Zigzag-edged Graphene Nanoribbons Research Articles

Axisymmetric All-Carbon Devices with High-Spin Filter Efficiency, Large-Spin Rectifying, and Strong-Spin Negative Differential Resistance Properties

We propose the perfect all-carbon axisymmetric spintronic devices consisting of a zigzag-edged trigonal graphene (ZTG) linked to left and right zigzag-edged graphene nanoribbons (ZGNR) electrodes via carbon atomic chains (CACs). To ensure the stability of the system, the edge carbon atoms are passivated by hydrogen atoms. The self-consistent density functional theory (DFT) calculations show that the simple all-carbon system possesses the prefect spin-filtering property at a wide voltage region from −1.0 to 1.0 V. More importantly, the proposed system can act as a perfect dual spin diode in the antiparallel (AP) spin configuration, and the single-spin rectifying ratio can reach 103. When we add the number of the CACs linked to the left ZGNR electrode, the device shows the obvious single-spin negative differential resistance (NDR) behavior, which originates from the appearance of the localized states in the region of the left ZGNR electrode and ZTG. Meanwhile, the perfect spin-filtering and dual spin-diode ...

First-principles study of the atomic structure and edge state of zigzag carbon nanoscrolls

The atomic structure and edge state of zigzag carbon nanoscrolls (ZCNSs) are investigated using first-principles calculations based on density functional theory. The results show a non-monotonic dependence of the total energy of ZCNS on the surface curvature due to a competition between the elasticity and the van der Waals interactions in a scroll. The edge states can be tuned by using different forms of edge hydrogenation and inner radius. It is found that the edge state range of monohydrogenated ZCNSs is smaller than that of monohydrogenated zigzag-edged graphene nanoribbons (ZGNRs), which is also verified using the tight-binding approximation. With the different edge hydrogenations, ZCNSs prefer the [Formula: see text] hybridization more than the [Formula: see text] one. Our present study could suggest the possibility of adjusting the electronic properties of ZCNSs and may provide potential applications in the electronic devices.

The integrated spintronic functionalities of an individual high-spin state spin-crossover molecule between graphene nanoribbon electrodes

The spin-polarized transport properties of a high-spin-state spin-crossover molecular junction with zigzag-edge graphene nanoribbon electrodes have been studied using density functional theory combined with the nonequilibrium Green’s-function formalism. The molecular junction presents integrated spintronic functionalities such as negative differential resistance behavior, spin filter and the spin rectifying effect, associated with the giant magnetoresistance effect by tuning the external magnetic field. Furthermore, the transport properties are almost unaffected by the electrode temperature. The microscopic mechanism of these functionalities is discussed. These results represent a step toward multifunctional molecular spintronic devices on the level of the individual spin-crossover molecule.

Magnetic transport properties of DBTAA-based nanodevices with graphene nanoribbon electrodes

Based on the density functional theory combined with the nonequilibrium Green’s function formalism, the magnetic transport properties of dibenzotetraaza[14]annulene (DBTAA) and transition metal (TM)-DBTAA (TM=Fe and Co) sandwiched between two ferromagnetic zigzag-edge graphene nanoribbon electrodes are investigated. The results show that giant magnetoresistance, spin-filtering and spin-polarized current rectifying effects can be realized simultaneously in the DBTAA system by modulating the external magnetic field. Introducing of TM atoms has obvious effects on these spin-related effects. The mechanisms of these intriguing phenomena are proposed and these phenomena would be instructive in the design of high-performance magnetic nanodevices.

Non-PGM ORR Catalyst Active-Site Screening

Non-platinum group metal (non-PGM) oxygen reduction reaction (ORR) catalysts, based on pyrolyzed C, N, and transition metal systems, may replace precious metal catalysts in Pt-based fuel cell cathodes in the future. Major barriers to the adoption of these catalyst materials include much needed improvements in both ORR activity as well as durability under operating conditions. Detailed knowledge of the atomic-scale structure of the active site(s) would aid in strategically synthesizing materials with a higher density of desired active sites (those with simultaneously high ORR activity and practical stability). Density functional theory (DFT) modeling has previously been utilized to consider both relative stability of atomic scale structures,1 as well as to evaluate ORR activity2-5. Through the use of easily calculated descriptors, high-throughput DFT-based materials searches are possible. Relative formation energies of structures can be calculated via DFT with uniform atomic reference states. These formation energies may serve as a descriptor of relative active-site structure stability. This approach also allows for consideration of shifts in chemical potentials that can affect relative stability. Once the relaxed structure has been simulated, calculation of an active site with a bound ORR intermediate allows for an estimate of the binding energy of that intermediate using a computational hydrogen electrode approach2. Previously, O binding has been utilized as an activity descriptor for ORR catalysts2 but for non-PGM catalysts, OH binding is often the potential determining step in the ORR reaction pathway. As such, for screening purposes, we utilize the OH binding energy as a descriptor for ORR activity. Ideally, this value should be close to 1.23 eV with values less than this indicating OH overbinding and values greater indicating underbinding. Scaling law relations6suggest that both O and OH binding energies should be equally valid activity descriptors. We have performed an initial screening of 22 proposed active-site structures containing C, Fe, N, and H, as suggested through previously established structural motifs7. Three graphene-like C host materials, a “bulk” periodic graphene sheet, a zig-zag edge graphene nanoribbon, and an arm-chair edge graphene nanoribbon were considered. Active-site structures with one and two N-coordinated Fe atoms were considered as embedded in each of the three host materials, acting as C substitutional defects. In some cases, iron replaced two carbons to give different Fe coordination symmetry. Direct comparisons between similar structures in different C host materials were made where possible. For the nanoribbon hosted active site structures, the N-coordinated Fe moiety was only considered at the nanoribbon edge. The outcomes of this study will be the focus of this talk. In addition to the previously reported Fe2N5(*OH) zig-zag edge active site structure,8 preliminary studies suggest that bulk and zig-zag edge Fe2N6 structures (Figure 1) also have favorable OH binding characteristics, suggesting further investigation of their full ORR reaction pathways is merited and will be discussed. In all three cases, Fe2N5(*OH) zig-zag edge, Fe2N6 bulk, and Fe2N6 zig-zag edge, Fe is five-fold coordinated with N and a neighboring Fe, suggesting such coordination is an important aspect for tuning OH binding energy. Fe-Fe bond distances of 2.20 Å, 2.25 Å, and 2.24 Å were calculated for the Fe2N5(*OH) zig-zag, Fe2N6 bulk, and Fe2N6 zig-zag structures, respectively. In the case of the Fe2N5(*OH) zig-zag structure, the Fe atoms are initially out of the graphene plane whereas in the Fe2N6structures, they are initially in the graphene plane. Both multi-metal structures show enhanced stability, compared to their single-Fe counterpart structures for a wide range of Fe chemical potentials. By combining these simulation data, all calculated using the same methodology, direct comparison of structure-function relations for activity and structural stability for a wide range of structures, including similar structures at bulk and multiple edges, will be available for comparison. References 1 Holby, E. F. & Taylor, C. D. Applied Physics Letters 101, 064102, (2012). 2 Norskov, J. K. et al. Journal of Physical Chemistry B 108, 17886-17892, (2004). 3 Studt, F. Catalysis Letters 143, 58-60, (2013). 4 Kattel, S., Atanassov, P. & Kiefer, B. The Journal of Physical Chemistry C 116, 17378-17383, (2012). 5 Kattel, S., Atanassov, P. & Kiefer, B. Physical Chemistry Chemical Physics 15, 148-153, (2013). 6 Abild-Pedersen, F. et al. Physical Review Letters 99, 016105, (2007). 7 Holby, E. F., Wu, G., Zelenay, P. & Taylor, C. D. The Journal of Physical Chemistry C 118, 14388-14393, (2014). 8 Holby, E. F. & Taylor, C. D. Scientific Reports 5, (2015). Figure 1. Model Fe2N6 structure at a zig-zag edge nanoribbon with bound OH ORR intermediate. Figure 1

The design of bipolar spin semiconductor based on zigzag–edge graphene nanoribbons

Using first-principle methods, we investigate the magnetism properties of zigzag–edge graphene nanoribbons (ZGNRs) with a nanopore, and find that edges of such a nanopore show important effects although the electron transport is mainly along the outer edge of ZGNR. The robust bipolar spin semiconductor can be obtained when the edges of such a nanopore are varied, where both spin states have a gap but can relatively shift. We speculate that bipolar spin semiconductor behavior is related with two factors: broken inner edge states and width of electrode. A series of models are considered: 6-ZGNRs with only one edge occupied by one triangle protrusion (TP), are connected with different width ZGNR electrodes. With the increase the electrode width along the TP edge direction, the systems show the following behavior in turn: spin metallicity, spin gapless semiconductor, and bipolar spin semiconductor. Finally, we show that the half-metallicity is realizable when electric fields are applied across the ZGNR with a nanopore, and their magnetic properties can be controlled by the external electric field. These findings suggest a new possibility for developing nanometer-scale carbon spintronic devices.

Two-dimensional Kagome phosphorus and its edge magnetism: a density functional theory study

By means of density functional theory calculations, we predict a new two-dimensional phosphorus allotrope with the Kagome-like lattice(Kagome-P). It is an indirect gap semiconductor with a band gap of 1.64 eV. The gap decreases sensitively with the compressive strain. In particular, shrinking the lattice beyond 13% can drive it into metallic state. In addition, both the AA and AB stacked Kagome-P multi-layer structures exhibit a bandgap much smaller than 1.64 eV. Edges in the Kagome-P monolayer probably suffer from the edge reconstruction. An isolated zigzag edge can induce antiferromagnetic (AF) ordering with a magnetic transition temperature of 23 K. More importantly, when applying a stretching strain beyond 4%, such an edge turns to possess a ferromagnetic ground state. A very narrow zigzag-edged Kagome-P ribbon displays the spin moment distribution similar to the zigzag-edged graphene nanoribbon because of the coupling between the opposites edges. But the inter-edge coupling in the Kagome-P ribbon vanishes more rapidly as the ribbon width increases. These properties make it a promising material in spintronics.

Tunable half-metallic properties and spin Seebeck effects in zigzag-edged graphene nanoribbons adsorbed with V atom or V-benzene compound

We investigate the spin transport and thermospin properties of zigzag-edged graphene nanoribbons (ZGNRs) adsorbed with a V atom or V-benzene compound using the spin-polarized density functional theory. It is found that the spin polarization of the transmittance at the Fermi level can be obviously enhanced as the V atom moves from the edge to middle hollow sites. In some cases, ZGNRs with V adsorption can possess half-metallicity with −100% spin polarization, which is mainly attributed to the appearance of a spin-up localized state from the d orbital of the V atom. When a benzene is covered on the V atom, the sign of spin polarization is altered, and even it approaches 100%. In addition, compared with the ideal ZGNRs, the spin thermoelectric effects of these adsorbed ZGNRs are obviously enhanced in some cases. Meantime, the magnitude of the spin Seebeck coefficient at the Fermi level is nearly equal to that of the charge Seebeck coefficient. Moreover, we also find that the spin thermoelectric properties are rather sensitive to the width of the nanoribbon, and the sign and magnitude of the spin Seebeck coefficient can be modified by the width. Therefore, these adsorbed ZGNRs can be used to design perfect spin-filtering systems and high-performance thermopsin devices.

Large rectification ratio induced by nitrogen (boron) doping in graphene nanoribbon electrodes for OPE junctions

Using nonequilibrium Green's function method combined with density functional theory, we present electronic transport properties for a molecular device constructed by an oligomeric phenylene ethynylenes (OPE) molecule between two zigzag-edge graphene nanoribbon (zGNR) electrodes. Nitrogen and boron dopants are introduced into one electrode, which makes the molecular junctions to have obvious rectifying behaviors. The conductance properties are closely related to the position and the kind of impurity atoms in electrodes. Moreover, the I–V curve of the junction, which is boron doping at the edge, displays unidirectional features in the studied bias region, and large rectification ratios up to 103 are obtained. The mechanism of the performance is analyzed in terms of transmission spectra combined with molecular projected self-consistent Hamiltonian and electrode band structures under different bias voltages. Symmetry analyses of the Bloch wave functions of the corresponding subbands and the frontier molecular orbitals are also applied to reveal the rectifying behavior.

Layer-selective half-metallicity in bilayer graphene nanoribbons.

Half-metallicity recently predicted in the zigzag-edge graphene nanoribbons (ZGNRs) and the hydrogenated carbon nanotubes (CNTs) enables fully spin-polarized electric currents, providing a basis for carbon-based spintronics. In both carbon systems, the half-metallicity arises from the edge-localized electron states under an electric field, lowering the critical electric field Dc for the half-metallicity being an issue in recent works on ZGNRs. A properly chosen direction of the electric field alone has been predicted to significantly reduce Dc in the hydrogenated CNTs, which in this work turned out to be the case in narrow bilayer ZGNRs (biZGNRs). Here, our simple model based on the electrostatic potential difference between the edges predicts that for wide biZGNRs of width greater than ~2.0 nm (10 zigzag carbon chains), only one layer of the biZGNRs becomes half-metallic leaving the other layer insulating as confirmed by our density functional theory (DFT) calculations. The electric field-induced switching of the spin-polarized current path is believed to open a new route to graphene-based spintronics applications.

Spin-dependent Seebeck effects in a graphene nanoribbon coupled to two square lattice ferromagnetic leads

We theoretically investigate spin-dependent Seebeck effects for a system consisting of a narrow graphene nanoribbon (GNR) contacted to square lattice ferromagnetic (FM) electrodes with noncollinear magnetic moments. Both zigzag-edge graphene nanoribbons (ZGNRs) and armchair-edge graphene nanoribbons (AGNRs) were considered. Compared with our previous work with two-dimensional honeycomb-lattice FM leads, a more realistic model of two-dimensional square-lattice FM electrodes is adopted here. Using the nonequilibrium Green's function method combining with the tight-binding Hamiltonian, it is demonstrated that both the charge Seebeck coefficient SC and the spin-dependent Seebeck coefficient SS strongly depend on the geometrical contact between the GNR and the leads. In our previous work, SC for a semiconducting 15-AGNR system near the Dirac point is two orders of magnitude larger than that of a metallic 17-AGNR system. However, SC is the same order of magnitude for both metallic 17-AGNR and semiconducting 15-AGNR systems in the present paper because of the lack of a transmission energy gap for the 15-AGNR system. Furthermore, the spin-dependent Seebeck coefficient SS for the systems with 20-ZGNR, 17-AGNR, and 15-AGNR is of the same order of magnitude and its maximum absolute value can reach 8 μV/K. The spin-dependent Seebeck effects are not very pronounced because the transmission coefficient weakly depends on spin orientation. Moreover, the spin-dependent Seebeck coefficient is further suppressed with increasing angle between the relative alignments of magnetization directions of the two leads. Additionally, the spin-dependent Seebeck coefficient can be strongly suppressed for larger disorder strength. The results obtained here may provide valuable theoretical guidance in the experimental design of heat spintronic devices.

Energetics, Electron States, and Magnetization in Nearly Zigzag-Edged Graphene Nano-Ribbons

Energetics, spin polarization, and electron states of graphene nano-ribbons (GNRs) are investigated by using the density functional theory. To consider realistic shapes of GNRs in nature, we focus on the nearly zigzag-edged GNRs (NZE-GNRs) which have finite-length zigzag edges connected by another short zigzag edge (kink). Through the systematic calculations, we have clarified roles of nano-shapes of NZE-GNRs in its stability, the spin polarization, and the electron states. Edge states localized near the zigzag edge, the flat bands, and the consequent spin-polarized ground states exist when NZE-GNRs have long enough zigzag edges or wide enough width. In contrast, we have found that the large kink size hinders their appearance. We have also calculated the edge formation energies of various NZE-GNRs. The relative stability of the edge is enhanced when the zigzag edge is short or the width is narrow.

The line-defect–induced valley filter in strained graphene

We theoretically demonstrate that a kind of extended line defect, an experimentally available topological defect in graphene lattice, can induce one-dimensional boundary states. And in the presence of the pseudomagnetic field generated by an inhomogeneous strain, such boundary states are valley chiral in the sense of electronic propagation direction being locked to its valley degree of freedom. Based on such an electronic characteristic, we further show that when the line defect is embedded in the strained graphene strip, the valley filtering and valley valve functions can be realized in the electronic transport process. Therefore, we argue that the line defect can play a key role in the field of graphene valleytronics, instead of a zigzag-edged graphene nanoribbon.

Effects of amino-nitro side groups on electron device of oligo p-phenylenevinylene molecular between ZGNR electrodes

We study the electronic transport property for a molecular device of an oligo (p-phenylenevinylene) (OPV) molecule without or with different side groups between two zigzag-edged graphene nanoribbon (ZGNR) electrodes. By using ab initio calculations based on density-functional theory, the effects of negative differential resistance (NDR) and spin-rectifying in I–V characteristics are revealed and explained for the proposed molecular device. Our analysis indicates that the NDR behavior comes from the conduction orbital being suppressed under certain bias voltage, while the rectifying effect is because of the asymmetry distribution of the highest occupied molecular orbital or the lowest unoccupied molecular one as well as the corresponding coupling between the molecule and electrodes. Interestingly, the transport property of the device can be improved by introducing amino-nitro side groups to the OPV molecule. The NDR behavior can be much enhanced for molecule with amino side group, and the rectification can be improved for molecule with amino and nitro side groups, respectively. In particular, the NDR behavior of peak to valley ratio can be much enhanced and a molecular rectifier which offers rectification ratio of more than three orders of magnitude up to 2863 by adding NH2 and NO2 side groups to OPV molecule, respectively.

Half-metallic properties, single-spin negative differential resistance, and large single-spin Seebeck effects induced by chemical doping in zigzag-edged graphene nanoribbons.

Ab initio calculations combining density-functional theory and nonequilibrium Green's function are performed to investigate the effects of either single B atom or single N atom dopant in zigzag-edged graphene nanoribbons (ZGNRs) with the ferromagnetic state on the spin-dependent transport properties and thermospin performances. A spin-up (spin-down) localized state near the Fermi level can be induced by these dopants, resulting in a half-metallic property with 100% negative (positive) spin polarization at the Fermi level due to the destructive quantum interference effects. In addition, the highly spin-polarized electric current in the low bias-voltage regime and single-spin negative differential resistance in the high bias-voltage regime are also observed in these doped ZGNRs. Moreover, the large spin-up (spin-down) Seebeck coefficient and the very weak spin-down (spin-up) Seebeck effect of the B(N)-doped ZGNRs near the Fermi level are simultaneously achieved, indicating that the spin Seebeck effect is comparable to the corresponding charge Seebeck effect.

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.

A first principle study on the spin transport properties in heterojunctions based on zigzag-edged graphene nanoribbons and graphitic carbon nitride nanoribbons

By using non-equilibrium Green’s functions (NEGF) and DFT, we investigate the spin-dependent electronic transport properties of two heterojunctions based on zigzag-edged graphene nanoribbons and graphitic carbon nitride nanoribbons.

Giant spin thermoelectric effects in all-carbon nanojunctions.

We examine the thermospin properties of an all-carbon nanojunction constructed by a graphene nanoflake (GNF) and zigzag-edged graphene nanoribbons (ZGNRs), bridged by the carbon atomic chains. The first-principles calculations show that the phonon thermal conductance is much weaker than the electron thermal conductance at the Fermi level, and even the former is a few percent of the latter in the low-temperature regime. In the meantime, the carbon-based device possesses an excellent spin transport property at the Fermi level due to the appearance of half-metallic property. Furthermore, the single-spin Seebeck coefficient has a larger value at the Fermi level. These facts ultimately result in a significant enhancement of spin thermoelectric figure of merit (FOM) ZST. By controlling the carbon-chain lengths and the temperature, the maximal value of ZST can reach 30. Moreover, we also find that the room temperature ZST displays an odd-even effect with the carbon-chain lengths, and it is always larger than the charge thermoelectric FOM ZCT.

Tuning spin polarization and spin transport of zigzag graphene nanoribbons by line defects.

From first-principles methods, the spin-dependent electronic properties of zigzag-edged graphene nanoribbons (ZGNRs) with a line defect (558-defect) are investigated systematically and compared to those of the pristine ZGNR. Results show that the line defect possesses an obvious tuning effect on the spin-polarization of the edge carbon atoms of the defective ZGNRs, and the spin-polarization and spin-transport are sensitive to the position of line defects. The defective ZGNRs can realize a transition from antiferromagnetism (AFM) to ferrimagnetism and ferromagnetism (FM) via changing the position of line defects from the center to the zigzag edge of ZGNRs. More importantly, when the line defect is located at the one edge, the defective ZGNRs exhibit the long-range magnetic ordering at edges with a high Curie temperature up to 276 K, and the defective ZGNR system can generate a high-performance spin-filter effect in the large bias range, 0.0-0.5 V. Such a sensitive modulation for the spin-polarization and spin-transport holds great promise for applications of the graphene-based systems in nano-scale spintronic devices.

Magnetic structure and magnetic transport properties of graphene nanoribbons with sawtooth zigzag edges.

The magnetic structure and magnetic transport properties of hydrogen-passivated sawtooth zigzag-edge graphene nanoribbons (STGNRs) are investigated theoretically. It is found that all-sized ground-state STGNRs are ferromagnetic and always feature magnetic semiconductor properties, whose spin splitting energy gap Eg changes periodically with the width of STGNRs. More importantly, for the STGNR based device, the dual spin-filtering effect with the perfect (100%) spin polarization and high-performance dual spin diode effect with a rectification ratio about 1010 can be predicted. Particularly, a highly effective spin-valve device is likely to be realized, which displays a giant magnetoresistace (MR) approaching 1010%, which is three orders magnitude higher than the value predicted based on the zigzag graphene nanoribbons and six orders magnitude higher than previously reported experimental values for the MgO tunnel junction. Our findings suggest that STGNRs might hold a significant promise for developing spintronic devices.