Interlayer Electric Field Research Articles

Electric-field tunable Type-I to Type-II band alignment transition in MoSe2/WS2 heterobilayers

Semiconductor heterojunctions are ubiquitous components of modern electronics. Their properties depend crucially on the band alignment at the interface, which may exhibit straddling gap (type-I), staggered gap (type-II) or broken gap (type-III). The distinct characteristics and applications associated with each alignment make it highly desirable to switch between them within a single material. Here we demonstrate an electrically tunable transition between type-I and type-II band alignments in MoSe2/WS2 heterobilayers by investigating their luminescence and photocurrent characteristics. In their intrinsic state, these heterobilayers exhibit a type-I band alignment, resulting in the dominant intralayer exciton luminescence from MoSe2. However, the application of a strong interlayer electric field induces a transition to a type-II band alignment, leading to pronounced interlayer exciton luminescence. Furthermore, the formation of the interlayer exciton state traps free carriers at the interface, leading to the suppression of interlayer photocurrent and highly nonlinear photocurrent-voltage characteristics. This breakthrough in electrical band alignment control, interlayer exciton manipulation, and carrier trapping heralds a new era of versatile optical and (opto)electronic devices composed of van der Waals heterostructures.

Janus molybdenum di-chalcogenides based van der Waals bilayers for supercapacitor electrode design- effects of interlayer stacking orientations on quantum capacitance

In this work, for the first time, the quantum capacitance of an artificial two-dimensional (2D) material system based on van der Waals (vdW) bilayer heterostructures of Janus Molybdenum Di-chalcogenides, MoXY (X≠Y, X/YS, Se, Te) have been extensively investigated for electrode design of electrical double-layer (EDL) supercapacitor. The effects of different interlayer stacking orientations on local charge distribution, interlayer charge transfer, energy band structure, and density of states (DOS) have been comprehensively analyzed and correlated with the excess charge density and quantum capacitance variation with local electrode potential. Furthermore, the total capacitance reduction with respect to EDL capacitance has been analyzed over a wide range of EDL capacitance, and the performance of vdW Janus bilayers has been systematically benchmarked against the natural monolayers/bilayers and Janus monolayers of Molybdenum Di-chalcogenides. The results demonstrate that the large interlayer charge transfer induced built-in interlayer electric field significantly reduces the energy band gap of vdW Janus bilayers for S–Te and Se–Te interlayer stacking, thus leading to a significantly superior quantum capacitance. Finally, the favorable valley-dependent effective mass and band-degeneracy in MoSTe/MoSeTe have greatly enhanced quantum capacitance in S–Te and Se–Te stacking, rendering these bilayers a highly attractive candidate for supercapacitor electrode design.

Band alignment of one-dimensional transition-metal dichalcogenide heterotubes.

One-dimensional (1D) van der Waals (vdW) heterotubes, where different kinds of 1D nanotubes coaxially nest inside each other, offer a flexible platform for promising applications. The various properties of these 1D heterotubes depend on their diameter. Here, we present a systematic theoretical investigation into the structural and electronic properties of two kinds of 1D transition-metal dichalcogenide (TMD) heterotubes. We demonstrate that the thermodynamic stability of 1D heterotubes is determined by their interlayer distance. Additionally, we establish that the band alignment transition changes from type I to type II in 1D TMD heterotubes. We identify two distinct transition mechanisms, originating from the exchange of either the valence band maximum or the conduction band minimum. According to an electrostatic model, the band alignment transition is attributed to the interlayer electric field effect, which depends on the heterotube diameter. The findings in this work provide valuable physical insights into the band alignment transition in 1D heterotubes and are instrumental for their potential applications in nanotechnology.

Ultraviolet interlayer excitons in bilayer WSe2.

Interlayer excitons in van der Waals heterostructures are fascinating for applications like exciton condensation, excitonic devices and moiré-induced quantum emitters. The study of these charge-transfer states has almost exclusively focused on band edges, limiting the spectral region to the near-infrared regime. Here we explore the above-gap analogues of interlayer excitons in bilayer WSe2 and identify both neutral and charged species emitting in the ultraviolet. Even though the transitions occur far above the band edge, the states remain metastable, exhibiting linewidths as narrow as 1.8 meV. These interlayer high-lying excitations have switchable dipole orientations and hence show prominent Stark splitting. The positive and negative interlayer high-lying trions exhibit significant binding energies of 20-30 meV, allowing for a broad tunability of transitions via electric fields and electrostatic doping. The Stark splitting of these trions serves as a highly accurate, built-in sensor for measuring interlayer electric field strengths, which are exceedingly difficult to quantify otherwise. Such excitonic complexes are further sensitive to the interlayer twist angle and offer opportunities to explore emergent moiré physics under electrical control. Our findings more than double the accessible energy range for applications based on interlayer excitons.

Utilizing Linear Polymers to Optimize Remanent Polarization and Construct Multilayer Interfaces to Obtain High-Efficient Poly(vinylidene fluoride)-Based Energy-Storage Composites.

Polyvinylidene fluoride (PVDF) has been widely studied as a ferroelectric polymer for energy dielectric applications. However, high-polarization PVDF has a low-efficiency issue, owing to high residual polarization. This study introduces highly insulating, low-loss linear polycarbonate (PC) into PVDF-based dielectrics. The PC layer optimizes the remanent polarization (Dr) of PVDF and maintains it within a small range, thus achieving a high charge-discharge efficiency. The multilayer structural design of PVDF-based dielectrics adjusts the interlayer electric field distribution. We have thoroughly studied the influence of the number and proportion of PC layers on the polarization and breakdown of the multilayer films as well as achieved collaborative regulation of dual parameters. Our results indicate that three layers of PC-PVDF-PC (CPC) films containing a large proportion of PC can polarize under high electric fields and maintain excellent charge-discharge efficiency, achieving an energy density and efficiency of 11.48 J/cm3 and 92.4%, respectively, under 610 kV/mm. The PVDF-based dielectrics prepared in this work are all organic films, and their flexibility and foldability are conducive to the preparation of flexible devices.

Enhancing UV-C Photoelectron Lifetimes for Avalanche-like Photocurrents in Carbon-Doped Bi3O4Cl Nanosheets.

Interlayer electric fields in two-dimensional (2D) materials create photoelectron protecting barriers useful to mitigate electron-hole recombination. However, tuning the interlayer electric field remains challenging. Here, carbon-doped Bi3O4Cl (C:Bi3O4Cl) nanosheets are synthesized using a gas phase protocol, and n-type carriers are acquired as confirmed by the transconductance polarity of nanosheet field effect transistors. Thin C:Bi3O4Cl nanosheets show excellent 266 nm photodetector figures of merit, and an avalanche-like photocurrent is demonstrated. Decaying behaviors of photoelectrons pumped by a 266 nm laser pulse (266 nm photoelectrons) are observed using transient absorption spectroscopy, and a significant 266 nm photoelectron lifetime quality in C:Bi3O4Cl is presented. Built C:Bi3O4Cl models suggest that the interlayer electric field can be boosted by two different carbon substitutions at the inner and outer bismuth sites. This work reports a facile approach to increase the interlayer electric field in Bi3O4Cl for future UV-C photodetector applications.

Role of interlayer and interfacial electric field generated during simultaneous HER and OER over Li intercalated [formula omitted] for efficient photocatalytic activity

Substantial variation in the photocatalytic activity on the interaction of reaction intermediates with the surface of the catalyst is governed by the charge transfer mechanism. Charge transfer within a stacked layer of photocatalyst and across the reactant-photocatalyst interface alters photocatalytic activity due to the generation of an electric field. This work addresses the synergistic effect of interlayer (Eil→) and interfacial (Eif→) electric field on the photocatalytic activity of Li-intercalated g−C3N4 (Li−CN) through the study of electronic and optical properties using density functional theory. Charge transfer analysis for simultaneous oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in the presence of two water molecules shows variation in built-in Eil→ of Li−CN as the surface reaction proceeds via the induced Eif→ generated by the reactants on Li−CN. This change in Eil→ alters the energy barrier of the next reaction step through directional charge transfer. The present study reveals the mutual interplay of Eil→, Eif→ and adsorption energy of reaction adsorbates for improved photocatalytic activity of Li−CN for efficient OER and HER.

Magnetic Field-Controlled Electrical Conductivity in AA Bilayer Graphene

We consider the effect of the external magnetic field on the in-plane conductivity in the AA-stacked bilayer graphene system in the strong excitonic condensate regime. We include the effects of the applied inter-layer electric field and the Coulomb interactions. The on-site and inter-layer Coulomb interactions were treated via the bilayer Hubbard model. Using the solutions for the physical parameters in the system, we calculate the in-plane conductivity of the bilayer graphene. By employing the Green-Kubo formalism for the polarization function in the system, we show that the conductivity in the AA bilayer system is fully controlled by the applied magnetic field. For the partial filling in the layers, the electrical conductivity is different for different spin orientations, and, at the high values of the magnetic field, only one component remains with the given spin orientation. Meanwhile, for the half-filling limit, there is no spin-splitting observed in the conductivity function. The theory evaluated here shows the new possibility for spin-controlled electronic transport in the excitonic bilayer graphene system.

Engineering two-dimensional SnC/HfSSe heterojunction as a direct Z-scheme photocatalyst for water splitting hydrogen evolution

Developing robust water-splitting photocatalyst remains a pivotal challenge for the conversion of solar energy to hydrogen fuel. Herein, we report SnC/HfSSe heterojunctions to be a competitive direct Z-scheme photocatalysts with HfSSe acting as an oxidant and SnC as a reductant for water-splitting hydrogen generation by employing the first-principles calculations. The photogenerated electron–hole migration logically follows the Z-scheme path due to the large interlayer built-in electric field, small interlayer bandgap, proper band edge alignment, and high interlayer carrier recombination. The overpotential is appropriate to the oxygen evolution reaction (OER), while the overpotential is slightly large to the hydrogen envolution reaction (HER) so that the incident photon energy above the band gap of the components is required for the HER. The heterojunctions have excellent visible light absorption, exhibiting the remarkable red shift and stronger light absorption with respect to the two components of the heterojunction. Moreover, the heterojunction exhibits an outstanding solar-to-hydrogen (STH) efficiency as high as 27% in extremely acidic solution.

Monolayers of Germanene/Janus Ga2SeTe van der Waals Heterostructures by First-Principles Calculations for High-Performance Optoelectronic Devices

The emerging Janus Ga2SeTe with an asymmetric structure has attracted a great deal of research interest. In this paper, we construct the van der Waals heterostructures (vdWHs) of monolayer Janus Ga2SeTe and germanene (Ge) and named their two stacking modes as Te/Ge vdWH and Se/Ge vdWH. We find that Ge/Ga2SeTe vdWHs in both stacking modes form n-type Schottky contacts. Furthermore, the Schottky barrier height and Schottky contact type in Ge/Ga2SeTe are modulated through interlayer distances and electric fields, which lead to a transition from n-type to p-type Schottky contacts or ohmic contacts. In particular, the weak interaction between the two monolayers leads to the creation of a band gap in the Dirac cone. When a compressive strain in the z direction is applied to the Se/Ge heterostructure, Ge has a larger band gap. The results of our work can provide valuable guidance for the design of controllable Schottky nanodevices and high-performance optoelectronic devices based on Ge/Ga2SeTe vdWHs.

Electric-Field-Tunable Spin Polarization and Carrier-Transport Anisotropy in an A-Type Antiferromagnetic van der Waals Bilayer

Two-dimensional (2D) magnetic semiconductor materials with the electric-field-tunable spin polarization and carrier-transport anisotropy are of great significance for the fundamental physics and device application. Here we propose a general strategy to tune the spin polarization and anisotropic effective mass in a 2D A-type antiferromagnetic (AFM) bilayer system. We take the A-type AFM bilayer $\mathrm{CrSBr}$ (with the intralayer ferromagnetic and interlayer AFM coupling) as an example to confirm this design principle. Firstly, a vertical electric field can lift the spin degeneracy in the A-type AFM bilayer $\mathrm{CrSBr}$. By flipping the direction of electric field, the spin polarization direction can be reversed. Secondly, in the A-type AFM bilayer $\mathrm{CrSBr}$ with an interlayer twist angle of 90\ifmmode^\circ\else\textdegree\fi{}, both the spin direction and the carrier-transport anisotropy can be tuned simultaneously by applying a vertical electric field due to the in-plane anisotropic carrier effective mass in each $\mathrm{CrSBr}$ monolayer. This opens an opportunity for controlling the spin polarization as well as carrier-transport anisotropy in the 2D magnetic van der Waals layered materials by interlayer twist and electric field, and provides an idea for utilizing A-type AFM semiconductors to design spintronic devices.

Bandgap engineering in BP/PtO2 van der Waals (vdW) hetero-bilayer using first-principles study

Based on the motivation of the recent advancement of the van der Waals heterostructure (vdW HBL), we have studied the tunable optoelectronic properties of the two-dimensional (2D) boron phosphide–platinum di-oxide (BP/PtO2) heterostructure using dispersion corrected density functional theory (DFT). Six different stackings are considered to stack the 2D BP upon 2D PtO2 and are tested through DFT. Phonon spectra and binding energy calculation validate the dynamical and chemical stability of the constructed heterostructures. It is found that HBL1, HBL3, and HBL4 have type-II indirect band gaps of 0.001, 0.027, and 0.021 eV, respectively whereas the other HBLs 2, 4, and 5 show a semiconductor–metal transition. The variation in the interlayer distances, cross-plane electric field, and biaxial strain can effectively tune the bandgap, although type-II band alignment remains unaffected in all cases. A large built-in electric field, of ∼15 eV in electrostatic potential between the 2D structures and type-II band alignment of the HBL, suggests efficient separation of charges in all the HBLs. The bandgaps are highly responsive to the interlayer distances, electric field, and biaxial strain in the HBL. It is found that the bandgap increases under the application of compressive strain and an external electric field along the negative z-direction up to −0.4 V/Å. Interestingly, a semiconductor–metal transition occurs for tensile strain and the external electric field along the positive z-direction. All HBLs have efficient optical absorption in the visible and UV portions of the solar spectra, which is highly anticipated for optoelectronics applications. These unrivaled properties of the vdW BP/PtO2 HBL that we have explored make them a promising candidate for nano-electronic devices and infrared detector applications.

Excimer Formation in the Non-Van-Der-Waals 2D Semiconductor Bi2 O2 Se.

The layered semiconductor Bi2 O2 Se is a promising new-type 2D material that holds layered structure via electrostatic forces instead of van der Waals (vdW) attractions. Aside from the huge success in device performance, the non-vdW nature in Bi2 O2 Se with a built-in interlayer electric field has also provided an appealing platform for investigating unique photoexcited carrier dynamics. Here, experimental evidence for the observation of excimers in multilayer Bi2 O2 Se nanosheets via transient absorption spectroscopy is presented. It is found that the excimer formation is the primary decay pathway of photoexcited excitons and three-stage excimer dynamics with corresponding time scales are established. Excitation-fluence-dependent excimer dynamics further suggest that the excimer is diffusive and its formation can be simply described as excitons relaxed to an excimer geometry. This work indicates the outstanding promise of unique excitonic processes in Bi2 O2 Se, which may motivate novel device designs.

A sensitive photodetector: Tuning the electronic structure of the Cu2O/MoS2 heterojunction by controlling the interlayer spacing or electric field

A sensitive photodetector: Tuning the electronic structure of the Cu2O/MoS2 heterojunction by controlling the interlayer spacing or electric field

Engineering Schottky barrier in vertical graphene/InN heterostructure

In recent years, van der Waals stacked heterostructures (vdWHs) have gained much study interest for enhancing nanoelectronics’ performance. We take the first-principles methods to study the interface contact characters of the graphene/InN vdWH under the interlayer coupling and electric field. The natural electronic characters of both the graphene and InN components keep intact at the equilibrium interlayer distance with an n-type Schottky contact. Impressively, it tunes to Ohmic contact when the interlayer distance increases to 4.73 Å or the applying negative electric field increases to −0.42 V/Å, while it converts to p-type Schottky contact under a positive electric field greater than 0.2 V/Å. These findings are of great significance for the application of graphene/InN vdWH in high-performance nanoelectronics.

First-principles calculations to investigate electronic properties of ZnO/PtSSe van der Waals heterostructure: Effects of vertical strain and electric field

In the present study, we report the electronic properties of ZnO/PtSSe van der Waals heterostructure by using the density functional theory. Two different stacking configurations ZnO/SePtS and ZnO/SPtSe are formed with very small lattice mismatch and they are confirmed to be stable through ab initio molecular dynamic simulations. Obtained results demonstrate that both ZnO/SePtS and ZnO/SPtSe stacking configurations are indirect semiconductors. At the ground state, band gaps of ZnO/SePtS and ZnO/SPtSe configurations are respectively 0.895 and 0.448 eV, that quite smaller than that of both ZnO and PtSSe monolayers. The band gap of heterostructure depends strongly on the interlayer distance and electric field. Our findings not only give insight into the physical properties of heterostructures but also open up possibilities for their application in electronic devices.

Electrically tunable and reversible magnetoelectric coupling in strained bilayer graphene

The valleys in hexagonal two-dimensional systems with broken inversion symmetry carry an intrinsic orbital magnetic moment. Despite this, such systems possess zero net magnetization unless additional symmetries are broken, since the contributions from both valleys cancel. A nonzero net magnetization can be induced through applying both uniaxial strain to break the rotational symmetry of the lattice and an in-plane electric field to break time-reversal symmetry owing to the resulting current. This creates a magnetoelectric effect whose strength is characterized by a magnetoelectric susceptibility, which describes the induced magnetization per unit applied in-plane electric field. Here, we predict the strength of this magnetoelectric susceptibility for Bernal-stacked bilayer graphene as a function of the magnitude and direction of strain, the chemical potential, and the interlayer electric field. We estimate that an orbital magnetization of ~5400 $\mu_{\text{B}}/\mu\text{m}^2$ can be achieved for 1% uniaxial strain and a 10 $\mu\text{A}$ bias current, which is almost three orders of magnitude larger than previously probed experimentally in strained monolayer MoS$_2$. We also identify regimes in which the magnetoelectric susceptibility not only switches sign upon reversal of the interlayer electric field but also in response to small changes in the carrier density. Taking advantage of this reversibility, we further show that it is experimentally feasible to probe the effect using scanning magnetometry.

Van der Waals heterostructure of graphene and germanane: tuning the ohmic contact by electrostatic gating and mechanical strain.

Recent exciting developments in synthesis and properties study of the Germanane (GeH) monolayer have inspired us to investigate the structural and electronic properties of the van der Waals GeH/Graphene (Gr) heterostructure by the first-principle approach. The stability of the GeH/Gr heterostructure is verified by calculating the phonon dispersion curves as well as by thermodynamic binding energy calculations. According to the band structure calculation, the GeH/Gr interface is n-type Ohmic. The effects of different interlayer distances and strains between the layers and the applied electric field on the interface have been investigated to gain insight into the van der Waals heterostructure modifications. An interlayer distance of 2.11 Å and compressive strain of 6% alter the contact from Ohmic to Schottky status, while the electric field can tune the GeH/Gr contact as p- or n-type, Ohmic, or Schottky. The average electrostatic potential of GeH/Gr and the Bader charge analysis have been used to explain the results obtained. Our theoretical study could provide a promising approach for improving the electronic performance of GeH/Gr-based nano-rectifiers.

Guidelines for Ferroelectric FET Reliability Optimization: Charge Matching

An optimization principle for ferroelectric FET (FeFET), centered around charge matching between the ferroelectric and its underlying semiconductor, is theoretically investigated. This letter shows that, by properly reducing the ferroelectric polarization charge and its background dielectric constant, charge matching can be improved to enable simultaneously: i) reduction of the interlayer and semiconductor electric fields during programming, reading, and retention, leading to prolonged endurance and retention; ii) improvement of the memory window; and iii) suppression of device-to-device variations by affording full polarization switching. These attributes provide an incentive for the presentation of the proposed guidelines for FeFET optimization as detailed in this letter.

Tunable nonlocal valley-entangled Cooper pair splitter realized in bilayer-graphene van der Waals spin valves

We investigate the Cooper pair splitting based on crossed Andreev reflection in a two-dimensional narrow superconductor coupled to two leads, where the two leads are van der Waals (vdW) spin valves formed by inserting bilayer graphene (BLG) between two antiparallel insulating ferromagnetic layers. In this BLG-based $n$-type vdW spin valve/superconductor/$p$-type vdW spin valve junction with edge-contact geometry, when controlling the Fermi energy and the interlayer electric field in BLG independently by top- and bottom-gate voltages in the vdW spin valve region, we first demonstrate that the two electrons of perfect Cooper pair splitting can remain nonlocal valley entanglement. This effect can be explained as follows: When both the two antiparallel insulating ferromagnetic layers and the interlayer electric field are taken into account, BLG exhibits a spin-splitting but valley-degenerate band gap near the Dirac point. Thus, our setups allow the Cooper pair in the BCS scenario to serve as a natural source of valley entanglement. We further predict large oscillations of cross conductance as a function of the doping of the superconductor, the junction length, and the bias voltage. Our results may provide a tool for realizing fast nonlocal switching of the valley-entanglement state.