Graphene-based Single-Molecule Junctions: analysis of electronic structure, transport properties and advantages of graphene electrodes

The approaching of intrinsic limits for the scaling of silicon-based semiconductor electronic devices no longer permits to easily pursue the Moore's law. The channel length is approaching the electron De Broglie wavelength in silicon, meaning that no control over the source to drain barrier is quantum mechanically possible for next future technological node scale lengths. In order to meet the demand for new high-performance and low-power consumption chips, molecular electronics is one among the possible solutions, with a renewed attention gained thanks to recent advancements in terms of novel quantum mechanical trasport phenomena like quantum interference. Molecular electronics aims at employing single molecules to implement electronic components: single-molecule junctions (SMJs) are the fundamental experimentally proven devices in molecular electronics, where molecules can be chosen ad hoc to obtain the designed features. SMJs are realized by chemically bonding molecules (usually organic molecules) between two electrodes (usually gold) via specifically chosen chemical connections or properly designed linkers. In literature there are several examples of metal and organic electrodes, including Graphene-based, but currently there are not many studies that have shown which material is the most suitable to obtain the desired conduction properties. In this Thesis, transport properties in various graphene structures are theoretically investigated through analytic models and ab initio simulations following a bottom-up approach. The electronic band structure is firstly derived with a tight binding method and the analytical solution is used in the Landauer-Buttiker model for conductance in graphene sheets and nanoribbons. After a state-of-art literature overview of nanofabrication processes for graphene break junctions, such structures are then used as electrodes in SMJs to understand the advantages and drawbacks w.r.t. metal-based electrodes. Thus, graphene-molecule-graphene single molecule junctions (GMG-SMJs) are simulated with QuantumATK, in particular ope3, ope5 and [3.3]pCp via amide linkages. The obtained results show that graphene electrodes, thanks to pi-conjugation continuity throughout the entire system, are the most natural electrodes for organic molecule-based devices, ensuring the persistence of the intrinsic properties of the molecules in contrast to metal electrodes.