Increasing Ribbon Width Research Articles

Quantum discord in zigzag graphene nanoribbons

Based on Hubbard model with the Hartree-Fock approximation, we study the properties of quantum discord (QD) between the nearest-neighbor sites A and B in zigzag graphene nanoribbons thermalized with a reservoir at temperature T. Several influences of the site position, on-site Coulomb repulsion U, temperature, and ribbon width on QD are discussed in detail. The results show that QD is robust against thermal fluctuations, and QD for the leg pairs along the zigzag chain near ribbon edges is always larger than that for the rung pairs linking two adjacent zigzag chains. QD for the rung pairs increases and then approaches to saturation as the ribbon width increases, where the velocity of saturation is strongly correlated to U. Moreover, for rung pairs the values of U at the QD peaks perform the scaling behaviors with increasing ribbon width.

Signature of excitonic insulators in phosphorene nanoribbons

Phosphorene is a recently developed two-dimensional (2D) material that has attracted tremendous attention because of its unique anisotropic optical properties and quasi-one-dimensional (1D) excitons. We use first-principles calculations combined with the maximally localized Wannier function tight binding Hamiltonian and Bethe–Salpeter equation (BSE) formalism to investigate quasiparticle effects of 2D and quasi-1D blue and black phosphorene nanoribbons. Our electronic structure calculations shows that both blue and black monolayered phases are semiconductors. On the other hand black phosphorene zigzag nanoribbons are metallic. Similar behavior is found for very thin blue phosphorene zig-zag and armchair nanoribbon. As a general behavior, the exciton binding energy decreases as the ribbon width increases, which highlights the importance of quantum confinement effects. The solution of the BSE shows that the blue phosphorene monolayer has an exciton binding energy four times higher than that of the black phosphorene counterpart. Furthermore, both monolayers show a different linear optical response with respect to light polarization, as black phosphorene is highly anisotropic. We find a similar, but less pronounced, optical anisotropy for blue phosphorene monolayer, caused exclusively by the quasi-particle effects. Finally, we show that some of the investigated nanoribbons show a spin-triplet excitonic insulator behavior, thus revealing exciting features of these nanoribbons and therefore provides important advances in the understanding of quasi-one dimensional phosphorus-based materials.

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.

Edge magnetism in zigzag graphene nanoribbons with Rashba spin–orbit coupling

We study the edge magnetism in zigzag graphene nanoribbons (ZGNRs) described by the mean-field Hubbard model with the controllable Rashba spin–orbit coupling (RSOC) at finite temperatures. The results show that the edge magnetic moments are robust against thermal fluctuation. Moreover, the magnitude of such edge magnetism can be significantly enhanced by the on-site Coulomb repulsion U, whereas RSOC yields the opposite effect. However, RSOC plays a crucial role in tuning the edge magnetism, which can drive the phase transition from the antiferromagnetic state into the ferromagnetic state in ZGNRs. When the ribbon width increases, the magnitudes of the edge magnetic moments under the weak U are apt to increase until reaching saturation. If U is large enough, the edge magnetism is almost independent of ribbon width. These versatile magnetic properties of ZGNRs could be useful in promising spintronics applications.

Giant enhancement of the axial Casimir force of a rotating particle near hyperbolic metasurface

We show that the axial Casimir force (ACF) acting on a rotating nanoparticle can also appear in the proximity of a hyperbolic metasurface made of graphene ribbons since it was first demonstrated existing near a bi-isotropic material. The ACF can be enhanced by several orders of magnitude compared with that in the bi-isotropic configuration. We numerically demonstrate that the largest ACF exerted on the particle can be achieved when the rotation direction of the nanoparticle is parallel to the hyperbolic metasurface. ACF is also determined by the azimuth angle between the rotating axis and the edge of the metasurface. The dependence of ACF on the chemical potential and ribbon width of graphene is also investigated in detail. ACF increases as the chemical potential increases for fixed ribbon width. For given chemical potential, ACF increases first and then decreases after reaching the maximum value when the ribbon width increases. The results extend the application of hyperbolic metasurfaces in regulating Casimir interactions.

Tunable magnetic and electronic properties of armchair BeN4 nanoribbons.

Recently, layered BeN4 as a novel Dirac semimetal has been fabricated (M. Bykov, T. Fedotenko, S. Chariton et al. Phys. Rev. Lett., 2021, 126, 175501). Motivated by the experiment, we perform first-principles calculations to predict the stability, magnetic configurations, and electronic structures of unsaturated BeN4 nanoribbons with an armchair-terminated edge. The magnetic interactions and electronic properties of BeN4 nanoribbons are sensitively influenced by the edge morphology. The BeN4 nanoribbons with both edges occupied by Be atoms undergo a transition from a ferromagnetic (FM) metal to an antiferromagnetic (AFM) semiconductor with the increase of ribbon width. The configurations with edges situated by Be and N atoms are FM/ferrimagnetic (FIM) metals or nearly half-metals, and the spin polarizability is as high as 85% when the ribbon width is N = 5. The nanoribbons with both edge sites occupied by pentagonal N atoms are nonmagnetic (NM), while the nanoribbons terminated by N atoms in a hexagonal ring are FM metals. We also explore the magnetic properties and band structures of BeN4 nanoribbons with hydrogen passivation. Our results open up a versatile edge engineering avenue to design BeN4-based spintronic and nanoelectronic devices.

Tight-binding studies of uniaxial strain in T-graphene nanoribbons

The role of uniaxial strain in armchair, T-graphene nanoribbons (ATGNRs) with symmetric and asymmetric structures is investigated using a nearest-neighbour, tight-binding (TB) model. ATGNRs with structural symmetry and two a sub-lattice structure exhibit Dirac points at zero strain. Application of uniaxial strain to these systems induces multiple Dirac points under compression (up to −20% strain), with the number of these points commensurate with the number of tetra-carbon base-units along the width of the unit cell, accounting also for the mirror symmetry of the structure. Under tensile, uniaxial strain (up to 20% extension), the induced asymmetry in the carbon tetrabond results in the number of Dirac points being reduced, although a minimum number are preserved due to the fundamental mirror-symmetry of the symmetric ATGNR. Asymmetric ATGNRs, which are semiconductors, are shown to have tunable band-gaps that decrease as a function of increasing ribbon width and uniaxial strain. Uniaxial strain induces a single Dirac point at the band edge of these systems under high compression (16%), with the closing of the band gap linked to symmetry-induced perturbations in the structure that override the symmetry-breaking, gap-opening mechanisms. In summary, the TB model shows ATGNRs to have suitable device features for flexible electronics applications, such as band-gap tuning, and for the strain engineering of relativistic properties.

Odd–even effect and bandgap modulation by C-H doping in armchair nanoribbons of monolayer WS[formula omitted

Breaking the symmetry in out-of-plane direction in monolayer structures to trigger peculiar electronic features has long been predicted. Inspired by the recent experimental progress on the doping of carbon-based units in monolayer transition-metal dichalcogenide (TMD), we investigate the electronic structure and transport of armchair WS2 nanoribbons doping with C-H units in one face, using first-principle calculations. An odd–even effect is found, where the ribbons doped with even number of C-H units exhibit semiconducting behavior, and that with even number are metallic. With the increase of ribbon width, the band gap is found to decrease smoothly and finally trigger a semiconducting-metallic transition. Moreover, a periodically oscillatory variation of band gap is observed. Analysis shows it is resulted by the asymmetric edge effect induced by Janus-face doping. Our findings provide another way to modulate the electronic structure of TMDs, and can be extended to other TMD structures.

Effects of strain and electric fields on the electronic transport properties of single-layer β12-borophene nanoribbons.

Motivated by recent experimental and theoretical research on a monolayer of boron atoms, borophene, the transmission probability and current-voltage characteristics of β12-borophene nanoribbons (BNRs) with zigzag and armchair edges have been calculated using the five-band tight-binding calculation, the Green's function approach, and the Landauer-Büttiker formalism. We focus on the effects of the geometrical parameters, perpendicular electric field, and external strain on the electronic transport properties of β12-BNRs by considering the effects of the substrate. Our calculations show that the transmission coefficient and current of the system decrease by increasing the channel length, whereas increasing ribbon width leads to an increment in the transmission probability as well as the I-V characteristic of β12-BNRs. Besides, the application of tensile strain causes a decrement in the current of the inversion symmetric model of the armchair β12-BNR, whereas the current increases in the presence of compressive strain. We also observed a dip in the transmission spectrum of the biased β12-BNR along the armchair direction which shows a metal-to-n-doped semiconductor phase transition in the device when applying a strong enough electric field. Moreover, the current of the inversion symmetric model of the β12-BNR with zigzag and armchair edges increases with the application of a perpendicular electric field, while in the case of the homogeneous model, the application of an electric field enhances the current of the β12-BNR only in the zigzag direction. These results provide insights for future experimental research and show that β12-BNRs are potential candidates for next-generation electronic devices.

Electronic structures and transport properties of the Bi2O2Se nanoribbons with different edge passivation types

The electronic structures and transport properties of edge-passivated Bi2O2Se nanoribbons have been investigated by combining the density functional theory and non-equilibrium Green’s functions. Compared with the bare Bi2O2Se nanoribbons, the stability of H, F, Cl-passivated Bi2O2Se nanoribbons is greatly improved. The band structures show that the bare Bi2O2Se nanoribbons are metallic whereas H, F, Cl-passivated ones are direct band gap semiconductors, and their band gaps slightly decrease as increasing ribbon width due to the quantum confinement effect. Meanwhile, the current-voltage characteristics of edge-passivated Bi2O2Se nanoribbons have also been obtained. The results show that not only the ribbon width, but also the edge passivation atom can effectively manipulate the electronic transport properties of Bi2O2Se nanoribbons. These findings can help us to make appropriate choices in edge passivation atom and ribbon width to adjust the electronic transport properties of Bi2O2Se nanoribbons.

Modulation of the optical properties of zigzag silicene nanoribbons by double carbon chains

Using first-principles calculations, we investigate the electronic and optical properties of zigzag silicene nanoribbons substituted with double carbon chains. The results show that the chains are pulled nearly straight and produce a rather obvious transverse contraction in the width direction of the ribbon. The double carbon chains introduce defect states that appear as two degenerate bands across the Fermi level. These nanoribbons are always metallic regardless of the position of the carbon chains or the ribbon width. Under the same bandwidth, the imaginary parts of the dielectric functions in the Ex and Ey directions reveal red- and blue-shifts, respectively, with increasing distance between the two C chains. The imaginary parts of the dielectric functions in the Ex and Ey directions reveal blue- and redshifts, respectively, with increasing ribbon width. Three major peaks in the imaginary part of the dielectric function correspond to the intrinsic plasma frequencies originating from electron transitions of silicon and carbon. Such excellent electronic and optical properties may lead to some important applications of the nanoribbons in short-wavelength optoelectronic devices.

The magnetism enhancement and spin transport in zigzag borophene nanoribbons edge-passivated by N atoms

We study the effects of edge passivation by N atoms on the magnetic and electrical properties of zigzag borophene nanoribbons (ZBNRs) employing the density functional theory combined with the non-equilibrium Green’s function method. Significant enhancement on the edge magnetism is found in general, in contrast to the magnetism suppression in hydrogenated ZBNRs, and half-metallicity can be observed in some special cases. Different from the graphene nanoribbons, the properties of ZBNRs are sensitive to the ribbon width, since their electronic structures fluctuate greatly with the width. Magnetism and conductivity change sharply and the ground state jumps back and forth between the ferromagnetic and antiferromagnetic configurations as the ribbon width increases. Spin-dependent negative differential resistivity may be realized in N-passivated ZBNRs.

The electronic and transport properties of zigzag β-antimonene nanoribbons

The electronic and transport properties of zigzag β-antimonene nanoribbons (Z- SbNRs) are investigated based on density function theory (DFT) and non-equilibrium Green's functions (NEGF). The calculated band structure of Z-SbNRs shows a typical semiconductor characteristic. For Z-SbNRs, the band gap decreases monotonically with the increase of ribbon width. Moreover, it is found that the width and the length of central scattering region have a great effect on the electronic transport property of Z-SbNRs. The I-V curve of the two-probe device shows the current is almost zero when the bias voltage is lower than the threshold voltage of 2.0 V. However, when the bias voltage is higher than the threshold voltage, the transport channels are opened and the current increases quickly.

Tunable direct-indirect band gaps of ZrSe2 nanoribbons

The atomic and electronic structures of armchair and zigzag ZrSe2 nanoribbons have been investigated systematically. Both the armchair and zigzag ZrSe2 nanoribbons are nonmagnetic semiconductors, while their bandgaps show quite different behaviors depending on the ribbon width. We find that all the zigzag ribbons possess direct energy gaps, which smoothly decline with the increasing ribbon width. On the other hand, energy gaps for the armchair ribbons change from direct gaps to indirect ones as the ribbon width increases and exhibit a width-dependent oscillation behavior. Moreover, the semiconducting behaviors and the bandgap types are robust, and they remain unchanged in bilayer and multilayer thin films with inter-layer interactions. These findings indicate that ZrSe2 nanoribbons are promising candidate materials for applications in nanoelectronic devices.

First principles study on the electronic structures and transport properties of armchair/zigzag edge hybridized graphene nanoribbons

Graphene nanoribbons (GNRs) can be mainly classified into armchair graphene nanoribbons (aGNRs) and zigzag graphene nanoribbons (zGNRs) by different edge chiral directions. In this work, by introducing Stone-Wales defects on the edges of the V-shaped aGNRs, we propose a kind of armchair/zigzag edge hybridized GNRs (a/zHGNRs) and using the density functional theory and the nonequilibrium Green's function method, the band structures and electronic transport properties of the a/zHGNRs have been calculated. Our results show that an indirect bandgap appears in the band structures of the a/zHGNRs, which is very different from the direct bandgap of aGNRs and gapless of zGNRs. We also find that the valance band is mainly derived from the armchair partial atoms on the hybridized edge, while the conduction band comes mainly from the zigzag partial atoms of the hybridized edge. Meanwhile, the bandgap also oscillates with a period of three when the ribbon width increases. In addition, our quantum transport calculations show that there is a remarkable transition between the semiconductor and the metal with different ribbon widths in the a/zHGNRs devices, and the corresponding physical analysis is given.

Magnetic Field Effects on Optical Conductivity of Doped Armchair Graphene Nanoribbon

We address the optical conductivity of armchair graphene nanoribbon within tight-binding model Hamiltonian using Green’s function approach. The possible effects of ribbon width, magnetic field, and chemical potential on optical absorption are investigated. We have also found the dependence of the optical conductivity for various ribbon widths and chemical potentials. Our results show a peak appears in the frequency dependence of optical conductivity of graphene nanoribbon. Upon increasing ribbon width, the height of the optical conductivity at frequency of peak increases. Also, applying the magnetic field causes an increase of the optical conductivity. However, upon more increase of the magnetic field leads to a decrease of the optical conductivity. Moreover, we study the effects of chemical potentials on frequency behavior of optical absorption of graphene nanoribbon for both metallic and insulator cases.

Thermal transport characterization of carbon and silicon doped stanene nanoribbon: an equilibrium molecular dynamics study.

Equilibrium molecular dynamics simulation has been carried out for the thermal transport characterization of nanometer sized carbon and silicon doped stanene nanoribbon (STNR). The thermal conduction properties of doped stanene nanostructures are yet to be explored and hence in this study, we have investigated the impact of carbon and silicon doping concentrations as well as doping patterns namely single doping, double doping and edge doping on the thermal conductivity of nanometer sized zigzag STNR. The room temperature thermal conductivities of 15 nm × 4 nm doped zigzag STNR at 2% carbon and silicon doping concentration are computed to be 9.31 ± 0.33 W m−1 K−1 and 7.57 ± 0.48 W m−1 K−1, respectively whereas the thermal conductivity for the pristine STNR of the same dimension is calculated as 1.204 ± 0.21 W m−1 K−1. We find that the thermal conductivity of both carbon and silicon doped STNR increases with the increasing doping concentration for both carbon and silicon doping. The magnitude of increase in STNR thermal conductivity due to carbon doping has been found to be greater than that of silicon doping. Different doping patterns manifest different degrees of change in doped STNR thermal conductivity. Double doping pattern for both carbon and silicon doping induces the largest extent of enhancement in doped STNR thermal conductivity followed by single doping pattern and edge doping pattern respectively. The temperature and width dependence of doped STNR thermal conductivity has also been studied. For a particular doping concentration, the thermal conductivity of both carbon and silicon doped STNR shows a monotonic decaying trend at elevated temperatures while an opposite pattern is observed for width variation i.e. thermal conductivity increases with the increase in ribbon width. Such comprehensive study on doped stanene would encourage further investigation on the proper optimization of thermal transport characteristics of stanene nanostructures and provide deep insight in realizing the potential application of doped STNR in thermoelectric as well as thermal management of stanene based nanoelectronic devices.

Edge modulation of electronics and transport properties of cliff-edge phosphorene nanoribbons

Based on the first-principles calculations, we study the electronic structures and transport properties of cliff-like edge phosphorene nanoribbons (CPNRs), considering different types of edge passivation. The band structures of bare CPNRs possess the metallic features; while hydrogen (H), fluorine (F), chlorine (Cl) and oxygen (O) atoms-passivated CPNRs are semiconductor materials, and the band gap values monotonically decrease when the ribbon width increases. Moreover, the H and F-passivated CPNRs exhibit the direct band gap characteristics, while the Cl and O-passivated cases show the features of indirect band gap. In addition, the edge passivated CPNRs are more energetically stable than bare edge case. Meanwhile, our results also show that the transport properties of the CPNRs can be obviously influenced by the different edge passivation.

Strain effect on SnS2 nanoribbons: Robust direct bandgap of zigzag-edge and sensitive indirect semiconductor with armchair-edge states

First-principles calculations are used to study the electronic properties and strain effect on the SnS2 nanoribbons with zigzag- and armchair-terminated edge states. Theoretical results show that bandgaps of both types of nanoribbons decrease monotonically with the increasing ribbon width. The indirect bandgap characteristic is reserved in ANRs, while ZNRs turn into direct bandgap semiconductors. Applying the uniaxial strain, the bandgap of 13-ANR is sensitive and can be modified significantly, while the 8-ZNR exhibits a 2.1eV robust direct bandgap at X point. Our calculations indicate that zigzag-edged SnS2 nanoribbons can be potential candidates in optoelectronics and photocatalyst.

Perfect spin filtering effect in ultrasmall helical zigzag graphene nanoribbons

The spin-polarized transport properties of helical zigzag graphene nanoribbons (ZGNRs) are investigated by first-principles calculations. It is found that although all helical ZGNRs have similar density of states and edge states, they show obviously different transport characteristics depending on the curling manners. ZGNRs curled along zigzag orientation exhibit perfect spin filtering effect with a large spin-split gap near the Fermi level, while ZGNRs curled along armchair orientation behave as conventional conductors for both two spin channels. The spin filtering effect will be weakened with the increase of either ribbon width or curling diameter. The results suggest that ultrasmall helical ZGNRs have important potential applications in spintronics and flexible electronics.