Narjes Gorjizadeh

The effects of defects on the conductance of graphene nanoribbons

The quantum conductance of graphene nanoribbons that include vacancy and adatom-vacancy defects is studied for both armchair and zigzag edge structures. The conductance is calculated by using the Greens function formalism combined with a tight-binding method for the description of the system. Our results reveal that, owing to the localized states that appear near the defect sites, the conductance of the defected nanoribbons generally decreases. We show that details of the conductance reduction depend on the structure of the defect, its distance from the ribbon edges, and the ribbon width. While some defect structures cause the conductance of the ribbon to vanish, some other defects have no effect on the conductance at the Fermi energy.

Chemical functionalization of graphene nanoribbons

We review the electronic properties of graphene nanoribbons functionalized by various elements and functional groups. Graphene nanoribbons are strips of graphene, the honeycomb lattice of carbon with sp2 hybridization. Basically nanoribbons can be classified into two categories, according to the geometry of their edge, armchair, and zigzag, which determine their electronic structure. Due to their fascinating electronic and magnetic properties many applications has been suggested for these materials. One of the major methods to use graphene nanoribbons in future applications is chemical functionalization of these materials to make an engineering on their band gap. In this review, we introduce various types of modifying graphene nanoribbons to meet their promising applications.

Non-coherent transport in carbon chains.

The effect of electron-phonon (e-ph) interaction on the conductance of carbon chains is investigated by a non-equilibrium Greens function technique combined with a four-orbitals-per-atom tight-binding Hamiltonian. The optimized structure of the chain is found to be the semiconducting polyyne type (···-C≡C-C≡C-···). Our results show that the conductance of a carbon chain attached to two fixed contacts decreases due to e-ph interaction, and this reduction is stronger for longitudinal phonon modes which decrease the hopping energy between carbon atoms. Study of individual phonon modes reveals that emission of longitudinal phonons is stronger than that of transverse modes at room temperature, while absorption of transverse phonons is dominant. Conductance at finite temperature is also studied by considering the overall phonon effects; this shows that the reduction of the conductance is stronger at higher temperatures. The results are explained on the basis of the unique features of the carbon chain band structure.

Electronic and Transport Properties of Defected Graphene Nanoribbons

Graphene nanoribbons, quasi-one-dimensional structures of carbon, are fascinating materials. These structures can be constructed as strips of graphene sheet, the two dimentional honeycomb lattice of carbon with sp2 hybridization. Geometrically two main types of slice can be cut from a graphene sheet, with zigzag edge and armchair edge (Niimi05; Kobayashi05). The edge geometry is the key parameter which determines the electronic properties of the nanoribbons. Although the two-dimensional graphene is a zero band-gap semi-metal, electronic structure of nanoribbons depend on their edge geomtery (Saito92; Klein94; Fujita96; Son06a). A simple tight-binding model with one orbital per atom predicts that zigzag nanoribbons are metallic. But density functional calculations shows that all graphene nanoribbons are semiconductors at their ground state with band gaps which depend on their width and edge geometry, closing at infinite width, i. e. infinite graphene. Moreover, the electronic structure of graphene nanoribbons can be modified by chemical functionalization, such as functionalization by various atomic sepcies or by functional groups (Maruyama04a; Gunlycke07; Hod07; Gorjizadeh08). A large variety of electronic and magnetic properties, such as semiconducting with a wide range of band gap, metallic, ferromagnetic, antiferromagnetic, half-metallic, half-semiconducting, can be obtained by chamical modifications of the nanoribbons. Modification of the edge or using an adsorbate or substitution of carbons of the nanoribbon with an appropriate host are different options of functionalizations of these materials. These properties, along with the ballistic electronic transport, and the quantum Hall effect (Novoselov05a; Zhang05) and high carrier mobility (Novoselov05a) cause these quasi-1D materials to be promising candidates for nanoelectronics applications (Novoselov04b; Son06b; Obradovic06; Li08; Hod09; Zhu10). Various junctions can be constructed by connecting nanoribbons of different widths and types with perfect atomic interface, and electronic device can be integrated on them by selective chemical funtionalization on a single nanoribbon sheet (Huang07; Yan07; Gorjizadeh08). In order to achieve their potential for these applications it is essential to have a better understanding of the electronic structure of graphene nanoribbons and have ability to control them. From a practical point of view, when nanoribbons are fabricated