Gapped ferromagnetic graphene nanoribbons

We theoretically design a graphene-based all-organic ferromagnetic semiconductor by terminating zigzag graphene nanoribbons (ZGNRs) with organic magnets. A large spin-split gap with a 100% spin polarized density of states near the Fermi energy is obtained, which is of potential application in spin transistors. The interactions among electron, spin and lattice degrees of freedom are studied using the first-principles calculations including non-collinear spin orientations. All of the calculations consistently demonstrate that although no d electrons existing, the antiferromagnetic π-π exchange together with the strong electron-lattice interactions between organic magnets and ZGNRs make the ground state ferromagnetic.     

On‐Surface Synthesis of NBN‐Doped Zigzag‐Edged Graphene Nanoribbons

We report the first bottom‐up synthesis of NBN‐doped zigzag‐edged GNRs (NBN‐ZGNR1 and NBN‐ZGNR2) through surface‐assisted polymerization and cyclodehydrogenation based on two U‐shaped molecular precursors with an NBN unit preinstalled at the zigzag edge. The resultant zigzag‐edge topologies of GNRs are elucidated by high‐resolution scanning tunneling microscopy (STM) in combination with noncontact atomic force microscopy (nc‐AFM). Scanning tunneling spectroscopy (STS) measurements and density functional theory (DFT) calculations reveal that the electronic structures of NBN‐ZGNR1 and NBN‐ZGNR2 are significantly different from those of their corresponding pristine fully‐carbon‐based ZGNRs. Additionally, DFT calculations predict that the electronic structures of NBN‐ZGNRs can be further tailored to be gapless and metallic through one‐electron oxidation of each NBN unit into the corresponding radical cations. This work reported herein provides a feasible strategy for the synthesis of GNRs with stable zigzag edges yet tunable electronic properties.     

Half-metallicity in graphene nanoribbons with topological defects at edge

We report first principles studies of zigzag edged graphene nanoribbons (ZGNR) with one edge partially covered by topological defects. With increasing coverage of an edge by pentagons and heptagons, which are two of the simplest topological defects possible in a graphenic lattice, ZGNRs evolve from a magnetic semiconductor to a ferromagnetic metal. This evolution can be intermediated by a narrow bandgap half-metallic phase, upon suitable concentration and conformation of defects at the edge. Spin-frustration induced by topological defects lead to substantial lowering of magnetic ordering and localization of defect-states in the vicinity of the defects. Dispersion of bands constituted by the defect-states within the bandgap of the corresponding unmodified ZGNR, leads to availability of energy windows for spin-polarized electron transport. Driven primarily by exchange interactions, the energy window for transport of electrons near Fermi energy, is consistently wider and more prevalent for the minority spin, in the entire class of ZGNRs with discontinuous patches of topological defects at an edge. Such defects have been widely predicted and observed to be naturally present at the interfaces in polycrystalline graphene, and can even be formed through chemical and physical processes. Our approach thus may lead to a feasible strategy to manifest workable half-metallicity in ZGNRs without involving non-carbon dopants or functional groups.     

Theoretical study of nitrogen‐doped graphene nanoflakes: Stability and spectroscopy depending on dopant types and flake sizes

As nitrogen‐doped graphene has been widely applied in optoelectronic devices and catalytic reactions, in this work we have investigated where the nitrogen atoms tend to reside in the material and how they affect the electron density and spectroscopic properties from a theoretical point of view. DFT calculations on N‐doped hexagonal and rectangular graphene nanoflakes (GNFs) showed that nitrogen atoms locating on zigzag edges are obviously more stable than those on armchair edges or inside flakes, and interestingly, the N‐hydrogenated pyridine moiety could be preferable to pure pyridine moiety in large models. The UV–vis absorption spectra of these nitrogen‐doped GNFs display strong dependence on flake sizes, where the larger flakes have their major peaks in lower energy ranges. Moreover, the spectra exhibit different connections to various dopant types and positions: the graphitic‐type dopant species present large variety in absorption profiles, while the pyridinic‐type ones show extraordinary uniform stability and spectra independent of dopant positions/numbers and hence are hardly distinguishable from each other. © 2018 Wiley Periodicals, Inc.     

Structural and electronic properties of graphene nanotube-nanoribbon hybrids

The structural and electronic properties of a hybrid of an armchair graphene nanotube and a zigzag graphene nanoribbon are investigated by first-principles spin-polarized calculations. These properties strongly depend either on the nanotube location or on the spin orientation. The interlayer spacing, the transverse distance from the center of the ribbon and the stacking configuration affect the electronic structures. The antiferromagnetic configuration has a lower total energy than the ferromagnetic one. The interlayer atomic interactions between the two subsystems would change the low energy dispersions, open subband spacings, and induce more band-edge states. Moreover, such interactions create an energy gap and break the spin degeneracy in the antiferromagnetic configuration. The band-edge-state energies are sensitive to the nanotube location.     

Half-metallicity in undoped and boron doped graphene nanoribbons in the presence of semilocal exchange-correlation interactions

We perform density functional calculations on one-dimensional zigzag edge graphene nanoribbons (ZGNRs) of different widths, with and without edge doping including semilocal exchange correlations. Our study reveals that, although the ground state of edge-passivated (with hydrogen) ZGNRs prefers to be anti-ferromagnetic, the doping of both of the edges with boron atoms stabilizes the system in a ferromagnetic ground state. Both the local and semilocal exchange correlations result in half-metallicity in edge-passivated ZGNRs at a finite cross-ribbon electric field. However, the ZGNR with boron edges shows half-metallic behavior irrespective of the ribbon width even in the absence of electric field and this property sustains for any field strength, opening a huge possibility of applications in spintronics.     

On-Surface Synthesis of NBN-Doped Zigzag-Edged Graphene Nanoribbons

We report the first bottom-up synthesis of NBN-doped zigzag-edged GNRs (NBN-ZGNR1 and NBN-ZGNR2) through surface-assisted polymerization and cyclodehydrogenation based on two U-shaped molecular precursors with an NBN unit preinstalled at the zigzag edge. The resultant zigzag-edge topologies of GNRs are elucidated by high-resolution scanning tunneling microscopy (STM) in combination with noncontact atomic force microscopy (nc-AFM). Scanning tunneling spectroscopy (STS) measurements and density functional theory (DFT) calculations reveal that the electronic structures of NBN-ZGNR1 and NBN-ZGNR2 are significantly different from those of their corresponding pristine fully-carbon-based ZGNRs. Additionally, DFT calculations predict that the electronic structures of NBN-ZGNRs can be further tailored to be gapless and metallic through one-electron oxidation of each NBN unit into the corresponding radical cations. This work reported herein provides a feasible strategy for the synthesis of GNRs with stable zigzag edges yet tunable electronic properties.     

Zigzag graphene nanoribbons: Flexible and robust transparent conductors

We report the effect of bending deformation on the optical and transport properties of zigzag graphene nanoribbons (ZGNRs) induced by the uniaxial strain using the first-principles method combined with non-equilibrium Green's function. The optical properties of ZGNRs in the region of visible light are almost unchanged under the uniaxial strain, whereas an absorption peak occurs at the infrared region for the bent ZGNRs under the transverses strain. The transport properties of ZGNRs under the transverses strain with the bending angle up to 65° remain almost the same as those of the flat one. The transmission coefficients around the Fermi level only slightly decrease when the bending angle further increases to 72.5°. Moreover, ZGNRs under the longitudinal strain show the same transmission conductance around the Fermi level as that of the flat one. The edge states of ZGNRs still behave as excellent ballistic transport channels under bending deformation, which makes them promising flexible and robust transparent conductors.     

First-Principles Study of Field Emission from Zigzag Graphene Nanoribbons Terminated with Ether Groups

Field emission properties of zigzag graphene nanoribbons terminated with C-O-C ether groups (including cyclic and alternative ether groups at edge, denoted as ZGNR-CE and ZGNR-AE) are studied by adopting a self-consistent method based on density functional theory calculation. The results show that the field emissions of these two nanoribbons are dominated by states around Brillouin zone center and close to Fermi level. Because of lower work function, the ZGNR-CE can produce much stronger emission current than recon-structed zigzag graphene nanoribbon. The ZGNR-AE has nearly completely spin-polarized emission current, although its emission current is not strong enough. It is also found that under the lower E-field, the uniaxial strain can effectively modulate their emission currents but the spin polarization of ZGNR-AE keeps unchanged with the varied strain. The under-lying mechanisms are revealed by combining the analyses of their work functions and bandstructures with edge dipole model.     

Quenching of local magnetic moment in oxygen adsorbed graphene nanoribbons

The electronic and magnetic properties of oxidized zigzag and armchair graphene nanoribbons, with hydrogen passivated edges, have been investigated from ab initio pseudopotential calculations within the density functional scheme. The oxygen molecule in its triplet state is adsorbed most stably at the edge of a zigzag nanoribbon. The Stoner metallic behavior of the ferromagnetic nanoribbons and the Slater insulating (ground state) behavior of the antiferromagnetic nanoribbons remain intact upon oxygen adsorption. The formation of a spin-paired C-O bond drastically reduces the local atomic magnetic moment of carbon at the edge of the ferromagnetic zigzag ribbon.     

First principles study of magnetism in nanographenes

Magnetism in nanographenes [also known as polycyclic aromatic hydrocarbons (PAHs)] is studied with first principles density functional calculations. We find that an antiferromagnetic (AFM) phase appears as the PAH reaches a certain size. This AFM phase in PAHs has the same origin as the one in infinitely long zigzag-edged graphene nanoribbons, namely, from the localized electronic state at the zigzag edge. The smallest PAH still having an AFM ground state is identified. With increased length of the zigzag edge, PAHs approach an infinitely long ribbon in terms of (1) the energetic ordering and difference among the AFM, ferromagnetic, and nonmagnetic phases and (2) the average local magnetic moment at the zigzag edges. These PAHs serve as ideal targets for chemical synthesis of nanographenes that possess magnetic properties. Moreover, our calculations support the interpretation that experimentally observed magnetism in activated carbon fibers originates from the zigzag edges of the nanographenes.     

First-principles study of the triwing graphene nanoribbons: junction-dependent electronic structures and electric field modulations

Using first-principles calculations, we investigate the structural, electronic and magnetic properties of triwing zigzag graphene nanoribbons (TW-ZGNRs), as well as the electric field effects on their electronic structures. The TW-ZGNRs have comparable energetic stabilities to the normal ZGNRs and exhibit fascinating junction-dependent electronic properties. With the sp(2) hybridized junctions, the TW-ZGNRs undergo a Peierls distortion and behave as ferromagnetic metals. While the TW-ZGNRs with sp(3) hybridized junctions become semiconductors, which have a ferrimagnetic ground state. An external electric field can further modulate the band structures of semiconducting TW-ZGNRs. The parallel electric field directly moves the flat bands around the Fermi level, while the perpendicular field controls the edge states at the ribbon wings. By these electric field modulations, the band gaps are effectively tuned and half-metallicity can be induced into TW-ZGNRs. Our studies demonstrate that the junctions play an important role in the electronic structures of TW-ZGNRs, which have well-tunable electronic and magnetic properties for potential applications in nanoelectronics and spintronics.     

Effects of Edge Oxidation on the Stability and Half‐Metallicity of Graphene Quantum Dots

A comprehensive first‐principles theoretical study of the electronic properties and half‐metallic nature of zigzag edge‐oxidized graphene quantum dots (GQDs) is carried out by using density functional theory (DFT) with the screened exchange hybrid functional of Heyd, Scuseria and Ernzerhof (HSE06). The oxidation schemes include ‐OH, ‐COOH and ‐COO groups. We identify oxidized GQDs whose opposite spins are localized at the two zigzag edges in an antiferromagnetic‐type configuration, showing a spin‐polarized ground state. Oxidized GQDs are more stable than the corresponding fully hydrogenated GQDs. The partially hydroxylated and carboxylated GQDs with the same size exhibit half‐metallic state under almost the same electric‐field intensity whereas fully oxidized GQDs behave as spin‐selective semiconductors. The electric‐field intensity inducing the half metal increases with the length of the partially oxidized GQDs, ranging from M=4 to 7.     

Molecular rectification in triangularly shaped graphene nanoribbons

We present a theoretical study of electron transport in tailored zigzag graphene nanoribbons (ZGNRs) with triangular structure using density functional theory together with the nonequilibrium Green's function formalism. We find significant rectification with a favorite electron transfer direction from the vertex to the right edge. The triangular ZGNR connecting to the electrode with one thiol group at each terminal shows an average rectification ratio of 8.4 over the bias range from ?1.0 to 1.0 V. This asymmetric electron transport property originates from nearly zero band gap of triangular ZGNR under negative bias, whereas a band gap opens under positive bias. When the molecule is connected to the electrode by multithiol groups, the current is enhanced due to strong interfacial coupling; however, the rectification ratio decreases. The simulation results indicate that the unique electronic states of triangular ZGNR are responsible for rectification, rather than the asymmetric anchoring groups. © 2012 Wiley Periodicals, Inc.     

Magnetism in Nonplanar Zigzag Edge Termini of Graphene Nanoribbons

Graphene nanoribbons (GNRs) and nanographenes synthesized by on-surface reactions using tailor-made molecular precursors offer an ideal playground for a study of magnetism towards nano-spintronics. Although the zigzag edge of GNRs has been known to host magnetism, the underlying metal substrates usually veil the edge-induced Kondo effect. Here, we report the on-surface synthesis of unprecedented, π-extended 7-armchair GNRs using 7-bromo-12-(10-bromoanthracen-9-yl)tetraphene as the precursor. Characterization by scanning tunneling microscopy/spectroscopy revealed unique rearrangement reactions leading to pentagon- or pentagon/heptagon-incorporated, nonplanar zigzag termini, which demonstrated Kondo resonances even on bare Au(111). Density functional theory calculations indicate that the nonplanar structure significantly reduces the interaction between the zigzag terminus and the Au(111) surface, leading to a recovery of the spin localization of the zigzag edge. Such a distortion of planar GNR structures offers a degree of freedom to control the magnetism on metal substrates.     

Edge state magnetism of single layer graphene nanostructures

We study edge state magnetism in graphene nanostructures using a mean field theory of the Hubbard model. We investigate how the magnetism of the zigzag edges of graphene is affected by the presence of other types of terminating edges and defects. By a detailed study of both regular shapes, such as polygonal nanodots and nanoribbons, and irregular shapes, we conclude that the magnetism in zigzag edges is very robust. Our calculations show that the zigzag edges that are longer than three to four repeat units are always magnetic, irrespective of other edges, regular or irregular. We, therefore, clearly demonstrate that the edge irregularities and defects of the bounding edges of graphene nanostructures do not destroy the edge state magnetism.     

Theoretical and experimental determination of the electronic structure of V(2)O(5), reduced V(2)O(5-x) and sodium intercalated NaV(2)O(5)

In this work the electronic structure of V(2)O(5), reduced V(2)O(5-x) (V(16)O(39)) and sodium intercalated NaV(2)O(5) has been studied by both theoretical and experimental methods. Theoretical band structure calculations have been performed using density functional methods (DFT). We have investigated the electron density distribution of the valence states, the total density of states (total DOS) and the partial valence band density of states (PVBDOS). Experimentally, amorphous V(2)O(5) thin films have been prepared by physical vapour deposition (PVD) on freshly cleaved highly oriented pyrolytic graphite (HOPG) substrates at room temperature with an initial oxygen understoichiometry of about 4%, resulting in a net stoichiometry of V(2)O(4.8). These films have been intercalated by sodium using vacuum deposition with subsequent spontaneous intercalation (NaV(2)O(5)) at room temperature. Resonant V3p-V3d photoelectron spectroscopy (ResPES) experiments have been performed to determine the PVBDOS focusing on the calculation of occupation numbers and the determination of effective oxidation state, reflecting ionicity and covalency of the V-O bonds. Using X-ray absorption near edge spectra (XANES) an attempt is made to visualize the changes in the unoccupied DOS due to sodium intercalation. For comparison measurements on nearly stoichiometric V(2)O(5) single crystals have been performed. The experimental data for the freshly cleaved and only marginally reduced V(2)O(5) single crystals and the NaV(2)O(5) results are in good agreement with the calculated values. The ResPES results for V(2)O(4.8) agree in principle with the calculations, but the trends in the change of the ionicity differ between experiment and theory. Experimentally we find partly occupied V 3d states above the oxygen 2p-like states and a band gap between these and the unoccupied states. In theory one finds this occupation scheme assuming oxygen vacancies in V(2)O(5) and by performing a spin-polarized calculation of an antiferromagnetic ordered NaV(2)O(5.).     

Length-Dependent Magnetic Evolution of Anthenes on Au(111)

Nanographenes with zigzag edges, for example, anthenes, exhibit a unique nonbonding π-electron state, which can be described as a spin-polarized edge state that yields specific magnetic ground state. However, prior researches on the magnetism of anthenes with varying lengths on a surface is lacking. This study systematically fabricated anthenes with inherent zigzag carbon atoms of different lengths ranging from bisanthene to hexanthene. Their magnetic evolution on the Au(111) surface was analyzed through bond-resolved scanning probe techniques and density functional theory calculations. The analyses revealed a transition in magnetic properties associated with the length of the anthenes, arising from the imbalance between hybridization energy and the Coulomb repulsion between valence electrons. With the increasing length of the anthenes, the ground state transforms gradually from a closed-shell to an antiferromagnetic open-shell singlet, exhibiting a weak exchange coupling of 4 meV and a charge transfer-induced doublet. Therefore, this study formulated a chemically tunable platform to explore size-dependent π magnetism at the atomic scale, providing a framework for research in organic spintronics.     

Graphene layer growth chemistry: five- and six-member ring flip reaction

Reaction pathways are presented for hydrogen-mediated isomerization of a five- and six-member carbon ring complex on the zigzag edge of a graphene layer. A new reaction sequence that reverses the orientation of the ring complex, or "flips" it, was identified. Competition between the flip reaction and the "ring separation" was examined. Ring separation is the reverse of the five- and six-member ring complex formation reaction, previously reported as "ring collision". The elementary steps of the pathways were analyzed using density functional theory (DFT). Rate coefficients were obtained by solution of the energy master equation and classical transition-state-theory utilizing the DFT energies, frequencies, and geometries. The results indicate that the flip reaction pathway dominates the separation reaction and should be competitive with other pathways important to the graphene zigzag edge growth in high-temperature environments.