Zigzag Silicene Nanoribbons Research Articles

Dual spin filtering and dual spin diode through a zigzag silicene nanoribbon

In this study, we investigate the spin-dependent electron transport properties of a zigzag silicene nanoribbon (ZSiNR) by using spin-polarized density functional theory and the non-equilibrium Green’s function method. The results show that such a ZSiNR with an FM configuration exists the perfect dual spin filtering and dual spin diode effects when the bias voltage was applied. These phenomena still exist in ZSiNRs with different widths. As the ribbon width increases, the progressively smaller bias voltage is required to trigger a polarized current. Therefore, such a ZSiNR in FM configuration is a good candidate for spintronic devices.

Effect of band bending on the valley-resolved transport through zigzag silicene nanoribbons subject to edge electric fields

This work examines the effect of band bending on the valley polarization of electron transport through zigzag silicene nanoribbons (ZSiNRs) subject to perpendicular edge electric fields. When the edge electric fields are antisymmetric, the induced edge potential (UAE) determines the Fermi energy (EF) at which the valley polarization PKK’ changes sign. In the band-bending region, PKK’ shows plateaus with values of −1/3, −1/5, −1/7, etc. The band bending and the energy region with PKK’ < 0 grow as UAE increases. When UAE = 0.63t (t = 1.6 eV) and |EF| < 0.625t, PKK’ < 0. When the edge electric fields are symmetric, a conductance gap appears near EF = 0, two energy regions with PKK’ = 0 arise in the band-bending region, and the induced edge potential USE dictates the Fermi energy at which PKK’ changes from 0 to non-zero values. With increasing USE, the band bending increases, and the width of the conductance gap and energy regions with PKK’ = 0 widen, and plateaus with PKK’ = ±1, ±1/3, ±1/5, etc. vanish one by one. When USE = 0.8t and |EF| < 0.625t, the valley polarization of electron transport disappears completely. As UAE or USE increases to a certain degree, the band bending diminishes quickly, and the case of small UAE or USE gradually recovers. According to the above results, ZSiNRs under symmetric or antisymmetric perpendicular edge electric fields have potential applications in manufacturing versatile valleytronic devices.

Structure and electrical properties of bilayer zigzag silicene nanoribbons depending on stone-wales and applied electric fields by SCC-DFTB method

The influence of geometry and electrical properties of Stone-Wales (SW) defects and an applied electric field on bilayer zigzag silicene nanoribbons (ZSiNRs) were investigated using the self-consistent charge density functional tight-binding (SCC-DFTB) method. The results show that concave deformation occurs at SW defects, leading to varying degrees of disruption in the original symmetrical structure. The overall stability of the marginal Stone-Wales (MSW) nanoribbons surpasses that of the central Stone-Wales (CSW) nanoribbons, SW-II type defects are prone to form in bilayer nanoribbons. The band gap of MSW defective nanoribbons expands significantly, realizing the transition from metallic to semi-metallic and semi-conductor properties. The electrons are transferred from the inner to the outer side of the nanoribbons, with a higher prevalence of defect nanoribbons compared to those that are perfect. The electric field intensity significantly alters the properties of both MSW-I and MSW-II defective nanoribbons.

Controllable spin transport in a zigzag silicene nanoribbon embedding multiple rectangular quantum dots

Based on the recursive Green-function method together with Landauer–Büttiker formalism, the spin-dependent transport properties of electrons in a zigzag silicene nanoribbon embedding multiple rectangular quantum dots (QDs) are investigated. According to an analysis of the energy band under the periodically distributed electric field and exchange ferromagnetic field, the parallel exchange field induced by the ferromagnetic insulators eliminates the spin degeneracy, which leads to spin-polarized transport in the proposed structure. By tuning a periodic electric field, we found the relationship between the number of QDs and the splitting peak for conductance in the anti-parallel exchange field. We discover the population of electrons near QDs by calculating the local density of states. The effect of the geometry of periodic QDs on the shift of resonance peak is evaluated. The spin polarization is further explored for various configurations of electric field and exchange field in order to manipulate the spin filtering more effectively. The results provide an avenue to design a controllable spin bandpass filter with the modulation of electric field and exchange field.

Transport properties of GaAs Co-doped H-passivated low-buckled and high-buckled zigzag silicene nanoribbon two probe devices

Transport properties of GaAs Co-doped H-passivated low-buckled and high-buckled zigzag silicene nanoribbon two probe 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.

DFT Analysis of Hydrogenated Zigzag Aluminum Nitride Nanoribbons for Spintronic Devices

Density functional theory (DFT) and nonequilibrium Green’s function (NEGF) framework are used to explore the structural, spin-polarized electronic, and spin-based transport properties of edge-hydrogenated zigzag aluminum nitride nanoribbons (ZAlNNRs). The proposed ZAlNNR is observed to be structurally stable and exhibits half-metallic nature in the magnetic state. The quantum transport property of the proposed two-terminal device model of 1H-AlN-1H demonstrates the bipolar spin-filter characteristics along with giant magnetoresistance (GMR), spin-based peak to valley current ratio (spin-PVCR), and spin-based rectification ratio (spin-RR) of the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sup> , 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> , and 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> , respectively. The calculated GMR and spin-RR of the 1H-AlN-1H device are 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> and 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> times higher than zigzag silicene nanoribbon (ZSiNR) and doped-zigzag graphene nanoribbon (doped ZGNR), respectively. The observed GMR, spin-PVCR, and spin-rectifying behavior of the reported ZAlNNR device could be deployed for multifunctional spintronic device applications.

Band Polarization Effect on the Kondo State in a Zigzag Silicene Nanoribbon.

Using the Numerical Renormalization Group method, we study the properties of a quantum impurity coupled to a zigzag silicene nanoribbon (ZSNR) that is subjected to the action of a magnetic field applied in a generic direction. We propose a simulation of what a scanning tunneling microscope will see when investigating the Kondo peak of a magnetic impurity coupled to the metallic edge of this topologically non-trivial nanoribbon. This system is subjected to an external magnetic field that polarizes the host much more strongly than the impurity. Thus, we are indirectly analyzing the ZSNR polarization through the STM analysis of the fate of the Kondo state subjected to the influence of the polarized conduction electron band. Our numerical simulations demonstrate that the spin-orbit-coupling-generated band polarization anisotropy is strong enough to have a qualitative effect on the Kondo peak for magnetic fields applied along different directions, suggesting that this contrast could be experimentally detected.

Investigating magneto-resistance in transition metals doped silicene nanoribbons

The Magneto Resistance (MR) is investigated in magnetic impurity-doped Zigzag Silicene Nano-Ribbons (ZSiNR) through Density Functional Theory (DFT) and Non-Equilibrium Green's Functions (NEGF) formalism using different spin configurations (Ferromagnetic (FM) and Antiferromagnetic (AFM)) in electrodes. The MR is calculated, and results show a GMR of about 2 ​× ​104% at 0.01V bias voltage for up-spin electrons in Co@ZSiNR structure and a total GMR of 2.6 ​× ​104% at 0.02V, which makes the proposed structures as possible candidates in spintronic device applications. The structures are modified by adding the impurities to the electrodes region, and removing the Hydrogen passivation. The GMR effect is investigated in these structures with different widths, which improves GMR ratios in certain widths.

Several semiconducting two-dimensional silicon nanosheets assembled from zigzag silicene nanoribbons.

Semiconducting two-dimensional intrinsic silicon nanosheets are ideal materials for many applications in modern industry, since they are the only ones that can match well with previous silicon components. However, such materials are still lacking, especially those with moderate band gaps. In this work, by using first-principles theory, a series of two-dimensional intrinsic silicon nanosheets are assembled from zigzag silicene nanoribbons with different widths. The result shows that all the nanosheets behave as semiconductors, although some of them possess small band gaps of dozens of meV. Two of them, individually assembled from the two narrowest zigzag silicene nanoribbons, possess the largest indirect band gaps of 0.20 and 0.26 eV, respectively. Under low compressive strain, these two nanosheets would turn into quasi-direct or direct band gap semiconductors, and the gaps increase up to 0.62 or 0.54 eV, respectively. Due to the electron transfer from three-fold to four-fold coordinated Si atoms, the charge carriers prefer to transport along the zigzag direction, and electrons and holes transport in the respective Si chains. Interestingly, the investigation of Poisson's ratio reveals that the assembled silicon nanosheets have a negative Poisson's ratio in certain strain ranges if the width n of zigzag silicene nanoribbons is even. This work provides a new approach to design semiconducting silicon nanosheets and benefits to the applications of two-dimensional silicon nanosheets in many electronic and mechanical fields.

Spin thermoelectric effects on aluminum or phosphorus doped zigzag silicene nanoribbons

Based on the density-functional theory (DFT) combined with nonequilibrium Green’s function (NGF), this paper investigates the effects of either single aluminum (Al) or single phosphorus (P) atom substitutions at different edge positions of zigzag-edged silicene nanoribbons (ZGNRs) in the ferromagnetic state on the spin-dependent transport properties and spin thermoelectric effects. It has been found that the spin polarization at the Fermi level can reach 100% or –100% in the doped ZSiNRs. Meanwhile, the spin-up Seebeck effect (for -100% case) and spin-down Seebeck effect (for 100% case) are also enhanced. Moreover, the spin Seebeck coefficient is much larger than the corresponding charge Seebeck coefficient at a special doping position and electron energy. Therefore, the study shows that the Al or P doped ZSiNRs can be used to prepare the ideal thermospin devices.

Photoirradiation and electric field tunable spin/charge transport in a Y-shaped silicene nanojunction

In this work, we propose a Y-shaped nanojunction consisting of zigzag silicene nanoribbons (ZSNRs), and study the charge/spin transport based on a tight-binding approach, Landauer formalism and non-equilibrium Green’s function method. The topological states with various edge modes, such as quantum spin Hall insulator, quantum Hall insulator and band insulator, can all be engineered by the electric field and photoirradiation in ZSNRs. Through tuning the edge states of the two output terminals, three types of charge/spin router can be achieved in the present nanojunction. The first is a charge current switcher, which switches the charge current from one output terminal to the other. The second is a spin current router, which shunts the spin-up and spin-down current into different output terminals. The last is a spin filter, which filters the spin-polarized current into a specific output terminal. The proposed Y-shaped nanojunction may have potential applications in electronic and spintronic nanodevices in the future.

Spin transport properties for B-doped zigzag silicene nanoribbons with different edge hydrogenations

Exploring silicon-based spin modulating junction is one of the most promising areas of spintronics. Using nonequilibrium Green’s function combined with density functional theory, a set of spin filters of hydrogenated zigzag silicene nanoribbons is designed by substituting a silicon atom with a boron one and the spin-correlated transport properties are studied. The results show that the spin polarization can be realized by structural symmetry breaking induced by boron doping. Remarkably, by tuning the edge hydrogenation, it is found that the spin filter efficiency can be varied from 30% to 58%. Moreover, it is also found and explained that the asymmetric hydrogenation can give rise to an obvious negative differential resistance which usually appears at weakly coupled junction. These findings indicate that the boron-doped ZSiNR is a promising material for spintronic applications.

Flat Zigzag Silicene Nanoribbon with Be Bridge.

The emergence of flat one- and two-dimensional materials, such as graphene and its nanoribbons, has promoted the rapid advance of the current nanotechnology. Silicene, a silicon analogue of graphene, has the great advantage of its compatibility with the present industrial processes based on silicon nanotechnology. The most significant issue for silicene is instability in the air due to the nonplanar puckered (buckled) structure. Another critical problem is that silicene is usually synthesized by epitaxial growth on a substrate, which strongly affects the π conjugated system of silicene. The fabrication of free-standing silicene with a planar configuration has long been pursued. Here, we report the strategy and design to realize the flat zigzag silicene nanoribbon. We theoretically investigated the stability of various silicene nanoribbons with substituents at the zigzag edges and found that zigzag silicene nanoribbons with beryllium (Be) bridges are very stable in a planar configuration. The obtained zigzag silicene nanoribbon has an indirect negative band gap and is nonmagnetic unlike the magnetic buckled silicene nanoribbons with zigzag edges. The linearly dispersive behavior of the π and π* bands associated with the out-of-plane 3psi and 2pBe orbitals is clearly observed, showing the existence of a Dirac point slightly above the Fermi level. We also observed that spin–orbit coupling induces a gap opening at the Dirac point.

Structural & electronic properties of zigzag silicene nanoribbons with symmetric/asymmetric edge passivations via fluorine and hydrogen

The intriguing properties of silicene nanoribbons indicate potential applications in nanoelectronic devices. The first-principles calculation based on density functional theory has been performed to investigate the structural, electronic and magnetic properties of asymmetric (sp3-sp2) and symmetric (sp3-sp3) edge functionalized zigzag silicene nanoribbons (ZSiNR). The structural analysis reveals that fully fluorinated ZSiNR for both asymmetric and symmetric edge functionalized ZSiNRs are energetically more stable irrespective of ribbon width. For selective structures, the magnetic ground state is found to be robust against thermal excitations at room temperature (25 meV) indicating their potential for practical applications. Moreover, bipolar magnetic semiconductor and half-metallic behavior have been observed in spin-based band structure calculations. Further, the spin-dependent transport properties of ZSiNR have been performed using the two-terminal device model which produces negative differential resistance (NDR) behavior due to spin-down current. This investigation reveals the immense capabilities of fluorine symmetric and asymmetric termination to modulate the electronic and spintronic properties of ZSiNR. Thus, the present work can pave the way for futuristic electronics and spintronics nanodevices.

Realizing stable half-metallicity in zigzag silicene nanoribbons with edge dihydrogenation and chemical doping

Although many schemes have been proposed to obtain full half-metallicity in zigzag silicene nanoribbons with edge monohydrogenation (H–H ZSiNRs) by chemical modification, the resulted negligible energy difference between the antiferromagnetic (AFM) and ferromagnetic (FM) configurations makes the half-metallicity hardly observable practically. In this work, based on density functional calculations, we find that the ZSiNRs with edge dihydrogenation (H2–H2 ZSiNRs) can be tuned to be half-metallic by replacing the central two zigzag Si chains with two zigzag Al–P chains, and more importantly, the FM–AFM energy difference is significantly increased compared with the H–H cases. The obtained half-metallicity originates from the different potential between two edges of the ribbon after doping, which results in the edge states of two spin channels shifting oppositely in energy. This mechanism is so robust that the half-metallicity can always be achieved, irrespective of the ribbon width. Our finding provides a fantastic way for achieving stable half-metallicity in ZSiNRs.

Tuning the electronic and magnetic properties of hydrogen saturated zigzag silicene nanoribbon doped with B atoms chain

We performed the first-principles calculations about the electronic and magnetic properties of hydrogen saturated zigzag silicene nanoribbon (HZSiNR) doped with a B atoms chain. The calculated results of formation energies indicate that the doped configurations possess thermodynamic stability. For HZSiNR doped with B atoms chain, a significant transition from semiconducting character to metallic character can occur, which is strongly originated from contributions of B impurities and its connected Si atoms. Moreover, the magnetic state can change from antiferromagnetic state to ferromagnetic state. Meanwhile, the local magnetic moments of Si atoms connected with B atoms are tremendously induced.

Even–odd chain effect and quantum pumping effect induced spatial spin filter on a Y-shaped zigzag silicene nanoribbon junction

We propose a possible spatial spin filter on a Y-shaped zigzag silicene nanoribbon (ZSNR) junction with different even(odd)-chain electrodes by applying three pumping electric fields. By means of the tight-binding model and Keldysh Green’s function method, we calculate the spin-dependent tunneling currents on the middle–left (M–L) and middle–right (M–R) terminals of Y-shaped ZSNR junction, and find that this spin pump not only can generate a pure spin current and a 100% polarized spin current at a zero bias, but also can spatially separate spin up and spin down currents on its even(odd)-chain M–L(R) terminals. The spatial spin filtering phenomena might come from the combination of the even–odd chain effect and single photon-assisted pumping process. The pumped spin up(down) current and its ON(OFF) transport states on M–L(R) terminal of the Y-shaped ZSNR junction can be modulated by almost all the device’s parameters such as the Fermi level, pumping frequency, spin–orbit coupling, staggered lattice potential and magnetization distribution. We also explore the possible way to close the spatial spin filtering behaviors in the device. Our findings might be useful for producing and separating 100% polarized spin up and down currents on multi-terminal silicene nanodevices.

Band bending and zero-conductance resonances controlled by edge electric fields in zigzag silicene nanoribbons

We study the band structure and transport property of a zigzag silicene nanoribbon when electric fields are applied to the edges. It is found that band bending could be induced and controlled by the antisymmetric edge fields, which can be understood based on the wave functions of the edge states. The highest valence band and the lowest conduction band coexist in the band-bending region. With the narrowing of edge potentials, the bending increases gradually. When the edge fields become symmetric, an asymmetric band gap at the Dirac points can be obtained due to the intrinsic spin-orbit interaction, suggesting a valley-polarized quantum spin Hall state. The gap could reach a maximum value rapidly and then decrease slowly as the electric fields increase. Due to the combining effect of the band bending, band-selective rule, and resonant states, many zero-conductance resonances and resonance peaks appear in different regions, which could be described by the Fano resonance effect. Furthermore, the band bending and zero-conductance resonances are robust against the Hubbard interaction. The Hubbard interaction could work as a spin-dependent edge field, together with the edge electric fields, leading to a spin-dependent band gap and various quantum phases such as metal and half-metal.

Electronic properties of zigzag silicene nanoribbons with single vacancy defect

&lt;p&gt;Silicene is envisaged as one of the two-dimensional (2D) materials for future nanoelectronic applications. In addition to its extraordinary electronic properties, it is predicted to be compatible with the silicon (Si) fabrication technology. By using nearest neighbour tight-binding (NNTB) approach, the electronic properties of zigzag silicene nanoribbons (ZSiNRs) with single vacancy (SV) defects are modelled and simulated. For 4-ZSiNR with L=2, the band structures and density of states (DOS) are computed based on SV incorporated ZSiNRs at varying defect locations. The results show that the SV defect will shift the band structure and increase the peak of DOS while the bandgap remain zero. This work provides a theoretical framework to understand the impact of SV defect which is an inevitable non-ideal effect during the fabrication of silicene nanoribbons (SiNRs).&lt;/p&gt;