Electron Transfer and Transmission at Molecule–Metal and Molecule–Semiconductor Interfaces

Abstract

This chapter continues our discussion of electron transfer processes, now focusing on the interface between molecular systems and solid conductors. Interest in such processes has recently surged within the emerging field of molecular electronics, itself part of a general multidisciplinary effort on nanotechnology. Notwithstanding new concepts, new experimental and theoretical methods, and new terminology, the start of this interest dates back to the early days of electrochemistry, marked by the famous experiments of Galvani and Volta in the late eighteenth century. The first part of this chapter discusses electron transfer in what might now be called “traditional” electrochemistry where the fundamental process is electron transfer between a molecule or a molecular ion and a metal electrode. The second part constitutes an introduction to molecular electronics, focusing on the problem of molecular conduction, which is essentially electron transfer (in this context better termed electron transmission) between two metal electrodes through a molecular layer or sometimes even a single molecule. In Chapter 16 we have focused on electron transfer processes of the following characteristics: (1) Two electronic states, one associated with the donor species, the other with the acceptor, are involved. (2) Energetics is determined by the electronic energies of the donor and acceptor states and by the electrostatic solvation of the initial and final charge distributions in their electronic and nuclear environments. (3) The energy barrier to the transfer process originates from the fact that electronic and nuclear motions occur on vastly different timescales. (4) Irreversibility is driven by nuclear relaxation about the initial and final electronic charge distributions. How will this change if one of the two electronic species is replaced by a metal? We can imagine an electron transfer process between a metal substrate and a molecule adsorbed on its surface, however the most common process of this kind takes place at the interface between a metal electrode and an electrolyte solution, where the molecular species is an ion residing in the electrolyte, near the metal surface. Electron transfer in this configuration is the fundamental process of electrochemistry.