nm, the conduction band minimum of InSe shifts upward and across the Fermi level, the conduction band minimum of InSe is closer to the Fermi level than the valence band maximum, which indicates that the n-type Schottky contact is formed. The Fermi level moves from the the conduction band minimum to the valence band maximum of InSe when the positive external electric field increases from 0.8 V/nm to 2 V/nm. The n-type Schottky barrier height exceeds the p-type Schottky barrier height gradually, which demonstrates that the positive external electric field transforms the n-type Schottky contact into the p-type Schottky contact at the graphene/InSe interface. When the positive external electric field exceeds 2 V/nm, the valence band of InSe moves upward and cross the Fermi level (<i>Φ</i><sub>Bp</sub> < 0), the ohmic contact is obtained again. Meanwhile, p-type (hole) doping in graphene is enhanced under negative external electric field and a large positive external electric field is required to achieve n-type (electron) doping in graphene. The external electric field can control not only the amount of charge transfer but also the direction of charge transfer at the graphene/InSe interface.

Trap‐Related Nonvolatile Negative Photoconductivity in a Single Ag@Al2O3 Hybrid Nanorod for a Photomemory with Light‐Writing and Bias‐Erasing

AbstractFor zero band‐gap metal Ag and ultrawide band‐gap Al2O3, it is difficult to produce impressive photoresponse to visible light due to the limitation of energy gaps. Herein, it is demonstrated that individual Ag@Al2O3 hybrid nanorods, synthesized by a two‐stage hydrothermal method and followed by thermal reduction annealing, can show excellent negative photoconduction of about 400 nm violet and 800 nm near‐infrared lights at room temperature. Moreover, the light‐induced high resistance state (HRS) is well maintained at relatively low operation bias after the removal of illumination, indicative of a nonvolatile memory effect. More importantly, the device is back to its initial low resistance state (LRS) after subsequently being applied a relatively large bias, suggestive of an erasable effect with large bias. In the hybrid nanorods, Ag nanoparticles serve as trap centers and can capture and store charges. Under the illumination of sub‐band‐gap light, trapped charges are excited, resulting in an HRS due to emptying the traps. On the contrary, a large external electric field triggers charges to be injected into traps in dark, resulting in an LRS. Regarding a superior negative photoswitching with light‐writing and bias‐erasing memory, Ag@Al2O3 nanohybrids have a tremendous potential in optical sensors and nonvolatile photomemory applications.

Solution-Processed Insulators for Flexible Metal-Insulator-Metal Structures

Repeatable switching hysterisis in metal-insulator-metal devices is commonly attributed to the motion of oxygen vacancies under a sufficiently large external electric field. The resulting memristive behaviour has become a compelling alternative to traditional non-volatile memory device architectures. A room-temperature process for the fabrication of a metal-insulator-metal structure employing niobium pentaoxide ( $$\hbox {Nb}_{2}\hbox {O}_{5}$$ ) as the active layer has been developed, without any annealing of the oxide film. Electrical characterization of the devices shows sharp switching of resistivity. The developed process is very simple, cost-effective and can be implemented on flexible substrates.

Electrified magnetic catalysis in three-dimensional topological insulators

The gap equations for the surface quasiparticle propagators in a slab of three-dimensional topological insulator in external electric and magnetic fields perpendicular to the slab surfaces are analyzed and solved. A different type of magnetic catalysis is revealed with the dynamical generation of both Haldane and Dirac gaps. Its characteristic feature manifests itself in the crucial role that the electric field plays in dynamical symmetry breaking and the generation of a Dirac gap in the slab. It is argued that, for a sufficiently large external electric field, the ground state of the system is a phase with a homogeneous surface charge density.

Electric control of inverted gap and hybridization gap in type-II InAs/GaSb quantum wells

The quantum spin Hall effect has been predicted theoretically and observed experimentally in InAs/GaSb quantum wells as a result of inverted band structures, for which electron bands in InAs layers are below heavy hole bands in GaSb layers in energy. The hybridization between electron bands and heavy hole bands leads to a hybridization gap away from k=0. A recent puzzling observation in experiments is that when the system is tuned to more inverted regime by a gate voltage (a larger inverted gap at k=0), the hybridization gap decreases. Motivated by this experiment, we explore the dependence of hybridization gap as a function of external electric fields based on the eight-band Kane model. We identify two regimes when varying electric fields: (1) both inverted and hybridization gaps increase and (2) inverted gap increases while hybridization gap decreases. Based on the effective model, we find that light-hole bands in GaSb layers play an important role in determining hybridization gap. In addition, a large external electric field can induce a strong Rashba splitting and also influence hybridization gap.

Microchannel Voltammetry in the Presence of Large External Voltages and Electric Fields.

The ability to perform electrochemistry in the presence of large voltages and electric field magnitudes without concern for the local potential has many possible applications in micro/nanofluidic assays and in capillary electrophoresis. Traditionally, electrochemistry in the presence of significant external electric fields has been dominated by end-channel detection for capillary and microchip electrophoresis detection. We describe novel instrumentation for potentiostatically controlled voltammetry that can be applied in the presence of high external voltages and electric fields. Cyclic voltammetry is demonstrated without significant shifts in the half-wave potential at working electrodes at local potentials of up to ∼1500 V and field strengths of up to 3000 V/cm, using a standard Ag/AgCl reference electrode.

A theoretical guide for fabricating a conductive molecular wire on a silicon surface via an in situ surface polymerization reaction.

It has been a long-standing goal to make conductive molecular wires or linear polymer chains on traditional semiconductors or insulator substrates to satisfy the ongoing miniaturization in electronic devices. Here, we have proposed a surface in situ polymerization reaction for a pre-absorbed molecule, 4-hydrazinyl-3-(pyridin-4-ylmethyl)-benzaldehyde (HPyMB), to produce a conductive molecular wire on a silicon surface. Our first-principles calculations show that HPyMB molecules can absorb alternatively on the exposed Si atoms created via ultrahigh vacuum scanning tunneling microscopy on a hydrogen passivated H-Si(001)2 × 1 surface along the [110] direction. The adsorption is exothermic and its generated energy is sufficient for the following intermolecular dehydration polymerization reaction to overcome the activation energy barriers and thereafter form a molecular wire on the surface. This polymerized molecular wire is mechanically stable since it is chemically bonded onto the surface. After polymerization, the system becomes conductive due to the charge transfer from the molecule-surface bonds to their pyridine rings. More importantly, by removing 1.1 electrons from the system, the surface polymer chain is the sole conductive channel. Furthermore, its conducting nature remains robust even under a large external electric field. Our findings open a new window for the fabrication of conductive molecular wires or polymer chains on semiconductor surfaces, and provide insights into the mechanism behind the molecular wire conductivity.

Ferroelectric domain evolution in 0.9625Bi0.5Na0.5TiO3−0.0375BiZn0.5Ti0.5O3 piezoceramic

The domain evolution of 0.9625Bi0.5Na0.5TiO3−0.0375BiZn0.5Ti0.5O3 ceramic is studied by piezoelectric force microscopy. Nanodomains are the most common in the as-prepared ceramic, and they can transform to monodomain in a micrometer-scale grain under a large external electric field, which corresponds to a maximum piezoelectric d33 coefficient. After the field is removed, the mondomain commonly decomposes gradually, and occasionally it transforms to periodic stripe-like domains. This experiment exhibits the domain evolution during poling and depoling processes of a normal ferroelectric ceramic, which is different from ergodic relaxors.

Theory of the SrTiO3surface state two-dimensional electron gas

We present a theory of the quasi two-dimensional electron gas (2DEG) systems which appear near the surface of SrTiO$_3$ when a large external electric field attracts carriers to the surface. We find that non-linear and non-local screening by the strongly polarizable SrTiO$_3$ lattice plays an essential role in determining 2DEG properties. The electronic structure always includes weakly bound bulk-like bands that extend over many SrTiO$_3$ layers. At 2D carrier-densities exceeding $\sim 10^{14} {\rm cm}^{-2}$ tightly bound bands emerge that are confined within a few layers of the surface.

Reversible, opto-mechanically induced spin-switching in a nanoribbon-spiropyran hybrid material

It has recently been shown that electronic transport in zigzag graphene nanoribbons becomes spin-polarized upon application of an electric field across the nanoribbon width. However, the electric fields required to experimentally induce this magnetic state are typically large and difficult to apply in practice. Here, using both first-principles density functional theory (DFT) and time-dependent DFT, we show that a new spiropyran-based, mechanochromic polymer noncovalently deposited on a nanoribbon can collectively function as a dual opto-mechanical switch for modulating its own spin-polarization. These calculations demonstrate that upon mechanical stress or photoabsorption, the spiropyran chromophore isomerizes from a closed-configuration ground-state to a zwitterionic excited-state, resulting in a large change in dipole moment that alters the electrostatic environment of the nanoribbon. We show that the electronic spin-distribution in the nanoribbon-spiropyran hybrid material can be reversibly modulated via noninvasive optical and mechanical stimuli without the need for large external electric fields. Our results suggest that the reversible spintronic properties inherent to the nanoribbon-spiropyran material allow the possibility of using this hybrid structure as a resettable, molecular-logic quantum sensor where opto-mechanical stimuli are used as inputs and the spin-polarized current induced in the nanoribbon substrate is the measured output.

Controlling Half-Metallicity of Graphene Nanoribbons by Using a Ferroelectric Polymer

On the basis of first-principles computational approaches, we present a new method to drive zigzag graphene nanoribons (ZGNRs) into the half-metallic state using a ferroelectric material, poly(vinylidene fluoride) (PVDF). Owing to strong dipole moments of PVDFs, the ground state of the ZGNR becomes half-metallic when a critical coverage of PVDFs is achieved on the ZGNR. Since ferroelectric polymers are physisorbed, the direction of the dipole field in PVDFs can be rotated by relatively small external electric fields, and the switching between half-metallic and insulating states may be achieved. Our results suggest that, without excessively large external gate electric fields, half-metallic states of ZGNRs are realizable through the deposition of ferroelectric polymers and their electronic and magnetic properties are controllable via noninvasive mutual interactions.

Simulation of water clusters in vapour, alkanes and polyethylenes

The Gibbs Ensemble Monte Carlo (GEMC) technique has been used to study the clustering of water in vapour, alkanes and polyethylene, where the water clusters are in equilibrium with liquid phase water. The effect of an external electric field and ionic impurities on the clustering of water in the hydrocarbons (alkanes and polyethylene) has also been studied. The simulations of water clustering in polyethylene were made more efficient by using a connectivity altering osmotic Gibbs ensemble method. It was found that trends in the size distribution of water clusters in the hydrocarbons are similar to those found in the pure vapour, but that fewer and smaller clusters are formed as the length of the hydrocarbon chain increased. Also, large external electric fields decrease the solubility of water in hydrocarbons, whereas the presence of ionic species dramatically increases the solubility.

Molecular Dynamics Simulation of Ion Emission from Nanodroplets of Ionic Liquids

The work reported here is motivated by a desire to fully understand the physics of colloidal thruster propulsion technology. Colloidal thruster technology is based on the electrostatic acceleration of small droplets and/or ions that are generated by feeding a conducting fluid through a small capillary and applying a large acceleration potential between the capillary and an extraction electrode. Maximum thrust is obtained when the device is operated in the droplet mode. However, by transitioning to an ion mode, the specific impulse can be greatly increased. Here, we use molecular modeling to study the dynamics of small droplets of ionic liquids (EMIM-BF4) in the presence of large external electric fields. Interatomic and molecular forces are described using a molecular mechanics force field, and the whole ion structure is included. Before an electric field is applied, minimization methods are used to determine the equilibrium structure of the droplet. An electric field is then applied and the resulting motions of droplets and individual ions are observed. In addition, the surface tension of the droplets is calculated from the dynamic simulations. Surface tensions calculated from the simulations agree with macroscopic experimental values within uncertainty. Thus, the results indicate that the Taylor expression for the critical electric field for droplet ionization is order-of-magnitude-correct.

Effects of an external electric field on the ferroelectric-paraelectric phase transition in polyvinylidene fluoride-trifluoroethylene copolymer Langmuir–Blodgett films

X-ray diffraction and capacitance measurements have been employed to study the structural and dielectric behavior of the ferroelectric-paraelectric phase transition under the influence of a large external electric field. The samples under study are ultrathin (15–100nm) Langmuir–Blodgett films of a copolymer of vinylidene fluoride (70%) with trifluoroethylene (30%) deposited on aluminum-coated silicon. In situ θ-2θ x-ray diffraction was used to measure the change in interlayer spacing perpendicular to the film surface, corresponding to the (110) direction and indicating that the polymer chains along (001) lie predominantly in the plane, while capacitance measurements were used to monitor the behavior of the dielectric constant of the film. Application of a large electric field, up to 265MV∕m, raises the phase transition temperature and can convert the nonpolar trans-gauche paraelectric phase to the polar all-trans ferroelectric phase in a reversible manner.

Photon propagation in magnetic and electric fields with scalar/pseudoscalar couplings: A new look

We consider the minimal coupling of two photons to neutral scalar and pseudoscalar fields, as, for instance, in the case of the Higgs boson and axion, respectively. In this framework, we analyze the photon dispersion relations in the presence of static and homogeneous external magnetic and electric fields, by taking into account the contribution of the imaginary part of the scalar/pseudoscalar self-energy. We show that this contribution cannot be neglected when it is of the same order as the photon-scalar/pseudoscalar mixing term. In addition to the usual lightlike photon propagation mode, with a refraction index $n>1$, a massive mode with mass of the order of the coupled boson mass can be induced, provided that the external field is above a particular critical value. Depending on the values of the external field, photon energy, and mass of the scalar/pseudoscalar particle, the scalar/pseudoscalar width could induce a sizable rate of photon splitting in two photons due to a strong resonant phenomenon. This effect has no practical laboratory applications for the Higgs physics due to the very large critical external magnetic or electric fields involved, for a photon energy of the order of a TeV. However, it can have relevant consequences in the axion physics or in any other scenario where light neutral scalar/pseudoscalar fields have minimal coupling with two photons.

Nonlinear electric field dependence of piezoresponse in epitaxial ferroelectric lead zirconate titanate thin films

Landau–Devonshire-type phenomenological thermodynamic theory is employed to explain the electric field dependence of piezoelectric properties of tetragonal single domain PbZrxTi1−xO3 (PZT). The strong nonlinearity of the converse piezoelectric coefficient under a large external electric field is proved to be intrinsic both in bulk crystal and epitaxial tetragonal PZT thin films. The tunability of piezoelectric responses by an external electric field and its dependence on the film/substrate misfit and elastic compliance of thin films are characterized quantitatively. The theoretical predictions are in good agreement with the experimental results of piezoresponse scanning microscopy. Due to the large tunability of piezoresponse in it, PZT 50/50 (x=0.5) epitaxial film is a promising candidate for use in future tunable devices.