Theoretical study of transport properties for selected single-molecule junnctions

Abstract

Molecular electronics, in which molecules are employed to build a functional electronic device, has become a promising technology since it is expected to provide an ultimate reduction in electronic device dimensions. Many research groups across the world are able to realize single-molecule devices and observe interesting transport characteristics. Among them, it can be found traditional semiconductor behaviour (as rectification), magnetoresistance behaviour or quantum interference (QI) effects. However, in spite of the achievements done so far in molecular electronics, there are a series of problems which have to be overtaken to finally incorporate this technology to actual everyday devices. So, the many possibilities which offer molecular electronics as well as the problems that must be left behind, increase the demand for understanding the quantum physics that dominates the transport properties at such nanoscale devices. In consequence, theoretical studies based on ab-initio methods and on transport theory (at equilibrium or at non equilibrium), are a necessary tool to grasp the physics of such nanodevices. In this work, it has been performed a theoretical research on various single-molecule devices in order to try to put some light in some of the different troubles in the molecular electronics field. In this respect, the search for new functionalities taking advantage the quantum character of the nanoscale devices, the reduction of the variability in single molecule junctions, the enhancement of the rectifier behaviour employing molecules or the study of new materials (graphene) as possible electrodes in next generation of molecular electronics devices, have been addressed. So, the results obtained show an example of how QI effects can be identified, tuned and exploited in a single-molecule junction made by metalloporphyrin molecules (chapter 6); propose a route to reduce variability that exploits the steric hindrance effect inherent to bulky groups (chapter 3); show how by exploiting carefully the molecular details, rectification ratios of order 300 can be achieved in molecular rectifiers (chapter 5) or show how planar molecules can help to alleviate the variability problem when graphene electrodes are used (chapter 4).