Molecular Tunnel Junctions Based on π-Conjugated Oligoacene Thiols and Dithiols between Ag, Au, and Pt Contacts: Effect of Surface Linking Group and Metal Work Function

The tunneling resistance and electronic structure of metal-molecule-metal junctions based onoligoacene (benzene, naphthalene, anthracene, and tetracene) thiol and dithiol molecules were measured and correlated using conducting probe atomic force microscopy (CP-AFM) in conjunction with ultraviolet photoelectron spectroscopy (UPS). Nanoscopic tunnel junctions (similar to 10 nm(2)) were formed by contacting oligoacene self-assembled monolayers (SAMs) on flat Ag, Au, or Pt substrates with metalized AFM tips (Ag, Au, or Pt). The low bias (<0.2 V) junction resistance (R) increased exponentially with molecular length (s), i.e., R = R-0 exp(beta s), where R-0 is the contact resistance and is the tunneling attenuation factor. The R-0 values for oligoacene dithiols were 2 orders of magnitude less than those of oligoacene thiols. Likewise, the beta value was 0.5 per ring (0.2 angstrom(-1)) for the dithiol series and 1.0 per ring (0.5 angstrom(-1)) for the monothiol series, demonstrating that beta is not simply a characteristic of the molecular backbone but is strongly affected by the number of chemical (metal-S) contacts. R-0 decreased strongly as the contact work function (Phi) increased for both monothiol and dithiol junctions, whereas 0 was independent of 4:1) within error. This divergent behavior was explained in terms of the metal-S bond dipoles and the electronic structure of the junction; namely, p is independent of contact type because of weak Fermi level pinning (UPS revealed E-F - E-HOMO varied only weakly with Phi), but R-0 varies strongly with contact type because of the strong metal S bond dipoles that are responsible for the Fermi level pinning. A previously published triple barrier model for molecular junctions was invoked to rationalize these results in which R-0 is determined by the contact barriers, which are proportional to the size of the interfacial bond dipoles, and 0 is determined by the bridge barrier, E-F - E-HOMO. Current-voltage (I - V) characteristics obtained over a larger voltage range 0-1 V revealed a characteristic transition voltage V-trans, at which the current increased more sharply with voltage. V-trans., values were generally >0.5 V and were well correlated with the bridge barrier E-F - E-HOMO. Overall, the combination of electronic structure determination by UPS with length- and work function-dependent transport measurements provides a remarkably comprehensive picture of tunneling transport in molecular junctions based on oligoacenes.