Angstrom-Scale Ruler Using Single Molecule Conductance Signatures

Experimental techniques that determine atomic arrangements in single metal–molecule–metal junctions will enable a mechanistic understanding and control of electronic properties on the nanoscale. Here, we develop a method to determine average gold and silver nanogap widths with Angstrom resolution using single molecule junction conductance and distance measurements of N,N′-diamino alkanes in a scanning tunneling microscope break junction setup. Our experiments are supported by density functional theory (DFT) calculations, which suggest that the alkane-conducting trans-configurations can be outcompeted by the nonconducting cis conformers when the nanogap distance is shorter than the length of the molecule. As a result, the distribution of binding geometries during the conductance plateau is peaked when the gap width is comparable to the molecule length. We apply this conductance ruler to determine the binding geometry of N,N′-oligophenyl amines which have been observed to have two distinct conductance signatures. Our measurements and DFT calculations show that for the high conductance geometry, oligophenyls preferentially bind away from the apex of the electrodes, so that the tip–tip nanogap distance is less than the full length of the molecule and the π-system can overlap with the electrodes. Significantly, our new conductance ruler method allows us to determine that the low conductance of oligophenyls occurs when the interelectrode distance is greater than the N–N length of the molecule, requiring two π–π stacked molecules to bridge the junction.