Ab-initio modelling of quantum transport in polycrystalline graphene

In this report, the electronic properties of graphene were studied with OpenMX, a software package for nano-scale material simulations based on DFT, norm-conserving pseudo-potentials, and pseudo-atomic localized basis functions. Graphene is a form of carbon consisting of atomically-thick planar sheets, with the atoms arranged in a honeycomb-shaped lattice. It exhibits outstanding mechanical, thermal, optical and electronic properties, arising from the confinement of electrons in two dimensions. It is especially known for its low energy excitations which are massless, chiral Dirac fermions. Graphene is much more efficient at conducting electrons than silicon and is currently investigated internationally for future technological applications. In this perspective, it is important to demonstrate the efficiency of OpenMX to reproduce the electronic properties of graphene and to determine its accuracy. Therefore, a particular hyperfine phenomenon is studied in graphene, the so-called spin-orbit coupling, an interaction between the electron spin and its orbital angular momentum. This was estimated to 60 μeV with a precision down to 1 μeV. In addition to graphene, two other allotropic forms of carbons were also investigated regarding their electronic properties, namely the penta-graphene and the hexagonal haeckelite. A tremendous enhancement was noticed for the spin-orbit coupling in penta-graphene, with an effect as intense as 2 meV, whereas we reported no notable effects for the hexagonal haeckelite. Another allotropic form of carbon is obtained when a slab of graphene is cut along a specific directions in order to form strips with ultra-thin width. Those ribbons were predicted to exhibit different electronic properties and particular quantum transport depending on the relative spin polarization at their edges. Different topological defects specific to carbon-based nanostructures were reviewed afterwards. Those defects were structurally optimized with OpenMX and then inserted in both one-dimensional nanoribbons and two-dimensional graphene single-layer. The electronic spin-dependent transmission for those system was investigated in search of new interesting features, such as spin-filter in spintronics or field-effect transistors in nanoelectronics. An excellent spin-valve was observed for a t5t7 di-vacancy embedded symmet- rically in a zigzag nanoribbon. Two highly symmetric and energetically stable planar periodic array of defects were introduced into two-dimensional graphene in order to connect two different lattice-oriented single-crystal domains. Two distinct transport behaviours were predicted for low-energy charge carriers depending on the grain boundary atomic structure, either perfect reflection, with a band gap of 1 eV, or high transparency, with a slight reduction of 20 % in transmission compared to pristine graphene.