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Here, uniform thin nanosheets’ film of mixed α- and β-Ni(OH)2 phases was cathodically electrodeposited onto nickel foam support and the fabricated electrode was utilized as binder-free electrode for supercapacitor applications. An additive-free aqueous electrolyte of 0.05 M nickel nitrate was used as the deposition bath, where the hydroxide film was prepared with only applying direct current of 10 mA/cm2 in a two-electrode system while a uniform electrical field was established through using two counter electrodes parallel to nickel foam cathode. The morphology, crystalline phase and also specific surface area of the deposited nickel hydroxide film onto Ni foam were analyzed using field-emission scanning electron (FE-SEM) and X-ray diffraction (XRD) techniques as well as BET and FT-IR spectroscopy. These analyses proved that a mixed alpha and beta phases of Ni(OH)2 with uniform nanosheets’ morphology are grown onto the nickel foam cathode. The electrochemical investigations using CV, GCD and electrochemical impedance spectroscopy (EIS) manifested the designed electrode provides specific capacitance of 2143 F g−1 at a current density of 0.5 A g−1 with high rate capability of 65% by increasing the current density from 0.5 to 20 A g−1 as well as 90% and 74% cycling stability at the current densities of 5 and 15 A/g, respectively. The excellent electrochemical performance of the as-prepared electrode is ascribed to the simultaneous deposition of both α and β phases through using a facile electrochemical deposition method. The presence of α and β phases guarantees the high specific capacitance and high cycling stability of the prepared electrode, respectively.

Here, we report a simple and green electrochemical route to fabricate a porous network of a Fe3O4 nanoparticle-porous reduced graphene oxide (p-rGO) nanocomposite supported on a nickel-foam substrate, which is directly used as a binder-free charge storage electrode. Through this method, pristine Fe3O4 NPs/Ni, p-rGO/Ni and Fe3O4 NPs@p-rGO/Ni electrodes are fabricated and compared. In the fabricated Fe3O4 NPs@p-rGO/Ni electrode, the porous rGO sheets served as a conductive network to facilitate the collection and transportation of electrons during the charge/discharge cycles, improving the conductivity of magnetite NPs and providing a larger specific surface area. As a result, the Fe3O4 NPs@p-rGO/Ni exhibited a specific capacitance of 1323 F g−1 at 0.5 A g−1 and 79% capacitance retention when the current density is increased 20 times, where the Fe3O4 NPs/Ni electrode showed low specific capacitance of 357 F g−1 and 43% capacity retention. Furthermore, the composite electrode kept 95.1% and 86.7% of its initial capacitances at the current densities of 1 and 4 A g−1, respectively, which were higher than those of a Fe3O4/NF electrode at similar loads (i.e. 80.4% and 65.9% capacitance retentions at 1 and 4 A g−1, respectively). These beneficial effects proved the synergistic contribution between p-rGO and Fe3O4. Hence, such ultrafine magnetite particles grown onto a porous reduced GO network directly imprinted onto a Ni substrate could be a promising candidate for high performance energy storage aims.

The spin-dependent electronic transport and thermoelectric properties for a sawtoothlike graphene nanoribbon between two ferromagnetic leads with parallel or antiparallel magnetic moments are investigated theoretically. Both Christmas-tree graphene nanoribbon and tree-saw graphene nanoribbon have been considered to exemplify the effect of the chirality. By using ab initio calculations combined with a non-equilibrium Green’s function method, we obtain the important result that these sawtoothlike graphene nanoribbon junctions, regardless of their chirality, can exhibit giant charge and spin Seebeck coefficients as well as high thermoelectric efficiency at room temperature by putting ferromagnetic stripes or magnetic fields on the nanoribbons. Specifically, large $$|S_\mathrm{C}|\,(\sim 5.8~\hbox {mV/K})$$, $$|S_\mathrm{S}|(\sim 1.8~\hbox {mV/K})$$, charge figure of merit $$ZT_\mathrm{C}$$ and spin figure of merit $$ZT_\mathrm{S}(\sim 20)$$ were found in these sawtoothlike graphene nanoribbon junctions. In addition, negative differential resistance and spin filtering were also realized in these sawtoothlike graphene nanoribbon junctions. The maximum of spin filtering efficiency can be up to 100%. Our studies suggest that sawtoothlike graphene nanoribbon junctions are suitable for use as highly efficient spin caloritronics at room temperature.

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The study on the electronic especially optical properties of zigzag-edged phosphorene nanoribbons (ZPNRs) in an antiferromagnetic state has been rarely involved. Here we theoretically study the electronic and optical properties of a ZPNR with N chains ( N -ZPNR) modulated by a vertical electric field (VEF). Within the Hubbard model Hamiltonian combined with the self-consistent calculation, and by adopting the Green’s function method, we obtain the band structure and corresponding conductance for the system. The corresponding optical absorption spectrum is further obtained based on the Kubo formula. It is interesting to find that there is an even–odd effect on the electronic and optical properties of N -ZPNR induced by the VEF, which may fundamentally originate from the parity of the wavefunctions. The VEF lifts the degeneracy of the two edge bands for 12-ZPNR, but it only bends and shifts the two edge bands without lifting the spin degeneracy for 13-ZPNR. We further present the spatial distribution of the corresponding wavefunction to understand this even–odd effect more intuitively. As expected, the VEF can separate the spin-up and -down optical absorption for 12-ZPNR, but it does not break the spin degeneracy of the optical absorption for 13-ZPNR. Our results may provide a further understanding on the electronic and optical properties of N -ZPNR with an antiferromagnetic state, and may be useful to identify the even–odd property of ZPNR samples by detecting the optical absorption in experiment.

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.

The extended line defect of graphene is an extraordinary candidate in valleytronics while the high valley polarization can only occur for electrons with high incidence angles which brings about tremendous challenges to experimental realization. In this paper, we propose a novel quantum mechanism to filter one conical valley state in the line defect of graphene by applying a local magnetic field. It is found that due to the movement of the Dirac points, the transmission profiles of the two valleys are shifted along the injection-angle axis at the same pace, resulting in the peak transmission of one valley state being reduced drastically while remaining unaffected for the other valley state, which induces nearly perfect valley polarization. The valley polarization effect can occur for all the incident angle and plays a key role in graphene valleytronics.

Here, we report a simple and green electrochemical route to fabricate a porous network of a Fe

We study the variation of electronic properties for armchair-edge phosphorene nanoribbons (APNRs) modulated by a transverse electric field. Within the tight-binding model Hamiltonian, and by solving the differential Schrodinger equation, we find that a band gap closure appears at the critical field due to the giant Stark effect for an APNR. The gap closure has no field polarity, and the gap varies quadratically for small fields but becomes linear for larger ones. We attribute the giant Stark effect to the broken edge degeneracy, i.e., the charge redistributions of the conduction band minimum and valence band maximum states localized at opposite edges induced by the field. By combined with the Green's function approach, it is shown that in the presence of the critical field a gap of density of states (DOS) disappears and a high value DOS turns up at the energy position of the band gap closure. Finally, as the field increases, we find the band gap decreases more rapidly and the gap closure occurs at smaller fields for wider ribbons. Both the band gap and DOS variations with the field show an insulator-metal transition induced by a transverse electric field for the APNR. Our results show that wider APNRs are more appreciable to design field-effect transistors.

By using first-principle calculations combining with nonequilibrium Green's function method, we have systematically investigated the electron transport and thermoelectric properties for an armchair-edge graphene nanoribbon (AGNR)/graphyne nanoribbon (GYNR) heterojunction with different edges and types of GYNRs. The results show that the phonon transport is dramatically suppressed by a few times of magnitude due to the phonon scattering by the AGNR–GYNR interface. Moreover, it is found that the enhancement of ZT can be effectively tuned by edges and types of GYNR, which offers an effective way to improve the thermoelectric performance of heterojunctions by choosing optimal edges and types of GYNR.

We study theoretically the spin-dependent transport and the current-induced spin transfer torque (STT) for a zigzag silicene nanoribbon (ZSiNR) with Anderson-type disorders between two ferromagnetic electrodes. By using the nonequilibrium Green's function method, it is predicted that the transport property and STT through the junction depend sensitively on the disorder, especially around the Dirac point. As a result, the conductance decreases and increases for two electrode in parallel and antiparallel configurations, respectively. Due to the disorder, the magnetoresistance (MR) decreases accordingly even within the energy regime for the perfect plateau without disorders. In addition, the conductance versus the relative angle of the magnetization shows a cosine-like behavior. The STT per unit of the bias voltage versus the angle of the magnetization exhibits a sine-like behavior, and versus the Fermi energy is antisymmetrical to the Dirac point and exhibits sharp peaks. Furthermore, the peaks of the STT are suppressed much as the disorder strength increases, especially around the Dirac point. The results obtained here may provide a valuable suggestion to experimentally design spin valve devices based on ZSiNR.

We theoretically investigate the valley polarization in silicene with two parallel line defects due to Rashba spin-orbit coupling (RSOC). It is found that as long as RSOC exceeds the intrinsic spin-orbit coupling (SOC), the transmission coefficients of the two valleys oscillate with the same periodicity and intensity, which consists of wide transmission peaks and zero-transmission plateaus. However, in the presence of a perpendicular electric field, the oscillation periodicity of the first valley increases, whereas that of the second valley shortens, generating the corresponding wide peak-zero plateau regions, where perfect valley polarization can be achieved. Moreover, the valley polarizability can be changed from 1 to −1 by controlling the strength of the electric field. Our findings establish a different route for generating valley-polarized current by purely electrical means and open the door for interesting applications of semiconductor valleytronics.

The α-T 3 lattice, a minimal model that presents flat bands, has sparked much interest in research but the finite-size effect and interaction has been rarely involved. Here we theoretically study the electronic structure and transport properties of zigzag-edge α-T 3 nanoribbons (ZαT 3NRs) with and without uniaxial strain, where the exemplary widths N = 40 and 41 for two series are considered. By adopting the mean-field Hubbard model combined with the nonequilibrium Green’s function method, we show that the spin-degenerate dispersionless flat band at the Fermi energy for the pristine ribbons is split into spin-up and -down flat bands under electron–electron Coulomb interaction. Specifically, the two bands are shifted toward in an opposite direction and away from the Fermi energy, which leads to an energy gap opening in the case of α ≠ 1. All three series of ZαT 3NRs with width N = 3n, 3n + 1, 3n + 2 (where n is a positive integer) exhibit an energy gap. This differs from the simple tight-binding calculations without considering electron–electron Coulomb interaction, for which the gap is always zero in the case of N = 3n + 1. Here, the origin of the energy gap for N = 3n + 1 arises from Coulomb repulsion between electrons. Importantly, the energy gap can be effectively manipulated by an uniaxial strain and Coulomb interaction if α ≠ 1. The gap linearly increases (decreases) when a tensile (compressive) strain increases, and it also monotonously increases as enhancing Coulomb interaction. Interestingly, a ground state of antiferromagnetic to ferromagnetic transition occurs when α increases from 0.8 to 1, leading to a semiconductor to metallic transition. Besides, the α-, strain- and interaction-dependent conductance is also explored. The findings here may be of importance in the band gap engineering and electromechanical applications of α-T 3 nanoribbon-based devices.

We design the piezoelectric photonic crystal (PPC) with periodically reversed ferroelectric domains on the base of the structure of the piezoelectric superlattice, and study theoretically the transmission properties of electromagnetic (EM) waves in the PPC through Transfer Matrix Method of the photonic crystal (PhC). We analyze the mechanism for the influence of the piezoelectric effect on the transmission properties of EM waves in the PPC. The piezoelectric effect can influence the transmission properties of EM waves by changing the dielectric properties of the PPC. And we discuss the difference between the PPC and PhC. In the low frequency region, the reflection spectrum of PPC appear the total reflection and zero refractive indexes, and which doesn't exist in the PhC. In the high frequency region, the frequency positions and widths of the photonic band gaps of the PPC are higher and wider than cases of the PhC, respectively. Some potential applications are also discussed.

We study the electronic properties of multilayer armchair phosphorene nanoribbons (APNRs) modulated by certain strains. The band structure is calculated by solving the corresponding differential Schrodinger equation, and the density of states (DOS) is obtained combining the Green's function method. It is found that the certain strain has similar effects on different layer APNRs. The vertical tensile strain along the z-direction, the compressive uniaxial strain along the y-direction and biaxial strain along the xy-direction can close the band gap. The gap disappears and some peaks appear around at the energy position of the band gap closure for the corresponding DOS. Further, it is shown that the band gaps depend linearly on the strain for all APNRs. However, the band structure and DOS for four layer APNR are most sensitive to the strains among all APNRs. Both the band gap and DOS variations with the strain show an insulator-metal transition induced by the strain for APNRs. Our results show that the four layer APNR are most suitable for the usage of electromechanical field effect transistors or detectors for detecting the strains.

we investigate the electronic and thermoelectric transport properties for a graphene–silicene–graphene (GSG) heterojunction with zigzag-edge nanoribbons under the modulation of the effective spin–orbit coupling (SOC) and potential energy. Using the nonequilibrium Green's function method, it is demonstrated that both the transmission coefficient T and the charge Seebeck coefficient S C display the oscillatory behavior and can be effectively modulated by effective SOC λ SO and the potential energy V 0 . Furthermore, the even–odd difference in transport and thermoelectric properties disappears in the GSG heterojunction. Additionally, the dependence of the transmission coefficient and the charge Seebeck coefficient on Anderson disorder strength has been studied.

The electronic and transport properties for armchair-edge silicene nanoribbons (ASiNR) with effective spin–orbit coupling and potential energy were investigated by using the non-equilibrium Green's function method. The energy gaps and the conductance for ASiNRs can be effectively modulated by effective spin–orbit coupling λ SO and the potential energy V 0 . With increasing λ SO , the energy gap for 6-ASiNR and 7-ASiNR decreases, while it remains zero for metallic 8-ASiNR. Interestingly, an energy gap appears for 8-ASiNR in presence of V 0 , which results in the appearance of a conductance gap for 8-ASiNR system. Additionally, the dependence of conductance on Anderson disorder strength has been studied.

We study the spin-dependent transport and thermoelectric properties of ferromagnetic zigzag-edge phosphorene nanoribbon with N chains (N-ZPNR) modulated by a perpendicular electric field (PEF). By adopting the Hubbard model Hamiltonian combined with the self-consistent calculation, and using the nonequilibrium Green's function method, we obtain the band structure, spin-dependent transmission coefficients and Seebeck coefficients. It is demonstrated that the bands are split with spin-up and -down ones for both 12- and 13-ZPNR in the ferromagnetic state, and both ZPNRs show spin-semiconducting behavior with the spin gap of value 1 eV. Interestingly, there is an even-odd effect on the spin-dependent transport and thermoelectric properties when a PEF is applied, which originates from the structure symmetry of N-ZPNR and can result in different behavior of edge states for even and odd cases. The electric field lifts the degeneracy of the two edge bands for both spin-up and -down channels in 12-ZPNR, however, it only bends the spin-up edge bands and shifts the spin-down ones upward without splitting in 13-ZPNR. In addition, we find that the spin Seebeck coefficients exhibit wide plateaus and their maximum values can reach unprecedented giant 5 mV K-1 for both ZPNRs. The spin Seebeck coefficients for both ZPNRs decrease gradually as temperature increasing. As the electric field increasing, the value of the plateau in the spin Seebeck coefficient for 12-ZPNR decreases gradually, however, the plateau almost has no variation for 13-ZPNR in this case. This different responses of spin Seebeck coefficients to the PEF between even and odd cases originate from the different edge state responses to the electric field, and we provide the explanation from the corresponding spin-up and spin-down Seebeck coefficients. Our results reveal that ferromagnetic ZPNRs are perfectly spin-polarized, and are promising candidates for spin caloritronics with high efficiency.

We study the electronic and transport properties of monolayer/bilayer/bilayer (ML/BL/BL) and monolayer/bilayer/monolayer (ML/BL/ML) zigzag-edge phosphorene nanoribbon (ZPNR) junctions modulated by a perpendicular electric field (PEF). Within the tight-binding model Hamiltonian and by adopting the nonequilibrium Green's function, it is demonstrated that some oscillating conductance plateaus appear for the junctions, and the conductances are suppressed by the applied PEF. Interestingly, the direction of the PEF has different influence on the conductance of both junctions. We further present separately the band structures of the left lead, the central region and right lead, to reveal the reason for different conductance behavior in detail. Our results show that the ML/BL/BL ZPNR junction may be more suitable for the usage of field effect transistor than the ML/BL/ML ZPNR one under a PEF. Moreover, a PEF can be applied to distinguish the bottom−bottom and bottom−up configurations for the ML/BL/ML ZPNR junction.

Using the nonequilibrium Green’s function method combined with the tight-binding Hamiltonian, we theoretically investigate the spin-dependent transmission probability and spin Seebeck coefficient of a crossed armchair-edge graphene nanoribbon (AGNR) superlattice p-n junction under a perpendicular magnetic field with a ferromagnetic insulator, where junction widths W1 of 40 and 41 are considered to exemplify the effect of semiconducting and metallic AGNRs, respectively. A pristine AGNR system is metallic when the transverse layer m = 3j + 2 with a positive integer j and an insulator otherwise. When stubs are present, a semiconducting AGNR junction with width W1 = 40 always shows metallic behavior regardless of the potential drop magnitude, magnetization strength, stub length, and perpendicular magnetic field strength. However, metallic or semiconducting behavior can be obtained from a metallic AGNR junction with W1 = 41 by adjusting these physical parameters. Furthermore, a metal-to-semiconductor transition can be obtained for both superlattice p-n junctions by adjusting the number of periods of the superlattice. In addition, the spin-dependent Seebeck coefficient and spin Seebeck coefficient of the two systems are of the same order of magnitude owing to the appearance of a transmission gap, and the maximum absolute value of the spin Seebeck coefficient reaches 370 µV/K when the optimized parameters are used. The calculated results offer new possibilities for designing electronic or heat-spintronic nanodevices based on the graphene superlattice p-n junction.

We theoretically calculate the spin-dependent transmission probability and spin Seebeck coefficient for a zigzag-edge graphene nanoribbon p-n junction with periodically attached stubs under a perpendicular magnetic field and a ferromagnetic insulator. By using the nonequilibrium Green's function method combining with the tight-binding Hamiltonian, it is demonstrated that the spin-dependent transmission probability and spin Seebeck coefficient for two types of superlattices can be modulated by the potential drop, the magnetization strength, the number of periods of the superlattice, the strength of the perpendicular magnetic field, and the Anderson disorder strength. Interestingly, a metal to semiconductor transition occurs as the number of the superlattice for a crossed superlattice p-n junction increases, and its spin Seebeck coefficient is much larger than that for the T-shaped one around the zero Fermi energy. Furthermore, the spin Seebeck coefficient for crossed systems can be much pronounced and their maximum absolute value can reach 528 μV [Formula: see text] by choosing optimized parameters. Besides, the spin Seebeck coefficient for crossed p-n junction is strongly enhanced around the zero Fermi energy for a weak magnetic field. Our results provide theoretical references for modulating the thermoelectric properties of a graphene superlattice p-n junction by tuning its geometric structure and physical parameters.

Prompted by recent reports on the electronic band structure of graphene-hexagonal indium chalcogenide bilayer, the electronic transport and thermoelectric properties for a superlattice junction based-graphene-In2Te2 nanoribbon bilayer are investigated theoretically. By using ab initio calculations combining with nonequilibrium Green's function method, our important result here is that these superlattice junctions, regardless of their chirality and width, can exhibit a metallic behavior, as well as large current with linear increase under low bias. This phenomenon of current is particularly significant for both 7-AGNRBL and 8-AGNRBL systems. Interestingly, the negative differential resistance can be achieved in 8-ZGNRBL junction. Furthermore, the conductance, electronic contribution to the thermal conductance and Seebeck coefficient are sensitive to the chirality and width of AGNRBL as well as the temperature. When the temperature rises, the electronic contribution to the thermal conductance rapidly increases, while the Seebeck coefficient sharply decreases.

Exploration a valid way to optimize the thermoelectric performance is one of mainstream in present thermoelectric research community. In this paper, taken γ-graphyne as a potential carbon-based thermoelectric material as an example, we investigate the influence of surface fluctuation disorder on the thermoelectric transport properties by means of a nonequilibrium Green's function method. The results show that the surface fluctuation disorder caused by the substrate roughness could dramatically improve the thermoelectric energy conversion efficiency of γ-graphyne nanoribbons, e.g., the thermoelectric figure of merit could approach 3.0 at room temperature (the length and width are about 55.41 nm and 1.45 nm). Such obvious enhancement mainly stems from the significant suppression of both phononic and electronic thermal conductance, but survived high Seebeck coefficient of pristine nanoribbon. The parameters of surface fluctuation disorder, i.e., fluctuation height and correlation length, on the thermoelectric properties of γ-graphyne nanoribbons are explored as well in this study. The findings presented in this work prove that the surface fluctuation disorder is indeed an efficient approach to boost the thermoelectric properties of γ-graphyne, and provide a guideline to design and fabricate new carbon-based thermoelectric devices.

Using the fully self-consistent non-equilibrium Greenʼs function (NEGF) method combined with density functional theory, we investigate numerically the electronic transport property for pristine and doped crossed graphene nanoribbon (GNR) junctions. It is demonstrated that in the case of zigzag interfaces, the I – V characteristics of the junction with or without doping always show semiconducting behavior, which is different from that in the case of armchair interfaces [Zhou, Liao, Zhou, Chen, Zhou, Eur. Phys. J. B 76 (2010) 421]. Interestingly, negative differential resistance (NDR) behavior can be clearly observed in a certain bias region for nitrogen-doped shoulder crossed junction. A mechanism for the NDR behavior is suggested.

We present ab initio calculations for spin-dependent electron transport in a molecular device constructed by two carbon chains capped with a phenyl ring, which is sandwiched between two zig-zag-edged graphene nanoribbon (ZGNR) electrodes, where the ZGNRs are modulated by external magnetic field. The coexistence of switching, dual spin-filtering effects, and negative differential resistance (NDR) in the model device is demonstrated with the theory of carbon π-electrons. Interestingly, a two-state molecular conformational switch can be realized by changing the orientation between the planes of phenyl ring and electrodes, where the majority-spin current modulation (ON/OFF ratio) is 170–479 within the considered bias window. Moreover, the device shows perfect dual spin-filtering effect and can generate and control a full dual spin-polarized current through either the source-drain voltage or magnetic configuration of the electrodes. The selective spin current is due to a dual selection rule, the symmetry match between two ZGNR electrodes spin channel, and the carbon chain’s spin selection in our system. In addition, the obvious NDR behavior has also been observed in our model.

The magnetotransport property for a monolayer graphene with two turnable magnetic barriers has been investigated by the transfer-matrix method. We show that the parameters of barrier height, width, and interval between two barriers affect the electron wave decaying length, which determine the conductance with parallel or antiparallel magnetization configuration, and consequently the tunneling magnetoresistance (TMR) for the system. Interestingly, a graphene attached by two barriers with different heights can produce a resonant TMR peak at low energy region one order of magnitude larger than that for the system with two same height barriers because that the asymmetry of magnetic barriers block the electron transmission in the case of antiparallel magnetization configuration. The results obtained here may be useful in understanding of electron tunneling in graphene and in designing of graphene-based nanodevices. (C) 2011 Elsevier B.V. All rights reserved.

The electronic transport property for a crossed junction of graphene nanoribbons with and without impurity doping is investigated numerically by a fully self-consistent non-equilibrium Green's function method combined with density functional theory. It is demonstrated that the transport property of the junction depends sensitively on both the dopant positions and the geometry of junction. Specifically, the I-V characteristics of the junction with either nitrogen- or boron-doped stems always show metallic behavior. However, the current strongly depends on the doping atomic species and sites, but slightly depends on the geometry of junction under small bias voltage. The findings here may be important in the design of graphene-based electronic devices for realizing on/off states.

The spin/valley polarization in silicene with extended line defects is investigated according to the chiral filtration mechanism. It is shown that the inner-built quantum Hall pseudo-edge states with identical chirality can serve as a chiral filter with a weak magnetic field and that the transmission process is restrained/strengthened for chiral states with reversed/identical chirality. With two parallel line defects, which act as natural chiral filtration, the filter effect is greatly enhanced, and 100% spin/valley polarization can be achieved.

We theoretically investigate the electronic transport properties through a rectangular potential barrier embedded in armchair-edge graphene nanoribbons (AGNRs) of various widths. Using the Landauer formula and Dirac equation with the continuity conditions for all segments of wave functions at the interfaces between regions inside and outside the barrier, we calculate analytically the conductance and Fano factor for the both metallic and semiconducting AGNRs, respectively. It is shown that, by some numerical examples, at Dirac point the both types of AGNRs own a minimum conductance associated with the maximum Fano factor. The results are discussed and compared with the previous relevant works.

We theoretically investigate the figure of merit ZT for a quantum wire side-coupled by a graphene sheet and sandwiched between two ferromagnetic electrodes with noncollinear magnetic moments. By using the nonequilibrium Green’s function combining with the tight-binding Hamiltonian, we demonstrate that the ZT for the system develops an oscillating behavior and weakly depends on the wire-graphene coupling strength as well as magnetic configuration of the leads. On the contrary, it is strongly dependent on temperature and the polarization strength of the leads. Importantly, the maximum value of ZT for the system without the polarization strength (p = 0) is about 1.1 at temperature k B T = 0.015Γ 0, which is in agreement with the experimental measurements for silicon nanowires.

We study the variation of electronic property for zigzag-edge phosphorene nanoribbons (ZPNRs) under a perpendicular electric field (PEF). Using the tight-binding Hamiltonian combined with the surface lattice Green's function (GF) approach, we show that the response of edge states to PEF for a N-ZPNR with even- or odd-N (number of zigzag chains) is qualitatively different. The field opens a gap between two edge bands near the Fermi energy for even-N ribbons, but for odd-N ones where the two edge bands are always nearly degenerated. This difference is originally from that the Stark-effect-induced energies at the upper and lower edges for even- and odd-N ZPNRs are different due to the peculiar lattice structure of phosphorene. In consequence, the electronic densities are more localized at the edges driven by the field for even-N ZPNRs but not for odd-N ones. This even–odd effect is also reflected in conductance, which indicates that the odd-N ZPNRs may be more suitable for the usage of field-effect transistor.

We investigate the tunneling transport for electrons on the surface of a topological insulator (TI) through an electrostatic potential barrier. By using the Dirac equation with the continuity conditions for all segments of wave functions at the interfaces between regions inside and outside the barrier, we calculate analytically the transmission probability and conductance for the system. It is demonstrated that, the Klein paradox can also been observed in the system same as in graphene system. Interestingly, the conductance reaches the minimum value when the incident electron energy is equal to the barrier strength. Moreover, with increasing barrier width, the conductance turns up some tunneling oscillation peaks, and larger barrier strength can cause lower conductance, shorter period but larger oscillation amplitude. The oscillation amplitude decreases as the barrier width increases, which is similar as that of the system consisting of the compressive uniaxial strain applied on a TI, but somewhat different from that of graphene system where the oscillation amplitude is a constant. The findings here imply that an electrostatic barrier can greatly influence the electron tunneling transport of the system, and may provide a new way to realize directional filtering of electrons.

A Z-shaped junction constructed by a few-nanometer-long armchair-edged boron nitride nanoribbon (ABNNR) sandwiched between two semi-infinite zigzag-edged BNNR electrodes with different hydrogen-passivated edge treatment is proposed, and its spin-dependent electronic transport is studied by ab initio calculations. It is found that a short ABNNR exhibits metallic behavior and can be used as a conduction channel. Interestingly, the spin-filtering and rectification effects exist in the junctions without any edge passivation or with boron-edge passivation. The analysis on the projected density of states and spatial distribution of molecular projected self-consistent Hamiltonian eigenstates gives an insight into the observed results for the system. Our results suggest that a BNNR-based nanodevices with spin-filtering and rectification effects may be synthesized from an hexagonal boron nitride sheet by properly tailoring and edge passivation.

The ballistic thermoelectric properties of γ-graphyne nanoribbons incorporating diamond-like quantum dots are theoretically investigated by means of a nonequilibrium Green's function. The calculation shows that as the diamond-like quantum dot is introduced, the thermoelectric conversion efficiency can be enhanced dramatically. Such enhancement mainly stems from the significant suppression of phononic and electronic thermal conductance and the reserved high Seebeck coefficient. Meanwhile, variation of the geometric size of the quantum dot will result in an obvious oscillatory behavior of thermoelectric performance, which is attributed to the resonant electronic conductance caused by the quasi-bound state in diamond-like quantum dots. When the nanoribbon is embedded with multi-quantum dots, we find that the thermoelectric efficiency can be improved further (the stable figure of merit can achieve 1.55 even at room temperature). These findings demonstrate the incorporation of diamond-like quantum dots is a viable approach to optimize the thermoelectric performance of γ-graphyne, and could provide helpful guidance for designing and fabricating nanoscale thermoelectric devices.

We study the transport property for armchair-edge graphene nanoribbons (AGNRs) with an adatom coupling to a semi-infinite quantum wire. Using the nonequilibrium Green's function approach with tight-binding approximation, we demonstrate that the tunneling current through the system is sensitively dependent on both the AGNR width and adatom location. Interestingly, when the adatom locates onto a carbon atom in the 3$j$th chain from the edge of a metallic AGNR, the system shows a transmission gap accompanied by a threshold voltage in $I-V$ curve like a semiconducting AGNR. This effect may be useful in scanning tunneling microscopy experimental characterization on graphene samples., Comment: 3 pages, 4 figures

We investigate the spin-dependent transport and spin thermopower for a zigzag silicene nanoribbon (ZSiNR) with two ends covered by ferromagnets (FMs) under the modulation of a perpendicular electric field, where we take the 6- and 7-ZSiNR to exemplify the effect of the even- and odd-N ZSiNRs, respectively. By using the nonequilibrium Green's function approach, it is demonstrated that a ZSiNR-based FM/normal/FM junction still shows an interesting symmetry-dependent property although the σ mirror plane is absent for any ZSiNR due to the buckled structure of silicene. The junction with even- or odd-N ZSiNR has very different transport and thermopower behavior, which is attributed to the different parity of π and [Formula: see text] band wavefunctions under the c 2 symmetry operation with respect to the centre axis between two edges, and is linked to the unique symmetry of the band structure for the ribbon. As a result, the magnetoresistance (MR) for the 6-ZSiNR junction with a 100% plateau around zero electron energy is observed, but the plateau is absent for the 7-ZSiNR one. In addition, the spin thermopower also displays the even-odd behaviour. The 6-ZSiNR junction is found to possess superior thermospin performance compared with the 7-ZSiNR one, and its spin thermopower can be improved by one order of magnitude in the absence of an electric field. As the strength of the field increases, the spin thermopower for the 6-ZSiNR junction dramatically decreases, while it notably enhances for the 7-ZSiNR one. Interestingly, the spin thermopower for both junctions is strongly dependent on the strength of magnetisation in FM, and it can be very pronounced with a maximum absolute value of 200 μV K(-1)by the optimisation of the parameters. However, with the increase in temperature, the spin thermopower for the 6-ZSiNR junction decreases, but the situation for the 7-ZSiNR one is opposite. Finally, the spin figure of merit for the 6-ZSiNR junction is found to be four orders of magnitude larger than that for the 7-ZSiNR one. This even-odd effect is common for N-SiNR, and the result can be regarded as an advance in the understanding of the characteristics of silicene and may be valuable for experimentally designing spin valve and heat spintronic devices based on silicene.

We theoretically investigate spin-dependent Seebeck effects for a system consisting of a narrow graphene nanoribbon (GNR) contacted to square lattice ferromagnetic (FM) electrodes with noncollinear magnetic moments. Both zigzag-edge graphene nanoribbons (ZGNRs) and armchair-edge graphene nanoribbons (AGNRs) were considered. Compared with our previous work with two-dimensional honeycomb-lattice FM leads, a more realistic model of two-dimensional square-lattice FM electrodes is adopted here. Using the nonequilibrium Green's function method combining with the tight-binding Hamiltonian, it is demonstrated that both the charge Seebeck coefficient SC and the spin-dependent Seebeck coefficient SS strongly depend on the geometrical contact between the GNR and the leads. In our previous work, SC for a semiconducting 15-AGNR system near the Dirac point is two orders of magnitude larger than that of a metallic 17-AGNR system. However, SC is the same order of magnitude for both metallic 17-AGNR and semiconducting 15-...

We investigate the energy band structure and the spin-dependent transport for a normal/ferromagnetic/normal two-dimension topological insulator (TI) junction. By diagonalizing Hamiltonian for the system, the band structure shows that the edge states on two sides are coupled resulting in a gap opening due to the transverse spatial confinement. Further, the exchange field induced by magnetic impurities can also modulate the band structure with two spin degenerate bands splitting. By using the nonequilibrium Green's function method, the dependence of spin-dependent conductance and spin-polarization on the Fermi energy, the exchange field strength and the ferromagnetic TI (FTI) length are also analyzed, respectively. Interestingly, the degenerate conductance plateaus for spin-up and -down channels are broken, and both the conductances are suppressed by magnetic impurities due to the time-reversal symmetry broken and inelastic scattering. The spin-dependent conductance shows different behaviors when the Fermi energy is tuned into different ranges. Moreover, the conductance can be fully spin polarized by tuning the Fermi energy and the exchange field strength, or by tuning the Fermi energy and the FTI length. Consequently, the junction can transform from a quantum spin Hall state to a quantum anomalous Hall state, which is very important to enable dissipationless charge current for designing perfect spin filter.

We theoretically investigate the thermoelectric transport through a quantum wire (QW) side-coupled by a graphene sheet and sandwiched between two ferromagnetic electrodes with noncollinear magnetic moments. By using the nonequilibrium Green's function combining with the tight-binding Hamiltonian, it is demonstrated that both the thermopower and the electronic contribution to the thermal conductance develop an oscillating behavior with resonances and antiresonances due to constructive and destructive interferences in the system, respectively. Interestingly, the thermopower changes its sign for even- or odd-number of atoms in the wire, and the thermal conductance is always positive with an even-odd behavior at zero energy level position of the quantum wire. Moreover, the thermopower and the thermal conductance are weakly dependent on the wire–graphene coupling strength as well as the relative magnetic configurations of leads. On the contrary, they are both strongly dependent on the temperature and the polarization strength of the leads. The results obtained here may provide a valuable theoretical guidance to experimentally design heat spintronic devices.

We theoretically investigate the thermoelectric property of a graphene sheet coupled to two ferromagnetic electrodes with noncollinear magnetic moments. By using the nonequilibrium Green's function combining with the tight-binding Hamiltonian, it is demonstrated that the thermopower weakly depends on the polarization strength as well as on magnetic configuration of the leads. On the contrary, the electronic contribution to the thermal conductance is sensitive to both the polarization strength and the relative alignment of magnetization directions. Furthermore, the thermopower shows a linear dependence on temperature and an inverse dependence on the Fermi energy in the graphene, which agrees well the recent experimental measurements.