Zigzag-edged Graphene Nanoribbons Research Articles

Topological Structure Realized in Cove-Edged Graphene Nanoribbons via Incorporation of Periodic Pentagon Rings.

Cove-edged zigzag graphene nanoribbons are predicted to show metallic, topological, or trivial semiconducting band structures, which are precisely determined by their cove offset positions at both edges as well as the ribbon width. However, due to the challenge of introducing coves into zigzag-edged graphene nanoribbons, only a few cove-edged graphene nanoribbons with trivial semiconducting bandgaps have been realized experimentally. Here, we report that the topological band structure can be realized in cove-edged graphene nanoribbons by embedding periodic pentagon rings on the cove edges through on-surface synthesis. Upon noncontact atomic force microscopy and scanning tunneling spectroscopy measurements, the chemical and electronic structures of cove-edged graphene nanoribbons with periodic pentagon rings have been characterized for different lengths. Combined with theoretical calculations, we find that upon inducing periodic pentagon rings the cove-edged graphene nanoribbons exhibit nontrivial topological structures. Our results provide insights for the design and understanding of the topological character in cove-edged graphene nanoribbons.

Rotating single molecule-based devices: Single-spin switching, negative differential electrical and thermoelectric resistance

Single molecule-based devices have been one of the most candidates to realize the miniaturization of traditional electronic devices. Here we investigate the spin-polarized transport properties of an iron-complex molecule sandwiched between two ferromagnetic zigzag-edged graphene nanoribbon electrodes based on the non-equilibrium Green’s function formalism combined with the density functional theory. When the iron-complex molecule rotates, a single-spin switching effect is observed at the Fermi level, and meanwhile a perfect spin filtering effect appears in the molecular device. As the electric bias increases, an obvious negative differential electrical resistance is observed. In addition, we also find a negative differential thermoelectric resistance in the absence of the electric bias under a small molecular rotation, and the sign and magnitude of the thermally driven current can be tuned by the temperature. These theoretical findings imply that the iron-complex molecular devices have potential applications in the next-generation spin electric and thermoelectric devices.

Effect of DNA bases on the thermoelectric performance in graphene nanoribbons

The thermoelectric transport properties of four kinds of DNA bases sandwiched between two zigzag-edge graphene nanoribbon electrodes are investigated by first-principles calculation. The modulation of quantum interference, molecular length, and molecular torsion on thermoelectric transport properties are studied. It is found that the thermopower and efficiency can be obviously enhanced in the guanine and electrode coupling geometry that manifests a quantum interference effect. A decrease of thermopower with a number of DNA bases has been found, while the efficiencies have a different trend with the increase of the number of different molecules. In addition, the thermopower and efficiency also can be considerably enhanced by the twisting angle between the molecule and graphene electrodes.

Effectuating tunable valley selection via multiterminal monolayer graphene devices

Valleytronics using two-dimensional materials opens unprecedented opportunities for information processing using a valley polarizer as a basic building block. Various methodologies, such as strain engineering, the inclusion of line defects, and the application of static magnetic fields, have been widely explored for creating valley polarization. However, these methods suffer from low transmission or lack of polarization directionality. To overcome the above limitations, we propose an all-electrical valley polarizer using zigzag edge graphene nanoribbons in a multiterminal device geometry. The proposed device can be gate-tuned to operate along two independent regimes: (i) a terminal-specific valley filter that utilizes band-structure engineering, and (ii) a parity-specific valley filter that exploits the parity selection rule in zigzag edge graphene. We show that the device exhibits intriguing physics in the multimode regime of operation affecting the valley polarization; we investigate various factors influencing polarization in wide-device geometries, such as optical analogs of graphene Dirac fermions, angle-selective transmission via $p\ensuremath{-}n$ junctions, and the localization of edge states. Furthermore, we evaluate the performance of the proposed device structures in the presence of Anderson short-range disorder at the edges and the bulk, and we find it to be resilient to edge disorder even for a higher disorder strength. The device geometry is optimized to achieve maximum valley polarization, thereby paving the way for a physics-based tunable valleytronic device.

Band structure engineering and transport properties of graphene/BN van der Waals heterostructures

It is an important topic in spintronics to find regulable half-metallic materials. In this work, by using density functional theory and nonequilibrium Green’s functional method, we find that the van der Waals heterostructures formed by zigzag-edged boron nitride nanoribbons and zigzag-edged graphene nanoribbons (ZGNRs) are spin splitting semiconductors. The energy band gap of the heterostructures can be regulated by applying a local voltage and the semiconductor–metal–semiconductor phase transition can be realized. Two spin-dependent transport devices are constructed based on the heterostructures. One device can generate a fully spin-polarized current within a small bias range, and the other device can generate high spin polarization within a large bias range. Moreover, based on zigzag-edged graphene/boron nitride nanoribbon (ZGBNNR) heterostructures, a dual-probe device with double vertical gates in the electrode regions is designed, which can form a metal–semiconductor–metal tunneling device. These findings indicate that ZGBNNRs can be used in excellent spintronic devices.

Cross-plane transport in cyclo[18]carbon-based molecular devices

The isolated cyclo[18]carbon (C18) has a vertically discrete π-electron conjugate system that possesses unique electronic properties. However, the exploration on cross-plane transport properties of C18-based molecular devices is still in a rudimentary stage. In this work, we propose a type of spin-filtering device with AA-stacked C18 as the central molecule sandwiched between two zigzag-edged graphene nanoribbons (ZGNRs) and investigate the cross-plane transport behaviors in terms of a nonequilibrium Green's function method and a density functional theory. We find that it exhibits the integrated effect of negative differential resistance, spin-filtering, switching, and half-metallic properties. Moreover, it shows conspicuous magnetoresistance (MR) and rectification effects. The MR can be up to 12 480% when it is clamped vertically between two ZGNRs, and the rectification ratio can reach 2926 through inserting a transition metal atom V. Our results provide an avenue for the design of next-generation carbon-based molecular spintronic devices.

Perylene-based molecular device: multifunctional spintronic and spin caloritronic applications.

Carbon-based magnetic molecular junctions are promising candidates for nanoscale spintronic applications because they are atomically thin and possess high stability and peculiar magnetism. Herein, based on first-principles and non-equilibrium Green's function, we designed a carbon-based molecular spintronic device composed of carbon atomic chains, zigzag-edged graphene nanoribbon (ZGNR), and a perylene molecule. Our results show that the device exhibits integrated spintronic and spin caloritronic functionalities, such as the bias-voltage driven spin filtering effect, negative differential resistance effect and giant magnetoresistance, temperature-gradient driven spin Seebeck effect, thermal spin filtering effect, high thermal magnetoresistance, and thermal colossal giant magnetoresistance. Furthermore, considering the phonon vibration effect, the spin and charge thermoelectric figure of merits (ZTsp and ZTch) can be enhanced and the peak of ZTsp is much larger than that of ZTch, indicating the excellent thermospin performance. The asymmetrical contact configuration between the carbon atomic chain and perylene/ZGNR inhibits the phonon thermal conductivity significantly, leading to the optimal ZTsp and ZTch of 2.4 and 0.5 at 300 K, respectively. These results suggest multifunctional spintronic and spin caloritronic applications for the perylene-based molecular device.

Band Structure and Quantum Transport of Bent Bilayer Graphene.

We investigate the band structures and transport properties of a zigzag-edged bent bilayer graphene nanoribbon under a uniform perpendicular magnetic field. Due to its unique geometry, the edge and interface states can be controlled by an electric field or local potential, and the conductance exhibits interesting quantized behavior. When Zeeman splitting is considered, the edge states are spin-filtered, and a weak quantum spin Hall (WQSH) phase appears. In the presence of an electric field or local potential, a WQSH-QH junction or WQSH-spin-unbalanced QSH junction can be achieved, respectively, while fully spin-polarized currents appear in the interface region. Zeeman splitting lifts the spin degeneracy, leading to a WQSH around zero energy with a quantized two-terminal conductance of 4e2/h, which is robust against weak nonmagnetic disorder. These results provide a way to manipulate the band structures and transport properties of the system using an electric field, local potential, and Zeeman splitting.

Multiple hydrogen bonds enhancing formaldehyde adsorption on functionalized zigzag graphene nanoribbons

In this work, based on density functional theory, the edge and the center of both armchair-edged graphene nanoribbons (AGNRs) and zigzag-edged graphene nanoribbons (ZGNRs) have been modified through five oxygen-containing groups (epoxy, methoxy, formyl, hydroxyl and carboxyl) to form functionalized AGNRs and ZGNRs, in which distorted surface of AGNRs can be observed. Computational results indicated that ZGNR is appealing choice to act as a candidate for designing formaldehyde (HCHO) sensor. Simulation results demonstrate that the interactions between HCHO and oxygen-containing ZGNRs were mainly determined by the multiple hydrogen bonds. The novel design of carboxyl modified ZGNRs on the center of surface with the largest adsorption energy of − 0.510 eV and the relatively long recovery time of 4.15 × 10-4 s could act as a physical sensor to detect HCHO molecules. Dynamics simulation results indicated that HCHO shows very strong hydrogen bonding interactions during competitive adsorption with N2 and O2 at 298.15 K and 308.15 K. Consequently, simulation results offer a molecular-level insight into the interactions between HCHO and functionalized ZGNR surfaces, and might provide useful information for the design of HCHO sensors.

Geometry dependent current–voltage characteristics of T-stub zigzag-edged graphene nanoribbon quantum waveguides

Geometry dependent current–voltage characteristics of T-stub zigzag-edged graphene nanoribbon quantum waveguides

All-carbon multifunctional molecular spintronic device: A first-principles study

Based on the first-principles density functional theory combined with the nonequilibrium Green’s function methodology, we have designed and investigated the spin-polarized transport properties of an all-carbon molecular device, in which a C60 fullerene is sandwiched between two ferromagnetic zigzag-edge graphene nanoribbon electrodes. The results show that the spin-polarized properties are strongly dependent on the external magnetic field modulation and spin-filtering, giant magnetoresistance and spin-rectifying effects are realized in this device. The mechanisms are proposed for these interesting phenomena. These results demonstrate that the designed all-carbon device holds great potentials in the development of multifunctional molecular spintronic device.

Tunable giant magnetoresistance ratio in bilayer CuPc molecular devices.

We investigate the influence of the distance between the buffer layer and the central molecule on the electrical transport, spin-filter transport, magnetoresistance effects and thermoelectric properties of a bilayer CuPc molecular device with V-shaped zigzag-edged graphene nanoribbon (VZGNR) electrodes by combining density functional theory and the non-equilibrium Green's function. The results show that the spin-dependent total conductance and spin filter efficiency of the bilayer CuPc molecular device reach a maximum with a parallel spin configuration (PC) when the carbon atom at the edge of the electrode is in the center of the carbon atom at the edge of the bilayer molecules due to the stronger coupling interaction between the double-layer molecules and the leads. Moreover, the spin polarization of the bilayer CuPc molecular device is reversed at certain distances; there is a minimum spin filter efficiency (SFE) of −99.93448% and a maximum SFE of 97.91% observed in the anti-parallel spin configuration (APC) of the device and there is a minimum SFE of −26.03175% and a maximum SFE of 99.99996% observed with the PC at zero bias. The SFE oscillates with increasing considered bias voltage in the PC and APC when the distances are d = 0 Å and d = −1.06 Å, and a negative differential resistance (NDR) effect was observed. For the PC and APC, there is a giant magnetoresistance (MR) effect and the MR ratio exceeds 5.21 × 107% (99.9996%), and the MR ratio oscillates with increasing considered bias voltage when d = 0 Å. The MR ratio could be reserved by applying a certain bias voltage. These transport behaviors can be well understood by analyzing the transmission spectra, projected density of states and scattering states. There are pure spin Seebeck coefficients and pure charge Seebeck coefficients at certain temperatures when the distances are certain values, which means that the corresponding temperature differences could produce pure spin current and pure charge current, respectively. Our results provide new ideas for designing ultrahigh-performance spintronic molecular devices.

Cove-Edged Graphene Nanoribbons with Incorporation of Periodic Zigzag-Edge Segments.

Structurally precision graphene nanoribbons (GNRs) are promising candidates for next-generation nanoelectronics due to their intriguing and tunable electronic structures. GNRs with hybrid edge structures often confer them unique geometries associated with exotic physicochemical properties. Herein, a novel type of cove-edged GNRs with periodic short zigzag-edge segments is demonstrated. The bandgap of this GNR family can be tuned using an interplay between the length of the zigzag segments and the distance of two adjacent cove units along the opposite edges, which can be converted from semiconducting to nearly metallic. A family member with periodic cove-zigzag edges based on N = 6 zigzag-edged GNR, namely 6-CZGNR-(2,1), is successfully synthesized in solution through the Scholl reaction of a unique snakelike polymer precursor (10) that is achieved by the Yamamoto coupling of a structurally flexible S-shaped phenanthrene-based monomer (1). The efficiency of cyclodehydrogenation of polymer 10 toward 6-CZGNR-(2,1) is validated by FT-IR, Raman, and UV-vis spectroscopies, as well as by the study of two representative model compounds (2 and 3). Remarkably, the resultant 6-CZGNR-(2,1) exhibits an extended and broad absorption in the near-infrared region with a record narrow optical bandgap of 0.99 eV among the reported solution-synthesized GNRs. Moreover, 6-CZGNR-(2,1) exhibits a high macroscopic carrier mobility of ∼20 cm2 V-1 s-1 determined by terahertz spectroscopy, primarily due to the intrinsically small effective mass (m*e = m*h = 0.17 m0), rendering this GNR a promising candidate for nanoelectronics.

Spin thermoelectric effects of new-style one-dimensional carbon-based nanomaterials

Based on first-principles methods, the authors of this paper investigate spin thermoelectric effects of one-dimensional spin-based devices consisting of zigzag-edged graphene nanoribbons (ZGNRs), carbon chains and graphene nanoflake. It is found that the spin-down transmission function is suppressed to zero, while the spin-up transmission function is about 0.25. Therefore, an ideal half-metallic property is achieved. In addition, the phonon thermal conductance is obviously smaller than the electronic thermal conductance. Meantime, the spin Seebeck effects are obviously enhanced at the low-temperature regime (about 80K), resulting in the fact that spin thermoelectric figure of merit can reach about 40. Moreover, the spin thermoelectric figure of merit is always larger than the corresponding charge thermoelectric figure of merit. Therefore, the study shows that they can be used to prepare the ideal thermospin devices.

Ultrahigh Spin Filter Efficiency, Giant Magnetoresistance and Large Spin Seebeck Coefficient in Monolayer and Bilayer Co-/Fe-/Cu-Phthalocyanine Molecular Devices.

The spin related electrical and thermoelectric properties of monolayer and bilayer MPc (M = Co, Fe, Cu) molecular devices in a parallel spin configuration (PC) and an anti-parallel spin configuration (APC) between the V-shaped zigzag-edged graphene nanoribbon electrodes and the center bilayer MPc molecules are investigated by combining the density functional theory and non-equilibrium Green’s function approaches. The results show that there is an ultrahigh spin filter efficiency exceeding 99.99995% and an ultra-large total conductance of 0.49996G0 for FePc-CoPc molecular devices in the PC and a nearly pure charge current at high temperature in the APC and a giant MR ratio exceeding 9.87 × 106% at a zero bias. In addition, there are pure spin currents for CuPc and FePc molecular devices in the PC, and an almost pure spin current for FePc molecular devices in the APC at some temperature. Meanwhile, there is a high SFE of about 99.99585% in the PC and a reserved SFE of about −19.533% in the APC and a maximum MR ratio of about 3.69 × 108% for the FePc molecular device. Our results predict that the monolayer and bilayer MPc (M = Co, Fe, Cu) molecular devices possess large advantages in designing high-performance electrical and spintronic molecular devices.

Designing multifunctional single-molecule devices by mononuclear or binuclear manganese phthalocyanines

Molecular-scale magnetic devices have attracted great attention of research in recent years. In this work, by using the non-equilibrium Green’s function (NEGF) method in combination with density functional theory (DFT), we investigated the spin transport properties of manganese phthalocyanine based spintronic devices constructed by a mononuclear manganese phthalocyanine (MnPc) or binuclear manganese phthalocyanine (Mn2Pc2) sandwiched between two zigzag-edge graphene nanoribbon (zGNR) electrodes. The calculation results show that both MnPc and Mn2Pc2 devices are good spin filters manifesting excellent spin filtering effect, which is originated from the distinct difference in the energy alignments of the spin-up and spin-down molecular electronic states with respect to the Fermi energy (EF) of zGNR electrodes. More interestingly, the spin polarization of the tunneling electrons through the devices is intimately opposite to that of Mn atom(s). Specifically, for a spin-up polarization of Mn in the devices, almost only the spin-down electrons are allowed to go through the device, and vice versa. In addition, spin filtering efficiency (SFE) of the devices is evidently enhanced in Mn2Pc2 devices in comparison with that of the MnPc devices, of which the value of SFE can reach almost 100%. This enhancement is attributed to the fact that the spin electronic states of Mn2Pc2 devices are closer to EF and there is a weaker localization in their spatial distributions induced by the external bias voltage. Furthermore, both MnPc and Mn2Pc2 devices can act as NOT logic gates by taking the spin polarization characteristics of Mn atom(s) and current of devices as input and output signals, respectively. This work demonstrates that mononuclear MnPc and binuclear Mn2Pc2 molecules have potential applications in designing multifunctional spintronic single-molecule devices.

Width Dependent Elastic Properties of Graphene Nanoribbons.

The mechanical response of graphene nanoribbons under uniaxial tension, as well as its dependence on the nanoribbon width, is presented by means of numerical simulations. Both armchair and zigzag edged graphene nanoribbons are considered. We discuss results obtained through two different theoretical approaches, viz. density functional methods and molecular dynamics atomistic simulations using empirical force fields especially designed to describe interactions within graphene sheets. Apart from the stress-strain curves, we calculate several elastic parameters, such as the Young’s modulus, the third-order elastic modulus, the intrinsic strength, the fracture strain, and the Poisson’s ratio versus strain, presenting their variation with the width of the nanoribbon.

A multi-functional spintronic device based on 1,4,5,8-naphthalenetetracarboxylic diimide

By using a combination of density functional theory (DFT) and nonequilibrium Green’s function (NEGF) method, spin-resolved electronic transport behaviors of the device made of 1,4,5,8-naphthalenetetracarboxylic diimide (NTCDI) connected to two semi−infinite zigzag edged graphene nanoribbon (ZGNR) electrodes was analyzed theoretically. The results prove that the device has negative differential resistance (NDR) effect and perfect spin filtering effect under parallel (P) and anti-parallel (AP) structures, and there are a large spin rectification and perfect dual-spin filtering effect in AP configuration. Besides, by regulating the relative dihedral angle between the NTCDI molecule and ZGNR electrode planes, an on/off ratio in the magnitude of 105 was also achieved, enabling molecular switching. To further validate these characteristics, we analyzed the band structures, spin-resolved transmission spectra, and molecular orbitals' spatial distribution within the bias window. The proposed configuration with the intriguing properties enables its potential application in multi-functional nanoelectronic devices.

Giant Switching Effect and Spintronic Transport Properties in Cyclo[18]carbon‐Based Molecular Devices

The isolated cyclo[18]carbon (C18), as a newly synthesized carbon allotrope, has potential to become the core unit of high‐end electronic devices due to its excellent physical properties. However, the research on C18 in molecular devices is still in the rudimentary stage. Herein, a novel molecular spin‐filtering device, formed by covalently sandwiching a C18 ring between two zigzag‐edged graphene nanoribbons (ZGNRs), is investigated based on the nonequilibrium Green's function method and density functional theory. It is found that the spin‐polarized current can be implemented and controlled through adjusting the magnetic configuration of left (right) electrode. Remarkably, a strong bimolecular configuration switching effect can be achieved by inserting a metal carbon chain between nonmagnetic ZGNRs and C18 (on/off ratio of ≈2737). The results suggest theoretical guidance for the design of next‐generation carbon‐based molecular spintronic devices.

Generation of pure spin current in graphene nanoribbons with continous antidots

Spin caloritronics, which combines the characteristics of thermoelectronics with the characteristics of spintronics, has a wide range of promising applications in high-speed and low-dissipation devices. In this paper, according to the density functional theory combined with nonequilibrium Green’s function method, we propose a scheme for generating pure spin current with spin dependent Seebeck effect in the zigzag-edged graphene nanoribbons by introducing continuous antidots (hexagonal defects). Specifically, by creating an antidot at one edge of the nanoribbon, an X-shape transmission spectrum around the Fermi level is formed, which results from the disrupted edge of the nanoribbon. The mechanism is well explained by the cooperation between the varying localization features of the eigenstates around the Fermi level for the unit cell and the scattering states at the Fermi level for the device. Therefore, the electrons of the two spin channels flow in the opposite directions under a temperature gradient, generating the spin current and charge current. By slightly tuning the chemical potential of the device, the charge current can be zero, while the spin current is not equal to zero. With the increase of adjacent antidot number along the width of the nanoribbons, the structures are more disrupted, thus promoting the pure spin current due to the increase of the spin Seebeck coefficient. However, for zigzag graphene nanoribbons with <i>W</i> zigzag carbon chains, the pure spin current decreases when the number of the antidots are more than (<i>W</i>/2–1), which results from the decrease of the spin conductance. So, the maximum pure spin current can be obtained when the number of the continuous antidots introduced along the width of nanoribbons reaches (<i>W</i>/2–1). These findings indicate a novel strategy for thermally generating the spin current by introducing continuous antidots along the nanoribbon bandwidth in zigzag graphene nanoribbons and will be greatly instructive in designing the graphene spintronic devices.