Zigzag Silicene Nanoribbons Research Articles

High hydrogen storage capacity in calcium‐decorated silicene nanostructures

We report our first‐principle's calculations on the possibility of Ca‐decorated silicene sheet and zigzag silicene nanoribbons (ZSiNRs) as hydrogen storage medium. We predict that Ca atoms prefer to disperse on the silicene or at the edges of ZSiNRs without clustering, due to the strong binding between Ca and Si atoms. By adsorbing Ca atoms on the both sides of silicene, the hydrogen storage capacity can reach to 6.17 wt% (gravimetric density) with an average adsorption energy of 0.265 eV H2−1, which are quite optimial for reversible hydrogen adsorption and desorption at ambient conditions. The hydrogen storage capacity can be further improved to 8.43 wt% with the average adsorption energy in the range of 0.182–0.269 eV H2−1 in the Ca‐decorated ZSiNRs. The adsorption of H2 on Ca‐decorated Si nanostructures is mainly dominated by polarization and the orbital hybridizations. These findings indicate that the Ca‐decorated silicene and ZSiNRs have potential application in hydrogen storage.

Spin-dependent Seebeck Effect, Thermal Colossal Magnetoresistance and Negative Differential Thermoelectric Resistance in Zigzag Silicene Nanoribbon Heterojunciton.

Spin-dependent Seebeck effect (SDSE) is one of hot topics in spin caloritronics, which examine the relationships between spin and heat transport in materials. Meanwhile, it is still a huge challenge to obtain thermally induced spin current nearly without thermal electron current. Here, we construct a hydrogen-terminated zigzag silicene nanoribbon heterojunction, and find that by applying a temperature difference between the source and the drain, spin-up and spin-down currents are generated and flow in opposite directions with nearly equal magnitudes, indicating that the thermal spin current dominates the carrier transport while the thermal electron current is much suppressed. By modulating the temperature, a pure thermal spin current can be achieved. Moreover, a thermoelectric rectifier and a negative differential thermoelectric resistance can be obtained in the thermal electron current. Through the analysis of the spin-dependent transport characteristics, a phase diagram containing various spin caloritronic phenomena is provided. In addition, a thermal magnetoresistance, which can reach infinity, is also obtained. Our results put forward an effective route to obtain a spin caloritronic material which can be applied in future low-power-consumption technology.

Controllable spin polarization and spin filtering in a zigzag silicene nanoribbon

Using non-equilibrium Green's function, we study the spin-dependent electron transport properties in a zigzag silicene nanoribbon. To produce and control spin polarization, it is assumed that two ferromagnetic strips are deposited on the both edges of the silicene nanoribbon and an electric field is perpendicularly applied to the nanoribbon plane. The spin polarization is studied for both parallel and anti-parallel configurations of exchange magnetic fields induced by the ferromagnetic strips. We find that complete spin polarization can take place in the presence of perpendicular electric field for anti-parallel configuration and the nanoribbon can work as a perfect spin filter. The spin direction of transmitted electrons can be easily changed from up to down and vice versa by reversing the electric field direction. For parallel configuration, perfect spin filtering can occur even in the absence of electric field. In this case, the spin direction can be changed by changing the electron energy. Finally, we investigate the effects of nonmagnetic Anderson disorder on spin dependent conductance and find that the perfect spin filtering properties of nanoribbon are destroyed by strong disorder, but the nanoribbon retains these properties in the presence of weak disorder.

Adsorption of Ti atoms on zigzag silicene nanoribbons: influence on electric, magnetic, and thermoelectric properties

We study the adsorption effects of Ti atoms on the physical properties of zigzag silicene nanoribbons using the density functional theory combined with the nonequilibrium Green’s function methods. The adsorption geometries, conductance spectra, current voltage curves, spin polarizations, magnetoresistance, and Seebeck coefficients are evaluated in different adsorption samples. Ti adatoms prefer sites inside the nanoribbons instead of on the edges. Two neighboring adatoms are attractively coupled and prefer being adsorbed on the same side. The giant magnetoresistance in nanoribbons of even width is usually greatly reduced, except in symmetric adsorption cases. Strong spin negative differential resistance phenomena can be observed and pure spin current can be produced by temperature gradient in specific cases.

Effect of SW defect on structural and transport properties of silicene nanoribbons

Using density functional theory and nonequilibrium Green's function technique, we performed theoretical investigations on the structural and transport properties of zigzag silicene nanoribbons (SiNRs) with Stone–Wales (SW) defect. The calculated formation energy is significantly lower than that of graphene and silicene, which implies the high stability of such defect in SiNRs. Negative differential resistance (NDR) can be observed within certain bias voltage range in both perfect and SW-defected SiNRs. In order to elucidate the mechanism, the NDR behavior, the transmission spectra and molecular projected self-consistent Hamiltonian (MPSH) states are discussed in details.

Electronic and magnetic properties of zigzag silicene nanoribbons with Stone–Wales defects

The structural, electronic, and magnetic properties of zigzag silicene nanoribbons (ZSiNRs) with Stone–Wales (SW) defects were investigated using first-principles calculations. We found that two types of SW defects (named SW-Ι and SW-ΙΙ) exist in ZSiNRs. The SW defect was found to be the most stable at the edge of the ZSiNR, independently of the defect orientation, even more stable than it is in an infinite silicene sheet. In addition, the ZSiNRs can transition from semiconductor to metal or half-metal by modifying the SW defect location and concentration. For the same defect concentration, the band structures influenced by the SW-Ι defect are more distinct than those influenced by the SW-ΙΙ when the SW defect is at the edge. The present study suggests the possibility of tuning the electronic properties of ZSiNRs using the SW defects and might motivate their potential application in nanoelectronics and spintronics.

Thermoelectric Properties of Doped Zigzag Silicene Nanoribbons

Thermoelectric properties of silicene nanoribbons doped with magnetic impurity atoms are investigated theoretically for both antiparallel and parallel orientations of the edge magnetic moments. Spin density distribution and transport parameters have been determined by ab-initio numerical methods based on the density functional theory. Doping with magnetic atoms considerably modi es the spin density distribution, leading to a ground state with a non-zero magnetic moment. Apart from this, the spin thermopower can be considerably enhanced by the impurity atoms.

Doping Effect on Spin Dependent Electron Transport in Zigzag Silicene Nanoribbons

Silicene has been recently synthesized in the form of nanoribbons on the anisotropic Ag (110) surface. The effects of disorder on silicene nanoribbons are expected to exhibit remarkable properties in the form of nanostructures. It has been found that the electronic structures of the doped zigzag silicene nanoribbons (ZSiNRs) are different from those of pristine ZSiNRs. In this paper, we study the spin dependent electron conductance of ZSiNRs substitutionally doped with Boron/Nitrogen (B/N) atoms by using Green's Function method based on Tight Binding approximation and Landauer-Buttiker formalism. B/N atoms place on different sites of ZSiNRs from edge to center. The B/N atoms have influence on spin dependent transport. Also, conductance varies with the position of B/N atoms. Our findings are comparable with recent DFT calculations. These doping effects can be used to design novel spintronic devices.

Magnetoelectric control of valley and spin in a silicene nanoribbon modulated by the magnetic superlattices

Abstract The control of valley and spin degrees of freedom and the transport properties of electrons in a zigzag silicene nanoribbon modulated by the magnetic superlattices are investigated theoretically. Due to the valley–spin locking effect in silicene, the valley degree of freedom can be controlled by magnetic means. The valley or/and spin selection induced by the exchange field result in the perfect spin–valley filter and tunneling magnetoresistance effect in the double ferromagnetic barriers on the surface of the silicene nanoribbon. It is more interesting that there are valley-resolved minigaps and minibands in the zigzag silicene nanoribbon modulated by the magnetic superlattices which give rise to the periodically modulated spin (or/and valley) polarization and tunneling magnetoresistance. The results obtained may have certain practical significance in applications for future valleytronic and spintronic devices.

Bipolar spin-filtering, rectifying and giant magnetoresistance effects in zigzag silicene nanoribbons with asymmetric edge hydrogenation

Using the nonequilibrium Green's function method and the spin-polarized density functional theory, the spin-dependent electronic transport properties of zigzag silicene nanoribbons (ZSiNRs) with asymmetric edge hydrogenation have been studied. The results show that there exists nearly 100% bipolar spin-filtering behavior in the ZSiNR-based devices with antiparallel spin configuration. Moreover, rectifying behavior and giant magnetoresistance are found in the devices. Our calculation suggests ZSiNRs with asymmetric edge hydrogenation as a promising candidate material for spintronics.

Electronic and spin transport properties in zigzag silicene nanoribbons with edge protrusions

We investigate the electronic transport properties of zigzag-edged silicene nanoribbons (ZSiNRs) with one or two protrusions at the edges using the density functional theory combined with nonequilibrium Green's function method. It is found that the protrusion generally breaks down the edge state along the same edge, which carries current in the junction. For the ZSiNR having an even number of zigzag chains in its width, the protrusions can increase the conductance except for the case of two symmetric protrusions. For ZSiNRs with an odd number of zigzag chains in its width, the introduction of edge protrusions can suppress currents. We also investigate the spin-dependent transport properties of ZSiNR-based devices with antiparallel (AP) magnetism configuration. Interestingly, only non- and symmetric-protrusion models with a width of an even number of zigzag chains show a perfect spin filter effect.

Valley-polarized insulating states in zigzag silicene nanoribbons

Valley-polarized insulating states are found numerically in zigzag-edged silicene nanoribbons (ZSiNRs) because of the strong spin-orbit couplings and the spin-polarized electronic structures. We further investigate the effects of the valley-polarization on the transport and optical properties of ZSiNRs: it splits the degeneracy of ZSiNRs’ conductance plateaus and offers valley-polarized transport channels; new optical activation modes appear for the singlet exciton states due to unequal cancellation of the contributions from different valleys.

Transport properties of bare and hydrogenated zigzag silicene nanoribbons: Negative differential resistances and perfect spin-filtering effects

Ab initio calculations are performed to investigate the spin-polarized transport properties of the bare and hydrogenated zigzag silicene nanoribbons (ZSiNRs). The results show that the ZSiNRs with symmetric (asymmetric) edges prefer the ferromagnetic (antiferromagnetic) as their ground states with the semiconductor properties, while the accordingly antiferromagnetic (ferromagnetic) states exhibit the metallic behaviors. These facts result in a giant magnetoresistance behavior between the ferromagnetic and antiferromagnetic states in the low bias-voltage regime. Moreover, in the ferromagnetic ZSiNRs with asymmetric edges, a perfect spin-filtering effect with 100% positive electric current polarization can be achieved by altering the bias voltage. In addition, we also find that the negative differential resistances prefer the metastable states. The findings here indicate that the asymmetric and symmetric ZSiNRs are promising materials for spintronic applications.

Vacancy Effects on Electric and Thermoelectric Properties of Zigzag Silicene Nanoribbons

We study the crystal reconstruction in the presence of monovacancies (MVs), divacancies (DVs) and linear vacancies (LVs) in a zigzag silicene nanoribbon (ZSiNR) with transversal symmetry. Their influence on the electric and thermoelectric properties is assessed by the density functional theory combined with the nonequilibrium Green's functions. In particular, we focus on the spin resolved conductance, magnetoresistance and current-voltage curves. A 5-atom-ring is formed in MVs, a 5-8-5 ring structure in DVs, and a 8-4-8-4 ring structure in LVs. The linear conductance becomes strongly spin dependent when the transversal symmetry is broken by vacancies especially if they are located on the ribbon's edges. The giant magnetoresistance can be smeared by asymmetric vacancies. Single spin negative differential resistance may appear in the presence of LVs and asymmetric MVs or DVs. A strong spin Seebeck effect is expected at room temperature in ZSiNRs with LVs.

Single and multiple doping effects on charge transport in zigzag silicene nanoribbons.

A non-equilibrium Green's function technique combined with density functional theory is used to study the spin-dependent electronic band structure and transport properties of zigzag silicene nanoribbons (ZSiNRs) doped with aluminum (Al) or phosphorus (P) atoms. The presence of a single Al or P atom induces quasibound states in ZSiNRs that can be observed as new dips in the electron conductance. The Al atom acts as an acceptor whereas the P atom acts as a donor if it is placed at the center of the ribbon. This behavior is reversed if the dopant is placed on the edges. Accordingly, an acceptor-donor transition is observed in ZSiNRs upon changing the dopant's position. Similar results are obtained if two silicon atoms are replaced by two impurities (Al or P atoms) but the conductance is generally modified due to the impurity-impurity interaction. If the doping breaks the twofold rotational symmetry about the central line, the transport becomes spin-dependent.

Spin effects in thermoelectric properties of Al- and P-doped zigzag silicene nanoribbons

Electric and thermoelectric properties of silicene nanoribbons doped with Al and P impurity atoms are investigated theoretically for both antiparallel and parallel orientations of the edge magnetic moments. In the former case, appropriately arranged impurities can lead to a net magnetic moment, and thus also to spin dependent transport and spin thermoelectric phenomena. In the latter case, in turn, spin thermoelectric effects also occur in the absence of impurities. To calculate spin density distribution and transport parameters we use ab initio numerical methods based on the density functional theory. The obtained results clearly show that the spin thermopower can be considerably enhanced by the impurities. This enhancement appears in certain regions of chemical potential.

Tailoring of the structural, energetic and electronic properties of silicene-based nanostructures

Silicene-based nanostructures are highly promising for future applications in electronics as silicon is an important element for conventional semiconductor industry. In our recent works, we have studied the effect of external electric filed, strain and metal adatoms on tailoring the structural, energetic and electronic properties of silicene-based nanostructures using first-principles and tight-binding methods. We find that half-metallicity can be realized in zigzag silicene nanoribbons when applying electric field across the nanoribbon width. Under small tensile strain, some armchair silicene nanoribbons are predicted to have linear energy dispersion around the Fermi level. And strain-induced structural phase transitions are observed in silicene bilayers in which strong covalent interlayer bonds form. When metal atoms are adsorbed on silicene, several metal adatoms obtain a larger binding energy than the cohesive energy of the bulk metal and the bonding between the metal atoms and silicene can be ionic or covalent.

Electronic structures of reconstructed zigzag silicene nanoribbons

Edge states and magnetism are crucial for spintronic applications of nanoribbons. Here, using first-principles calculations, we explore structural stabilities and electronic properties of zigzag silicene nanoribbons (ZSiNRs) with Klein and pentagon-heptagon reconstructions. Comparing to unreconstructed zigzag edges, deformed bare pentagon-heptagon ones are favored under H-poor conditions, while H-rich surroundings stabilize di-hydrogenated Klein edges. These Klein edges have analogous magnetism to zigzag ones, which also possess the electric-field-induced half-metallicity of nanoribbons. Moreover, diverse magnetic states can be achieved by asymmetric Klein and zigzag edges into ZSiNRs, which could be transformed from antiferromagnetic-semiconductors to bipolar spin-gapless-semiconductors and ferromagnetic-metals depending on edge hydrogenations.

Nitrogen and Boron substitutional doped zigzag silicene nanoribbons: Ab initio investigation

We performed a spin polarized density-function theory study of the stabilities, electronic and magnetic properties of zigzag silicene nanoribbons (ZSiNRs) substitutionally doped with a single N or B atom located at various sites ranging from edge to center of the ribbon. From minimization of the formation energy, it is found that the substitutional doping is favorable at edge of the ribbon. A single N or B atom substitution one edge Si atom of ZSiNRs can greatly suppress the spin-polarizations of the impurity atom site and its vicinity region, and leads to a transition from antiferromagnetic (AFM) state to ferromagnetic (FM) state, which is attributed to the splitting of the original spin degenerate edge bands. A single N atom doped ZSiNRs still keep semiconductor property but a single B atom doped ZSiNRs exhibit a half-metallic character. Our results reveal that substitution doped ZSiNRs have potential applications in Si-based nanoelectronics, such as field effect transisitors (FETs), negative differential resistance (NDR) and spin filter (SF) devices.

Half-metallicity in aluminum-doped zigzag silicene nanoribbons

The spin-dependent electronic structures of aluminum-(Al) doped zigzag silicene nanoribbons (ZSiNRs) are investigated by first-principles calculations. When ZSiNRs are substitutionally doped by a single Al atom on different sites in every three primitive cells, they become half-metallic in some cases, a property that can be used in spintronic devices. More interestingly, spin-down electrons can be transported at the Fermi energy when the Al atom is placed on the sub-edge site. In contrast, spin-up electrons can be transported at the Fermi energy when the ZSiNRs are doped on sites near their centre. The magnetic moment on the edge is considerably suppressed if the Al atom is doped on edge or near-edge sites. Similar results are obtained for a phosphorus-(P) and boron-(B) doped ZSiNR. When two or more Si atoms are replaced by Al atoms, in general the half-metallic behaviour is replaced by a metallic, spin gapless semiconducting or semiconducting one. When a line of six Si atoms, along the ribbon's width, are replaced by Al atoms, the spin resolution of the band structure is suppressed and the system becomes nonmagnetic.