External Vertical Electric Field Research Articles

External electric field assisted energy band gap modulation and optical properties of SiGe/AsSb heterobilayers

In this work, first-principle calculations within Density Functional Theory (DFT) were performed to study the structural, electronic, and optical properties of eight different bilayer van der Waals (vdW) heterostructures composed of SiGe and AsSb monolayers. These heterobilayers exhibit large energy bandgap (90 meV–459 meV) with linear energy–momentum relationship, implying high charge carrier mobility. Application of external vertical electric field can effectively modulate the energy bandgap of these heterobilayers from 64 meV to 459 meV, depending on stacking orders. Our study shows that a moderate amount of electric field can almost linearly tune the energy bandgap, however, semiconductor to metal transition occurs beyond a certain limit. The mechanism of energy bandgap tuning by the external electric field was examined by studying differential charge density. The optical properties such as complex dielectric function, electron energy loss function, complex refractive index, reflectivity, absorption coefficient, and optical conductivity of these SiGe/AsSb heterobilayers are studied for parallelly and perpendicularly polarized incident light. Our research demonstrates that these heterostructures exhibit variable plasma frequency depending on the stacking order of the heterostructures, anisotropic reflectivity i.e., birefringence properties in response to the incident light's polarization. Also, they exhibit high refractive indices (∼3) and absorption coefficients (∼105 cm−1) from visible to ultraviolet incident light frequencies. Our study proposes several SiGe/AsSb heterobilayers with unique electronic and optical properties which may have potential applications in nanoscale electronic and optoelectronic applications.

Van der Waals heterostructures of AlAs and InSe: Stacking-dependent Raman spectra and electric field dependence of electronic properties

In the present work, the electronic and vibrational properties of a van der Waals type heterostructure, composed of single layers of AlAs and InSe, are investigated using density functional theory (DFT)-based first-principles calculations. Vibrational analyses reveal that dynamically stable single layers of AlAs and InSe form van der Waals type heterostructure which is shown to exhibit stacking-dependent Raman spectra by means of the frequency shifts. According to our findings, a type-II band alignment with a direct band gap of 1.84 eV is found in the ground state stacking of AlAs/InSe vertical heterostructure, in contrast to the indirect band gap behaviors of each individual layer. Moreover, the application of an external vertical electric field shows that the both band alignment type and the electronic behavior of the heterostructure can be tuned. The heterostructure is found to exhibit direct to indirect band gap transition under negative electric field as well as a transition from type-II to type-I heterojunction under negative fields up to 0.3 V/Å. The stronger fields along the same direction results in overlapping of valence states of each layer and lead to a non-linear change of the energy band gap. Overall, the predicted van der Waals type heterobilayer of InSe and AlAs with stacking-dependent vibrational features and well-controlled electronic properties under external field is shown to be potential candidate for optical and optoelectronic applications.

Ideal two-dimensional quantum spin Hall insulators MgA2Te4 (A = Ga, In) with Rashba spin splitting and tunable properties.

For decades, topological insulators have played a pivotal role in fundamental condensed-matter physics owing to their distinctive edge states and electronic properties. Here, based on in-depth first-principles calculations, we investigate the MgA2Te4 (A = Ga, In) structures belonging to the MA2Z4 2D material family. Among them, the topological insulator MgGaInTe4 exhibits band inversion and a sizeable bandgap of up to 60.8 meV which satisfies the requirement for room-temperature realization. Under the spin-orbit coupling effect, MgGaInTe4 with inversion asymmetry undergoes Rashba spin splitting. The Rashba-like and Dirac-type edge states emerge from different terminals along (010) for MgGaInTe4. The external vertical electric field is verified to modulate the inverted bandgap and topological state of MgGaInTe4 by converting a nontrivial state to a trivial state and MgIn2Te4 with the original trivial state to a nontrivial one. Accordingly, MgGaInTe4 and MgIn2Te4 have significant potential for application in topological quantum field-effect transistors. Our research identifies that the MgA2Te4 (A = Ga, In) structures have huge potential to be candidate 2D materials for spintronics and topological quantum devices.

Manipulation of valley-pseudospin in 2D WTe2/CrI3 van der Waals heterostructure by magnetic proximity effect

Lifting the valley degeneracy is an essential condition for exploiting the valley degrees of freedom to process and store information in valleytronics. The magnetic proximity effect (MPE) has been proven to be an effective method for introducing a magnetic field via interlayer exchange interaction, which can be conveniently achieved by two-dimensional (2D) van der Waals (vdW) heterostructure engineering. We have investigated the electronic properties and valley physics of 2D WTe2/CrI3 heterostructure using first-principles calculations. The results show that a significant valley splitting of 34.01 meV can be obtained in the WTe2/CrI3 heterostructure due to the simultaneous breaking of inversion and time-reversal symmetry, which sensitively depends on the atomic overlap of the projected position in the specific stacking model. In addition, we also show that the valley splitting can be enhanced by applying a vertical external electric field (E-field) or reducing the interlayer distance. More than that, it can be boosted by an order of magnitude (∼115.25 meV) through Cr ion intercalation. This is attributed to that the self-intercalated Cr ion has ferromagnetic (FM) ordering and perpendicular magnetic anisotropy due to the exchange coupling of the CrI3 layer. This result provides fundamental insights and valuable guidance for the application of 2D WTe2/CrI3 heterostructure in novel spintronic and valleytronic devices.

Improvement of sensing performance of SO2, SOF2, and SO2F2 gases on monolayer and bilayer HfSe2 under electric field

In this contribution, we use DFT simulations to gain a systematic understanding of how an external vertical electric field (E-field) affects the improvement of the sensing performance of SF6 gases on monolayer and bilayer HfSe2. The individual layered HfSe2 nanosheets' predicted band gaps and lattice constants agree with previously published results, demonstrating the validity of our theoretical framework and computational details. In the current study, it is examined the viability of stacked HfSe2 nanosheets as a candidate for detection SO2, SOF2, and SO2F2. An external E-field can facilitate the adsorption and the stability of SF6 products onto monolayer and bilayer HfSe2 nanosheets. The highest adsorption energies of SO2, SOF2, and SO2F2 on bilayer HfSe2 occurred at an E-field intensity of roughly −0.41 V/A0, with values −1.46, −1.39, and −1.16 eV, respectively. The electronic structures of the layered HfSe2 systems were seen to change as a result of the adsorption of gas molecules, where all the monolayer HfSe2 systems containing adsorbed molecules exhibited semiconductor properties with various band gaps, barring the monolayer HfSe2/SO2 system. Also, all bilayer HfSe2 systems exhibit metallic behavior in all values of E-field. Overall, these study's findings demonstrated that monolayer and bilayer HfSe2 might be utilized in techniques for the adsorption and detection of SF6 products. That bilayer HfSe2 also works to remove these gases.

Theoretical Design of an Mox W1-x S2 /Graphene (x=0.25/0.75) Heterojunction with Adjustable Band Gap: Potential Candidate Materials for the Next Generation of Optoelectronic Devices.

Multicomponent two-dimensional (2D) transition metal dichalcogenides (TMDCs) semiconductors based on adjustable band gap are increasingly used to design optoelectronic devices with specific spectral response. Here, we have designed the Mox W1-x S2 /graphene heterostructure with adjustable band gap by adopting the combination idea of alloying and multiple heterogeneous recombination. The contact type, stability and photoelectric properties of Mox W1-x S2 /graphene heterojunction were investigated theoretically. At the same time, by applying external vertical electric field to Mox W1-x S2 /graphene, the regulate of heterojunction Schottky contact type was realized. The results show that Mox W1-x S2 /graphene heterojunction has broad application prospects in the field of photocatalysis and Schottky devices, and is suitable for being a potential candidate material for next generation of optoelectronic devices. The design of Mox W1-x S2 /graphene heterostructure enables it to obtain the advanced characteristics that are lacking in the one-component intrinsic 2D TMDCs semiconductors or graphene materials, and provides a theoretical basis for the experimental preparation of such heterojunctions.

The impact of an external electric field on the Rashba effect in two-dimensional hybrid perovskites

The Rashba effect in 2D hybrid perovskites could be tunned by adjusting the vertical external electric field.

Tunable electronic properties and negative differential resistance effect of the intrinsic type-III ZrS[formula omitted]/WTe[formula omitted] van der Waals heterostructure

Van der Waals heterostructure (vdWH) has attracted considerable interest because of its ability to combine the excellent properties of stacked materials to suit different application requirements. The limited presence of type-III vdWH is a serious barrier for the development of two-dimensional materials in the field of tunneling field effect transistors. Herein, we propose a novel ZrS2/WTe2 vdWH with type-III band alignment and investigate its structural, electronic and quantum transport properties using first-principles calculations. The physical mechanism of type-III band alignment is analyzed. The electronic structures of this vdWH can be adjusted to enhance the band-to-band tunneling effect or achieve an interesting transition from type-III to type-II band alignment via external vertical electric field and different strains. Finally, we study the quantum transport properties and confirm again the type-III band alignment. The I–V curve shows apparent negative differential resistance (NDR) behaviors under low bias voltages, which provides substantial evidence supporting the applicability of this vdWH to low power devices. Our study not only expands the field of type-III band alignment heterostructures in 2D materials, but also provides a remarkable candidate material with tunable electronic properties and the favorable NDR effect for experimental designs and practical applications of novel multi-functional nanoelectronic devices.

Effects of edge hydrogenation and applied electric field on the structure and electrical properties of zigzag silicene nanoribbons by SCC-DFTB calculations

The effects of edge hydrogenation and applied electric field in different directions on the geometrical structure and electrical properties of zigzag silicene nanoribbons (ZSiNRs) with different period widths (width = 2,3,4) were investigated by using a self-consistent charge density functional tight-binding (SCC-DFTB) method. The results show that the degree of warpage of the ZSiNRs is significantly changed by hydrogenation, while the bond length and bond angle are less affected. The edge hydrogenation reduces the Si-dangling bonds in the nanoribbons, which increased the binding energy of nanoribbon and enhanced system stability. In the absence of an electric field, the unhydrogenated nanoribbons with different period widths all exhibit semiconducting properties, and the hydrogenated nanoribbons exhibit metallic or semi-metallic properties. Under the vertical external electric field in the z-direction, the stability and energy gap of the unhydrogenated nanoribbons are sensitive to the change of strength, and the oscillation fluctuation is large. With the increase of strength, the atoms in the outermost four layers of the nanoribbons show obvious charge gain and loss. When the vertical electric field is applied to the hydrogenated nanoribbons, the energy gap changes are related to the period width. Their electrical properties realized the conversion between the semi-metal properties and the direct band gap semiconductor properties. Charge transfer occurs between the adjacent two layers of atoms. Under the same electric field strength, the amount of charge transfer increases from left to right along the direction of the width of the nanoribbon; at the same time, with the increase of the electric field strength, the amount of charge transfer also presents an increasing trend.

Polarizability, plasmons, and screening in 1T′-MoS2 with tilted Dirac bands

In the presence of an external vertical electric field and strain, it is evident that 1T′-MoS2 exhibits tilted Dirac bands which are valley-spin-polarized. Additionally, this material experiences a topological phase change between a topological insulator and band insulator for a critical value of the electric field. Using linear response theory, we calculated the polarization function which is in turn employed to obtain the dielectric function. This latter quantity is subsequently utilized in calculations to determine the plasmons dispersion relation, their decay rate and impurity screening corresponding to various levels of doping, the critical applied vertical electric field strengths and the spin-orbit coupling gap in 1T′-MoS2 with tilted Dirac bands.

Electric field tunable electronic properties of antimonene/graphyne van der Waals heterostructure

The 2D antimonene and graphyne monolayers have been intensively studied due to their unique physical, electronic and chemical properties. In this work, a novel antimonene/graphyne (Sb/Gy) vdW heterostructure has been designed and investigated to explore the 2D/2D hybrid effects on electronic and interface properties by the first-principle calculations. The results indicate that the Sb/Gy vdW heterostructure displays small lattice mismatch, superior thermodynamic stability and semiconductivity with a staggered type-Ⅱ band alignment. The band structure of the Sb/Gy vdW heterostructure can be modulated by external vertical electric field, which undergoes transition from indirect to direct semiconductor under positive external vertical electric field. Furthermore, an interesting transformation from type-II to type-III band alignment is observed upon the applying of negative electric field. These findings can provide a good guidance for the applications of the Sb/Gy vdW heterostructures for nanoelectronic and optoelectronic devices. • Antimonene/graphyne vdW heterostructure possesses small lattice mismatch and superior thermodynamic stability. • Antimonene/graphyne vdW heterostructure is a typical type-II semiconductor with a small indirect bandgap. • External electric fields can tailor the electronic structures and interface properties of the heterostructure. • Semiconductor-semimetal and indirect-direct semiconductor transition are observed under external vertical electric field.

Second harmonic helicity and Faraday rotation in gated single-layer 1T′−WTe2

Single-layer of 1T$'$ phase of WTe$_2$ provides a rich platform for exotic physical properties such as nonlinear Hall effect and high-temperature quantum spin hall transport. Utilizing a continuum model and diagrammatic method, we calculate the second harmonic conductivity of monolayer 1T$'$-WTe$_2$ modulated by an external vertical electric field and electron doping. We obtain a finite helicity and Faraday rotation for the second harmonic signal in response to linearly polarized incident light in the presence of time-reversal symmetry. The second harmonic signal's helicity is highly controllable by altering the bias potential and serves as an optical indicator of the nonlinear Hall current. Our study motivates future experimental investigation for the helicity spectroscopy of two dimensional materials.

Electric field induced pure spin-photo current in zigzag stanene and germanene nanoribbons

The spin-photo current in single layer stanene and germanene under a linearly polarized light is theoretically investigated based on the tight-binding Hamiltonian combined with the nonequilibrium Green’s function at room temperature. The results show that by considering the simultaneous effect of the linear illumination and a vertical external electric field without any magnetic exchange element, pure spin-photo current without charge current is generated in two-dimensional lattices with a large intrinsic spin–orbit coupling. The necessity of enhanced spin life-time for detection of spin polarization can be explained by spin-valley locking concept. Spin-valley locking arises in buckled two-dimensional materials as a result of the large spin–orbit coupling and electric-field reversible valley spin polarization. Equal absorption of the linearly illumination at both valleys with different spin polarization, leads to pure spin-photo current injection. In addition, an acceptable photoresponsivity has been reported in a broad range of photon energy. The numerical results indicate high quantum efficiency with a maximum of nearly 83% and 50% for stanene and germanene, respectively. This work may pave theoretical reference toward design of new spin-optoelectronic devices based on satanene and germanene junctions with high performance.

Two-dimensional graphyne–graphene heterostructure for all-carbon transistors

Semiconducting graphyne is a two-dimensional (2D) carbon allotrope with high mobility, which is promising for next generation all-carbon field effect transistors (FETs). In this work, the electronic properties of van der Waals heterostructure consists of 2D graphyne and graphene (GY/G) were studied from first-principles calculations. It is found that the band dispersion of isolated graphene and graphyne remain intact after they were stacked together. Due to the charge transfer from graphene to graphyne, the Fermi level of the GY/G heterostructure crosses the VB of graphene and the CB of graphyne. As a result, n-type Ohmic contact with zero Schottky barrier height (SBH) is obtained in GY/G based FETs. Moreover, the electron tunneling from graphene to graphyne is found to be efficient. Therefore, excellent electron transport properties can be expected in GY/G based FETs. Lastly, it is demonstrated that the SBH in the GY/G heterostructure can be tune by applying a vertical external electric field or doping, and the transition from n-type to p-type contact can be realized. These results show that GY/G is potentially suitable for 2D FETs, and provide insights into the development of all-carbon electronic devices.

Sliding ferroelectricity in 2D van der Waals materials: Related physics and future opportunities

Near the 100th anniversary of the discovery of ferroelectricity, so-called sliding ferroelectricity has been proposed and confirmed recently in a series of experiments that have stimulated remarkable interest. Such ferroelectricity exists widely and exists only in two-dimensional (2D) van der Waals stacked layers, where the vertical electric polarization is switched by in-plane interlayer sliding. Reciprocally, interlayer sliding and the “ripplocation” domain wall can be driven by an external vertical electric field. The unique combination of intralayer stiffness and interlayer slipperiness of 2D van der Waals layers greatly facilitates such switching while still maintaining environmental and mechanical robustness at ambient conditions. In this perspective, we discuss the progress and future opportunities in this behavior. The origin of such ferroelectricity as well as a general rule for judging its existence are summarized, where the vertical stacking sequence is crucial for its formation. This discovery broadens 2D ferroelectrics from very few material candidates to most of the known 2D materials. Their low switching barriers enable high-speed data writing with low energy cost. Related physics like Moiré ferroelectricity, the ferroelectric nonlinear anomalous Hall effect, and multiferroic coupling are discussed. For 2D valleytronics, nontrivial band topology and superconductivity, their possible couplings with sliding ferroelectricity via certain stacking or Moiré ferroelectricity also deserve interest. We provide critical reviews on the current challenges in this emerging area.

Electric field and strain induced gap modifications in multilayered GaN

Two-dimensional materials with a wide bandgap characteristic are critical to developing novel nanoscaled electronic devices. This study investigates the layer, in-plane biaxial strain, and vertical external electric field effects on the stability and electronic properties of multilayered GaN employing the density functional theory. Using structural optimization and phonon-mode calculations, the GaN structures with one to five layers are found to be stable and refer to the Born–Oppenheimer local minima. Under dimensionality effect, GaN unveils a semiconducting feature with an indirect bandgap at the HSE06 level that changes from 3.37eV for a monolayer to 2.79eV for a five-layered structure as the quantum confinement effect is reduced. The calculations demonstrate that the indirect bandgap of multilayered GaN decreases linearly when increasing the tensile strain. However, compressive strain engineering would induce an indirect-to-direct gap transition. Additionally, when the strength of the applied perpendicular external electric field to the multilayered GaN surface is increased, the GaN bandgap rapidly disappears due to the field-induced near free electronic gas. Calculated carrier effective mass indicate that both external effects impact charge carrier mobility as well. Such significant changes in the electronic properties under external excitations show that multilayered GaN shows promise in configurable nanoelectronic devices.

Enhancement of nitride-gas sensing performance of SiC7 monolayer induced by external electric field

The geometrics, adsorption energy, charge transfer, electronic and optical properties of molecules (NO, NH3, NO2, H2, N2, O2 and CO2) adsorbed on the SiC7 monolayer with and without electric field were studied using first principles calculations. Our results show that the SiC7 monolayer has high sensitivity and selectivity toward NO, NH3, and NO2 monitoring. However, the strong interaction between the three molecules and monolayer makes it difficult for molecules desorption from the SiC7 monolayer, and resists the monolayer as a reusable gas sensor. When an external vertical electric field is applied, the adsorption energy can be decreased obviously, thereby effectively decrease the recovery time. In addition, the applied electric field can also effectively modify the electronic and optical properties of molecules adsorbed on the SiC7 monolayer. Our results indicate that the gas-sensing performance of the SiC7 monolayer toward NO, NO2, and NH3 can be significantly improved by applying an external electric field. Therefore, it is predicted that the SiC7 monolayer is a promising candidate for reusable gas sensors with high sensitivity and selectively for detecting NO and NH3 gases at room temperature with the applied electric field, and a disposable gas sensor for detecting NO2 gas.

The tuning on the magnetism and the electronic structures of monolayer Ti2N MXene by electric field

By first-principles calculations, the ground state of monolayer Ti2N is predicted to be ferromagnetic (FM) metal. Applying a vertical external electric field, we investigated the variation of its lattice constants, the magnetic moments (MMs) of Ti atoms, the band structures and the magnetic anisotropy energy (MAE). In addition, using mean-field approximation method, the Curie temperature (TC) of monolayer Ti2N is estimated to be 39 K. With the increase of the electric field, the exchange parameters decrease, which results in the reduction of TC. Our research provides an important theoretical basis for the manipulation of magnetic MXene nano-devices.

Anisotropic longitudinal optical conductivities of tilted Dirac bands in 1T′−MoS2

1T$^\prime$-MoS$_2$ exhibits valley-spin-polarized tilted Dirac bands in the presence of external vertical electric field and undergoes a topological phase transition between the topological insulator and band insulator around the critical value of the electric field. Within the linear response theory, we theoretically investigate the anisotropic longitudinal optical conductivities of tilted Dirac bands in both undoped and doped 1T$^\prime$-MoS$_2$, including the effects of the vertical electric field. The influence of the spin-orbit coupling gap, band tilting, and vertical electric field on the optical conductivities of tilted Dirac bands is revealed. A theoretical scheme for probing the topological phase transition in 1T$^\prime$-MoS$_2$ via exotic behaviors of longitudinal optical conductivities is proposed. The results for 1T$^\prime$-MoS$_2$ are expected to be qualitatively valid for other monolayer tilted gapped Dirac materials, such as $\alpha$-SnS$_2$, TaCoTe$_2$, and TaIrTe$_4$, due to the similarity in their band structures.

Electronic dispersions of a stable twisted bilayer phosphorene in 2O-tαP phase

It is reported for the electronic properties of an in-plane twisted bilayer phosphorene, known as the 2O-tαP phase, and the only dynamically stable phase beyond the AB stacking. This was achieved using first-principles calculations, a generalized empirical tight-binding model inclusive of electric field effects, and a two-parameter low energy effective model, the latter two providing an efficient scheme for nanoelectronics related applications. The tight-binding model reproduces a global fit to the first-principles dispersion, and the low energy model provides more accurate near-gap bands. Both are orders-of-magnitude faster and less memory-intensive than performing first-principles calculations. The twisted 2O-tαP structure possesses a direct bandgap of 1.27 eV, larger than that of the shifted AB structure (1.03 eV). The hole and electron polar effective mass anisotropy ratios are 27.34 and 1.95, respectively. An important observation is that the layer twisting results in the removal of Dirac cones as a reflection of a different band topology compared to the AB one, while the twofold degeneracy at the Brillouin zone boundary and the symmetry of the energy surface are both broken by an external vertical electric field. With an increasing electric field strength, a decreasing bandgap and an increasing energy difference between the valence band maximum and the twisted band point are both predicted by the tight-binding model and the low energy model.