Tunable gap in stable arsenene nanoribbons opens the door to electronic applications.

Arsenic has been predicted to present significantly more diverse 2D phases than other elemental compounds like graphene. While practical applications must be based on finite arsenene samples, like nanoribbons, theory has so far focused on the infinite sheet. Our ab initio simulations show the clear contrast between the properties of arsenene nanoribbons and those of the monolayer, ranging from phase stability to electronic structure. We include nanoribbons derived from the buckled, puckered and square/octagon structures of bulk arsenene. The flexibility afforded by different parent structures, widths and edge passivations leads to a rich variety of semiconducting structures with tunable gaps.

What will freestanding borophene nanoribbons look like? An analysis of their possible structures, magnetism and transport properties.

We report a density-functional theory study of the stability and electronic structure of two recently proposed borophene sheets with Pmmn symmetry and nonzero thickness. We then investigate nanoribbons (BNRs) derived from these nanostructures, with particular attention to technologically relevant properties like magnetism and electronic transport. We consider two perpendicular directions for the edges of the stripes as well as different lateral widths. We show that the Pmmn8 sheet, with 8 atoms in its unit cell and generated by two interpenetrating lattices, has a larger binding energy than the Pmmn2 sheet, with only 2 atoms per unit cell. We also use their phonon spectra to show that the mechanical stability of the Pmmn8 sheet is superior to that of the Pmmn2 sheet. Nanoribbons derived from Pmmn8 are not only more stable than those derived from Pmmn2, but also more interesting from the technological point of view. We find a rich variety of magnetic solutions, depending on the borophene "mother structure", edge orientation, width and, in the case of Pmmn8-derived BNRs, the sublattice of edge atoms. We show that one can build BNRs with magnetic moment in both, one or none of the edges, as well as with parallel or antiparallel magnetic coupling between the edges when magnetic; moreover, their electronic character can be semiconducting, metallic or half-metallic, creating a perfect spin valve at low bias. These different behaviors are reflected in their densities of states, spin density and electronic transport coefficients, which are analyzed in detail. Our work provides a complete overview of what one may expect if nanoribbons are cut out from Pmmn sheets with a view to potential technological applications.

Spin-polarized transport in hydrogen-passivated graphene and silicene nanoribbons with magnetic transition-metal substituents.

We present a systematic theoretical study of the electronic transport in hydrogen passivated zigzag graphene and silicene nanoribbons with between zero and four neighboring H atoms on one edge replaced by magnetic transition metals (Fe, Co, and Ni). The calculations were performed using equilibrium transport and density-functional theory with the generalized gradient approximation to exchange and correlation. We considered the magnetic moments of the two edges aligned both ferromagnetically (Ferro-F form) and antiferromagnetically (Ferro-A form). The Ferro-A graphene-based ribbons were all semiconducting and would support moderate spin-polarized currents of either sign by applying positive or negative gate voltages. The Ferro-F graphene-based ribbons were all metallic; the most interesting for possible spintronic applications being that with a single Ni atom, in which strong spin-filtering at low bias resulted from a deep trough in the transmission of one spin component around the Fermi level. By contrast, in the Si-based analog this trough was split, partially eliminating the polarization of the current. This splitting was found to be related to the buckled structure of the Si-based nanoribbon, which has its origin in its preference for sp(3)-like hybridization.

Spin currents and filtering behavior in zigzag graphene nanoribbons with adsorbed molybdenum chains.

By means of density-functional-theoretic calculations, we investigated the structural, electronic and transport properties of hydrogen-passivated zigzag graphene nanoribbons (ZGNRs) on which a one-atom-thick Mo chain was adsorbed (with or without one or two missing atoms), or in which the passivating hydrogen atoms were replaced by Mo atoms. Mo-passivated ZGNRs proved to be nonmagnetic. ZGNRs with an adsorbed defect-free Mo chain were most stable with the Mo atoms forming dimers above edge bay sites, which suppressed the magnetic moments of the C atoms in that half of the ribbon; around the Fermi level of these systems, each spin component had a transmission channel via the Mo spz band and one had an additional channel created by polarization of the ZGNR π* band, leading to a net spin current. The absence of an Mo dimer from an Mo chain adsorbed at the ZGNR edge made the system a perfect spin filter at low voltage bias by suppressing the Mo spz band channels. Thus this last kind of hybrid system is a potential spin valve.

Spin-dependent electronic conduction along zigzag graphene nanoribbons bearing adsorbed Ni and Fe nanostructures.

Using SMEAGOL, an ab initio computational method that combines the non-equilibrium Green's function formalism with density-functional theory, we calculated spin-specific electronic conduction in systems consisting of single Fen and Nin nanostructures (n = 1-4) adsorbed on a hydrogen-passivated zigzag graphene nanoribbon. For each cluster we considered both ferromagnetically and antiferromagnetically coupled ribbon edges (Ferro-F and Ferro-A systems, respectively). Adstructures located laterally on Ferro-A ribbons caused significant transmittance loss at energies 0.6-0.25 eV below the Fermi level for one spin and 0.2-0.4 eV above the Fermi level for the other, allowing the potential use of these systems in transistors to create a moderately spin-polarized current of one or the other sign depending on the gate voltage. Ni3 and Ni4 clusters located at the centre of Ferro-F ribbons exhibited a strong spin-filtering effect in a narrow energy window around the Fermi level.

Electronic structure and transport properties of monatomic Fe chains in a vacuum and anchored to a graphene nanoribbon.

The electronic structure and transport properties of monatomic Fe wires of different characteristics are studied within the density functional theory. In both equidistant and dimerized (more stable) isolated wires, magnetism plays an important role since it leads to different shapes of the transmission coefficients for each spin component. In equidistant wires, electron localization around the Fermi level leads to symmetry breaking between d(xy) and d(x(2)-y(2)) bands. The main effect of the structural dimerization is to decrease the number of channels available for the minority spin component. When anchored to the edges of a graphene nanoribbon, the dimerization of the chain is preserved, despite the hybridization of the d states of Fe with the C atoms which gives way to a reduction in the number of d channels around the Fermi level. Most conduction is then led by an electronic channel from the ribbon and the sp(z) bands from the Fe wires. Suggestions to improve the spintronic ability of Fe wires are proposed.

Structure and electronic properties of molybdenum monatomic wires encapsulated in carbon nanotubes.

Monatomic chains of molybdenum encapsulated in single-walled carbon nanotubes (CNTs) of different chiralities are investigated using density functional theory. We determine the optimal size of the CNT for encapsulating a single atomic wire, as well as the most stable atomic arrangement adopted by the wire. We also study the transport properties in the ballistic regime by computing the transmission coefficients and tracing them back to the electronic conduction channels of the wire and the host. We predict that CNTs of appropriate radii encapsulating a Mo wire have metallic behavior, even if both the nanotube and the wire are insulators. Therefore, encapsulation of Mo wires in CNTs is a way to create conductive quasi-one-dimensional hybrid nanostructures.

Impact of dimerization and stretching on the transport properties of molybdenum atomic wires.

We study the electrical and transport properties of monatomic Mo wires with different structural characteristics. We consider first periodic wires with interatomic distances ranging between the dimerized wire to that formed by equidistant atoms. We find that the dimerized case has a gap in the electronic structure which makes it insulating, as opposed to the equidistant or near-equidistant cases which are metallic. We also simulate two conducting one-dimensional Mo electrodes separated by a scattering region which contains a number of dimers between 1 and 6. The I-V characteristics strongly depend on the number of dimers and vary from ohmic to tunneling, with the presence of different gaps. We also find that stretched chains are ferromagnetic.

Density functional theory based screening of ternary alkali-transition metal borohydrides: a computational material design project.

We present a computational screening study of ternary metal borohydrides for reversible hydrogen storage based on density functional theory. We investigate the stability and decomposition of alloys containing 1 alkali metal atom, Li, Na, or K (M(1)); and 1 alkali, alkaline earth or 3d/4d transition metal atom (M(2)) plus two to five (BH(4))(-) groups, i.e., M(1)M(2)(BH(4))(2-5), using a number of model structures with trigonal, tetrahedral, octahedral, and free coordination of the metal borohydride complexes. Of the over 700 investigated structures, about 20 were predicted to form potentially stable alloys with promising decomposition energies. The M(1)(Al/Mn/Fe)(BH(4))(4), (Li/Na)Zn(BH(4))(3), and (Na/K)(Ni/Co)(BH(4))(3) alloys are found to be the most promising, followed by selected M(1)(Nb/Rh)(BH(4))(4) alloys.