Reliable formation of single molecule junctions with air-stable diphenylphosphine linkers

We measure the conductance of single Au−molecule−Au junctions with a series of air-stable diphenylphosphine-terminated molecules using the scanning tunneling microscope-based break junction technique. Thousands of conductance versus displacement traces collected for each molecule are used to statistically analyze junction conductance and evolution upon elongation. Measured conductances for a series of alkane-based molecules exhibit an exponential decrease with increasing length, as expected for saturated molecules, with a tunneling decay constant of 0.98 ± 0.04. Measurements of junction elongation indicate strong metal−molecule binding, with a length that increases with the number of methylene groups in the backbone. Measured conductance histograms for four molecules with short, unsaturated backbones (e.g., benzene) are much broader with less well-defined peaks. These measurements are supported by density function theory calculations. The phosphine binds selectively to under-coordinated gold atoms through a donor−acceptor bond with a binding energy of about 1 eV. The calculated tunnel coupling correlates very well with experiment.