External Transverse Electric Field Research Articles

First-principles study of electric field effect and spin-polarization transport properties of zigzag α-2 graphyne nanoribbons

Combining the density functional theory with the non-equilibrium Green's function, we have investigated the external transverse electric field effect of the electronic structures and spin-polarization transport properties within zigzag α-2 graphyne nanoribbons (zα-2GYNRs). The external transverse electric field can induce remarkable spin-polarized and half-metallicity behaviors in our system when the electric field is lower than 0.4 V/Å with the anti-ferromagnetic state. Particularly, we find that a nonmagnetic semiconductor feature can be obtained when the electric field is larger than 0.5 V/Å. Furthermore, analyzing the current-voltage characteristics of the zα-2GYNRs device, some interesting phenomena such as the excellent spin filtering and negative differential resistance have been found. Our results indicate that the external transverse electric field is a practical method for band modulation of the α-2 graphyne, and the design of spin-resolved devices based on zα-2GYNRs can realize multifunctional spin-dependent transport features.

Polyaniline (C3N) nanoribbons: Magnetic metal, semiconductor, and half-metal

A two-dimensional polyaniline sheet has been recently synthesized and found that it is a semiconductor with an indirect band gap. Polyaniline nanoribbons decomposed from the two-dimensional polyaniline sheet (C3N sheet) are investigated using a density functional theory. The existence of nitrogen atoms in the edge of the ribbons increases the stability and magnetization of the ribbons and make them different from graphene nanoribbons. Unsaturated nanoribbons are magnetic metals so that the armchair C3N nanoribbons are gap-less spin semiconductors in the antiferromagnetic state and half-metals in the ferromagnetic state. A transition from a metal to semiconductor is observed in the armchair C3N nanoribbons when the edge atoms are passivated by hydrogen. The band gap of the hydrogen saturated armchair C3N nanoribbons can be controlled using an external transverse electric field so that its magnitude is dependent on the direction of the electric field. Being a metal or semiconductor in hydrogen saturated zigzag C3N nanoribbons is strongly dependent on the edge atoms so that just ribbons having nitrogen atoms in both edges are semiconductor. An external electric field cannot induce any spin polarization in the zigzag nanoribbons, which is in contrast with what was observed in zigzag graphene nanoribbons.

Electronic, magnetic and transport properties of silicene armchair nanoribbons substituted with monomer and dimer of Fe atom

This study employed density functional theory calculations to investigate the structural, electronic and magnetic properties of an armchair silicene nanoribbon (ASiNR) substituted with a monomer and a dimer of Fe atom. As a result, the direct band gap of pristine ASiNR turns into a smaller indirect band gap by substituting an Fe atom in the proper position. The magnetic moment of doped Fe reduces and the structure keeps its nonmagnetic property. The substitution of the Fe-dimer can change the pristine ASiNR from a nonmagnetic semiconductor to a magnetic half-metal, which is favorable for spintronic devices. Two external electric fields were applied to the structure substituted with the Fe-dimer and electronic properties were studied in this situation. It was shown that the Fe-dimer substituted ASiNR is such a versatile material that a band gap can be tuned by using an external transverse electric field. Furthermore, the transport properties of these two structures were studied with non-equilibrium Greens function formalism. It is intriguing that single-spin negative differential resistance was observed in the Fe-dimer doped ASiNR.

Band-gap modulation of carbon nanotubes with Haeckelite structure under a transverse electric field: A first principle study

We have addressed the effect of transverse electric field on chiral carbon nanotubes with two types of Haeckelite structure. We have successfully verified that chiral (10, 5) carbon nanotube with different types of Haeckelite structure react quite differently in response to a transverse electric field. The band gap varies quadratically for small electric fields and becomes monotonically linear upon increasing the transverse electric filed. Two main interesting phenomena were observed when a transverse electric field applied on the nanotubes with Haeckelite structures. First of all, the semiconductor-metal transition existed regardless Haeckelite structure type; although some variations in the band-gap between the pristine nanotubes and nanotubes with both types of Haeckelites structure. This variation is an indication for strong effect caused by introducing this special type of defects to the nanotube surface. Second, the variations in the band-gap for the pristine nanotube and the two nanotubes with Haeckelite structure have the same trend, even though each type of defects have distinct band gap values under various strengths of the external transverse electric field.

Half metal phase in the zigzag phosphorene nanoribbon

Exploring half-metallic nanostructures is a crucial solution for developing high-performance spintronic devices. Black phosphorene is an emerging two-dimensional material possessing strong anisotropic band structure and high mobility. Based on the first principles calculations, we investigated the electronic and magnetic properties of zigzag phosphorene nanoribbons (ZPNRs) with three different functionalization groups (OH/CN, OH/NO2, NH2/NO2) at the edges. We find that the interplay between edge functionalization and edge oxidation can induce the half metal phase in the ZPNRs, and the half metal phase can be controlled by the external transverse in-plane electric field and the proportion of the functional groups and edge oxidation. The results may pave a new way to construst nanoscale spintronic devices based on black phosphorene nanoribbons.

Effect of carrier doping and external electric field on the optical properties of graphene quantum dots

In this paper, we demonstrate that the optical properties of finite-sized graphene quantum dots can be effectively controlled by doping it with different types of charge carriers (electron/hole). In addition, the role played by a suitably directed external electric field on the optical absorption of charge-doped graphene quantum dots have also been elucidated. The computations have been performed on diamond-shaped graphene quantum dot (DQD) within the framework of the Pariser-Parr-Pople (PPP) model Hamiltonian, which takes into account long-range Coulomb interactions. Our results reveal that the energy band-gap increases when the DQD is doped with holes while it decreases on doping it with electrons. Further, the optical absorption spectra of DQD exhibits red/blue-shift on doping with electrons/holes. Our computations also indicate that the application of external transverse electric field results in a substantial blue-shift of the optical spectrum for charge-doped DQD. However, it is observed that the influence of charge-doping is more prominent in tuning the optical properties of finite-sized graphene quantum dots as compared to externally applied electric field. Thus, tailoring the optical properties of finite-sized graphene quantum dots by manipulative doping with charge carriers and suitably aligned external electric field can greatly enhance its potential application in designing nano-photonic devices.

On-Demand Spin-Orbit Interaction from Which-Layer Tunability in Bilayer Graphene.

Spin-orbit interaction (SOI) that is gate-tunable over a broad range is essential to exploiting novel spin phenomena. Achieving this regime has remained elusive because of the weakness of the underlying relativistic coupling and lack of its tunability in solids. Here we outline a general strategy that enables exceptionally high tunability of SOI through creating a which-layer spin-orbit field inhomogeneity in graphene multilayers. An external transverse electric field is applied to shift carriers between the layers with strong and weak SOI. Because graphene layers are separated by subnanometer scales, exceptionally high tunability of SOI can be achieved through a minute carrier displacement. A detailed analysis of the experimentally relevant case of bilayer graphene on a semiconducting transition metal dichalchogenide substrate is presented. In this system, a complete tunability of SOI amounting to its ON/OFF switching can be achieved. New opportunities for spin control are exemplified with electrically driven spin resonance and topological phases with different quantized intrinsic valley Hall conductivities.

Proximity Effects in Bilayer Graphene on Monolayer WSe_{2}: Field-Effect Spin Valley Locking, Spin-Orbit Valve, and Spin Transistor.

Proximity orbital and spin-orbit effects of bilayer graphene on monolayer WSe_{2} are investigated from first principles. We find that the built-in electric field induces an orbital band gap of about 10meV in bilayer graphene. Remarkably, the proximity spin-orbit splitting for holes is 2 orders of magnitude-the spin-orbit splitting of the valence band at K is about 2meV-more than for electrons. Effectively, holes experience spin valley locking due to the strong proximity of the lower graphene layer to WSe_{2}. However, applying an external transverse electric field of some 1 V/nm, countering the built-in field of the heterostructure, completely reverses this effect and allows, instead of holes, electrons to be spin valley locked with 2meV spin-orbit splitting. Such a behavior constitutes a highly efficient field-effect spin-orbit valve, making bilayer graphene on WSe_{2} a potential platform for a field-effect spin transistor.

Electronic properties of zigzag and armchair graphene nanoribbons in the external electric and magnetic fields

We explore, numerically, some electronic properties of zigzag and armchair graphene nanoribbons under the external perpendicular magnetic field and transverse electric field. Our results, in the magnetic field only, indicate that numerical Landau levels deviate from the Dirac Landau levels formula for higher levels and quantum Hall conductance curve of armchair nanoribbon shows oscillatory behavior in the high gate voltage. In the presence of transverse electric field only, it is shown that the electric dipole moment of zigzag nanoribbon increases abruptly versus the electric field in the range of low-intensity electric fields while for armchair nanoribbon this varies very slowly. This variation in stronger electric fields is staircase for armchair nanoribbon while it is smoothly for zigzag nanoribbon. In the presence of electric and magnetic fields, there are electrons and holes as charge carrier in the same proportions. Conducting electrons make a round current in the half of nanoribbons while conducting holes make a round current in the other half. Electronic vortices, which are static in the presence of magnetic field only, move along nanoribbons in the effect of the transverse electric field. By considering the curve of electric dipole moment versus the electric field, it is found that magnetic field increases the electric susceptibility of nanoribbons in the low-intensity electric fields substantially and creates considerable electric susceptibilities in several higher electric fields. So these indicate that the magnetic field increases the electric sensitivity of graphene nanoribbons.

Enhancement of adsorption and diffusion of lithium in single-walled carbon nanotubes by external electric field

Effects of an external transverse electric field on the adsorption and diffusion of Li atoms on the single-walled carbon nanotubes (CNTs) were investigated using density functional theory. Results showed that the adsorption energy was significantly enhanced by applying the electric field. As the external electric field was increased from 0.0 to 0.6 V/A, the adsorption energies were decreased from −1.37 to −2.31, −1.32 to −2.46, and −1.33 to −2.63 eV for the Li atoms adsorbed on (6,6), (8,8), and (10,10) CNTs, respectively. Meanwhile, the diffusion barriers of the Li atoms on the CNTs were also decreased as the external electric field was applied. When the external electric field was increased from 0.0 to 0.6 V/A, the energy barriers were decreased from 0.42, 0.40, and 0.39 eV to 0.20, 0.17, and 0.15 eV for Li diffusion in the (6,6), (8,8), and (10,10) CNTs, respectively. The results proved that an external electric field can be applied to enhance the adsorption and diffusion of Li atoms on the CNTs (used as the anode) for lithium ion batteries.

Inducing half-metallicity with enhanced stability in zigzag graphene nanoribbons via fluorine passivation

Half metals are the primary ingredients for the realization of novel spintronic devices. In the present work, by employing density functional theory based first-principles calculation, we predict half metallic behavior in fluorine passivated zigzag graphene nanoribbons (F-ZGNR). Four different structures have been investigated viz. one edge F passivated ZGNR (F-ZGNR-1), both edges F passivated ZGNR (F-ZGNR-2), F passivation on alternate sites in first configuration (alt-1) and F passivation on alternate sites in second configuration (alt-2). Interestingly, it is noticed that F passivation is analogous to H passivation (pristine), however, F-ZGNR are reckoned energetically more stable than pristine ones. An spin induced band gap is noticed for all F-ZGNR irrespective of their widths although its magnitude is slightly less than the pristine counterparts. With an external transverse electric field, ribbons undergo semiconducting to half metallic transformation. The observed half metallic character with enhanced stability present F-ZGNR as a better candidate than pristine ZGNR towards the realization of upcoming spintronic devices.

Channeling potential in single-walled carbon nanotubes: The effect of radial deformation

We study the effect of radial deformation in single-walled carbon nanotubes (SWCNTs), due to one external factor, on the channeling potential. The calculations covered the channeling potential for positrons of 100MeV move along the z-axis, which is the axis of the radially deformed SWCNTs (6, 0), (8, 0) under external mechanical stress at different values for the induced strain and also for radially deformed SWCNT (5, 5) under external transverse electric field of 1.8 and 2.6V/Å. The calculations executed according to the continuum model approximation given by Lindhard for the case of an axial channeling in single crystals. The results of the calculations in this work agreed well with previous calculations depending on the equilibrium electron density in perfect carbon nanotubes. It has been found that, for perfect nanotubes, the channeling potential, i.e., the potential at any point (x, y) in a plane normal to the nanotube axis (xy-plane), is a function of the distance from the nanotube center whatever the (x, y) coordinate and hence, it could be expressed in terms of one independent variable. On the other hand, in radially deformed SWCNTs, the channeling potential was found to be a function of two independent variables (x, y) and could be given here by a general formula in terms of fitting parameters for each nanotube with chiral index (n, m). The obtained formula has been used in plotting the contour plot for the channeling potential.

Tuning the electronic properties of single-walled SiC nanotubes by external electric field

The electronic properties of SiC nanotubes (SiCNTs) under external transverse electric field were investigated using density functional theory. The pristine SiCNTs were semiconductors with band-gaps of 2.03, 2.17 and 2.25eV for (6,6), (8,8) and (10,10) SiCNTs, respectively. It was found the band gaps was reduced with the external transverse electric filed applied. The (8,8) and (10,10) SiCNTs changed from semiconductor to metals as the intensity of electric field reached 0.7 and 0.5V/Å. The results indicate that the electronic properties of SiCNTs can be tuned by the transvers electric field with integrality of the nanotubes.

Structural and electronic properties of zigzag InP nanoribbons with Stone–Wales type defects

By means of density-functional-theoretic calculations, we investigate the structural and electronic properties of a hexagonal InP sheet and of hydrogen-passivated zigzag InP nanoribbons (ZInPNRs) with Stone–Wales (SW)-type defects. Our results show that the influence of this kind of defect is not limited to the defected region but it leads to the formation of ripples that extend across the systems, in keeping with the results obtained recently for graphene and silicene sheets. The presence of SW defects in ZInPNRs causes an appreciable broadening of the band gap and transforms the indirect-bandgap perfect ZInPNR into a direct-bandgap semiconductor. An external transverse electric field, regardless of its direction, reduces the gap in both the perfect and defective ZInPNRs.

Fundamental insights into the electronic structure of zigzag MoS2 nanoribbons.

The structural and electronic properties of zigzag MoS2 nanoribbons are investigated using first-principles density functional theory. Our models are motivated by the experimental observations, in which both Mo edges are terminated by S atoms. Our calculations show that the edge can introduce some extra states into the energy gap, which lead nanoribbons to exhibit a metallic characteristic. Such extra states around the Fermi level are flat or dispersed. Through detailed analyses, we identify and discriminate them based on the major contributors. By applying an external transverse electric field, Eext the extra states around the Fermi level can shift apparently, especially for those attributed to Mo-edge atoms. It can be explained by the charge redistribution in the MoS2 nanoribbons due to Eext. In addition, the nanoribbon can be changed from metal to an n/p-type semiconductor according to different edge hydrogenation. After full edge hydrogenation, we observe a characteristic of anti-bonding orbitals between H and S atoms at the Mo-edge. Interestingly, the energy of anti-bonding orbitals and electric conductivity of nanoribbons can be tailored by Eext. The results suggest a strategy controlling the performance of MoS2 for hydrogen evolution.

Electronic and transport properties of phosphorene nanoribbons

By combining density functional theory and nonequilibrium Green's function, we study the electronic and transport properties of monolayer black phosphorus nanoribbons (PNRs). First, we investigate the band gap of PNRs and its modulation by the ribbon width and an external transverse electric field. Our calculations indicate a giant Stark effect in PNRs, which can switch on transport channels of semiconducting PNRs under low bias, inducing an insulator-metal transition. Next, we study the transport channels in PNRs via the calculations of the current density and local electron transmission pathway. In contrast to graphene and ${\mathrm{MoS}}_{2}$ nanoribbons, the carrier transport channels under low bias are mainly located in the interior of both armchair and zigzag PNRs, and immune to a small amount of edge defects. Last, a device of the PNR-based dual-gate field-effect transistor, with high on/off ratio of ${10}^{3}$, is proposed based on the giant electric-field tuning effect.

Modulation of the band structure in bilayer zigzag graphene nanoribbons on hexagonal boron nitride using the force and electric fields

Modulation of semiconductor–halfmetal–metal transition in the antiferromagnetic (AF) ordering of bilayer zigzag graphene nanoribbons (BZGNRs) on hexagonal boron nitride (h-BN) heterostructure using the external force field Fext and transverse electric fields Eext (in the presence of interaction with the substrate) was performed within the framework of the density functional theory (DFT). We established critical values of Eext and interlayer distance in the bilayer providing for semiconductor–halfmetal–metal transition in one of electron spin configurations. Our calculations also show that the energy gap Eg in the AF-BZGNR/h-BN(0001) heterostructure can be controlled in a wide range of the Fext and Eext. This makes the AF-8-ZGNR/h-BN(0001) heterostructure being potentially promising for application in spintronic devices.

Electronic and magnetic properties of armchair MoS2 nanoribbons under both external strain and electric field, studied by first principles calculations

The electronic and magnetic properties of armchair edge MoS2 nanoribbons (MoS2-ANRs) underboth the external strain and transverse electric field (Et) have been systematically investigated by using the first-principles calculations. It is found that: (1) If no electric field is applied, an interesting structural phase transition would appear under a large tensile strain, leading to a new phase MoS2-A'NR, and inducing a big jump peak of the band gap in the transition region. But, the band gap response to compressive strains is much different from that to tensile strain, showing no the structural phase transition. (2) Under the small tensile strains (<10%), the combined Et and tensile strain give rise to a positive superposition (resonant) effect on the band gap reduction at low Et (<3 V/nm), and oppositely a negative superposition (antiresonant) one at high Et (>4 V/nm). On the other hand, the external compressive strains have always presented the resonant effect on the band gap reduction, induced by the electric field. (3) After the structural phase transition, an external large tensile strain could greatly reduce the critical field Etc causing the band gap closure, and make the system become a ferromagnetic (FM) metal at a relative low Et (e.g., <4 V/nm), which is very helpful for its promising applications in nano-mechanical spintronics devices. (4) At high Et (>10 V/nm), the magnetic moments of both the MoS2-ANR and MoS2-A'NR in their FM states could be enhanced greatly by a tensile strain. Our numerical results of effectively tuning physical properties of MoS2-ANRs by combined external strain and electric field may open their new potential applications in nanoelectronics and spintronics.

Spin-semiconducting properties in silicene nanoribbons.

We have investigated the relative stabilities and electronic properties of silicene nanoribbons with sawtooth edges (SSiNRs) by first-principles calculations. The SSiNR is more stable than the zigzag silicene nanoribbon (ZSiNR) and has a ferromagnetic ground state with an intrinsic energy gap between majority and minority spin-polarized bands, which shows that SSiNR is a spin-semiconductor. Under an external transverse electric field, the energy gap decreases and even vanishes. Meanwhile, the charge densities of the two edge bands near the Fermi level become spatially separated at different edges. We find also that the electric field-induced features can be achieved by a suitable uniaxial compressive strain. This can be understood from the effect of the Wilson transition. At last, the electronic structures of SSiNRs tuned by electric field and strain together are studied, showing that a small tensile strain makes the SSiNRs more sensitive to the electric field. These results suggest that the electric field or/and strain modulated SSiNRs have potential applications in silicon-based spintronic nanodevices.

Electronic Properties of SiNTs Under External Electric and Magnetic Fields Using the Tight-Binding Method

We investigated the electronic properties of silicon nanotubes (SiNTs) under external transverse electric fields and axial magnetic fields using the tight-binding approximation. It was found that, after switching on the electric and magnetic fields, band modifications such as distortion of degeneracy, change in energy dispersion and subband spacing, and bandgap size reduction occur. The bandgap of silicon gear-like nanotubes (Si g-NTs) decreases linearly with increasing electric field strength, but the bandgap for silicon hexagonal nanotubes (Si h-NTs) first increases and then decreases (metallic) or first remains constant and then decreases (semiconducting). Our results show that the bandgap of Si h-NTs is very sensitive to both electric and magnetic fields, unlike Si g-NTs, which are more sensitive to electric than magnetic fields.