The measurement of the electrical properties of molecules, down to the single molecule level, has become an experimental reality in recent years. A number of methods are now available for experimentally achieving this feat. The common aim of these methods is to entrap a single or small numbers of molecules between a pair of metallic contacts. This topical review focuses on describing and comparing experimental methods for entrapping and measuring the electrical properties of single molecules in metallic contact gaps. After describing the methods, reasons are tendered for apparent discrepancies in the literature between measured single molecule conductance values, with a focus on the most widely studied alkanedithiol system. Illustrative examples are then presented of the determination of the electrical properties of a range of single molecular systems, in order to highlight the progress which has been made in recent years.

One of the challenging goals of molecular electronics is to wire exactly one molecule between two electrodes. This is generally nontrivial under ambient conditions. We describe a new and straightforward protocol for unambiguously isolating a single organic molecule on a metal surface and wiring it inside a nanojunction under ambient conditions. Our strategy employs C(60) terminal groups which act as molecular beacons allowing molecules to be visualized and individually targeted on a gold surface using an scanning tunneling microscope. After isolating one molecule, we then use the C(60) groups as alligator clips to wire it between the tip and surface. Once wired, we can monitor how the conductance of a purely one molecule junction evolves with time, stretch the molecule in the junction, observing characteristic current plateaus upon elongation, and also perform direct I-V spectroscopy. By characterizing and controlling the junction, we can draw stronger conclusions about the observed variation in molecular conductance than was previously possible.

We study the formation mechanism of molecular junctions using break-junction experiments. We explore the contribution of gold-atom rearrangements in the electrodes by analyzing the junction stretching length, the length of individual plateaus, and the length of the gold one-atom contacts. Comparing the results for alkane dithiols and diamines, we conclude that thiols affect gold electrode dynamics significantly more than amines. This is a vital factor to be considered when comparing different binding groups.