Quantification of Ready-Made Molecular Bilayer Junctions Having Large Structural Uncertainty

A ready-made procedure for the preparation of molecular junctions at ambient conditions is reported. Junctions were constructed from bilayers of alkyl thiols at the interfaces between gold flakes and stationary gold strips. Addition of an amine or carboxylic acid end group to the alkyl thiols drastically increased the resistance (up to TΩ over a couple of nanometers only) and the breakdown voltage of the bilayer junction. These junctions pose a severe quantification challenge because of the large uncertainty regarding their microscopic morphology. A novel quantification procedure is proposed that replaces highly nonlinear tunneling or super-exchange current voltage relations by an effective analytical relation of two characteristic parameters: equilibrium conductance and shape factor. Correlating these factors over a series of systematically varying junctions allows us to evaluate the effective contact area for transfer. This approach was also extended to the field- emission regime. Within the large spreading of the data, our analyses show that low-bias transfer occurs via most of the contact area while field emission is limited to “hot spots” of a few nanometers wide and only couple of angstroms long. The extracted length for charge transfer was only 1/4 to 1/3 of the nominal bilayer thickness, except for the polar interfaces, which were considerably thicker. The effect of polar end groups on the bilayer thickness is presumably due to weak repulsive forces at the bilayer interface, preventing the collapse of the bilayer.