Graphene, a single two-dimensional sheet of carbon atoms with an arrangement mimicking the honeycomb hexagonal architecture, has captured immense interest of the scientific community since its isolation in 2004. Besides its extraordinarily high electrical conductivity and surface area, graphene shows a long spin lifetime and limited hyperfine interactions, which favors its potential exploitation in spintronic and biomedical applications, provided it can be made magnetic. However, pristine graphene is diamagnetic in nature due to solely sp2 hybridization. Thus, various attempts have been proposed to imprint magnetic features into graphene. The present review focuses on a systematic classification and physicochemical description of approaches leading to equip graphene with magnetic properties. These include introduction of point and line defects into graphene lattices, spatial confinement and edge engineering, doping of graphene lattice with foreign atoms, and sp3 functionalization. Each magnetism-imprinting strategy is discussed in detail including identification of roles of various internal and external parameters in the induced magnetic regimes, with assessment of their robustness. Moreover, emergence of magnetism in graphene analogues and related 2D materials such as transition metal dichalcogenides, metal halides, metal dinitrides, MXenes, hexagonal boron nitride, and other organic compounds is also reviewed. Since the magnetic features of graphene can be readily masked by the presence of magnetic residues from synthesis itself or sample handling, the issue of magnetic impurities and correct data interpretations is also addressed. Finally, current problems and challenges in magnetism of graphene and related 2D materials and future potential applications are also highlighted.

Emerging chemical strategies for imprinting magnetism in graphene and related 2D materials for spintronic and biomedical applications

We report ab initio analysis of current-voltage (I-V) characteristics of carbon nanotubes with nitrogen substitution doping. For zigzag semiconducting tubes doping with a single N impurity increases current flow and, for small radii tubes, narrows the current gap. Doping a N impurity per nanotube unit cell generates a metallic transport behavior. Nonlinear I-V characteristics set in at high bias and a negative differential resistance region is observed for the doped tubes. These behaviors can be well understood from the alignment/misalignment of the current carrying bands in the nanotube leads due to the applied bias voltage. For a armchair metallic nanotube, a reduction of current is observed with substitutional doping due to elastic backscattering by the impurity.

Current-voltage characteristics of carbon nanotubes with substitutional Nitrogen

The groundbreaking works in graphene and graphene nanoribbons (GNRs) over the past decade, and the very recent discovery of borophene naturally draw attention to the yet-to-be-explored borophene nanoribbons (BNRs). We herein report a density functional theory (DFT) study of the electronic and magnetic properties of BNRs. The foci are the impact of orientation (denoted as BxNRs and ByNRs with their respective periodic orientations along x- and y-axis), ribbon width (Nx, Ny=4–15), and hydrogenation effects on the geometric, electronic and magnetic properties of BNRs. We found that the anisotropic quasi-planar geometric structure of BNR and the edge states largely govern its electronic and magnetic properties. In particular, pristine ByNRs adopt a magnetic ground state, either anti-ferromagnetic (AFM) or ferromagnetic (FM) depending on the ribbon width, while pristine BxNRs are non-magnetic (NM). Upon hydrogenation, all BNRs exhibit NM. Interestingly, both pristine and hydrogenated ByNRs undergo a metal-semiconductor-metal transition at Ny=7, while all BxNRs remain metallic.

Electronic and magnetic properties of pristine and hydrogenated borophene nanoribbons

We present first-principles calculations of quantum transport which show that the resistance of metallic carbon nanotubes can be changed dramatically with homogeneous transverse electric fields if the nanotubes have impurities or defects. The change of the resistance is predicted to range over more than 2 orders of magnitude with experimentally attainable electric fields. This novel property has its origin that backscattering of conduction electrons by impurities or defects in the nanotubes is strongly dependent on the strength and/or direction of the applied electric fields. We expect this property to open a path to new device applications of metallic carbon nanotubes.

http://absimage.aps.org/image/MAR06/MWS_MAR06-2005-005501.pdf

Electrical Switching in Metallic Carbon Nanotubes

Germanene exhibits extremely high mobility, massless fermion behavior, and strong spin–orbit coupling drawing tremendous interest for high performance devices. It has a buckled two-dimensional structure, but not the intrinsic energy band gap and structural stability required for logic and switching devices. Application of a perpendicular electric field, surface adsorption, confinement of an armchair nanoribbon structure and edge functionalization are methods used to open a band gap. Edge functionalization of armchair germanene nanoribbons (AGeNRs) has the potential to achieve a range of band gaps. The edge atoms of AGeNRs are passivated with hydrogen (–H and –2H) or halogen (–F, –Cl, –OH, –2F, –2Cl) atoms. Using density functional theory calculations, we found that edge functionalized AGeNRs had band gaps as small as 0.012 eV when functionalized by –2H and as high as 0.84 eV with –2F. Formation energy studies revealed that AGeNRs produced a more stable structure under fluorine functionalization. Simulation results suggest that the electronic structure of germanene is similar to graphene and silicene. A spin-polarized density functional theory (DFT) study of electronic and magnetic properties of pristine, chemically functionalized and doped AGeNRs and zigzag nanoribbons (ZGeNRs) was performed. Formation energy studies revealed that the Ge atoms at the edge of the ribbon prefer to be replaced by impurity atoms. Doping can change the semiconducting behaviour of AGeNRs to metal behaviour due to the half-filled band making it useful for negative differential resistance (NDR) devices. In the case of ZGeNRs, single N or B doping transformed them from anti-ferromagnetic (AFM) semiconducting to ferromagnetic (FM) semiconducting or half-metal. These magnetic and electronic properties make edge functionalized doped AGeNRs and ZGeNRs promising for use in field effect transistors (FETs) and spintronics. Finally, energy band gap tuning of AGeNRs and ZGeNRs using edge functionalization may open a new route to integrate germanene in logic and high performance switching devices.

/pdf/edge-functionalized-germanene-nanoribbons-impact-on-4u4i4yskgy.pdf

Edge functionalized germanene nanoribbons: impact on electronic and magnetic properties

The dihydrogenation effects in the zigzag-edged graphene nanoribbons (ZGNRs) have been systematically investigated by first-principles calculations. Due to the dihydrogenation, the edges effectively turn to the so-called Klein edges, which results in localized edge states in (0,$\frac{2}{3}\ensuremath{\pi}$) and extended bulk states in ($\frac{2}{3}\ensuremath{\pi},\ensuremath{\pi}$). Compared with monohydrogenation, the edge magnetic moment is substantially increased and the edge states get much more delocalized, which results in the most attractive observation that the energy difference between the antiferromagnetic (AFM) and ferromagnetic (FM) configurations is greatly increased by nearly one order of magnitude from the general several meV and thus the AFM ground state of certain ZGNRs becomes stable at room temperature. This suggests that the dihydrogenated ZGNRs are more promising than monohydrogenated ones for spintronic devices. Our finding provides an enlighening hint for stablizing the ground AFM state in ZGNRs.

Stabilizing the ground state in zigzag-edged graphene nanoribbons by dihydrogenation

Band gap engineering in armchair-edged graphene nanoribbons by edge dihydrogenation

B, C and N adatoms effects on the transport properties in zigzag graphene nanoribbons

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Stabilizing the ground state in zigzag-edged graphene nanoribbons by dihydrogenation

Band gap engineering in armchair-edged graphene nanoribbons by edge dihydrogenation

B, C and N adatoms effects on the transport properties in zigzag graphene nanoribbons