Single-molecule nanoelectronics

Molecular electronics has its genesis in the idea that electron transfer in molecules, so central to chemical reactions, might form a basis for building electronic components on a molecular scale. At the time that Aviram and Ratner proposed a mechanism for a molecular diode, this was far from an obvious idea, and one with enormous potential impact, because the scale of electronic devices was measured in microns or even millimeters. In the four decades that have passed since the publication of their paper, the electronics industry has come a long way. At the time of writing, Intel is making silicon devices with dimensions down to 11 nm, achieved with optical lithography! Nowadays, less mileage is to be gained from the relative smallness of single-molecule electronic devices. Molecular films play critical roles in the electronics industry, in displays, flexible electronics, and sensors. But that is not the focus of this chapter. Here, we are concerned with measurements on and applications of single-molecule devices. The historic goal of molecular electronics has been to make better, smaller transistors for denser computer circuits. There are three main reasons why molecular electronic circuits by themselves seem unlikely to bring revolutionary new capabilities to the computer industry. The first, alluded to above, is that conventional semiconductor manufacturing is not so far from achieving molecular scale densities. The second is that the promise of 3D fabrication, offered by molecular self-assembly, is also being overtaken by advances in conventional device architectures that now enable remarkably complex stacking and interconnection of circuit elements. And the third is the oft-cited problem of the statistics of small numbers-How small can a semiconductor element be and still contain one doping atom? As we shall see, the same considerations apply to molecules, with polarization heterogeneities and fluctuations replacing doping as the problem.