Graphene Ribbons Terminated by Nanotubes

We study by density functional and large scale tight-binding transport calculations the electronic structure, magnetism and transport properties of the recently proposed graphene ribbons with edges rolled to form nanotubes. Edges with armchair nanotubes present magnetic moments localized either in the tube or the ribbon and metallic or half-metallic character, depending on the symmetry of the junction. These properties have potential for spin valve and spin filter devices with advantages over other proposed systems. Edges with zigzag nanotubes are either metallic or semiconducting without affecting the intrinsic mobility of the ribbon. By varying the type and size of the nanotubes and ribbons offers the possibility to tailor the magnetic and transport properties, making these systems very promising for applications. The atomic structure of graphene edges is important for the determination of the electronic and magnetic properties of graphene, especially for narrow graphene nanoribbons[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]. Recent theoretical work[13] on the stability of different graphene edges structures has shown that graphene edges can fold back on themselves and reconstruct as nanotubes, with low formation energy (see atomic structures in figure 1). In this article, we show that, beside protecting the edges from contamination and reconstructions, nanotubes at the edges may lead to magnetism and are not detrimental for the electronic mobility despite the row of sp hybridized atoms at the ribbon-tube junction. We study the electronic and magnetic properties of these systems by a combination of density functional theory (DFT) and large scale tight binding (TB) simulations of transport properties. Our calculations suggest that these systems could be used for a variety of applications that we sketch in figure 2. We consider systems formed by a nanoribbon terminated on both sides by the same armchair (AC) or zigzag (ZZ) nanotube. We notice that a ribbon with AC edges is terminated by ZZ nanotubes and a ribbon with ZZ edges is terminated by AC nanotubes. Nanoribbons terminated by AC nanotubes present interesting magnetic properties. By rolling the ZZ edges of a nanoribbon, two types of AC nanotubes can be formed, as shown in figure 1. If the atoms at the nanoribbon ZZ edge scroll and bind to the same sublattice sites within the nanoribbon, the formed AC nanotube has Electronic, Magnetic and Transport Properties of Graphene Ribbons Terminated by Nanotubes2 A-line A -l in e B -l in e A lin e B lin e A-line