Contact resistance in metal-molecule-metal junctions based on aliphatic SAMs: Effects of surface linker and metal work function

Using conducting probe atomic force microscopy (CP-AFM), we have formed molecular tunnel junctions consisting of alkanethiols and alkane isonitrile self-assembled monolayers sandwiched between gold, platinum, silver, and palladium contacts. We have measured the resistance of these junctions at low bias (dV/dI |_V=0) as a function of alkane chain length. Extrapolation to zero chain length gives the contact resistance, R₀ . R₀ is strongly dependent on the type of metal used for the contacts and decreases with increasing metal work function; that is, R_0,Ag > R_0,Au > R_0,pd > R_0,pt. R₀ is ∼ 10% smaller for Au junctions with isonitrile versus thiol surface linkers. We conclude that the Fermi level of the junction lies much closer to the HOMO than to the LUMO.