First-principles calculations of spin-dependent conductance of graphene flakes

Using ab initio density-functional theory and quantum transport calculations based on nonequilibrium Green’s function formalism we study structural, electronic, and transport properties of hydrogen-terminated short graphene nanoribbons (graphene flakes) and their functionalization with vanadium atoms. Rectangular graphene flakes are stable, having geometric and electronic structures quite similar to that of extended graphene nanoribbons. We show that a spin-polarized current can be produced by pure hydrogenated rectangular graphene flakes by exploiting the spatially separated edge states of the flake using asymmetric nonmagnetic contacts. Functionalization of the graphene flake with magnetic adatoms such as vanadium also leads to spin-polarized currents even with symmetric contacts. We observe and discuss sharp discontinuities in the transmission spectra which arise from Fano resonances of localized states in the flake.

Figure 1

(Color online) Geometry and spin-dependent charge density of the graphene flake cut from 4-ZGNR. The edges are saturated with hydrogen atoms. Green (dark) and yellow (light) regions denote the local majority-spin types of the charge density. Possible adsorption sites of adatoms are labeled as E1 and H1–H4. Electrodes (carbon chain or gold bar) are attached to the flake along directions A and Z1–Z4.

Figure 2

(Color online) Spin-dependent transmission spectrum of the graphene flake without (top panels) and with (middle panels) hydrogen saturation of the edge atoms. The carbon linear-chain electrodes are connected to the Z1 sites (see Fig. 1). The spectra when the carbon-chain electrodes are replaced by gold leads are shown in the bottom panels. Fermi level is set to zero.

Figure 3

(Color online) Spin-dependent transmission spectra of the hydrogen-saturated graphene flake when the electrodes make partial contact at the Z1 and Z2 sites (top panels) and when the contacts are uniform (bottom panels). Fermi level is set to zero.

Figure 4

(Color online) Majority-spin type of local charge density of the flake with single vanadium adatoms. The upper zigzag edge of the flake was spin-up polarized before the adsorption. The vanadium atom has a down magnetic moment in (a) and (c) and an up moment in (b) and (d). The lowest energy configuration for a single vanadium atom is shown in (a).

Figure 5

(Color online) Spin-dependent transmission spectra of the hydrogen-saturated graphene flake with an adsorbed vanadium atom at the E1 site in its ground state. Top panels: the electrodes make partial contacts at the Z1 sites. Bottom panels: when the contacts are uniform.

Figure 6

(Color online) Spin polarization in the transmission spectra of the graphene flake. The type and contact geometries of the electrodes are given as insets. In the lowest panel there is a single vanadium adatom in its ground state.

Figure 7

(Color online) LDOS isosurfaces calculated for particular energy values of the up-spin transmission spectra of partially contacted graphene flake. The $f$ value next to each plot is the maximum value of LDOS obtained in units of states/$\left({\text{Å}}^3\text{eV}\right)$ and can be taken as a measure of relative magnitudes among the plots. The plotted isosurfaces correspond to LDOS values of $f\times {10}^{-2}$.