Length dependence of charge transport in nanoscopic molecular junctions incorporating a series of rigid thiol-terminated norbornylogs

Four tetrathiol-terminated norbornane homologues were synthesized and self-assembled monolayers (SAMs) of these molecules were formed on Au via adsorption from CH ₂Cl ₂. SAMs were characterized structurally via spectroscopic ellipsometry (SE), reflection-absorption infrared spectroscopy (RAIRS), Rutherford backscattering spectrometry (RBS), and X-ray photoelectron spectroscopy (XPS). Results of these analyses show that the rigid norbornylogs form monolayers that have a surface coverage slightly lower than that of alkanethiols, and that they exhibit a nonmonotonic dependence of film thickness on molecular length. Nanoscale molecular junctions incorporating these SAMs were formed and characterized electrically using conducting probe atomic force microscopy (CP-AFM). The resistances of these junctions scale exponentially with the contour length of the molecules, with β = 0.9 Å ^-1, consistent with a nonresonant tunneling mechanism. Further, the resistance of norbornyl SAMs correlates well with the resistance of alkanedithiol SAMs of similar length, suggesting that the norbornyl molecules form sulfur-metal bonds on both ends of the junction.