In-plane Electric Field Research Articles

Bright and uniform light emission from stretchable, dual-channel energy conversion systems: Simultaneous harnessing of electrical and mechanical excitations

Recently, significant progress has been made in the development of new techniques for the fabrication of mechanically durable, bright, and deformable electroluminescent devices, leading to the emergence of various technologies, such as soft robots, actuators, flexible/stretchable/wearable electronics, and self-healable devices. However, these devices mostly possess coplanar structures, wherein the internally generated light must be transmitted through at least one of the electrodes, and require a thin emissive layer (EML), causing low brightness and less applicability in soft devices. This is particularly challenging in the case of stretchable electroluminescent devices, which require electrodes exhibiting both high transmittance and low resistance even in the stretchable state because thin EMLs have low tolerance to external mechanical deformations. Herein, we report in-plane electric-field-driven, stretchable alternating-current electroluminescent devices with high brightness by utilizing a thick EML comprising multiple parallelly patterned silver nanowires embedded in a zinc-sulfide-embedded polydimethylsiloxane layer. Since the device is driven by an internal in-plane electric field, it can utilize a thick EML without using planar electrodes. At an electric field of 8 V/μm, the device showed 3.8 times higher electroluminescence luminance than a thin coplanar-structured device and achieved a maximum brightness of 1324 cd/m2 (at 9.12 V/μm), suggesting that the electric field expands throughout the thick EML. Furthermore, the device exhibited strong mechanoluminescence and good durability of dual-channel luminescence under simultaneous electromechanical stimulation. We believe that our results represent a breakthrough in electroluminescence and mechanoluminescence research and provide important insights into the development of sustainable and stretchable devices with high brightness.

Monolayer CeI2 : An intrinsic room-temperature ferrovalley semiconductor

Two-dimensional ferrovalley semiconductors with robust room-temperature ferromagnetism and sizable valley polarization hold great prospects for future miniature information storage devices. As a new member of the ferroic family, however, such ferrovalley materials have rarely been reported. By first-principles calculations, we identify that monolayer $\mathrm{Ce}{\mathrm{I}}_{\text{2}}$ is an intrinsic ferromagnetic semiconductor and exhibits excellent ambient stability, strong easy in-plane magnetocrystalline anisotropy, and a high magnetic transition temperature up to 374 K. The ferromagnetism is found to arise from the hybridization of Ce-$4f/5d$ and I-$5p$ orbitals. When monolayer $\mathrm{Ce}{\mathrm{I}}_{\text{2}}$ is magnetized toward the off-plane $z$ direction, a spontaneous valley polarization as large as 208 meV in the top valence band can be achieved due to the simultaneous breaking of both inversion symmetry and time-reversal symmetry, which is further verified by the perturbation theory of spin-orbital coupling. Also, the anomalous valley Hall effect can be observed under an in-plane electrical field due to the robust valley-contrasting Berry curvature. Overall, the combination of intrinsic semiconducting ferromagnetism and spontaneous valley polarization renders monolayer $\mathrm{Ce}{\mathrm{I}}_{\text{2}}$ a compelling room-temperature ferrovalley semiconductor for potential applications in nanoscale spintronics and valleytronics.

Topological magnetic hysteresis in single crystals of CeAgSb2 ferromagnet

Closed-topology magnetic domains are usually observed in thin films and in an applied magnetic field. Here we report the observation of rectangular cross-section tubular ferromagnetic domains in thick single crystals of CeAgSb2 in zero applied field. Relatively low exchange energy, small net magnetic moment, and anisotropic in-plane crystal electric fields lower the domain wall energy and allow for the formation of the closed-topology patterns. The tubular domain structure irreversibly transforms into a dendritic pattern upon cycling the magnetic field. This transition between closed and open topologies results in a ‘topological magnetic hysteresis’— the actual hysteresis in magnetization, not due to the imperfections and pinning, but due to the difference in the pattern morphology. Similar physics was suggested before in pure type-I superconductors and is believed to be a generic feature of other nonlinear single (present case), or two-phase (type-I superconductor) systems where the effects similar to demagnetization (shape-dependent macroscopic variation of properties) lead to pattern formation.

Tunable electronic structure and Rashba spin splitting of WSe2/WS2 van der Waals heterostructure via strain and electric-field

Suitable band structure, effective carrier separation and spin-charge locking are critical for further development and application of nanoscale spintronic devices. In this work, the effects of in-plane biaxial strain and electric field on the electronic structures and Rashba spin splitting of WSe 2 /WS 2 van der Waals heterostructure are investigated by the first principles calculations. The results show that considering the SOC effect, an observable Dresselhaus spin splitting of 424.5 meV and Rashba spin splitting can be observed at the K and Γ point in the valence band, respectively. At the same time, unexpected circle-type Berry curvature occurs due to a broken mirror structure for the WSe 2 /WS 2 heterostructure. A largest Rashba parameter α R with 0.503 eV Å can be achieved at the strain of −4% and the positive electronic field tend to weaken the strength of Rashba spin splitting. The linear Rashba spin splitting strength of WSe 2 /WS 2 heterostructure is tried to clarify, and coexistence of valley-contrasting property and Rashba effect enables spintronics and valleytronics application can be achieved. These results provide a way to engineer the electronic structure and Rashba spin splitting of WSe 2 /WS 2 heterostructure and open up the possibilities for exploring spin logic electronic devices as well as valleytronic devices. • Effects of strain and electric field on the electronic structures of WSe2/WS2van der Waals heterostructure are studied. • The WSe 2 /WS 2 heterostructure exhibits type-II band alignment and tunable Rashba spin splitting. • Unexpected circle-type Berry curvature occurs due to broken mirror structure. • A largest Rashba parameter α R of 0.507 eV Å can be obtained at the strain of −4%.

Resistive switching effect in the n-InGaAs/GaAs heterostructures with double tunnel-coupled quantum wells

The electric conductivity behavior in the single and double tunnel-coupled quantum wells (QW) with different doping profile caused by the impact of short pulses of the strong longitudinal (in the quantum wells plane) electric field has been investigated. It is established that at low temperatures (4 K) after such an impact the long-term metastable state with increased electric conductance may be observed in the case of the asymmetric QW couple with the impurity delta shaped-layer in the narrower QW. It is not observed in the structures with other configurations. The observed effect is explained by the model accounting for metastable changes in the electron energy states spectrum in the studied structures caused by the strong electric field pulses.

Control of photocurrent and multi-state memory by polar order engineering in 2H-stacked α-In2Se3 ferroelectric

Controlling the polar order in ferroelectric materials may enrich the diversity of their property and functionality, offering new opportunities for the design of novel electronic and optoelectronic devices. In this paper, we report a planar multi-state memory device built upon a two-dimensional (2D) van der Waals layered ferroelectric material, 2H α-In2Se3. Three (high, median and low) resistance states are demonstrated to be interconvertible in this device with a fast switching speed, excellent endurance and retention performances via the modulation of the polar order of the ferroelectric α-In2Se3 layers under an in-plane electric field. Remarkably, reversible switching between the median-resistance state and the low-resistance state can be achieved by an ultralow electric field of 1–2 orders of magnitude smaller than the reported values in other 2D ferroelectric material-based memory devices. Furthermore, the three different polar order states are discovered to exhibit distinctive photo-responses. These results demonstrate great potentials of α-In2Se3 in nonvolatile high-density memory and advanced optoelectronic device applications.

In-plane electric and exchange fields engineered corner state bismuthene quantum dots

The energy spectrum of bismuthene quantum dots in the presence of in-plane electric and exchange fields are investigated by the tight-binding method. In the case of an in-plane exchange field, bismuthine is a second-order topological insulator and supports corner states in triangular and hexagonal quantum dots with zigzag edges. The corner states in hexagonal bismuthene quantum dots are more stable than the corner states in triangular quantum dots. In hexagonal quantum dots, the corner states are degenerate at zero energy as the exchange field h > 0.18 eV. However, the degeneracy of corner states can be destroyed by the electrical fields, and the energy levels of corner states oscillate with respect to the electric field direction. In addition, the on-site disorders will not destroy the corner states but lead to the separation of these states around zero energy.

Intrinsic spin, valley and piezoelectric polarizations in room-temperature ferrovalley Janus TiXY (XY = SCl and SeBr) monolayers.

Two-dimensional room-temperature Janus ferrovalley semiconductors with large spin, valley and piezoelectric polarizations provide fertile platforms for designing multifunctional nanodevices. Little research has been reported to date on such materials. Here, using first-principles calculations, we predict two dynamically stable Janus titanium chalcohalide (TiSCl and TiSeBr) monolayers, which are excellent piezoelectric ferrovalley semiconductors with in-plane magnetization and high magnetic transition temperatures (738 and 884 K). When an extrinsic magnetic field is used to force the magnetization along the out-of-plane direction, a large valley polarization (64 and 146 meV) can be generated in the highest valence band with a large spin-orbit coupling by the breaking of time-reversal and space-inversion symmetry, which can be further clarified by a two-band k·p model. This robust valley-contrasting physics characterized by the valley-dependent Berry curvature leads to the anomalous valley Hall effect. It can be observed by suitable hole doping or light irradiation under an in-plane electric field. Besides, we find that the missing mirror symmetry results in giant out-of-plane piezoelectric polarization (2.05 and 2.04 pm V-1). These outstanding properties give the Janus TiSCl and TiSeBr monolayers potential for a wide variety of applications in nanoelectronics, spintronics, valleytronics, piezoelectrics and other demanding areas.

Conduction band-edge valley splitting in two-dimensional ferroelectric AgBiP2S6 by magnetic doping: towards electron valley-polarized transport.

Two-dimensional valleytronic systems, using the valley index of carriers to perform logic operations, serves as the basis of the next-generation information technologies. For efficient use of the valley degree of freedom, the major challenge currently is to lift the valley degeneracy to achieve valley splitting. In this work, using first-principles calculations, we propose that valley splitting can be readily achieved in a ferroelectric AgBiP2S6 monolayer by TM doping (TM = V, Cr, Mn, Fe, Co, and Ni), which is highly feasible in experiments. In sharp contrast to most previous reports of valley-related features in the valence band-edge, the pristine AgBiP2S6 monolayer has a direct band-gap located at K/K′ points of the Brillouin zone and harbors strong coupled spin and valley physics around the conduction band-edge, due to inversion symmetry breaking combined with strong spin–orbit coupling. By TM-doping, the local magnetic moment can be introduced into the system, which can destroy the valley degeneration of the conduction band-edge and induce valley splitting. Especially in a V-doped system, accompanied with a large valley splitting (26.8 meV), there is a serious n-type doping in AgBiP2S6. The efficient electron-doping moves the Fermi level just located between the conduction band minimum of the K/K′ valleys, which is suitable for valley-polarized transport. Moreover, the valley-polarized index can be flipped by applying a small magnetic field to rotate the magnetocrystalline direction. The magnitude of valley splitting relies on the strength of orbital hybridization between the TM-d and Bi-p states and can be tuned continually by applying biaxial strain. Under an in-plane electric field, such valley degeneracy breaking would give rise to the long-sought anomalous valley Hall effect, which is crucial to design a valleytronic device.

Electric Field Influenced Coordinate Jump of the Guiding Center and Magnetotransport

We derived an electrical current formula in the presence of a strong out of plane magnetic field and an in plane electric field, and within two dimensional disordered systems. This current is originated from the guiding center coordinate jump. At a strong magnetic field regime, the current can be pictured as the migration of the center coordinates. During the electron impurity scattering, the guiding centers suddenly shift their coordinates. Because of the electric field, the coordinate shift accumulatively contributes to a longitudinal current. During the scattering, the value of cyclotron radius changes, which compensates for the change of the electric potential energy during the coordinate jump. The diversion of cyclotron radius is the classical manifestation of electric field dependent broadening and shifting of the Landau levels.

Promoted photocarrier separation by dipole engineering in two-dimensional perovskite/C2N van der Waals heterostructures.

Due to the aggravation of environmental pollution and the energy crisis, it is urgent to develop and design environment-friendly and efficient photocatalysts for water splitting. van der Waals heterostructures composed of different two-dimensional materials offer an easily accessible way to combine properties of individual materials for applications. Herein, a novel Cs3Bi2I9/C2N heterostructure is proposed through first-principles calculations. The structural, electronic, and optical properties, as well as the charge transfer mechanism at the interface of Cs3Bi2I9/C2N are systematically investigated. Due to the difference between the work functions of Cs3Bi2I9 and C2N monolayers, when they are constructed into heterostructures, redistribution of charge occurs in the whole structure, and some of the charge transfer occurs at the interface due to the formation of an internal electric field. The band structure of Cs3Bi2I9/C2N has type-II band alignment, and the band edge position as well as the band-gap value of the heterostructure are suitable for visible light water splitting. The in-plane biaxial strain, interfacial spacing, and external electric field can effectively modulate the electronic structure and photocatalytic performance of the heterostructure. Under certain conditions, the heterostructure can be changed from type-II to type-I band alignment, accompanied by the transition from an indirect band-gap semiconductor to a direct band-gap semiconductor. Moreover, the intrinsic anion defect (I vacancy) at different positions, as donor defects, can introduce defect levels near the conduction band edge, which affects the transition of photogenerated carriers in these systems. Our findings provide a theoretical design for strategies to improve the performance of two-dimensional perovskites/C2N in photocatalytic and optoelectronic applications.

Intrinsic valley polarization and anomalous valley hall effect in single-layer 2H-FeCl2

Valley, as a new degree of freedom for electrons, has drawn considerable attention due to its significant potential for encoding and storing information. Lifting the energy degeneracy to achieve valley polarization is necessary for realizing valleytronic devices. Here, on the basis of first-principles calculations, we show that single-layer FeCl 2 exhibits a large spontaneous valley polarization (∼101 meV) arising from the broken time-reversal symmetry and spin-orbital coupling, which can be continuously tuned by varying the direction of magnetic crystalline. By employing the perturbation theory, the underlying physical mechanism is unveiled. Moreover, the coupling between valley degree of freedom and ferromagnetic order could generate a spin- and valley-polarized anomalous Hall current in the presence of the in-plane electric field, facilitating its experimental exploration and practical applications.

Spin State Switching in Heptauthrene Nanostructure by Electric Field: Computational Study.

Recent experimental studies proved the presence of the triplet spin state in atomically precise heptauthrene nanostructure of nanographene type (composed of two interconnected triangles with zigzag edge). In the paper, we report the computational study predicting the possibility of controlling this spin state with an external in-plane electric field by causing the spin switching. We construct and discuss the ground state magnetic phase diagram involving (triplet) state, antiferromagnetic state and non-magnetic state and predict the switching possibility with the critical electric field of the order of 0.1 V/Å. We discuss the spin distribution across the nanostructure, finding its concentration along the longest zigzag edge. To model our system of interest, we use the mean-field Hubbard Hamiltonian, taking into account the in-plane external electric field as well as the in-plane magnetic field (in a form of the exchange field from the substrate). We also assess the effect of uniaxial strain on the magnetic phase diagram.

Metamorphosis of trilobite-like drops on a surface: Electrically driven fingering

The experimental evidence reveals that sessile drops on a dielectric horizontal substrate subjected to sub-critical in-plane electric field acquire steady-state configurations where a balance between the pulling-outwards electric Maxwell stresses and the restoring surface tension has been attained. On the other hand, the experiments show that in supercritical electric field the Maxwell stresses become dominant and not only stretch the drop as a whole but also trigger growth of multiple fingers crawling toward electrodes on both sides of the drop. This makes the drops with fingers stretched along the electric field lines similar to some trilobites known from their imprints in petrified sediments studied in paleontology. It is shown experimentally and theoretically that fingers are triggered during the encounters of the spreading drop outlines with minor surface imperfections. Such surface defects (existing originally or pre-notched on purpose) result in fingers which can grow being directed by the electric-field lines. The present work details multiple experimental observations of the trilobite-like fingering with several types of commercially available paints (colloidal dispersions) and also provides a theoretical framework for this novel type of fingering.

Vertical Cylinder-to-Lamella Transition in Thin Block Copolymer Films Induced by In-Plane Electric Field.

Morphological transition between hexagonal and lamellar patterns in thin polystyrene–block–poly(4-vinyl pyridine) films simultaneously exposed to a strong in-plane electric field and saturated solvent vapor is studied with atomic force and scanning electron microscopy. In these conditions, standing cylinders made of 4-vinyl pyridine blocks arrange into threads up to tens of microns long along the field direction and then partially merge into standing lamellas. In the course of rearrangement, the copolymer remains strongly segregated, with the minor component domains keeping connectivity between the film surfaces. The ordering tendency becomes more pronounced if the cylinders are doped with Au nanorods, which can increase their dielectric permittivity. Non-selective chloroform vapor works particularly well, though it causes partial etching of the indium tin oxide cathode. On the contrary, 1,4-dioxane vapor selective to polystyrene matrix does not allow for any morphological changes.

Network model and four-terminal transport in minimally twisted bilayer graphene

We construct a two-channel scattering model for the triangular network of valley Hall states in interlayer-biased minimally twisted bilayer graphene from symmetry arguments and investigate electronic transport in a four-terminal setup. In the absence of forward scattering, a single phenomenological parameter tunes the network between a triplet of chiral zigzag modes and pseudo-Landau levels. Moreover, the chiral zigzag modes give rise to robust Aharonov-Bohm resonances in the longitudinal conductance in the presence of a perpendicular magnetic field or an in-plane electric field. Interestingly, we find that when both a magnetic field and an in-plane electric field are applied, the resonances of different zigzag branches split depending on their propagation direction relative to the in-plane electric field. We further demonstrate that while the Hall response vanishes in the chiral zigzag regime, a finite Hall response is obtained without destroying the Aharonov-Bohm resonances in the longitudinal response, by weakly coupling different zigzag branches, which also gives rise to Hofstadter physics at accessible magnetic fields.

Evidence for Local Spots of Viscous Electron Flow in Graphene at Moderate Mobility.

Dominating electron-electron scattering enables viscous electron flow exhibiting hydrodynamic current density patterns, such as Poiseuille profiles or vortices. The viscous regime has recently been observed in graphene by nonlocal transport experiments and mapping of the Poiseuille profile. Herein, we probe the current-induced surface potential maps of graphene field-effect transistors with moderate mobility using scanning probe microscopy at room temperature. We discover micrometer-sized large areas appearing close to charge neutrality that show current-induced electric fields opposing the externally applied field. By estimating the local scattering lengths from the gate dependence of local in-plane electric fields, we find that electron-electron scattering dominates in these areas as expected for viscous flow. Moreover, we suppress the inverted fields by artificially decreasing the electron-disorder scattering length via mild ion bombardment. These results imply that viscous electron flow is omnipresent in graphene devices, even at moderate mobility.

Donor-Acceptor structural polymeric carbon nitride with in-plane electric field accelerating charge separation for efficient photocatalytic hydrogen evolution

The realization of efficient photocatalytic hydrogen evolution based on polymeric carbon nitride (PCN) is severely hampered by the insufficient separation of photogenerated electrons and holes. Herein, we successfully introduced Benzo[b]thiophene-2-carboxylic acid (BTH) as donor parts through supramolecular self-assembly into the framework of PCN to form D-A structure to promote the photoexciton separation efficiency. The SCN-BTH structure shows the improved separation efficiency of charge carriers and shorter diffusion path resulting in hydrogen evolution rate up to 3950 μmol/g/h, which is 13.3 times higher than that of bulk carbon nitride (BCN) under visible light irradiation (λ > 420 nm). Further density functional theory (DFT) calculation reveals that SCN-BTH structure exhibits strong spatial separation of lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) compared to that of BCN with the even distribution. In addition, electrostatic potential diagram indicates the in-plane polarized electric field in SCN-BTH embedded with the D-A structure can drive the photo-generated electrons and holes toward reverse direction, thereby accelerating the separation of carriers. All those factors synergistically facilitated the charge separation and transport process which are responsible for the improved photocatalytic performance.

Stabilization of correlated ferroelectric and ferromagnetic domain structures in BiFe0.9Co0.1O3 films

Co-substituted bismuth ferrite, BiFe0.9Co0.1O3, thin film is a promising candidate material for low-power-consumption nonvolatile magnetic memory devices because magnetization reversal by an electric field has been directly observed at room temperature. However, this phenomenon takes place in a limited region where the ferroelectric domain is composed of two polarization variants forming stripe patterns. We demonstrate the extension of the stripe domain structure to the entire electrically poled region by utilizing an effective in-plane electric field called a trailing field. A one-to-one correlation between ferroelectric and ferromagnetic domains was also demonstrated.

Evolution and Shape of Two-Dimensional Stokesian Drops under the Action of Surface Tension and Electric Field: Linear and Nonlinear Theory and Experiment

The creeping-flow theory describing evolution and steady-state shape of two-dimensional ionic-conductor drops under the action of surface tension and the subcritical (in terms of the electric Bond number) electric field imposed in the substrate plane is developed. On the other hand, the experimental data are acquired for drops impacted or softly deposited on dielectric surfaces of different wettability and subjected to an in-plane subcritical electric field. Even though the experimental situation involves viscous friction of drops with the substrates and wettability-driven motion of the contact line, the comparison to the theory reveals that it can accurately describe the steady-state drop shape on a non-wettable substrate. In the latter case, the drop is sufficiently raised above the substrate, which diminishes the three-dimensional effects, making the two-dimensional description (lacking the no-slip condition at the substrate and wettability-driven motion of the contact line) relevant. Accordingly, it is demonstrated how the subcritical electric field deforms the initially circular drops until an elongated steady-state configuration is reached. In particular, the surface tension tends to round off the non-circular drops stretched by the electric Maxwell stresses imposed by the electrodes. A more pronounced substrate wettability leads to more elongated steady-state configurations observed experimentally than those predicted by the two-dimensional theory. The latter cases reveal significant three-dimensional effects in the electrically driven drop stretching. In the supercritical electric fields (corresponding to the supercritical electric Bond numbers), the electrical stretching of drops predicted by the present linearized two-dimensional theory results in splitting into two separate droplets. This scenario is corroborated by the predictions of the fully nonlinear results for similar electrically stretched bubbles in the creeping-flow regime available in the literature as well as by the present experimental results on a substrate with slip.