FIRST-PRINCIPLES INVESTIGATION OF THE ELECTRONIC AND TRANSPORT PROPERTIES OF CARBON ATOM WIRES

The last years have witnessed an ever increasing interest in the field of molecular electronics, in which molecules connected with two or more electrodes are used as main building blocks to fabricate the components of integrated circuits. The achievement of high performance interconnects is one of the key step for the realization of such devices. In this context, conjugated carbon wires are the most promising candidates to this role due to their structural stability and their experimental feasibility. In particular we investigated the transport properties of systems where a freestanding conjugated carbon chain is contacted with two electrodes in order achieve a two-terminal device. Although some important issues had already been investigated in this field when starting this project, information were very scattered and a well-rounded perspective of the key aspects affecting the electron transport was missing. This thesis represents one attempt to fill this gap first in the case of Ag-chain-Ag systems and then for graphene-chain-graphene devices. Specifically we focused on the role that the chain-electrode junction plays in determining the characteristics of such systems, finding that the contact geometry strongly affect the transport properties. In fact, for Ag-chain-Ag systems, qualitative different transport properties have been found depending on the adsorption site on the silver surface, while results showed only a weak dependence on the chain length. Moreover we observed that those results can be extended to "tip-like" contacts, mimicking a more realistic electrodic surface. As opposite, linking atoms such as sulfur or silicon strongly influence the conductance. We further demonstrated that the vibrational motion of the atoms has negligible effects on the transport properties for a very large range of temperature. Finally we also critically assessed the reliability of our results using the optical properties of the gas-phase molecules to benchmark the adopted setup. The results obtained for graphene-chain-graphene devices confirmed that the junction plays a central role in determining the strength of the electrode-chain coupling, and in turn, the conductance. We first analyzed in details the transport properties of single chains connected to ideal electrodes, investigating their geometrical and electronic structure in order to establish a connection with their transport properties. Furthermore, at odds with the above mentioned metal-C-metal systems, we found that "tip-like" junctions here have dramatic effects on the transport properties. More importantly, we investigated the possibility to observe quantum interference, chemical gating and conductance switching behaviors in such systems.