ELECTRONIC TRANSPORT IN GRAPHENE-BASED NANO JUNCTIONS

Aim of this thesis is the theoretical investigation of these aspects. We start addressing the band gap opening in graphene upon adsorption of transition metal atoms. Depending on the TM considered, we observe the opening of gaps with different widths and energies in the two spin components. Furthermore, the fact that some of these gaps comprise the Fermi level allows to speculate that the electron transport through these systems should display spin-dependent behavior. We then tackle the more complex topic of electron transport with particular attention to the spin dependent properties. In this case we have to deal with open, non periodic systems whose electronic properties cannot be easily obtained with standard methods. For the calculation of the charge transport we make use of the Non Equilibrium Green's Function approach, that provides a rigorous description of quantum transport allowing the self-consistent calculation of the charge density under a bias voltage. Within this framework, we investigate two different types of graphene junctions. The first type is made by a graphene sheet with transition metal atoms adsorbed in a regular array on a finite region. Our results show that with Fe adatoms the currents of the two spin components are dramatically different displaying a 100% polarization for all the biases considered because of a complete quenching of the minority current. Also for Ti we find a similar behavior but with an opposite polarization: in this case in fact we observe a damping of the majority current. Co adsorption induces a polarization analogous to that of Fe but less intense. The second system analyzed is a molecular magnetic junction where a Fe porphyrin molecule is connected with graphene electrodes. While in the pristine case no relevant effects can be pointed out, when the electrodes are doped with boron atoms a non negligible current polarization is observed. The doping with nitrogen atoms gives instead rise to a different effect, namely Negative Differential Resistance, with a reduction of the current for increasing voltages. Finally, we address the problem of the possible employment of this molecular junction in a gas sensor device investigating the changes induced in the charge transport by the adsorption of two gas molecules, O2 and CO. The only relevant effect observed is the quenching of the polarization in the B-doped system.