Properties Of Zigzag Graphene Nanoribbons Research Articles

Impact of uniaxial strain on physical properties of zigzag graphene nanoribbons with topological defects

The synergistic regulation mechanism of uniaxial strain, topological defects, edge passivation atom and nanoribbon width on the geometric and electronic structures of zigzag graphene nanoribbons have been studied systematically by first-principles. It is found that the average formation energy and strain energy of X-N 1 N 2-LD-ZGNR (X = H, F and O, as well as, N 1 = N 2 = 3, 4 and 5) increase with the increase of uniaxial strain, and this relationship is also dependent of edge passivation atom species and nanoribbon width. And the edge of 55-LD-ZGNR passivating with O and F atoms is more beneficial than H atom for system stability. The stress–strain curve shows that the limiting strain of zigzag graphene nanoribbon depends on edge passivation atom species and nanoribbon width. The Young’s modulus in the case of ε > 3% and Poisson’s ratio except O-33-LD-ZGNR at ε = 1% of X-N 1 N 2-LD-ZGNR decrease with the increase of the tensile strain, and is dependent of nanoribbon width and edge atom species. And O-55-LD-ZGNR is easier than F-55-LD-ZGNR and H-55-LD-ZGNR to be stretched or compressed. The magnetism is induced in both H-55-LD-ZGNR and F-55-LD-ZGNR, and remains with the increases of uniaxial tension strain. What is more, magnetic property of O-55-LD-ZGNR can be regulated by applying uniaxial strain, and the band gap of the O-N 1 N 2-LD-ZGNR (N 1 = N 2 = 3, 4 and 5) system can be regulated by adjusting the uniaxial tensile strain and nanoribbon width. Our research provides a new method to open the graphene band gap, which can provide some new theoretical guidance for the application of graphene in electronic devices and other fields. The band gap of the O-LD-ZGNDR system is opened as the uniaxial tensile strain increases.

Spin thermoelectric properties induced by hydrogen impurities in zigzag graphene nanoribbons.

This study investigates the impact of hydrogen impurities on the spin-dependent thermoelectric properties of zigzag graphene nanoribbons (ZGNRs) through density functional theory and the Landauer-Büttiker formula. Hydrogenation induces a net magnetism with a localized spin-dependent band around the Fermi energy in ZGNRs with different spin configurations, such as antiferromagnetic and ferromagnetic states. The results reveal spin-semiconducting behavior with a tunable energy gap and fully spin-polarized states in certain energy ranges. Application of a thermal gradient induces a thermal spin current, leading to the emergence of the spin Seebeck effect (SSE). By strategically placing a single hydrogen atom in various positions within the ZGNRs, we demonstrate that the hydrogen impurity's location significantly influences the spin-dependent thermoelectric properties, offering opportunities for enhanced thermoelectric performance through engineering. The observed spin thermocurrent and SSE in different impurity locations, considering both ferromagnetic and antiferromagnetic spin configurations, highlight the potential of hydrogenated ZGNRs in spin-dependent thermoelectric devices.

Spin Seebeck effect and thermal properties of zigzag graphene nanoribbons with edge magnetism

Zigzag graphene nanoribbons (GNRs), demonstrating edge magnetism, are fascinating materials for electronic and spintronic applications. In this paper, we investigate with explicit consideration of electron-phonon scattering the Seebeck effect and thermal conductivity at various carrier densities in zigzag GNRs, which undergo antiferromagnetic, ferromagnetic, and paramagnetic transitions. Seebeck coefficients are spin dependent in ferromagnetic zigzag GNRs and can have opposite signs for the majority and the minority spin carriers, which enables the separation of spin carriers spatially under a thermal gradient. The electronic thermal conductivity is small compared to the lattice thermal conductivity, except for GNRs in the ferromagnetic state. A 100% increase in thermal conductivity is expected at antiferromagnetic to ferromagnetic transition. These results demonstrate that zigzag GNRs are also materials of high interest for spin caloritronics.

Effects of metallic electrodes on the thermoelectric properties of zigzag graphene nanoribbons with periodic vacancies

We theoretically analyze the thermoelectric properties of graphene quantum dot arrays (GQDAs) with line- or surface-contacted metal electrodes. Such GQDAs are realized as zigzag graphene nanoribbons (ZGNRs) with periodic vacancies. Gaps and minibands are formed in these GQDAs, which can have metallic and semiconducting phases. The electronic states of the first conduction (valence) miniband with nonlinear dispersion may have long coherent lengths along the zigzag edge direction. With line-contacted metal electrodes, the GQDAs have the characteristics of serially coupled quantum dots (SCQDs) if the armchair edge atoms of the ZGNRs are coupled to the electrodes. By contrast, the GQDAs have the characteristics of parallel quantum dots if the zigzag edge atoms are coupled to the electrodes. The maximum thermoelectric power factors of SCQDs with line-contacted electrodes of Cu, Au, Pt, Pd, or Ti at room temperature were similar or greater than 0.186 nW K−1; their figures of merit were greater than three. GQDAs with line-contacted metal electrodes have much better thermoelectric performance than surface contacted metal electrodes.

Optical spectra of zigzag graphene nanoribbons: a first-principles study

The electrical and optical properties of zigzag graphene nanoribbons is simulated using Density Functional Theory (DFT). It is found that n-ZGNRs (n = 18, 22, 26, 30) have metallic behavior. The results reveal that when the nanoribbon width increases, the first optical bandgap and optical absorption peaks diminish. Furthermore, at energies greater than around 1.40 eV, n-ZGNRs (n = 18, 22, 26, 30) have a negative refractive index for the z polarization. The maximum optical reflection peaks of n-ZGNRs (n = 18, 22, 26, 30) occur at about 5 eV for y polarization. Furthermore, unlike the y polarization, the z components of optical reflectivity are positioned at lower energies. The results show that the optical extinction of the simulated ZGNRs increases with ribbon width. The main reason is that the larger supercells have a higher mass density. For ∼ 0–4.50 eV, n-ZGNRs (n = 18, 22, 26, 30) support the conditions for the transverse-electric (TE) mode. Additionally, for higher than around 4.50 eV, n-ZGNRs (n = 18, 22, 26, 30) supplies the transverse-magnetic (TM) plasmons.

Zigzag graphene nanoribbons separated by F/O as interconnects for all-carbon integrated circuits: A DFT investigation

Adsorbing atom is a promising technology for graphene nanoribbons separation. Using a combination of density functional theory (DFT) and non-equilibrium Green's function (NEGF), we investigated stability and electronic transport properties of zigzag graphene nanoribbons (ZGNRs) that adsorb a row of fluorine or oxygen atoms on the central zigzag carbon chain. Results indicate that all ZGNRs after adsorption can exist stably. Adsorbing separation has excellent current work in comparison with the results of mechanical separation. Transmission pathways of electrons and local density of states (LDOS) reveal that fluorinated carbon chain can prevent electrons moving. Two sections on the ZGNR are separated by the row of fluorine atoms that are shown to function as a transport boundary between them. However, the row of oxygen atoms cannot separate ZGNR. Electrons tend to move around oxygen atoms and the lone-pair electrons of oxygen atoms participate in the transport of electrons, resulting in an improvement of carrier mobility. Realizing this adsorbing separation of ZGNR will provide new ideas for interconnects wiring in all-carbon integrated circuits.

Impurity Substitution Enhances Thermoelectric Figure of Merit in Zigzag Graphene Nanoribbons

The thermoelectric properties of zigzag graphene nanoribbons (ZGNRs) are sensitive to chemical modification. In this study, we employed density functional theory (DFT) combined with the nonequilibrium green’s function (NEGF) formalism to investigate the thermoelectric properties of a ZGNR system by impurity substitution of single and double nitrogen (N) atoms into the edge of the nanoribbon. N-doping changes the electronic transmission probability near the Fermi energy and suppresses the phononic transmission. This results in a modified electrical conductance, thermal conductance, and thermopower. Ultimately, simultaneous increase of the thermopower and suppression of the electron and phonon contributions to the thermal conductance leads to the significant enhancement of the figure of merit in the perturbed (i.e., doped) system compared to the unperturbed (i.e., nondoped) system. Increasing the number of dopants not only changes the nature of transport and the sign of thermopower but also further suppresses the electron and phonon contributions to the thermal conductance, resulting in an enhanced thermoelectric figure of merit. Our results may be relevant for the development of ZGNR devices with enhanced thermoelectric efficiency.

Valley-resolved transport in zigzag graphene nanoribbon junctions

Applying the transfer matrix and Green’s function methods, we study the valley-resolved transport properties of zigzag graphene nanoribbon (ZGNR) junctions. The width of the left and right ZGNRs are N L and N R , and N L ≥ N R . The step/dip positions of the conductance G, the intravalley transmission coefficients (T KK and ), and the valley polarization efficiency correspond to the subband edges of the right/left ZGNR that are controlled by N R /N L . The intervalley transmission coefficients ( and ) exhibit peaks at most of the subband edge of the left and right ZGNRs. In the bulk gap of the right ZGNR, =0, and = ±1, the valley polarization is well preserved. As N R increases, the energy region for = ±1 decreases. When N L is fixed and N R decreases, G, T KK , and exhibit more and more dips, and the peaks of () become more and more high, especially when (N L − N R )/2 is odd. These characters are quite useful for manipulating the valley dependent transport properties of carriers in ZGNR junctions by modulating N L or N R , and our results are helpful to the design of valleytronics based on ZGNR junctions.

Ab Initio Study of Electronic Properties of Zigzag Graphene Nanoribbon Doped with B, N, and P

Graphene, the mother of carbon nanostructures with its 2D character, has been extensively studied for its outstanding structure, electronic, optical, and magnetic properties. A great deal of effort has been placed on tuning the bandgap of graphene nanoribbons which are quasi-one-dimensional structures of graphene. The present work envisages ab initio calculations on the geometry and electronic properties of zigzag graphene nanoribbon ZGNR (N5) together with Boron (B), Nitrogen (N), and Phosphorus (P) as dopants. The band structure of ZGNR (N5) shows a metallic (Eg = 0 eV) transition, whereas, B, N, P doped ZGNR (N5) systems show a semiconducting (Eg > 0 eV) behavior in the band structure. The influence of doping on the Fermi level together with variations in the structure and electronic properties has been investigated. The study was carried out by varying the concentrations of B, N, P dopants from 1.11% to 4.44% in ZGNR (N5) systems and will be utilized to fabricate ZGNR (N5) based nanoelectronic devices.

Controlling electronic and optical properties of zigzag graphene nanoribbons by a modulated electric field: significance of σ bands

We investigate the peculiar electronic and optical properties of zigzag graphene nanoribbons (GNRs) under the effects of uniform and modulated electric fields. The hybridization between the four atomic orbitals ( s , p x , p y , p z ) is considered by means of the tight-binding model calculations. Both the π bands originating from the p z orbital and σ bands arising from the ( s , p x , p y ) orbitals play critical roles in the electronic structure and thus the absorption spectra. They determine the Fermi level and the available excitation channels between the occupied and unoccupied states. We show that an in-plane electric field in the form of modulated potential can significantly modify the electronic and optical properties of zigzag GNRs. The field-induced modification in energy dispersion, band-edge states, and band gap gives rise to rich absorption spectra with diverse structures. Our theoretical prediction could be verified by angle-resolved photoemission spectroscopies and optical spectroscopies.

The Strain-Tuned Spin Seebeck Effect, Spin Polarization, and Giant Magnetoresistance of a Graphene Nanobubble in Zigzag Graphene Nanoribbons.

By using first-principle calculations combined with the non-equilibrium Green’s function approach, we studied the spin caloritronic properties of zigzag graphene nanoribbons with a nanobubble at the edge (NB-ZGNRs). The thermal spin-polarized currents can be induced by a temperature difference, and the spin Seebeck effect is found in the nanoribbon. The spin polarization, magnetoresistance, and Seebeck coefficients are discussed, which are strongly affected and can be tuned by the geometrical strain. Moreover, some novel spin caloritronic devices are designed, such as a device that generates bidirectional perfect spin currents and thermally induced giant magnetoresistances. Our results open up the possibility of tuning the spin caloritronic properties of the NB-ZGNR-based devices by changing the elastic strain on the graphene nanobubble.

Impact of Topological Edge Defects on Spin Transport Properties of Zigzag Graphene Nanoribbons

Herein, the spin‐dependent transport properties of a zigzag graphene nanoribbon (zGNR) with edges decorated with a fluoranthene group are studied. Atomically perfect zigzag edge, including phenyl‐edge functionalization, was synthesized by Ruffieux et al. in a bottom‐up chemical technique. By performing nearest‐neighbor tight‐binding model in combination with Landauer formalism and Green's function approach, as well as considering Coulomb electronic interaction computed both with density functional calculations and mean‐field approximation of the Hubbard model, the magnetic and spin‐resolved transmissions are studied. Both the model Hamiltonian and density functional theory (DFT) descriptions yield very similar results. Importantly, by generating a special supercell, three different antiferromagnetic alignments for phenyl‐edge zGNR have been calculated, which was not developed in previous studies. The results show that by adding pentagon rings on both sides of the zGNR, the conductance loses its step‐like behavior. Our structure of interest is a semiconductor with a transport gap of 0.56 eV. This kind of defective structure will enable the spin‐feature characteristic, such as spin filtering, and add the spin degree of freedom to graphene‐based logic devices.

Probing divacancy defects in a zigzag graphene nanoribbon through an RKKY exchange interaction

We investigate the effects of vacancy defects on the electronic and magnetic properties of zigzag graphene nanoribbons (zGNRs) by making use of the Green’s function formalism in combination with a tight-binding Hamiltonian. We explain the evolution of indirect exchange coupling, known as the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction, including single, double, and multiple 5-8-5 divacancy (DV) defects. Our numerical calculations show that changes in the electronic structure and exchange coupling of zGNRs depend to a significant degree on the location of DV defects with respect to the ribbon edges and the number of DV defects. In the case where both impurities are located on the edge, the magnitude of the exchange coupling is several orders of magnitude higher than when they are placed on the interior of the nanoribbon. Furthermore, a periodic DV causes a dramatic change in the magnetic ground state of the ribbon. In the limit of a high vacancy potential, the strength of the RKKY interaction is approximately independent of the Fermi energy. We demonstrate that, on the one hand, the defect engineering of atomic vacancies is a promising way to modify the magnetic properties of graphene nanoribbons, and, on the other hand, unusual RKKY oscillations around the DVs are a new technique for directly probing the local vacancies in a zGNR through the RKKY exchange interaction.

Tuning flexoelectricty and electronic properties of zig-zag graphene nanoribbons by functionalization

The flexoelectric and electronic properties of zig-zag graphene nanoribbons are explored under mechanical bending using state of the art first principles calculations. A linear dependence of the bending induced out of plane polarization on the applied strain gradient is found. The inferior flexoelectric properties of graphene nanoribbons can be improved by more than two orders of magnitude by hydrogen and fluorine functionalization (CH and CF nanoribbons). A large out of plane flexoelectric effect is predicted for CF nanoribbons. The origin of this enhancement lies in the electro-negativity difference between carbon and fluorine atoms, which breaks the out of plane charge symmetry even for a small strain gradient. The flexoelectric effect can be further improved by co-functionalization with hydrogen and fluorine (CHF Janus-type nanoribbon), where a spontaneous out of plane dipole moment is formed even for flat nanoribbons. We also find that bending can control the charge localization of valence band maxima and therefore enables the tuning of the hole effective masses and band gaps. These results present an important advance towards the understanding of flexoelectric and electronic properties of hydrogen and fluorine functionalized graphene nanoribbons, which can have important implications for flexible electronic applications.

Effects of local shear strain on the zigzag graphene nanoribbon with a topological line defect

Abstract First-principles density functional theory calculations are performed to study the effects of local shear strain on the structural, mechanical and electronic properties of zigzag graphene nanoribbon with a topological line defect (LD-ZGNR). By changing the nanoribbon width N (N is the number of zigzag chains in the left or right ribbons of the topological line defect) and deformation positions, the optimal position is obtained. The mechanical properties of the LD-ZGNR such as stress-strain curves exhibit nonlinear behavior with strain, and the shear modulus is strongly dependent on the nanoribbon width N and deformation positions. Furthermore, the strong interactions among atoms of the LD-ZGNR under local shear strain induce dramatic changes to the electronic properties of the system. This work reveals mechanism of LD-ZGNR under local shear strain, and provides new idea and more accurate data for the design and application of graphene flexible elastic electronic devices.

Spin-dependent band-gap driven by nitrogen and oxygen functional groups in zigzag graphene nanoribbons

We investigated the role of nitrogen and oxygen functional groups on the electronic and magnetic properties of zigzag graphene nanoribbons (ZGNRs) using spin-polarized first-principles density functional theory calculations. Twenty four functional groups of nitrogen, oxygen, and nitrogen-oxygen configurations were attached to the edge of ZGNRs. We analyze the band structure, spin-resolved bandgap, charge transfer, magnetic ordering, and binding energy. Similar to the pristine ZGNR, an indirect bandgap semiconducting behavior dominated in functionalized ribbons, except for carbonyl, pyrane, and pyridinium groups, that displays a metallic behavior. Spin-resolved band gap calculations revealed a strong dependence on the functional groups. Carbonyl groups showed a half-metallicity behavior with only spin up states at the Fermi level. In general, the functionalized ribbons displayed ferromagnetic alignment along the edges, but antiferromagnetic alignment between them, resulting in a total null magnetization. We showed some exceptions, the carbonyl, pyrane, lactam, pyridine N-oxide, and pyridinium functional groups induced significant changes in the magnetic arrangement, not only along the edge where they were attached but also to the opposite side. We have also incorporated a pentagon-heptagon pair or a single pentagon defect at the edge to explore the quinone, pyrone, and pyridone functional groups.

Controlling the electronic properties of zigzag graphene nanoribbon using amino acids and oxygen molecule-A first principles DFT study

The density functional study on the functionalization of zigzag graphene nanoribbon [ZGNR(6,0)] was carried out by using the amino acids l-Serine and l-Valine with/without O2 molecule. Valine is an essential amino acid with hydrophobic character; Serine is a non-essential amino acid with polar nature. The functionalized systems were named as ZGNR-Ser, ZGNR-Val, ZGNR-SerO2, ZGNR-ValO2 respectively. The structural stability was analyzed by taking the difference in total energy between the pre and post optimized structures until the change comes to be of the order of 10−2 to 10−3 eV. The optimization was performed by using the local density approximation (LDA) with a k- points sampling of 1x1x150. The electronic properties were studied by using the partial charges, chemical potential, electrostatic field, electrostatic strain, static dielectric constant, the density of states, and band structure analysis. A direct relation between electrostatic energy and total energy was observed in all the systems. The band structure results envision that the ZGNR-SerO2, ZGNR-ValO2 systems show a semiconducting behavior, whereas the ZGNR-Ser, ZGNR-Val systems show a metallic behavior. The chemical potential analysis suggests a blue shift for the ZGNR-SerO2, ZGNR-ValO2 in comparison to the non‑oxygen counterparts. The present study indicates the possibility of tuning the electronic properties through covalent functionalization using the amino acids and O2 molecule and enunciates the option of using them in bioimaging, drug delivery, or theranostic applications.

Influence of single vacancy defect at varying length on electronic properties of zigzag graphene nanoribbons

Graphene, identified in 2004, is now an established two-dimensional (2D) material due to its outstanding physical and electronic characteristics namely its superior electrical conductivity. Graphene is a zero-gap material that has linear dispersion with electron-hole symmetry. As pristine sheet, it cannot be utilized in digital logic application without the induction of a band gap inside the band structure. In our work, the modeling and simulation of graphene nanoribbons (GNRs) are carried out to determine its electronics properties that are benchmarked with other published simulation data. A 4-Zigzag GNRs (4-ZGNRs) under different length are utilized. A single vacancy defects is introduced at various positions inside the atomic structure. The theoretical model is implemented based on single-neighbour tight binding technique coupled with a non-equilibrium Green’s function formalism. The single vacancy defects are represented by the elimination of tight binding energies in the Hamiltonian matrix. Subsequently, these matrix elements are utilized to compute dispersion relation and density of states (DOS) through Green’s function. It is found that single vacancy defects at different positions in 4-ZGNRs’ atomic structure under varying length has no significant impacts on the sub-band structure but these vacancies impact the DOS that are computed throught Green’s function approach.

Influence of single vacancy defect at varying length on electronic properties of zigzag graphene nanoribbons

Graphene, identified in 2004, is now an established two-dimensional (2D) material due to its outstanding physical and electronic characteristics namely its superior electrical conductivity. Graphene is a zero-gap material that has linear dispersion with electron-hole symmetry. As pristine sheet, it cannot be utilized in digital logic application without the induction of a band gap inside the band structure. In our work, the modeling and simulation of graphene nanoribbons (GNRs) are carried out to determine its electronics properties that are benchmarked with other published simulation data. A 4-Zigzag GNRs (4-ZGNRs) under different length are utilized. A single vacancy defects is introduced at various positions inside the atomic structure. The theoretical model is implemented based on single-neighbour tight binding technique coupled with a non-equilibrium Green’s function formalism. The single vacancy defects are represented by the elimination of tight binding energies in the Hamiltonian matrix. Subsequently, these matrix elements are utilized to compute dispersion relation and density of states (DOS) through Green’s function. It is found that single vacancy defects at different positions in 4-ZGNRs’ atomic structure under varying length has no significant impacts on the sub-band structure but these vacancies impact the DOS that are computed throught Green’s function approach.

Spin-dependent properties in zigzag graphene nanoribbons with phenyl-edge defects

Recent outbreaks in bottom-up chemical techniques have demonstrated the synthesis of atomically perfect zigzag graphene nanoribbons (ZGNRs) and of their corresponding analogs with phenyl-edge functionalization [P. Ruffieux et al., Nature 531, 489 (2016)]. Since spin-polarized currents can be generated at the edges of the ZGNRs, the control of phenyl decoration at the zigzag edges could accurately tune the electronic properties of these carbon-based nanoribbons. In the present paper, using first-principles calculations and a Landauer-B\uttiker approach, the electronic and magnetic properties as well as the spin-resolved transmissions are investigated in various phenyl-edge-modified ZGNRs. The understanding of the spin-dependent transport properties in relation to the atomic structure opens the way to design devices such as spin valves for future spintronics applications.