Zigzag-edged Nanoribbons Research Articles

Enhanced magnetism derived from pore-edge spins in thin Fe3GeTe2 nanomeshes

The growth of two-dimensional van der Waals magnetic materials presents attractive opportunities for exploring new physical phenomena and valuable applications. Among these materials, Fe3GeTe2(FGT) exhibits a variety of remarkable properties and has garnered significant attention. Herein, we have for the first time created a nanomesh structure-a honeycomb-like array of hexagonal nanopores-with the zigzag pore-edge atomic structure on thin FGT flakes with and without oxidation of the pore edges. It is revealed that the magnitude of ferromagnetism (FM) significantly increases in both samples compared with bulk flakes without nanomeshes. Critical temperature annealing results in the formation of zigzag pore edges and interpore zigzag-edge nanoribbons. We unveil that the non-oxide (O) termination of the Fe dangling bonds on these zigzag edges enhances FM behavior, while O-termination suppresses this FM by introducing antiferromagnetic behavior through edge O-Fe coupling. FGT nanomeshes hold promise for the creation of strong FM and their effective application in magnetic and spintronic systems.

Andreev reflection in graphene nanoribbons induced by d-wave superconductors

Honeycomb and square lattices are combined as a tight-binding model to examine the Andreev reflection in graphene nanoribbons induced by a superconductor. The superconducting symmetry is assumed to be the d-wave. The zero-bias tunneling conductance peak, which is generally produced by the d-wave superconductor, is absent for the nanoribbons under conditions similar to those when a quantum wire is the normal conductor. For the anisotropic superconductivity, propagating modes appear in the superconductor even for biases below the top of the superconducting energy gap. Features appear in the conductance at the subgap population thresholds of these propagating modes as a finite-size effect of the lattice system. The surface Andreev bound states responsible for the zero-bias anomaly also cause transport resonances in the vicinity of the zero bias despite the aforementioned destruction of the anomaly. The conductance spectra revealing these excitation behaviors are fairly unchanged regardless of the presence of a hopping barrier at the interface with the superconductor. The insensitivity to the interface scattering highlights the fact that barrier-less situation cannot be realized for the model due to the heterogeneous lattice. Concerning specular Andreev reflection, the wavefunction parity gives rise to its blocking for single-mode zigzag-edged nanoribbons. The blocking is suppressed when the anisotropic superconductivity is asymmetric for the nanoribbons.

Comparative Studies of Graphene and Phosphorene Zigzag Edge Nanoribbons with Antidots

Recently, there is much interest in narrow graphene‐like nanoribbons in view of their exceptional electronic and magnetic properties. Of special interest are ultranarrow zigzag edge nanoribbons because they can have magnetic moments at the edges. Of course, it is also well known that if there are antidots (nanopores) in a system, its physical properties are strongly affected, too. In this letter, the comparative studies of graphene and phosphorene nanoribbons are presented, with emphasis on edge as well as antidot effects. It is shown that phosphorene has quite large magnetic moments on both external and internal zigzag edges and it is a half‐metal for certain electron concentrations. So phosphorene is a very attractive material for spintronic applications.

Enhancement of thermoelectric efficiency ofT−HfSe2via nanostructuring

In this work, ab initio calculations based on density functional theory and the Landauer formalism are carried out to investigate ballistic thermoelectric properties of $T\text{\ensuremath{-}}\mathrm{Hf}{\mathrm{Se}}_{2}$ nanoribbons (NRs). The zigzag-edged NRs are metallic, and they are not included in this study. The armchair NRs possess two types of edge symmetries depending on the number of atoms present in a row; odd-numbered NRs have mirror symmetry, whereas the even-numbered NRs have glide reflection symmetry. The armchair-edged NRs are dynamically stable and show semiconducting properties with varying band gap values in the infrared and visible regions. Detailed transport analyses show that the $n$-type Seebeck coefficient and the power factor differ because of the structural symmetry, whereas the $p$-type thermoelectric coefficients are not significantly influenced. It is shown that the phonon thermal conductance is reduced to a third of its two-dimensional value via nanostructuring. The $p$-type Seebeck coefficient and the power factor for $T$-phase $\mathrm{Hf}{\mathrm{Se}}_{2}$ are enhanced in NRs. We report that the $p$-type $ZT$ value of $\mathrm{Hf}{\mathrm{Se}}_{2}$ NRs at 300 and 800 K are enhanced by factors of 4 and 3, respectively.

Switching characteristics of anthraquinone molecular devices based on graphene electrodes

With the development of microelectronics and the miniaturization of electronic devices, the use of molecular materials to construct various components in electronic circuits has become a most likely development trend. Compared with silicon-based semiconductor components, molecular electronic device has the advantages of small size, high integration, low energy consumption and fast response. In recent years, more and more molecules have been used to design molecular devices such as molecular diodes, molecular switches, molecular field effect transistors and molecular memories. In this paper, sandwich structure devices based on graphene nanoribbon electrodes are constructed. The first-principles calculation method combining density functional theory and non-equilibrium Green’s function is adopted to design the molecular devices with functional characteristics. The effects of redox reactions on the electrical transport properties of molecular devices are systematically discussed. The main research contents of this paper are as follows. The switching characteristics of an anthraquinone molecular device based on graphene electrode are studied. The zigzag-edge nanoribbons and armchair-edge graphene nanoribbons are selected as electrodes. Considering the two isomers of anthraquinone (HQ) and anthraquinone (AQ) molecules in the redox reaction, the double electrode molecular junction is constructed. The effects of redox reaction and electrode structure on the switching characteristics of anthraquinone molecular devices are discussed. It is found that the current in the HQ configuration is significantly greater than that in the AQ configuration, regardless of the zigzag-edge graphene electrode or the armchair-edge graphene electrode. That is, under the redox reaction, the anthraquinone molecules show significant switching characteristics. The switching ratio of zigzag-edge graphene electrode is selected to reach a maximum of 3125, and that of armchair-edge graphene electrode is selected to maximum of 1538. In addition, when the armchair-edge graphene is used as an electrode in the HQ configuration, the negative differential resistance is obviously between 0.7 and 0.9 V.

Energetics, electronic and magnetic properties of vanadium diselenide Nanoribbons: A first-principles study

Using the first-principles method based on density functional theory, we studied the structural, magnetic and electronic properties of 2H/1T-VSe 2 nanoribbons. The ground states of armchair-edge and zigzag-edge nanoribbons are obtained. With width increasing, the edge stabilities are nearly unchanged in H phase nanoribbons, whereas the edge stabilities of T phase nanoribbons increase. Meanwhile, the average magnetic moment per V atom grows monotonously in the ferromagnetic nanoribbons but fluctuates with the width parity in the antiferromagnetic armchair-edge nanoribbons. Due to strong confinement effects and magnetic coupling, the electronic structures of VSe 2 nanoribbons are sensitive to both the magnetic state and width, resulting in the nanoribbons vary in semiconductor, semi-metal and metal. These findings suggest that VSe 2 nanoribbons are promising candidates for spintronic applications. • 2H and 1T-VSe 2 nanoribbons show distinct width-dependency in stabilities. • The energetic more favorable edge-chirality structure is armchair for 2H and zigzag for 1T nanoribbons. • With width increasing, the average magnetic moment grows monotonously in the ferromagnetic. • Width can tune the electronic properties of VSe 2 nanoribbons change in semiconductors, semi-metals and metals.

Free-standing Monatomic Thick Two-dimensional Gold.

Monolayer metal membranes have attracted research attention owing to their fascinating physical properties. Unlike layered materials with weak interlayer van der Waals bonding, metallic monolayer membranes are difficult to exfoliate due to strong metallic bonding between layers. Here, we fabricate free-standing monatomic-thick Au membranes and nanoribbons framed in bulk crystals using in situ dealloying inside transmission electron microscope. The Au membranes are robust under high energy electron beam. Monatomic-thick nanoribbons with a minimal width of 0.6 nm are observed. First-principles calculations reveal that zigzag-edged nanoribbons are ferromagnetic with magnetic moments ranging 0.38-0.51 μB per unit-cell for a width less than 0.9 nm. In addition, a linear relationship between the bond length and the coordination number of atoms is directly investigated using atomic resolution images of monolayer and bilayer Au membranes. This work provides a pathway for direct fabrication of metal membranes and nanoribbons and to achieve novel physical properties.

Perfect negative differential resistance, spin-filter and spin-rectification transport behaviors in zigzag-edged δ-graphyne nanoribbon-based magnetic devices

Many layered graphene and graphene-like two-dimensional carbon materials have been successively proposed in theory and experiment owing to their exceptional properties and potential applications. By employing the first-principles study, we find that a new graphene-like material, δ-graphyne, whose properties of the zigzag-edged nanoribbon are very similar to those ones of zigzag-edged graphene case. The band structure clearly exhibits a metallic property in non-magnetic state. And we can see a visible spin splitting within the ferromagnetic state, however, spin degeneracy within the antiferromagnetic state. To this end, here we propose a molecular junction based on the zigzag-δ-graphyne nanoribbon symmetrically with additional phenyl rings at both edges. The computational results imply that the device has many good electron transport performances, such as negative differential resistance, spin-filtering and rectification effects, and so on. In particular, the spin-filtering efficiency can be up to 99%, and the maximum of the rectification ratio reaches up to 106%. The mechanisms for these effects are revealed and discussed in terms of the band structure, spin-resolved electron transmission spectrum, the local density of state and the transmission pathway. Therefore, our research results can provide a basis for the experimental preparation of the δ-graphyne-based junction.

Magneto-electronic properties of InSe nanoribbons terminated with non-metallic atoms and its strain modulation

Employing the first-principles calculation based on the density functional theory, the geometries, magneto-electronicproperties, and strain effects of the zigzag-edged InSe nanoribbons with the Se-edge saturated by H atoms and In-edge terminated by various non-metallic elements <i>X</i> (<i>X</i> = H, B, N, P, F and Cl) are studied. The calculated formation energy and Forcite annealing simulations show that the H-ZN(7)-X has a stable geometry. For F- and Cl- terminated ribbons, they have a magnetic metallic property similar to that in the case of H termination, and for the N termination the nanoribbon has the strongest magnetic property. However, the B and P terminations cause the magnetic properties at the ribbon edge to completely disappear, particularly when the mechanical strain is applied. The magnetic stability of H-ZN(7)-N is enhanced, and the spin polarization efficiency (SP) at the Fermi level can be effectively modulated in a range from zero to 92%, which means that it is possible to design a mechanical switch for controlling the spin transport at low bias. The strain modulating mechanism is related to the fact that the variation of strain-induced bond length leads the unpaired electrons to be redistributed or disappear. The magnetic properties of N-ZN(7)-N are mainly derived from the <i>p</i> orbitals of In, Se and N atoms, thus it is very important to develop non-transition metal magnetic materials.

Graphene Logic Gates

Graphene is a biocompatible material that can be incorporated safely into living tissue. This property makes graphene an ideal material for bioelectronics applications. The main obstacle for using graphene as a material for bioelectronic circuits is the lack of a bandgap, which results in noneffective current switching. Here, we use rectangular L-shaped graphene nanoribbons as a building block for graphene logic gates. Electrons are initially transported along the zigzag-edged nanoribbon and then the transport direction changes by 90°, resulting in transport along the armchair edge. Our computations showed that electron scattering because of this change in the direction causes the appearance of a pseudobandgap, which is large enough for logic operations. This pseudobandgap appears as a zero-conductance region for electron energies near the Fermi level. We propose an and , an or, and a not logic gate and use tight-binding Hamiltonians and nonequilibrium Greens functions to show that these designs can reproduce effectively the desired logic operations.

Optical Properties of a Single Carbon Chain-Doped Silicene Nanoribbon

Using first-principles spin polarization density function theory calculations, we have studied the electronic and optical properties of zigzag-edge silicene nanoribbons (ZSiNRs) doped with a single carbon chain. Because of the doped carbon chain, there are several defect states in the band structures of ZSiNRs across the Fermi level, and the ferromagnetic ground state is metallic. The dielectric functions in all three dimensions are completely different from each other, and thus the system exhibits strong optical anisotropism. The carbon chain influenced the dielectric functions most at low energy. The first peak in the E//x direction of the dielectric spectrum exhibits a significant blueshift, and its value has changed as well. The main absorption wavelength depends on the polarization direction of the incident light, but occurs within the UV region for all polarization directions. The peaks of the energy loss spectra correspond to the trailing edges in the reflectivity spectrum, and the highest peak corresponds to a plasmon frequency. Our results could be useful for investigating nanodevices based on silicene nanoribbons.

Conductance of L-shaped and T-shaped graphene nanoribbons

Rectangular graphene nanoribbons with two contacts have been studied extensively as parts of several nanoelectronic devices. L-shaped and T-shaped graphene nanoribbons received a little or no attention until now. In this letter, we present computations of the conductance of L-shaped graphene nanoribbons with two contacts and of T-shaped graphene nanoribbons with tree contacts. We used tight-binding Hamiltonians and non-equilibrium Green's functions to compute their conductance. In L and T-shaped nanoribbons electrons are initially transported along a zigzag-edged nanoribbon, and then the direction of their motion changes by 90O and are transported along an armchair-edged nanoribbon. Our results show that this change in direction of motion results in a zero-conductance region that extents about 0.25 eV below and above the Fermi level. This zero conductance region is large enough to cause current switching and, because of this, L and T-shaped graphene nanoribbons can be used as building blocks for logic gates.

Hydrogen Adsorption on Nearly Zigzag-Edged Nanoribbons: A Density Functional Theory Study

The realistic shapes of N doped graphene nanoribbons (GNRs) can be realized by considering nearly zigzag-edged (NZE) imperfections and pyridine defects (3NV). The paper focuses on NZE-GNRs with 3NV that is populated by Scandium abbreviated as Sc/NZE-3NVGNRs. Systematic calculations have clarified roles of the nano-shapes of NZE-3NVGNRs in its formation, energetics, stability and electron states functionalized with Sc using density functional theory (DFT) formalisms. According to DFT calculations, the magnitude of the spin that is attributed to the rise of magnetic order is closely linked to the altered shape of the ribbon edges. Also, calculations show that the stability of Sc functionalization at the 3NV and NZE site is thermodynamically stable and is dictated by a strong binding energy (BE). The magnitude of the BE is enhanced when the zigzag edge is short or the ribbon width is narrow, suggesting a reduced clustering of Sc atoms over the Sc-doped NZE-3NVGNRs. Results also show that as the length of the zigzag edge in Sc/NZE-3NVGNRs increases it creates considerable distortion on the appearance of the structure. Finally, the Sc/NZE-3NVGNRs as a potential candidate for hydrogen storage was evaluated and it was found that it could adsorb multiple hydrogen molecules.

Spin-charge transport properties for graphene/graphyne zigzag-edged nanoribbon heterojunctions: A first-principles study

Graphene and graphyne hold great potential for spintronic device applications due to their exceptionally good electrical and mechanical properties in the pristine form. Recently, considering these 2D carbon allotropes have similar bond lengths and unit cell shapes, ensuring a perfectly matched interface between them and construction of graphene/graphyne heterojunctions feasible. To this end, we study the spin-charge transport properties for monolayer graphene/graphyne zigzag-edged nanoribbon heterojunctions by employing the ab initio calculations, where two types of γ- and (6,6,12)-graphynes are considered, respectively. Our important result here is that these heterojunctions can exhibit an interesting variation of magnetoresistane by applying ferromagnetic stripes or external magnetic fields onto the ribbons to initially orient their spin configuration. Specifically, the magnetoresistance ratio can be up to 104% for graphene/γ-graphyne heterojunction, but only 300% for the graphene/(6,6,12)-graphyne one. In addition, the effects of spin-filtering and negative differential resistance are also observed in those heterojunctions. The maximum of spin filtering efficiency can be up to 99%. The mechanisms are revealed and analyzed by the connection of these effects to the evolution of the spin-resolved electron transmission spectra and pathways around the Fermi level at zero bias.

Room-temperature ferromagnetism from an array of asymmetric zigzag-edge nanoribbons in a graphene junction

Room-temperature ferromagnetism in graphene layers with defects has been experimentally measured. Despite disagreement around the intrinsic origin of carbon magnetism, experimental evidence has supported the existence of paramagnetism or ferromagnetism in carbon materials. Convincing theoretical explanations, however, have not yet been proposed. In this work, density functional theory calculations were used to suggest a plausible explanation for this phenomenon as it is observed at the zigzag grain boundaries of a mismatched single-double-single-layer graphene junction. We identified asymmetric zigzag-edge graphene nanoribbons that display ferromagnetic properties in a graphene junction structure. Two ferromagnetic asymmetric zigzag graphene nanoribbons displayed antiferromagnetic coupling in a defect-free structure at the grain boundary. The introduction of a vacancy or N-substitutional defect was found to destroy the magnetism on one side only; the nanoribbon on the other side continued to display a large ferromagnetic exchange coupling. The ferromagnetic nanoribbon in the junction was ferromagnetically correlated with other nanoribbons in the two-dimensional junction array, yielding a Curie temperature well-above room temperature. Moreover, the ferromagnetic correlation was observed regardless of the arrangement of the magnetic layers, enabling ferromagnetic ordering within the graphene junction array.

Half Metallic Behavior in H-Terminated Zigzag BC<sub>2</sub>N Nanoribbons

We study structural, stability, electronic and magnetic properties of zigzag-edged BC2N nanoribbons (ZBC2NNRs) with H-termination in the view of first principles calculations. Four kinds of edge arrangements are considered, labeled as B-C, N-C, N-B, and C-C. Interestingly, we find these four types of H-terminated ZBC2NNRs have various electronic structures. The half-metal and semi-metal are obtained depending on the edge atom alignment. The B-C and N-C ZBC2NNRs with H-termination are half-metals with antiferromagnetic (AFM) ground states. The magnetic moments of the antiferromagnetic (AFM) state always prefer to locate at the ribbon edges. However, the N-B and C-C ZBC2NNRs show spin-unpolarized semi-metallic behaviors at ground states. Our results suggest that the H-terminated ZBC2NNRs can be a promising candidate material in nanoelectronics and nanospintronics.

Capabilities of transition metals in retarding the bonding of carbon atoms to minimize dendritic graphene.

The avoidance of growing dendritic graphene on the copper substrate during the chemical vapor deposition process is greatly desired. Here we have identified a mechanism, in which (1) transition metal plates placed inside the copper pockets reduce the majority of active carbon atoms to eventually suppress the graphene growth rate, and (2) transition metals etch graphene C-C bonds along defective edges to grow into zigzag-edge ending domains with higher priorities. Via isotopic labeling of the methane method, we have observed bright-dark-alternating hexagonal-shaped rings, which are shown in Raman mapping images. Under a hydrogen atmosphere, we are capable of acquiring hexagonal openings within graphene domains by means of transition-metal-driven catalytic etching. This methodology may work as a simple and convenient way to determine graphene size and crystal orientation, and may enable the etching of graphene into smooth and ordered zigzag edge nanoribbons without compromising the quality of graphene.

A planar carbon allotrope with linear bipentagon-octagon and hexagon arrangement

A two-dimensional (2D) metallic carbon allotrope is proposed, which consists of linearly aligned bipentagon-octagon and hexagon rings in a planar sheet. The relatively high percentage of hexagon and the regular arrangement of the polygons make it energetically more favorable than most of other predicted 2D carbon allotropes. Phonon dispersions without negative frequencies also indicate its stability. Electronic structure calculations show that its metallic nature is mainly due to the atoms shared by the pentagon, hexagon and octagon. Its lattice thermal conductivity is only about one fifth of that of graphene. Armchair- and zigzag-edged nanoribbons of this structure are also studied. The former is metallic while the latter has a small band gap due to the spin-polarized edge states. The appropriate band gap and the significantly reduced thermal conductivity suggest potential applications in thermoelectricity.

Proposal for realizing the quantum spin Hall phase in a gapped graphene bilayer

Quantum spin Hall (QSH) insulators with gapless edge states have potential applications in designing low-dissipation devices. In spite of many predictions, to verify the QSH phase in graphene layered materials experimentally is still difficult due to the obstacle in achieving spin-orbit coupling strong enough. We propose a Rashba system of graphene bilayer gapped by dielectric layers and show it can host a valley-polarized QSH phase even when the Rashba interaction approached zero. Such a system exhibits asymmetric topological quantum phase transitions under opposite interlayer biases, due to the dielectric-potential induced inversion asymmetry in the absence of interlayer bias. Specifically, the quantum valley Hall phase exists in zigzag-edged nanoribbons under the bias in one direction but is absent under the reverse bias. These topological phenomena can be well understood by the competition among the dielectric-induced potential, Rashba interaction, and interlayer bias in modulating the bulk band gap. Moreover, the phase diagram is given and the corresponding phase boundaries are derived analytically. Our findings provide a possible way to detect the QSH-related asymmetric topological quantum phenomena in graphene bilayer based on the current experimental technology.

Electronic and thermoelectric transport properties for a zigzag graphene–silicene–graphene heterojunction modulated by external field

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 SC display the oscillatory behavior and can be effectively modulated by effective SOC λSO and the potential energy V0. 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.