Infoscience

Thesis

The role of disorder in the electronic and transport properties of monolayer and bilayer graphene

This thesis is devoted to the computational study of the electronic and transport properties of monolayer and bilayer graphene in the presence of disorder arising from both topological and point defects. Among the former, we study grain boundaries in monolayer graphene and stacking domain boundaries in bilayer graphene, whereas among the latter we study hydrogen atoms covalently bound on the graphene crystal lattice. The electronic spectrum of disordered graphene has been studied within a tight-binding framework, which has been coupled to the Landauer-Büttiker theory and Green¿s function techniques in order to have access to the properties of coherent transport of graphene charge carriers. We assess the low-energy equilibrium structures of defective graphene by a combination of ab initio density functional theory, classical potentials, and Monte Carlo methods. We study periodic grain boundaries in monolayer graphene and individuate two classes of defects with opposite effects in terms of scattering of low-energy charge carriers. One class, unexpectedly, is highly reflecting in the limit of low defect density, whereas another is highly transparent. Subsequently, we study disordered grain boundaries in order to predict the intrinsic conductance of realistic polycrystalline graphene samples. In two related works, conducted in collaboration with experimentalists, we identify the atomic structure of periodic grain boundaries imaged by scanning tunneling microscopy, and discuss the valley-filtering capabilities of a line defect of graphene that can be grown in a controllable manner. Next, we investigate the electronic transport of graphene with realistic hydrogen adsorbates, whose equilibrium configurations are obtained by means of Monte Carlo simulations. We find that the conductance of graphene dramatically increases upon formation of cluster adatoms, which we predict to happen spontaneously at room temperature. This is due to the non- resonant nature of a large fraction of hydrogen clusters in the room-temperature distribution, which we further elucidate by means of an analytically solvable model. Finally, we study the behavior, in terms of structural and electronic properties, of twisted bilayer graphene in the limit of zero twist angle. We find a critical angle below which the system arranges in a triangular superlattice of Bernal-stacking domains, separated by a hexagonal network of stacking domain boundaries. The presence of stacking domain boundaries is at the base of our interpretation of an experiment reporting oscillations in the electrical conductance of bilayer graphene subjected to mechanical indentation.

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