| AVS 58th Annual International Symposium and Exhibition | |
| Surface Science Division | Tuesday Sessions |
| Session SS+EM-TuA |
| Session: | Organic Electronic Interfaces |
| Presenter: | Justin Kreil, The University of Alabama |
| Authors: | J. Kreil, The University of Alabama P. LeClair, The University of Alabama G.J. Szulczewski, The University of Alabama |
| Correspondent: | Click to Email |
A series of self-assembled monolayers were prepared from para-substituted benzoic acids (X-C6H5CO2H where X = H, F, Cl, Br, I, and CN) onto oxidized Al films and characterized by x-ray photoelectron spectroscopy and contact angle measurements. The acids adsorb to the oxide as a carboxylate group with the plane of the aromatic ring largely perpendicular to the surface, which places the para-substitutent away from the surface. Tunnel junctions were made by vapor deposition of Ag and Pb films as the top electrodes onto the monolayers. Four point probe electrical measurements were made from 4 to 300K. At 4 K the superconducting gap of Pb was observed and unequivocally demonstrates tunneling through a barrier without metallic shorts. When Ag was the top electrode, differential conductance [G(V) = dI /dV] measurements at 4 K showed a quadratic dependence on the applied bias voltage and no zero-bias anomalies. These low temperature measurements suggest the monolayers form pin-hole free tunnel barriers. Two trends emerge when comparing G(V) versus bias voltage for junctions with Ag and Pb top electrodes. When Pb was the top electrode the minimum in G(V) versus bias voltage is offset from zero bias for each monolayer, which scales in a systematic manner with the polarity of the para C-X bond. However, when Ag was the top electrode there was no offset the tunneling conductance. The origin of the different tunneling behavior observed for Pb and Ag top electrodes will be discussed in detail.