Size modulation and defects in graphene based ribbons : magnetism and charge confinement

In this thesis, we investigated the effects of vacancy and heterojunction formation on electronic and magnetic properties of graphene nanoribbons (GNRs) by using first principles pseudopotential plane wave method within Density Functional Theory. Graphene based materials are expected to be very important in future technology. Through understanding of all the factors which influence their physical properties is essential. We have shown that electronic and magnetic properties of graphene nanoribbons can be affected by defect-induced itinerant states. The band gaps of armchair nanoribbons can be modified by hydrogen saturated holes. Defects due to periodically repeating vacancies or divacancies induce metallization, as well as magnetization in non-magnetic semiconducting nanoribbons due to the spin-polarization of local defect states. Antiferromagnetic ground state of semiconducting zigzag ribbons can change to ferrimagnetic state upon creation of vacancy defects, which reconstruct and interact with edge states. Even more remarkable is that all these effects of vacancy defects are found to depend on their geometry and position relative to edges. We also predicted that periodically repeated junctions of graphene ribbons of different widths form multiple quantum well structures having confined states. These quantum structures are unique, since both constituents of heterostructures are of the same material. The width as well as the band gap, for specific superlattices are modulated in direct space. Orientation of constituent nanoribbons, their widths, lengths and the symmetry of the junction are some of the crucial structural parameters to engineer electronic properties of these systems. Our further studies on nanoribbons and nanoribbon superlattices showed the strong dependence of band gaps and magnetic moments on applied uniaxial stress. This thesis presents an extensive study of size modulation and defect formation on graphene nanoribbons.