Edges Of Zigzag Graphene Nanoribbons Research Articles

Different noncollinear magnetizations on two edges of zigzag graphene nanoribbons* *Project supported by the National Natural Science Foundation of China (Grant Nos. NSFC11804158, NSFC61974067, NSFC91750112, and NSFC11947101).

Based on density functional theory and non-equilibrium Green’s function method, we studied noncollinear magnetism and spin transport in a 180° domain wall made of zigzag graphene nanoribbon (ZGNR) with different noncollinear magnetic profiles on the top and bottom edges. Our results show that a helical domain wall on the top (bottom) edge and an abrupt domain wall on the bottom (top) edge can survive in the ZGNR. This indicates that such characteristic magnetization distribution can be obtained by some means, e.g., the introduction of impurity on one edge. Compared to a wide ZGNR, a narrow ZGNR presents obvious coupling between the two edges which changes the magnetization and transmission greatly. As for the above-mentioned distinct magnetic profile, the spin transport is blocked in the abrupt domain wall due to strong spin flip scattering while remains unaffected in the helical domain wall due to the spin mixing effect. We deduce a formula of the transmission for various magnetic profiles of the ZGNRs. A new result based on this formula is that the transmission at the Fermi level can be zero, one, and two by tuning the edge magnetization. Our results provide insights into the noncollinear spin transport of the ZGNR-based devices.

Negative differential resistance in nanoscale heterostructures based on zigzag graphene nanoribbons anti-symmetrically decorated with BN

Abstract Nanoscale 2 dimensional (2D) heterostructures based on zigzag graphene nanoribbons (ZGNRs) anti-symmetrically decorated with boron nitride (BN), are proposed as resonant tunneling diodes (RTD). Boron nitride has been utilized to decorate two edges of zigzag graphene nanoribbons in order to create required bandgap difference for constructing double barrier quantum well heterostructure. Nonequilibrium Green's function (NEGF) method including tight-binding (TB) Hamiltonian is employed to obtain transmission versus energy (T-E) curves and current versus bias voltage (I-V) characteristics of proposed heterostructures. Effect of structural parameters on the I-V characteristic is investigated. Proposed RTDs clearly show negative differential resistance (NDR) behavior in their I-V characteristic. Moreover, it is shown that the NDR behavior of proposed heterostructures can be precisely tuned by modulating structural dimensions.

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

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

Functionalization of zigzag graphene nanoribbon with DNA nucleobases-A DFT study

The density functional theory (DFT) calculations on the chemical functionalization of zigzag graphene nanoribbon (ZGNR) with DNA (Deoxyribonucleic acid) nucleobases were carried out using the local density approximation (DFT-LDA). The binding energies, charge transfer, equilibrium geometries and band gap variations of bare ZGNR (b-ZGNR) and nucleobase functionalized ZGNR (nb-ZGNR), were examined at an electron temperature of 300 K. The functionalization of ZGNR with nucleobases adenine (Anb-ZGNR) and cytosine (Cnb-ZGNR) reveals a significant structural deformation compared to bare ZGNR(b-ZGNR).Our findings suggest that functionalization at the edge of ZGNR is effective in tuning its electronic and structural properties through significant enhancement of dipole moment, reduction in %s character and structural asymmetry, thereby suggesting the possibilities of using functionalized ZGNR as nanocarriers in biomedical applications.

Non-collinear magnetism and electronic transport of boron or nitrogen doped zigzag graphene nanoribbon

Zigzag graphene nanoribbon (ZGNR) is important for novel carbon-based spintronic applications. Currently, most of ZGNR spintronic studies focus on the collinear magnetism where the up-spin and down-spin are separated clearly. But in some cases, e.g. doping and adsorption, the magnetization profile can be modulated and thus noncollinear magnetism can occur. In order to shed light on possible noncollinear magnetism in ZGNR, we study non-collinear magnetism and electronic transport of boron or nitrogen-doped zigzag graphene nanoribbon based on noncollinear density functional theory and non-equilibrium Green's function method. For pristine ZGNR, our results show that the ZGNR presents helical magnetization distribution due to noncollinear magnetization in left and right lead. As the ZGNR is doped with boron and nitrogen atoms, the ZGNR shows a characteristic two-zone feature in the magnetization distribution. Near the dopant site, the magnetic moment of carbon atom is small. However, the magnetic moments of carbon atoms in the left (right) region of dopant are close to those of the left (right) lead. Such a feature provides the possibility of constructing domain walls with various widths on the edge of ZGNR. Moreover, the transmission at the Fermi level (<i>E</i> = 0 eV) decreases with the increase of relative angle between magnetizations of left and right lead, indicating that the spin-flip scattering dominates the electronic transport. However, at <i>E</i> = ±0.65 eV, there is a transmission dip with low transmission, which implies that the dopant induces the strong backscattering. To understand the origin of this dip, we calculate the density of states (DOS) and project the DOS onto each atom of doped ZGNR. The projected DOS shows a large and broad peak at <i>E</i> = −0.65 eV for N-doped ZGNR but at <i>E</i> = +0.65 eV for B-doped ZGNR. The consistency between the position of dip in transmission and the position of peak in DOS indicates that the transmission dip mentioned above is attributed to strong backscattering from the dopant-induced bound state. Our theoretical results are expected to be useful for understanding the noncollinear magnetism and spin scattering in the doped ZGNR-based devices. Also, our work provides a considerable insight into the design of ZGNR-based nanoelectronic devices, such as the transistor based on spin transfer torque effect.

First-principle investigations for electronic transport in nitrogen-doped disconnected zigzag graphene nanoribbons

Development of novel electronic devices is an area of active research which reveals the potential of organic nano-materials towards efficient nano-devices application. In this letter, we have investigated the electronic transport properties of nitrogen (N) doped disconnected zigzag graphene nanoribbons (ZGNR). Six different configurations viz. N1, N2, N3, N4, N5 and N6 were studied to reveal the effect of N-impurity on tunnelling dependent electron transport. It is predicted that the magnitude of current through these disconnected ZGNR devices is a function of tunnelling width, however, the qualitative behavior is quite similar. Further, a negative differential resistance (NDR) was observed for pristine nanoribbons which revokes upon N doping except for N3 configuration. The magnitude of tunnelling current is governed by the position of N impurities and can be further enhanced via selective doping of N atoms in the disconnected edges of ZGNR. Moreover, while operating in low bias regime (below 0.5 V), all the considered structures exhibit linear IV characteristics. Present findings may find applications in upcoming organic devices working on tunnelling phenomena.

Spin-Negative Differential Resistance in Zigzag Graphene Nanoribbons with Side-Attached Porphine Molecule

An isolated roughness at the edge of zigzag graphene nanoribbons (zGNRs) breaks the natural bipartite symmetry in the graphene lattice, which might lead to magnetic frustration. Interestingly, the coupling between delocalized states of the roughness and the extended states of the zGNRs induces quantum phenomena, such as destructive quantum interference (DQI), which might play an important role to allow or forbid electrons to transfer through a system. In this work, the electronic and spin-polarized transport properties of the porphine molecule attached to the 6zGNR via different β- and meso- connections are studied using nonequilibrium Green’s function calculations performed within the density functional theory framework (NEGF-DFT). Here, we demonstrated that the porphine/6zGNRs present in the FM ground state for all the couplings and the hybridization between porphine and 6zGNR disturb the bipartite graphene lattice, leading to spin-frustration. Particularly, the porphine molecule introduces Fano-antires...

Origin of spin polarization in an edge boron doped zigzag graphene nanoribbon: a potential spin filter

To realize a graphene based spintronic device, the prime challenge is to control the electronic structure of edges. In this work we find the origin of the spin filtering property in edge boron doped zigzag graphene nanoribbons (ZGNRs) and provide a guide to preparing a graphene based next-generation spin filter based device. Here, we unveil the role of orbitals (p-electron) to tune the electronic, magnetic and transport properties of edge B doped ZGNRs. When all the edge carbon atoms at one of the edges of ZGNRs are replaced by B (100% edge B doping), the system undergoes a semiconductor to metal transition. The role of passivation of the edge with single/double atomic hydrogen on the electronic properties and its relation with the p-electron is correlated in-depth. 50% edge B doped ZGNRs (50% of the edge C atoms at one of the edges are replaced by B) also show half-metallicity when the doped edge is left unpassivated. The half-metallic systems show 100% spin filtering efficiency for a wide range of bias voltages. Zero-bias transmission function of the other configurations shows asymmetric behavior for the up and down spin channels, thereby indicating their possible application potential in nano-spintronics.

Spin polarization properties of thiophene molecule adsorbed to the edge of zigzag graphene nanoribbon

Spin polarization properties of thiophene molecule adsorbed to the edge of zigzag graphene nanoribbon (GNR) are studied based on density functional theory calculations. It is shown that there is charge transfer between the thiophene molecule and the zigzag GNR, which changes the bond lengths of the thiophene molecule. Another consequence of the charge transfer is the spontaneous spin polarization of the thiophene molecule. The mechanism of the spontaneous spin polarization is also discussed based on the analysis of the density of states. It is found that the p orbitals of the carbon atoms and the sulfur atom in thiophene molecule become spin split near the Fermi energy, so preferential fillings of the spin-splitted p orbitals make the thiophene molecule spin polarized. This spontaneous spin polarization of thiophene molecules adsorbed to the zigzag GNR could be a new route to graphene-based organic spintronic devices.

Controlling Electronic Structure and Transport Properties of Zigzag Graphene Nanoribbons by Edge Functionalization with Fluorine

In this work, we report a detailed study of the electronic structure and transport properties of mono- and difluorinated edges of zigzag graphene nanoribbons (ZGNR) using density functional theory (DFT). The calculated formation energies at 0 K indicate that the stability of the nanoribbons increases with the increase in the concentration of difluorinated edge C atoms along with an interesting variation of the energy gaps between 0.0 to 0.66 eV depending on the concentration. This gives a possibility of tuning the band gaps by controlling the concentration of F for terminating the edges of the nanoribbons. The DFT results have been reproduced by density functional tight binding method. Using the nonequilibrium Green functional method, we have calculated the transmission coefficients of several mono- and difluorinated ZGNR as a function of unit cell size and degree of homogeneous disorder caused by the random placement of mono and difluorinated C atoms at the edges.

Voltage-driven spintronic logic gates in graphene nanoribbons.

Electronic devices lose efficacy due to quantum effect when the line-width of gate decreases to sub-10 nm. Spintronics overcome this bottleneck and logic gates are building blocks of integrated circuits. Thus, it is essential to control electronic transport of opposite spins for designing a spintronic logic gate, and spin-selective semiconductors are natural candidates such as zigzag graphene nanoribbons (ZGNR) whose edges are ferromagnetically ordered and antiferromagnetically coupled with each other. Moreover, it is necessary to sandwich ZGNR between two ferromagnetic electrodes for making a spintronic logic gate and also necessary to apply magnetic field to change the spin orientation for modulating the spin transport. By first principle calculations, we propose a method to manipulate the spin transport in graphene nanoribbons with electric field only, instead of magnetic field. We find that metal gates with specific bias nearby edges of ZGNR build up an in-plane inhomogeneous electric field which modulates the spin transport by localizing the spin density in device. The specific manipulation of spin transport we have proposed doesn't need spin-charge conversion for output and suggests a possible base for designing spintronic integrated circuit in atomic scale.

Spin transport properties of a carbon nanotube/zigzag graphene nanoribbon junction: a first principles investigation

A carbon nanotube (CNT)/zigzag graphene nanoribbons (ZGNRs) junctions has been proposed and investigated by first-principles calculations. The results show that large spin polarization of currents would be achieved when only one edge of ZGNR is coupled to the other lead. By virtue of spatial separation of edge state in two spin channel, one of those channels is opened at certain energy range and gives rise to spin-polarized currents under a low bias. This feature is stable whenever the ZGNR lead is under the antiferromagnetic ground states or is under the ferromagnetic states. Our findings indicate that this approach is simple and efficient for spintronics design.

Electrical control of the spin polarization of a current in “pure-carbon” systems based on partially hydrogenated graphene nanoribbon

Controlling a spin current by electrical means and eliminating the use of ferromagnetic contacts becomes a focus of research in spintronics, as compared with conventional magnetic control methods, electrical one could reduce the dimensions and energy consumption of integrated devices. Inspired by recent progress of controlling the hydrogenation on graphene [Xie et al., Appl. Phys. Lett. 98, 193113 (2011)], we investigate the electronic structure and spin-current transport of partially hydrogenated zigzag graphene nanoribbon (ZGNR) with various hydrogenation geometries, through first-principles calculations. It is found that for ZGNR in ferromagnetic edge-coupling state, near-edge hydrogenation would suppress the magnetization on the edge of ZGNR, and lower down the transmission around EF to zero except two peaks, which reside discretely on both sides of EF with opposite spins. Based on this feature, we propose and demonstrate a three-terminal device, where the spin polarization of the current can be modulated by gate voltage (Vg) to vary from (almost) 100% to −100%, which could serve as a perfect electrically-controlled “pure-carbon” dual-spin filter. Especially, the spin polarization varies gradually with Vg, so a current with any ratio of spin-up to spin-down electron numbers can be achieved. Moreover, the influences of ZGNR width and hydrogenation-region length on the system's performance are also discussed and a large range of ZGNR configurations are found to be suitable for the application of such a device.

Spin-polarized transport in zigzag graphene nanoribbons adsorbing nonmagnetic atomic chain

The spin-polarized transport properties of nonmagnetic (metallic Al and nonmetallic C) atomic chains adsorbed on zigzag graphene nanoribbons (ZGNRs) are investigated by the density functional theory (DFT) combined with the nonequilibrium Green’s function method. We find that the spin polarization of ZGNRs is sensitive to the adsorption sites and atomic types of the chains. As an Al chain is adsorbed on the middle of ZGNR, no spin-polarized transport arises. As the Al chain is adsorbed on the edge of ZGNR, high spin polarization is produced around the Fermi level. The different transport behaviors are originated from the fact that the edge adsorption of Al chain breaks the magnetization symmetry of two edges while the middle adsorption of Al chain only modifies the magnetizations of two edges equally. More prominent spin polarization is generated as a C chain is adsorbed on the edge of ZGNR. The complete spin polarization emerges not only around the Fermi level but also far from the Fermi level, owing to the edge states and the localized states. These results indicate that one can effectively modulate the spin-polarized transports of ZGNRs through adsorbing different nonmagnetic atomic chains.

Spin-polarized transport properties of Fe atomic chain adsorbed on zigzag graphene nanoribbons

The spin-polarized transport properties of Fe atomic chain adsorbed on zigzag graphene nanoribbons (ZGNRs) are investigated using the density-functional theory in combination with the nonequilibrium Green's function method. We find that the Fe chain has drastic effects on spin-polarized transport properties of ZGNRs compared with a single Fe atom adsorbed on the ZGNRs. When the Fe chain is adsorbed on the centre of the ZGNR, the original semiconductor transforms into metal, showing a very wide range of spin-polarized transport. Particularly, the spin polarization around the Fermi level is up to 100%. This is because the adsorbed Fe chain not only induces many localized states but also has effects on the edge states of ZGNR, which can effectively modulate the spin-polarized transports. The spin polarization of ZGNRs is sensitive to the adsorption site of the Fe chain. When the Fe chain is adsorbed on the edge of ZGNR, the spin degeneracy of conductance is completely broken. The spin polarization is found to be more pronounced because the edge state of one edge is destroyed by the additional Fe chain. These results have direct implications for the control of the spin-dependent conductance in ZGNRs with the adsorption of Fe chains.

Beryllium and boron decoration forms planar tetracoordinate carbon strips at the edge of graphene nanoribbons

Graphene has been viewed as one of the most promising materials in many fields. Recently, it has been found that by using Cu-decoration at the edge of zigzag graphene nanoribbons (ZGNR), a novel kind of planar tetracoordinate carbon (ptC) strip can be formed. In this paper, we investigate the edge-decoration of armchair graphene nanoribbons (AGNR) by various atom types and find that two new kinds of ptC strip can be effectively formed by using Be or B decoration. For the Be-decorated AGNR, the edge Be atoms take the form of a "zigzag-like" chain, and all the edge C atoms have a ptC nature. However, for the B-decorated AGNR, the edge B atoms form an infinite yet "fractured" chain consisting of separate B(4)-subunits, which results in only 50% of the edge C atoms being ptCs, in contrast with Be-decorated AGNR and Cu-decorated ZGNR. The high thermal stability of both types of ptC-based AGNR is indicated by isomeric sampling and molecular dynamics simulations.

Spontaneous edge-defect formation and defect-induced conductance suppression in graphene nanoribbons

We present a first-principles study of the migration and recombination of edge defects (carbon adatom and/or vacancy) and their influence on electrical conductance in zigzag graphene nanoribbons (ZGNRs). It is found that at room temperature, the adatom is quite mobile while the vacancy is almost immobile along the edge of ZGNRs. The recombination of an adatom-vacancy pair leads to a pentagon-heptagon ring defect structure having a lower energy than the perfect edge, implying that such an edge defect can be formed spontaneously. This edge defect can suppresses the conductance of ZGNRs drastically, which provides some useful hints for understanding the observed semiconducting behavior of the fabricated narrow GNRs.