Whence Molecular Electronics?

The drive toward yet further miniaturization of silicon-based electronics has led to a revival of efforts to build devices with molecular-scale components. The field of molecular electronics is teeming with results, rationalizations, and speculations [HN1]. Some claims may have been exaggerated, but news stories of a crisis in the field (1) are premature. Reports of passive molecular electronics devices, such as tunnel junctions and rectifiers, as well as of active devices, for example, single-molecule transistors and molecular switch tunnel junctions, have withstood scientific scrutiny. Simple molecular electronic devices usually consist of organic molecules sandwiched between conducting electrodes. According to early predictions, such devices could show electron tunneling (2) [HN2] or one-way flow of current (rectification) [HN3] through the molecule (3). In most tunneling junctions, linear alkanes are sandwiched between metal electrodes. Measurements over the past 25 years (4, 5) have largely validated McConnell's prediction (2) that the tunnel current depends exponentially on the length of the molecules between conducting electrodes [HN4]. In rectifiers, a molecule composed of an electron donor, a bridge, and an electron acceptor is extended between two electrodes (see the first figure, top panel). Experiments (6, 7) have again validated the early prediction by Aviram and Ratner (3) [HN5]