Armchair Nanoribbons Research Articles

Confinement and edge effects on atomic collapse in graphene nanoribbons

Atomic collapse in graphene nanoribbons behaves in a fundamentally different way as compared to monolayer graphene, due to the presence of multiple energy bands and the effect of edges. For armchair nanoribbons we find that bound states gradually transform into atomic collapse states with increasing impurity charge. This is very different in zig-zag nanoribbons where multiple quasi-one-dimensional \emph{bound states} are found that originates from the zero energy zig-zag edge states. They are a consequence of the flat band and the electron distribution of these bound states exhibits two peaks. The lowest energy edge state transforms from a bound state into an atomic collapse resonance and shows a distinct relocalization from the edge to the impurity position with increasing impurity charge.

The unique carrier mobility of monolayer Janus MoSSe nanoribbons: a first-principles study.

Charge-carrier mobility is a determining factor of the transport properties of semiconductor materials and is strongly related to the optoelectronic performance of nanoscale devices. Here, we investigate the electronic properties and charge carrier mobility of monolayer Janus MoSSe nanoribbons by means of first-principles simulations coupled with deformation potential theory. These simulations indicate that zigzag nanoribbons are metallic. Conversely, armchair nanoribbons are semiconducting and show oscillations in the calculated band gap as a function of edge-width according to the 3p < 3p + 1 < 3p + 2 rule, with p being the integer number of repeat units along the non-periodic direction of the nanoribbon. Although the charge-carrier mobility of armchair nanoribbons oscillates with the edge-width, its magnitude is comparable to its two-dimensional sheet counterpart. A robust room-temperature carrier mobility is calculated for 3.5 nm armchair nanoribbons with values ranging from 50 cm2 V-1 s-1 to 250 cm2 V-1 s-1 for electrons (e) and holes (h), respectively. A comparison of these values with the results for periodic flat sheet (e: 73.8 cm2 V-1 s-1; h: 157.2 cm2 V-1 s-1) reveals enhanced (suppressed) hole (electron) mobility in the Janus MoSSe nanoribbons. This is in contrast to what was previously found for MoS2 nanoribbons, namely larger mobility for electrons in comparison with holes. These differences are rationalized on the basis of the different structures, edge electronic states and deformation potentials present in the MoSSe nanoribbons. The present results provide the guidelines for the structural and electronic engineering of MoSSe nanoribbon edges towards tailored electron transport properties.

Topological phase transition and conductance in topological crystalline insulator with honeycomb lattice

Topological crystalline insulators (TCI) represent a new state of quantum matter which is topologically protected by crystalline symmetry. We construct a minimal tight-binding model for 2D thin TCI films with C 3 -symmetry honeycomb lattice. Four Dirac cones are observed at the high-symmetry points K, K′ and Γ points, considering the basic term spin-orbit interaction. By introducing the mass term, Dirac gaps can be opened to trigger the topological phase transition. We then numerically investigate the band structure, conductance and local density of state of the TCI films consisting of zigzag and armchair nanoribbons. The results indicate that there is a one-to-one correspondence between gapless topological edge states and nonzero mirror-Chern number. Finally, we apply an external electric field perpendicular to the TCI films, which breaks mirror symmetry. As a result, the nontrivial edge modes are destroyed, and the conductance is turned off by the electric field.

The effect of edge passivation with different atoms on ZrSe2 nanoribbons

In recent years, several two-dimensional (2D) materials with semiconducting electronic properties have been introduced. The ZrSe2 (Zirconium diselenide) is one of the best materials to replace the silicon in nanoelectronics due to its proper bandgap. In this research, we study the electronic properties of the armchair and zigzag ZrSe2 nanoribbons (AZSNRs and ZZSNRs). Moreover, we have investigated the effect of edge passivation of two 3AZSNR and 3ZZSNR (the ribbon width is 3) structures with hydrogen (H) and oxygen (O) atoms and also both of them (H/O) concurrently. By calculating the cohesive energy of all structures, we deduce that all zigzag and armchair structures with different edge passivations are stable and energy favorable. Also the edge passivation with H–O atoms can change the electronic properties of 3ZZSNR structure significantly, and the structure behavior changes from semiconductor to metallic. In the case of the armchair structures, the edge passivated structure with O atoms (3AZSNR-O) is the most stable and feasible to fabricate in nanoscale experiments. These results show that the ZrSe2 nanoribbons with different edge passivations have potential applications in nanoelectronics.

Gap opening in graphene nanoribbons by application of simple shear strain and in-plane electric field

The effects of shear strain and applied in plane electric field on the electronic properties of monolayer graphene nanoribbons (GNRs) are theoretically investigated. Band structures and the probability densities are calculated within the tight-binding model and the mechanical stresses submitted to the GNRs are taken into account by using the theory of linear elasticity with joint modifications in the elongation of the nearest-neighbor vectors and the modification of the hopping parameters. The energy gaps for specific widths of (semiconducting) armchair nanoribbons are verified also in the presence of either strain or field, whereas zigzag nanoribbons are metallic for any value of strain and exhibit a small gap for any value of field. However, our results demonstrate that when both strain and electric field are combined, a significant energy gap is always observed in the band structure, for any width or edge type of the ribbon. Moreover, the obtained total wave function is asymmetric along the ribbon width due to the applied electric field that pushes the electrons to one side of the ribbon and, under shear strain, a peak at the center of the ribbon in the spatial distribution is also observed owing to the preferable localization around the almost undeformed carbon bonds at ribbon center.

Excitons and plasmons of graphene nanoribbons in infrared frequencies in an effective-mass approximation

Effects of electron-electron interactions on the optical response of graphene nanoribbons are theoretically investigated in an effective-mass approximation in a comprehensive manner. In optical absorption spectra of armchair and zigzag nanoribbons without and with doping, excitons, which are bound states of electrons and holes, and plasmons manifest themselves as various prominent peaks. For light polarized parallel to nanoribbons, exciton peaks at high energies split because of interactions with dark excitons, to which optical transition is prohibited. For light polarized perpendicular to nanoribbons, in nondoped semiconducting armchair nanoribbons, moderate exciton peaks appear while in doped armchair and zigzag nanoribbons, only when the Fermi energy crosses at least two energy bands, large plasmon peaks occur because of a nature of Dirac electrons. These peaks can be assigned to specific optical transitions in energy bands. The optical absorption peaks arising from the excitons and plasmons in a wide range of categories of nanoribbons approximately correspond to those of carbon nanotubes by appropriate scaling of energy and the wave vector.

Electronic structures of WS2 armchair nanoribbons doped with transition metals

Armchair WS2 nanoribbons are semiconductors with band gaps close to 0.5 eV. If some of the W atoms in the ribbon are replaced by transition metals, the impurity states can tremendously affect the overall electronic structure of the doped ribbon. By using first-principles calculations based on density functional theory, we investigated substitutional doping of Ti, V, Cr, Mn, Fe, and Co at various positions on WS2 ribbons of different widths. We found that Fe-doped ribbons can have two-channel conduction in the middle segment of the ribbon and at the edges, carrying opposite spins separately. Many Co-doped ribbons are transformed into spin filters that exhibit 100% spin-polarized conduction. These results will be useful for spintronics and nanoelectronic circuit design.

Studies of 2D Bulk and Nanoribbon Band Structures in MoxW1–xS2 Alloy System Using Full sp3d5 Tight‐Binding Model

A full sp3d5 tight‐binding model (TBM) is used to calculate the band structure of MoxW1–xS2 alloy monolayers and armchair nanoribbons. First, the parameters required for the alloy system are extracted from density functional theory. Then, TBM is used to enable a larger computational domain for the alloy system. Herein, the electronic properties of MoxW1–xS2 alloys, such as the band structure, bandgap, and density‐of‐states effective mass, are determined computationally. These properties could be useful in the design of future devices.

Structural stability, magneto-electronic properties, and tuning effects for transition metal-doped net-Y nanoribbons

Low-dimensional carbon allotropes have attracted much attention due to their great potential for applications in future electronic and magnetic devices. Recently, new graphene-like nanoribbons consisting of four-, six-, and eight-membered carbon rings have been synthesized successfully, and a related two-dimensional structure net-Y was subsequently proposed. Here, we report theoretical investigations on the structural stability and magneto-electronic properties for the net-Y armchair nanoribbons doped with low-concentration transition metal (TM) atoms (Mn, Ni, Fe, Co, and V), especially focusing on their electric-magnetic and mechano-magnetic coupling effects. The calculated binding energy and molecular dynamics simulation confirm that all these ribbons hold a high stability. Particularly, by TM-doping, the intrinsic non-magnetic semiconducting ribbon can be converted to a bipolar magnetic semiconductor (BMS), a half metal (HM), or a spin-degenerate magnetic semiconductor (MSC), presenting rich magnetic features. Meanwhile, this magnetism is flexibly tunable by the transverse electric field or stretched strain. For example, a low electric field can transform the ribbon into the HM phase, and the mechanic strain can realize a continuous magnetic phase transition among BMS, magnetic metal (MS), MSC, HM, and half semiconductor. These findings indicate that the TM-doped net-Y ribbon possesses favorable magnetism, which might have promising applications in magnetic nanodevices.

Decay, amplification and absorption of initial terahertz pulse in resonant two-level medium based on noninteracting array of zigzag nanotubes and armchair nanoribbons

Theoretical investigation and computer simulation of the evolution of terahertz (THz) radiation in a two-level resonance gain medium was carried out on the basis of Maxwell–Bloch equations, using the slowly varying amplitude approximation. The gain medium is based on a system of noninteracting parallel-oriented metallic carbon nanotubes and nanoribbons. Considering the Maxwell–Garnett theory (MGT), the effective dielectric function (EDF) of the medium was obtained. Due to the large nanotube dipole moment (or large value of oscillator strength) not only THz radiation occurs, but also a totality of periodic pulses generation. Results of simulation totally coincide with the theoretical ones. The equation for the phase by analogy with the self-induced transparency (SIT), taking into account the dissipative processes, was obtained. The obtained equation describes a mathematical pendulum, where friction force is proportional to the velocity and directed to the opposite of particle movement. The friction leads to the fact that the pendulum makes a full rotation around the axis or decays, depending on the absorption properties of a gain medium. It was shown that at the certain properties of a gain medium, the equation coincides with the well-known Sine-Gordon equation. It was obtained, that at the initial pumping [Formula: see text] m[Formula: see text] and at the volume fraction of nanoparticles about 10[Formula: see text], the stationary value of radiation energy in a resonator is about 1 J/m3.

Towards spin quantum materials: Structure and potential energy profiles of weakly interacting arrays of iron porphyrin complexes at graphene armchair nanoribbon

The atomic structure, the rates of atomic spin polarization and potential energy profiles of heterostructure composed by iron porphyrin deposited on armchair graphene nanoribbon (FeP/AGNR) were investigated using density functional theory (DFT). The multiplicity of the Fe+2 ion in iron porphyrin for all possible types of coordination was determined as a triplet. Two possible η2 and η6 types of FeP coordination on AGNR were located with 0.43 and 0.15 kcal/mol forward and reversed potential barriers and 39% and 61% Arrhenius populations, respectively. It was found that FeP/AGNR can be used for perspective spin quantum applications due to localized spin-flip transition in IR A spectral region.

The structure, and electronic and magnetic properties of MX (M=GA, IN; X=S, SE, TE) nanoribbons

We used the first principle of density functional theory to perform detailed calculations regarding the structure, and the electronic and magnetic properties of MX (M[Formula: see text]=[Formula: see text]Ga, In; X[Formula: see text]=[Formula: see text]S, Se, Te) nanoribbons. The armchair nanoribbons (ARNs) are nonmagnetic semiconductors, which have even or odd oscillations of bandgaps. All small-sized zigzag nanoribbons (ZRNs) were found to break the six-membered ring structure and move to the center, thereby exhibiting nonmagnetic semiconductor behavior owing to the quantum confinement effect. However, among the large ZRNs, which are all metals, MTe ZRNs are nonmagnetic; this differs from the case of graphene, MoS2 and Ti2CO2 nanoribbons. MX (M[Formula: see text]=[Formula: see text]Ga, In; X[Formula: see text]=[Formula: see text]S, Se) ZRNs exhibited ferromagnetism owing to the presence of the unpaired electrons on the metal-edge side and the magnetic moment of each pair of molecules, which was controlled by the size of the nanoribbons. The results provided a theoretical reference that can be used in the future to produce MX materials for application in low-dimensional semiconductor devices, spin electron transport devices and new magnetoresistance devices.

Electronic structures and charge carrier mobilities of boron-graphdiyne sheet and nanoribbons

Recently, a novel carbon based nanomaterial, boron-graphdiyne (BGDY) was experimentally realized through a bottom-to-up synthetic strategy, which is a π -conjugated structure comprised of all sp -hybridized carbon skeleton and boron heteroatoms in a well-organized two-dimensional (2D) molecular plane, and has been demonstrated to exhibit good semiconducting properties with excellent conductivity. The successful synthesis of this new graphyne material consequently raise the importance of the evaluation of its intrinsic properties, especially the electronic transport properties. In this paper, we firstly investigate the geometry and electronic structure of BGDY sheet based on density functional theory (DFT) calculations, then combined with the Boltzmann transport equation with relaxation time approximation, we predict its charge mobility. It's found that monolayer BGDY is a direct band-gap semiconductor with a moderate bandgap of about 1.26 eV, more importantly, both the electron and hole mobilities are isotopic and high, especially, the electron mobilities can approach as high as 10 5 cm 2 V −1 s −1 . Furthermore, we report the energy band gaps and carrier mobilities of two types of BGYD nanoribbons, called zigzag and armchair nanoribbons. Our numerical calculations show that all the nanoribbons showing semiconducting property and their mobilities are comparable to or even larger than that of BGDY sheet. • The charge mobility of a newly synthesized carbon allotrope, the boron-graphdiyne (BGDY) sheet was predicted based on first-principle calculations. • Monolayer BGDY is semiconductor with a moderate direct bandgap of 1.26 eV, both the electron and hole mobilities are isotopic and high. • All the 1D boron-graphdiyne nanoribbons (BGDYNRs) are found possessing semiconducting property. • The carrier mobilities of BGDYNRs are found comparable to or even larger than that of monolayer BGDY sheet.

DFT investigation of H2S adsorption on graphenenanosheets and nanoribbons: Comparative study

Graphenenanosheet (GNS), armchair graphenenanoribbon (AGNR), and zigzag graphenenanoribbon (ZGNR) systems were investigated by first principle calculations using the density functional theory (DFT). The DFT calculations explored the potential of utilization of these materials as gas sensors to detect hydrogen sulfide (H 2 S) gas. H 2 S gas adsorption was explored using: the adsorption energy (E ads ), adsorption distance (D), charge transfer (ΔQ), density of states (DOS), and band structure of the generated systems before and after adsorption of H 2 S. The results showed that E ads of bare ZGNR was the highest of −0.171 eV as compared with GNS and AGNR. The surfaces of GNS, AGNR, and ZGNR have been modified with epoxy and then with a hydroxyl groups. The adsorption capacity of the three systems has been enhanced after the modifications with both the epoxy and hydroxyl groups. Based on the adsorption energy and charge transfer results, hydroxyl modified ZGNR system can be used effectively for detection applications of H 2 S since it exhibits the highest charge transfer and large adsorption energy. • Graphene nanosheet, armchair nanoribbon, and zigzag nanoribbon were investigated by DFT. • The materials were then used as gas sensors to detect H 2 S gas. • Adsorption energy, adsorption distance, charge transfer, density of states, and band structure were investigated. • The surfaces of graphene systems were modified with epoxy and hydroxyl groups. • The adsorption capacity of the three systems has been enhanced after the modifications.

Analytical study of the electronic and optical properties of the armchair MoS2 nanoribbons

In this paper, a tight binding approach is employed to compute the electronic properties of the armchair MoS2 nanoribbon. Among the three distinct groups of pristine armchair nanoribbons, the family with index N = 3p+2, (p being an integer), renders the highest band gap energy. Furthermore, having the Hamiltonian matrix and using the imaginary part of the dielectric function, the optical properties of the armchair nanoribbons are calculated for different widths. The transitions between subbands of the armchair MoS2 nanoribbons lead to peaks in the dielectric function. Simulation results illustrate how ribbon's width, may be tuned to design nanoribbon's band gap for a specific optoelectronic or digital electronic application.

Optical properties of zigzag and armchair ZnO nanoribbons

Abstract We conducted extensive density functional theory calculations within GGA + U approach to explore the geometry, electronic structure and optical properties of armchair and zigzag zinc oxide nanoribbons as a function of width. Our results show that single layered armchair ZnONRs are semiconductors, whereas the zigzag counterparts are metallic. However, the electronic energy gap in armchair configurations decreases considerably when the width of the ribbon increase. The first absorption peak of the armchair NRs is placed along the in-plane polarization and inside the visible spectral range; while it occurs at energies less than 1.2 eV for zigzag NRs. Moreover, the first absorption peak of both zigzag and armchair configurations is located in the ultraviolet region along the out-of-plane polarization. In addition, for all cases, the optical spectra are highly anisotropic along the in-plane and out-of-plane directions. These diverse electronic and optical properties might open ZnONR materials great possibility in future optoelectronic applications.

A computational study of electric and magnetic properties of silicon carbon (SiC) armchair nanoribbons

SiC is a two-dimensional semiconductor with a direct band gap. Many authors studied its electric and magnetic properties in sheet and ribbon shapes. The previous papers have shown that SiC sheet and full saturated armchair ribbons are non-magnetic (NM) semiconductor while zigzag ribbons are ferromagnetic (FM) semiconductor. In this paper, we saturate only C (CH) and Si (SiH) atoms of armchair ribbon by hydrogen and study its electric and magnetic properties by density functional theory (DFT). Our results show that CH, saturated carbon atoms in armchair ribbon, can show both FM and antiferromagnetic (AFM) semiconductor states while SiH, saturated silicon atoms in armchair ribbon, can show both FM half-metal and AFM metallic states. We found that the total magnetic moment of the unit cell is about 2 μB. Our results confirm that armchair ribbons can show many different magnetic states only by hydrogen saturation.

The effect of strain on the zigzag and armchair phosphorene nanoribbon

Abstract In this work, the band structure and electronic properties of different nanoribbons of phosphorene along two typical crystal directions of armchair (AM) and zigzag (ZZ) were investigated under compression and tensile strain by density functional theory. The results were compared to the band structure and electronic properties of phosphorene monolayer at similar conditions. The electronic properties of armchair nanoribbons revealed to be resistant to tensile strain in the direction of the armchair, consistent to phosphorene monolayer, but they undergo band structure change due to the applied compression strain at armchair direction. By comparing the effects of strain on the zigzag nanoribbon and phosphorene monolayer, it was found that the zigzag direction neither tolerates the tensile strain nor resists compression strain.

Quantum Anomalous Hall Effects in Graphene from Proximity-Induced Uniform and Staggered Spin-Orbit and Exchange Coupling.

We investigate an effective model of proximity modified graphene (or symmetrylike materials) with broken time-reversal symmetry. We predict the appearance of quantum anomalous Hall phases by computing bulk band gap and Chern numbers for benchmark combinations of system parameters. Allowing for staggered exchange field enables quantum anomalous Hall effect in flat graphene with Chern number C=1. We explicitly show edge states in zigzag and armchair nanoribbons and explore their localization behavior. Remarkably, the combination of staggered intrinsic spin-orbit and uniform exchange coupling gives topologically protected (unlike in time-reversal systems) pseudohelical states, whose spin is opposite in opposite zigzag edges. Rotating the magnetization from out of plane to in plane makes the system trivial, allowing us to control topological phase transitions. We also propose, using density functional theory, a material platform-graphene on Ising antiferromagnet MnPSe_{3}-to realize staggered exchange (pseudospin Zeeman) coupling.

Computational study of spin caloritronics in a pristine and defective antimonene nanoribbon

In this paper, by using first-principle density functional theory (DFT) combined with non-equilibrium Green's function (NEGF), thermally induced spin current in zigzag and armchair Antimonene Nanoribbon (SbNR) is investigated. Also, we obtain higher spin current in Armchair nanoribbon (ANR) than zigzag nanoribbon (ZNR), because the start energy of transmission for ANR is closer to the Fermi level than ZNR. The results show that the device has a perfect spin Seebeck effect under temperature difference without gate voltage or bias voltage. For the ANR configuration, the competition between spin up holes and spin down electrons leads to negative differential behavior of charge current, which is essential for the design of thermal transistors. We also investigate the effect of a single vacancy and double vacancy on the structural and electronic properties of ZNR and ANR. The results show that vacancy in ZNR leads to a decrease in spin currents, whereas in ANR, spin currents increase. Double vacancy in ANR can significantly enhance spin up current up to about 2.5 times the pristine one. The results verify the Armchair Nanoribbon as a promising candidate for spin caloritronics devices, which can be used in future low power consumption technology.