Armchair-edge Graphene Nanoribbons Research Articles

Magnetic structure and magnetic transport characteristics of nanostructures based on armchair-edged graphene nanoribbons

AGNR–Mn–F2 is shown to be an excellent half-metal with a wide bandgap and a stable magnetic ordering at a very high Curie temperature, as well as being predicted that this structure can stably exist in experiment. And it also displays outstanding magnetic device natures.

First-principles study of electronic transport properties of graphene nanoribbons with pentagon-heptagon (5-7) line defects

ABSTRACTIn this study, we investigated the influence of line defects consisting of pentagon-heptagon (5-7) pairs on the electronic transport properties of zigzag-edged and armchair-edged graphene nanoribbons (GNRs). Using the first-principles density functional theory, we study their electronic properties. To investigate their current-voltage (I-V) characteristics at low bias voltage (∼ 1 meV), we use the nonequilibrium Green’s function method. As a result, we found that the conductance of the GNRs having a connected line defect between source and drain shows better performance than that of the ideal zigzag-edged GNRs (ZGNRs). A detailed investigation of the transmission spectra and the wave function around the Fermi level reveals that the line defects arranged along the transport direction work similar to an edge state of the ZGNRs and can be an additional conduction channel. Our results suggest that such a line defect can be effective for low-resistance GNR interconnects.

First-principles study of chemical-edge-doping effect on transport properties of armchair-edge graphene nanoribbons

We have investigated the electronic transport of graphene nanoribbons (GNRs) with armchair edges containing boron, nitrogen, or oxygen, using the first-principles density functional theory. We found that such dopants at edge sites provided the n- or p-type doping effect for armchair graphene nanoribbons (AGNRs). As a result, even under an applied bias potential lower than the band gap energy of pristine AGNR, the nitrogen edge-doped GNRs show a large conductance that is almost the same as that of the ideal zigzag GNRs.

Spin-Polarized Transport Through a Zigzag-Edge Graphene Flake Embedded between Two Armchair Nanoribbon Electrodes

We study the coherent spin-polarized transport through a zigzag-edge graphene flake (ZGF), using Hubbard model in the nearest neighbor approximation within the framework of the Green function’s technique and Landauer formalism. The system considered consists of electrode/(ZGF)/electrode, in which the electrodes are chosen to be armchair nanoribbons. The study was performed for two types of electrodes i.e., armchair-edge graphene nanoribbons (AGNRs) and armchair-edge boron-nitride nanoribbons (ABNNRs). Our calculations of the electronic and transport properties of these systems show that both systems posses spin filtering properties, while the spin filtering is higher in the full graphene system than the second system with boron nitride nanoribbon electrodes. The spin filtering in these systems is due to the interaction of the spin of the carriers with the local zigzag-edge magnetism in ZGF. However, the reduction of the spin filtering property in the case of the system with boron-nitride nanoribbon electrodes could be due to the decrease in the effective spin interaction of boron atoms in the contact sites and magnetization of the zigzag-edge of the ZGF.

Edge-modified zigzag-shaped graphene nanoribbons: Structure and electronic properties

Control of the band gap of graphene nanoribbons is an important problem for the fabrication of effective radiation detectors and transducers operating in different frequency ranges. The periodic edge-modified zigzag-shaped graphene nanoribbon (GNR) provides two additional parameters for controlling the band gap of these structures, i.e., two GNR arms. The dependence of the band gap E g on these parameters is investigated using the π-electron tight-binding method. For the considered nanoribbons, oscillations of the band gap E g as a function of the nanoribbon width are observed not only in the case of armchair-edge graphene nanoribbons (as for conventional graphene nanoribbons) but also for zigzag GNR edges. It is shown that the change in the band gap E g due to the variation in the length of one GNR arm is several times smaller than that due to the variation in the nanoribbon width, which provides the possibility for a smooth tuning of the band gap in the energy spectrum of the considered graphene nanoribbons.

Giant Rectification Ratios of Azulene-like Dipole Molecular Junctions Induced by Chemical Doping in Armchair-Edged Graphene Nanoribbon Electrodes

Electron transport properties of an azulene-like dipole molecule anchored with carbon atomic chains sandwiched between two graphene nanoribbon (GNR) electrodes are theoretically investigated at the ab initio level. The molecular junctions are constructed with a strategy of modulating symmetry of Bloch wave functions. The chemical doping in an armchair-edged GNR is shown to play a significant role in determining the conductance behavior and rectifying performance of the molecular junctions. Giant rectification ratios up to 104 at low bias voltages are obtained for the molecular junctions with asymmetric arrangement of undoped zGNR and doped aGNR electrodes. The boron (aluminum) dopants in the aGNR electrode induce a better rectifying performance for the molecular junctions than the respective nitrogen (phosphorus) dopants. Moreover, the boron or nitrogen doping is more advantageous than the respective aluminum or phosphorus doping in view of improving rectifying behaviors of the molecular junctions. Taking...

Graphene nanopore field effect transistors

Graphene holds great promise for replacing conventional Si material in field effect transistors (FETs) due to its high carrier mobility. Previously proposed graphene FETs either suffer from low ON-state current resulting from constrained channel width or require complex fabrication processes for edge-defecting or doping. Here, we propose an alternative graphene FET structure created on intrinsic metallic armchair-edged graphene nanoribbons with uniform width, where the channel region is made semiconducting by drilling a pore in the interior, and the two ends of the nanoribbon act naturally as connecting electrodes. The proposed GNP-FETs have high ON-state currents due to seamless atomic interface between the channel and electrodes and are able to be created with arbitrarily wide ribbons. In addition, the performance of GNP-FETs can be tuned by varying pore size and ribbon width. As a result, their performance and fabrication process are more predictable and controllable in comparison to schemes based on edge-defects and doping. Using first-principle transport calculations, we show that GNP-FETs can achieve competitive leakage current of ∼70 pA, subthreshold swing of ∼60 mV/decade, and significantly improved On/Off current ratios on the order of 105 as compared with other forms of graphene FETs.

Seebeck effects in a graphene nanoribbon coupled to two ferromagnetic leads

We theoretically investigate the Seebeck effects for the system of a narrow graphene nanoribbon between two ferromagnetic (FM) electrodes with noncollinear magnetic moments. Both zigzag-edge graphene nanoribbons (ZGNRs) and armchair-edge graphene nanoribbons (AGNRs) have been considered. By using the nonequilibrium Green's function method combining with the tight-binding Hamiltonian, it is demonstrated that, the Seebeck coefficients are sensitive to the chirality and width of the nanoribbon in the absence of magnetic field. Compared with 22-ZGNR and metallic 17-AGNR systems, semiconducting 15-AGNR system is found to posses superior thermoelectric performance, its Seebeck coefficients can be improved by two orders of magnitude. Meanwhile, the Seebeck coefficients for both 22-ZGNR and metallic 17-AGNR systems are the same order as that of graphene system. Furthermore, the Seebeck coefficients are strongly dependent on the magnetization M as well as magnetic configuration of the two FM leads. Particularly, the Seebeck coefficient drastically enhances when the magnetization of the two FM leads is in antiparallel configuration. Interestingly, the Seebeck coefficient for both 22-ZGNR and metallic 17-AGNR systems increases with increasing temperature T, while it decreases with increasing T for semiconducting 15-AGNR system. Moreover, the dependence Seebeck coefficients on magnetic flux ϕ show an oscillation behavior. The results obtained here may provide a valuable theoretical guidance to experimentally design heat spintronic devices.

Excitonic effects of E11, E22, and E33 in armchair-edged graphene nanoribbons

We explore excitonic effects of E11, E22, and E33, which are excitons formed between the three highest valence subbands and the three lowest conduction ones, in armchair-edged graphene nanoribbons by applying the extended tight-binding model including electron-electron interactions. Our results show that the excitation energies and the binding energies decrease inversely with the ribbon widths and can be classified into three categories based on their width indices. We found the relation between the band structures and the binding energies and explained some recent observations of strong excitonic effects in graphene.

Electron transport channels and their manipulation by impurity in armchair-edge graphene nanoribbons

Under the scheme of the nonequilibrium Green’s function combined with the tight-binding approximation, we study electron transport properties in different atomic chains of an armchair-edged graphene nanoribbon (AGNR) and their manipulation using a single substitutional impurity atom. By calculation and analysis of the local bond currents between nearest atom sites in the AGNR, we find that electron transport along two armchair-edged chains is more active than that along other chains for any clean AGNR. For a metallic AGNR, interestingly, there exists a series of parallel distributed major channels for the low-energy electron transport. Further, the transport properties of these channels can be manipulated by a single substitutional impurity atom with different strength and locating position, e.g., a suitable impurity can cause a selected channel to be closed completely while others still open. However, in the high-energy regime these independent channels disappear, and a metallic AGNR becomes entirely metallic in this case. The findings here suggest that an AGNR may be used as a multi-channel plane material in the future nanoelectronic technology.

Electronic transport in three-terminal triangular carbon nanopatches

The electronic transport properties of three-terminal graphene-based triangular patches are investigated using a combination of semi-empirical tight-binding calculations and Green’s function-based transport theory within Landauer’s framework. The junctions are composed of a triangular structure based on armchair edged graphene nanoribbons. We show how details of the central region influence the resonant electronic transport across the triangular patches and highlight the unique features of the current flow as a function of geometry. These properties indicate an array of functionalities for the development of carbon-based complex nanocircuits and operational devices at the nanoscale.

Variable range of the RKKY interaction in edged graphene

The indirect exchange interaction is one of the key factors in determining the overall alignment of magnetic impurities embedded in metallic host materials. In this work we examine the range of this interaction in magnetically doped graphene systems in the presence of armchair edges using a combination of analytical and numerical Green function approaches. We consider both a semi-infinite sheet of graphene with a single armchair edge, and also quasi-one-dimensional armchair-edged graphene nanoribbons (GNRs). While we find signals of the bulk decay rate in semi-infinite graphene and signals of the expected one-dimensional decay rate in GNRs, we also find an unusually rapid decay for certain instances in both, which manifests itself whenever the impurities are located at sites which are a multiple of three atoms from the edge. This decay behavior emerges from both the analytic and numerical calculations, and the result for semi-infinite graphene can be interpreted as an intermediate case between ribbon and bulk systems.

Bias Dependence of Rectifying Direction in a Diblock Co-oligomer Molecule with Graphene Nanoribbon Electrodes

By applying nonequilibrium Green’s function method in combination with density functional theory, we study the rectifying properties of dipyrimidinyl-diphenyl co-oligomer molecules embedded in a carbon atomic chain sandwiched between two graphene nanoribbon (GNR) electrodes. Both the length of the carbon atomic chains and the edge geometry of the graphene nanoribbon electrodes are shown to play a significant role in determining the conductance behavior and rectifying performance of the molecular devices. As for GNRs with zigzag edges, the parallel (perpendicular) conformation between the principal plane of the molecule and the zigzag-edged GNR electrode is observed to be dependent on the odd (even) number of carbon atoms in the carbon chain, whereas for armchair-edged GNRs the parallel (perpendicular) case corresponds to an even (odd) number of carbon atoms. Taking an asymmetric arrangement of armchair and zigzag GNR electrodes, we demonstrate a molecular device having very interesting rectifying behaviors with marked rectification ratios at low bias voltages and inversion of rectifying direction when the bias voltage is large. Analysis of the transmission coefficients and molecular projected self-consistent Hamiltonian as well as band structures of the electrodes under various external bias voltages reveals an underlying mechanism of the observed results.

Localized vibrational, edges and breathing modes of graphene nanoribbons with topological line defects

Peculiar vibrational modes of graphene nanoribbons (GNRs) with topological line defects were presented. We find that phonon dispersion relations of the topological defective GNRs are more similar to those of perfect armchair-edge GNR than to zigzag-edge GNR in spite of their zigzag edge. All vibrational modes at Γ point are assigned in detail by analyzing their eigenvectors and are presented by video. Three types of characteristic vibrational modes, namely, localized vibrational modes in defect sites, edges, and breathing modes, are observed. Five localized vibrational modes near the defect sites are found to be robust against the width of the topological line-defective GNR. The Raman D’ band just originates from one localized mode, 1622 cm-1. The vibrational mode is sensitive to symmetry. The edge modes are related with structural symmetry but not with widths. Two edge modes are asymmetrical and only one is symmetrical. The breathing modes are inversely proportional to the width for wide-defect GNRs, and inversely proportional to the square root of the width for narrow-defect GNRs. The breathing mode frequencies of defective GNRs are slightly higher than those of perfect GNRs. These vibrational modes may be useful in the manipulation of thermal conductance and implementation of single phonon storage.

Effects of size on the structure and the electronic properties of graphene nanoribbons

In view of the current interest in graphene, especially graphene nanoribbons (GNRs), we carried out investigations using the molecular orbital cluster approach to find suitable models for use in simulations involving GNRs. Two types of nanoribbons were modeled, namely armchair- and zigzag-edged GNRs. We used four properties in determining the suitable models: molecular orbitals, spin densities, charges, and bond lengths. Among these criteria, it was found that size has a highly prominent effect on the molecular orbitals and bond lengths. The results show that models of sizes C126H32 and C110H30 are needed to represent the electronic and structural properties of infinite zigzag-edged and armchair-edged GNRs.

Quantum capacitance of the armchair-edge graphene nanoribbon

The quantum capacitance, an important parameter in the design of nanoscale devices, is derived for armchair-edge single-layer graphene nanoribbon with semiconducting property. The quantum capacitance oscillations are found and these capacitance oscillations originate from the lateral quantum confinement in graphene nanoribbon. Detailed studies of the capacitance oscillations demonstrate that the local channel electrostatic potential at the capacitance peak, the height and the number of the capacitance peak strongly depend on the width, especially a few nanometres, of the armchair-edge graphene nanoribbon. It implies that the capacitance oscillations observed in the experiments can be utilized to measure the width of graphene nanoribbon. The results also show that the capacitance oscillations are not seen when the width is larger than 30 nm.

Nanostructural Effects on Thermoelectric Power of Graphene Nanoribbons

Nanostructural effects on the thermoelectric power of graphene nanoribbons (GNRs) are revealed through first-principles simulation based on the density functional theory combined with nonequilibrium Green's function theory. The thermoelectric power of GNRs exhibits essentially different behavior depending on their edge structure and ribbon width. For zigzag-edged GNRs, the thermoelectric power shows a peculiar energy dependence originating from edge-localized electronic states with energy near the Fermi level. On the other hand, for armchair-edged GNRs (AGNRs), the thermoelectric power is classified into three categories depending on the ribbon width. Among AGNRs with similar ribbon width, an AGNR belonging to a category satisfying mod(Na, 3)=1 displays the largest thermoelectric power, where Na is the integer determining the ribbon width of an AGNR.

Aligning the Band Gap of Graphene Nanoribbons by Monomer Doping

Silicon-based field-effect transistors (FETs) are the building blocks of modern digital logic circuitry and therefore part of virtually every electronic device available today. Over the past decades, continuous downscaling of existing designs has met the rising performance requirements, but as the size of FETs approaches the regime of atomic structures, new concepts are required to maintain the current pace at which microelectronics is developing. At small gate channel lengths, the applicability of quantum mechanical principles results in several so-called short-channel effects (e.g. reduced carrier mobility). Since its experimental realization in 2004, graphene has been discussed intensively as a substitute for doped silicon in FETs because of its high charge carrier mobility and its unsurpassably low thickness. Graphene transistors have even been realized but cannot be put into the “off” state because of the lack of a band gap. However, there are concepts available for opening a band gap, for example, applying strain along the sheet or biasing bilayers of graphene. Also, lateral confinement in quasione-dimensional graphene nanoribbons (GNRs) leads to a band gap, which furthermore is highly sensitive to the width and edge shape of the GNR, thus opening possibilities to tailor the electronic properties of a device. Indeed, FETs built from nanoribbons show much higher on/off-ratios than graphene transistors, which makes them more suitable for integration into logic devices. However, the ability to control the electronic properties is essential: while the size of the gap can be engineered by varying the nanoribbon widths, the alignment of the GNR band structure with respect to the Fermi level of a metal electrode is equally important. Such a shifting of the entire band structure is observed both in two-dimensional graphene as well as in chemically synthesized or lithographically patterned GNRs upon doping, particularly with nitrogen atoms. Using present doping techniques, the distribution of dopant atoms will not be well-defined on the nanoscale and the band gap shift upon nitrogen doping depends on the site of the N atom, that is, the bonding configuration to neighboring carbon atoms. Generally, for doped and pristine GNRs, fabrication remains a challenge as well-established top-down approaches using lithography or unzipping of carbon nanotubes yield relatively wide ribbons with an undetermined edge structure. Particularly for small widths on the order of a few nanometers (where the band gap reaches a technologically relevant size) atomically precise edges are necessary and can be realized using Br-substituted precursor molecules, which are thermally activated on a surface and—in a bottom-up synthesis— covalently assemble to a specific nanostructure. In this study, we employed the latter approach to prepare GNRs with an atomically precise edge structure and doping pattern through polymerization of specific monomers directly on the Au(111) surface and studied the position and size of the band gap of these GNRs with surface-sensitive electron spectroscopies. Besides straight armchair edge GNRs, another type of chevron-shaped nanoribbons has previously been fabricated using an on-surface reaction. In this process adsorption of several layers of 6,11-dibromo-1,2,3,4-tetraphenyl-triphenylene (monomer 1 in Scheme 1) on Au(111) and heating at 250 8C leads to desorption of the second and higher layers as well as halogen dissociation and coupling of the resulting activated biradical monomers, yielding a sterically crowded and hence twisted polyphenylene. In a second heating step at 440 8C, this polymer undergoes a subsequent cyclodehydrogenation reaction providing access to the desired chevronshaped GNR with armchair edges. Selective substitution of the parent monomer 1 with either one or two N atoms provided monomers 2 and 3, respectively, which were used to generate GNRs with different doping levels (exemplarily shown for the doubly N doped GNR 5 in Scheme 1) and accordingly with potentially different electronic structure properties. Synthesis of the new monomers 2 and 3 was accomplished by Diels–Alder reactions of an appropriate cyclopentadienone with either mixed phenylpyridyl-acetylene or bispyridylacetylene, followed by immediate cheletropic CO extrusion (for details see the Supporting Information). [*] C. Bronner, S. Stremlau, A. Haase, Prof. Dr. P. Tegeder Fachbereich Physik, Freie Universit t Berlin Arnimallee 14, 14195 Berlin (Germany) E-mail: bronner@zedat.fu-berlin.de petra.tegeder@physik.fu-berlin.de

Twisting effects on energy band structures and transmission behaviors of graphene nanoribbons

By using the first-principles method based on the density-functional theory, twisting- deformation-dependent electrical characteristics of graphene nanoribbons (GNRs) are studied systematically. It is shown that the energy gap of the zigzag-edge graphene nanoribbon (ZGNR) is the most insensitive to twisting deformation, and it almost keeps metallicity unchanged, next is the armchair-edge graphene nanoribbons (AGNRs) by width W=3p-1 (p is a positive integer), and its gap has only a small change when twisting deformation occurs. However, the gap of AGNR with width W=3p+1 is extremely sensitive to twisting deformation, and it can display a variation from wide-gap semiconductor to moderate-gap semiconductor, quasi-metal, and metal, next is AGNR with W=3p. In other words, the larger the band gap for GNR in the absence of twisting deformation, the more significant the change (becoming small) of its band gap with twisting deformation. Additionally, for the whole electronic structure and transmission behavior, one can find that there is a much larger influence under twisting deformation in AGNR than in ZGNR. These studies suggest that it is necessary to take the effect of twisting deformation on the electrical characteristics into account in designing GNR-based nanodevices.

Rolling effects on electronic characteristics for graphene nanoribbons

Graphene nanoribbons (GNRs) are important nanomaterials. A carbon nanotube can be viewed as a GNR rolled into a seamless cylinder. By using the first-principles method based on the density-functional theory, the rolling deformation-dependent electronic characteristics of GNRs, including the band structure (particularly the bandgap), density of states (DOS), and transmission spectrum, are studied systematically. It is found that before all types of GNRs are rolled into carbon nanotubes, they are not sensitive to the rolling deformations, which means that for electronic structures and transport properties, GNRs have a very strong ability to resist the rolling deformations. After GNRs are rolled into nanotubes, zigzag-edge GNRs (ZGNRs) and armchair-edge GNRs (AGNRs) present distinct differences in property, ZGNRs almost maintain unchanged metallic behaviors or become quasi-metallic. But for AGNRs, their electronic characteristics experience large variations, and transformations occur between the quasi-metal and semiconductor with various bandgaps, which might be closely related to the periodical boundary conduction along the direction of tubular circumference of a carbon nanotube and variation of quantum confinement. These studies presented here are of significance for understanding the rolling effects on electronic characteristic and relationship of electronic characteristics between GNRs and carbon nanotubes (structure-property relationship).