Half-metallicity In Graphene Nanoribbons Research Articles

Edge-insensitive magnetism and half metallicity in graphene nanoribbons

Realizing magnetism in graphene nanostructures is a decade-long challenge. The magnetic edge state and half metallicity in zigzag graphene nanoribbons are particularly promising (Son et al 2006 Nature 444 347). However, its experimental realization has been hindered by the stringent requirement of the mono-hydrogenated zigzag edge. Using first-principle calculations, we predict that free-carrier doping can overcome this challenge and realize ferromagnetism and half-metallicity in narrow graphene nanoribbons of general types of edge structures. This magnetism exists within the density range of gate-doping experiments (~1013 cm−2) and has large spin polarization energy up to 17 meV per carrier, which induces a Zeeman splitting equivalent to an external magnetic field of a few hundred Tesla. Moreover, we trace the formation of this edge-insensitive magnetism to the quantum confinement of the electronic state near the band edge and reveal the scaling law of magnetism versus the ribbon width. Our findings suggest that combining doping with quantum confinement could be a general tool to realize transition-metal-free magnetism in light-element nanostructures.

Bias induced ferromagnetism and half-metallicity in graphene nano-ribbons

Towards spin selective electronics made of three coordinated carbon atoms, here we computationally propose robust and reversibly bias driven evolution of pristine undoped graphene nano-ribbons(GNR) into ferromagnetic-semiconductor, metal or a half metal, irrespective of their edge configurations. The evolution is a result of a rare ferromagnetic(FM) order emerging among nearest neighbouring(n-n) sites, in positively biased regions in their in-homogeneous bias unit-cells, in attempt to cooperatively minimise on-site Coulomb repulsion and kinetic energy, while maximising localization of electrons at the positively biased sites. The phenomenon appears to be a general property of in-homogeneously biased Coulomb correlated bipartite systems. Consequences are particularly rich in zigzag edged graphene nano-ribbons(ZGNR) due to the contest of bias driven n-n FM order and the inter-edge antiferromagnetic order inherent to ZGNRs, leading to systematic closing of gap for one of the spins, amounting to bias controlled unmissable opening of window for FM-semiconducting and half-metallic transport.

A New Paradigm to Half-Metallicity in Graphene Nanoribbons.

In contrast to the well-recognized transverse-electric-field-induced half-metallicity in zigzag graphene nanoribbons, here, we demonstrate by first-principles calculations that zigzag graphene nanoribbons sandwiched between hexagonal boron nitride nanoribbons or sheets can be tuned into half-metal simply by a bias voltage or a moderate compressive strain. The half-metallicity is attributed to an enhanced coupling effect of spontaneous polarization and asymmetrical exchange correlation along the ribbon width. The findings should open a viable route for efficient spin-resolved band engineering in graphene-based devices that are compatible with the current technology of the semiconductor industry.

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.

Half-metallicity of graphene nanoribbons and related systems: a new quantum mechanical El Dorado for nanotechnologies … or a hype for materials scientists?

In this work we discuss in some computational and analytical details the issue of half-metallicity in zig-zag graphene nanoribbons and nanoislands of finite width, i.e. the coexistence of metallic nature for electrons with one spin orientation and insulating nature for the electrons of opposite spin, which has been recently predicted from so-called first-principle calculations employing Density Functional Theory. It is mathematically demonstrated and computationally verified that, within the framework of non-relativistic and time-independent quantum mechanics, like the size-extensive spin-contamination to which it relates, half-metallicity is nothing else than a methodological artefact, due to a too approximate treatment of electron correlation in the electronic ground state.

Half-metallicity in graphene nanoribbons with topological line defects

First-principles calculations have been performed to investigate the electronic properties of graphene nanoribbons with topological line defects composed of octagons and fused pentagons. We find that the edge-passivated zigzag graphene nanoribbons (ZGNRs) with the line defects along the edge show half-metallicity as the line defect is close to one edge. The electronic properties of the ZGNRs with line defects can be tuned by changing the ribbon width and the position of the line defect. When the position of the line defect changes, there are transitions from an antiferromagnetic semiconductor to an antiferromagnetic half-metal, and then to a ferromagnetic metal, suggesting the potential applications of the system in spintronic devices.

Building Half-Metallicity in Graphene Nanoribbons by Direct Control over Edge States Occupation

Electronic structures of zigzag edged graphene nanoribbons (ZGNRs) doped with boron (B) or nitrogen (N) atoms are investigated by spin polarized first-principles calculations. We find that ZGNRs can be tuned to be either semiconducting, half-metallic, or metallic by controlling the distance of the impurity atoms to the edges. A new scheme is identified to achieve full half-metallicity in ZGNRs by doping B atom at one edge and N atom at the other. We find that the origin of the half-metallicity is due to interaction between the edge states and B/N atoms which results in direct control over the electron occupation of the edge states. This mechanism is so robust that full half-metallicity can always be produced in ZGNRs irrespective of the ribbon width, which opens new possibilities for applications of ZGNRs in spintronic devices.

Nonlocal Exchange Interaction Removes Half-Metallicity in Graphene Nanoribbons

Band gap studies of zigzag-edge graphene ribbons are presented. While earlier calculations at LDA level show that zigzag-edge graphene ribbons become half-metallic when cross-ribbon electric fields are applied, our calculations with hybrid density functional demonstrate that finite graphene ribbons behave as half-semiconductors. The spin-dependent band gap can be changed in a wide range, making possible many applications in spintronics.