Intrinsic Half-Metallicity in Modified Graphene Nanoribbons

We perform first-principles calculations based on density functional theory to study quasi-one-dimensional edge-passivated (with hydrogen) zigzag graphene nanoribbons of various widths with chemical dopants, boron and nitrogen, keeping the whole system isoelectronic. The gradual increase in doping concentration takes the system finally to zigzag boron nitride nanoribbons (ZBNNRs). Our study reveals that for all doping concentrations the systems stabilize in antiferromagnetic ground states. Doping concentrations and dopant positions regulate the electronic structure of the nanoribbons, exhibiting both semiconducting and half-metallic behaviors as a response to the external electric field. Interestingly, our results show that ZBNNRs with a terminating polyacene unit exhibit half-metallicity irrespective of the ribbon width as well as applied electric field, opening a huge possibility in spintronics device applications.

Figure 1

Schematic representation of the systems considered. The upper device is fabricated with a hydrogen passivated 8-ZGNR (8 zigzag chains) without any doping. The lower device depicts the doping positions in an 8-ZGNR, replacing two middle zigzag carbon chains by isoelectronic boron-nitrogen chains. Doping concentration increases gradually along the directions of solid arrows. Electric field (dashed arrows) has been applied along the cross-ribbon width, which is the $y$-axis direction.

Figure 2

The density of states as a function of energy, scaled with respect to the Fermi energy for the 8-ZGNR with (a1) 2, (b1) 4, and (c1) 6 zigzag carbon chains replaced by the zigzag boron-nitrogen chains at the middle of the nanoribbon. The spin density profile of these systems are shown in (a2), (b2) and (c2), respectively.

Figure 3

The density of states as a function of energy, scaled with respect to the Fermi energy for (a) 8, (b) 12, and (c) 24 ZBNNRs with terminating polyacene unit.

Figure 4

Contour plot of the density of states as a function of energy, scaled with respect to the Fermi energy for (a) conducting and (b) insulating spin channels of the 8-ZBNNR with a terminating polyacene unit with varying electric field.

Figure 5

The spin polarized band structure (5 valence $+5$ conduction) of the 8-ZBNNR with a terminating polyacene unit, as obtained from (a) SIESTA and (b) VASP calculations for (i) up and (ii) down spins separately. The Fermi energy ($E_F$) is set to zero.