Foundations of Molecular Electronics – Charge Transport in Molecular Conduction Junctions

Abstract

The most fundamental structure involved in molecular electronics is a molecular transport junction, consisting of one (ideally) or more molecules extending between two electrodes. These junctions combine the fundamental process of intramolecular electron transfer with the mixing of molecular and continuum levels at the electrodes and the nonequilibrium process of voltage-driven currents. Much of this book is devoted to the complicated but significant behaviors that arise from this conjunction. This introductory chapter attempts to sketch some of the principles and also some of the unresolved issues that characterize molecular transport junctions. Sections 1.2-1.4 deal with fundamental ideas. These include an appropriate theoretical formulation of the conductance calculation in terms of non-equilibrium Green's functions, the relationship between junction conductance and nonadiabatic electron transfer rates in the same molecular entities, and the role and magnitude of interactions between the dynamics of the transferring electronic charges and the nuclear degrees of freedom. Section 1.5 addresses some of the outstanding and difficult issues in understanding junction transport, including geometry and its change with voltage, the electrostatic profile under applied voltage, electronic structure models and their limitations, and fluctuations and switching phenomena in junctions. Molecular electronics is an area of very rapidly growing scientific and applied interest and activity. While the technological drivers, including materials, electrochemical, biological, sensing, memory and logic applications are all important, the fundamental issues involved in the nonequilibrium responses of molecule-based hybrid materials to applied electromagnetic fields is the fundamental driver for this science. In this sense, molecular electronics is a sort of spectroscopy. Due to the intensity and quality of the research being done in the area, the community may soon be able to understand molecular transport spectroscopy at a level of depth and sophistication almost comparable to other, more traditional spectroscopies. Contemporary research in the area, as exemplified in this book and at the Dresden conference that initiated the book, is driving in that direction.