Molecular Transport Junctions: Clearing Mists

What is the electronic conductance of a junction containing a single molecule attached at each end to metal electrodes? This simplest of questions in molecular electronics has proved surprisingly difficult to answer. In a 2003 review of the field, Salomon et al. showed that for one simple molecule, 1,8-octanedithiol attached to gold electrodes, the reported experimental values of low-bias conductance varied by over five orders of magnitude. It is difficult to discuss agreement with theory in this context. The main message of the present overview is that for 10 out of 12 molecules studied using repeated break junction methods with statistical analysis, agreement between experiment and density functional theory (DFT) calculations is within an order of magnitude (Table 1). In the worst case (1,4-phenyldithiol), it is within a factor of 50. Furthermore, some of these measurements were recently extended to include temperature effects. Overall, there is broad agreement between the data and calculations based on tunnel transport in the Landauer–Imry regime. The remaining disagreements may reflect interesting and important physics (discussed below), but the situation represents a sea change from what prevailed two years ago. Impressive though these achievements are, they bear on a problem of somewhat limited technological importance: In tunnel transport the molecule acts merely as a scatterer of electrons, much as any dielectric barrier would (although this molecular entity provides elastic scattering (Landauer–Imry) channels that facilitate transport compared to a vacuum gap). Functional molecular devices require active control of the electronic properties of the molecule, for example, switching conductance through control of the geometry and/or the oxidation state of the molecule. Single-molecule measurements have made progress here too, but much work is needed for development of a computationally accurate first-principles theory of transport that involves charge localization on molecules (i.e., reduction and oxidation).