Self-assembled monolayers of tetrathiafulvalene derivatives on Au(111): Organization and electrical properties

Self-assembled monolayers (SAMs) formed by dissymmetric tetrathiafulvalene (TTF) derivatives on gold (111) are prepared, and the structural and electrical properties have been studied. The TTFs are united to the metal through two S-Au bonds while the other "end" bears two long alkyl chains. Observations based on a combination of surface characterization techniques including ellipsometry, FTIR, contact angle, STM, and AFM suggest that these monolayers are disordered or at least loosely packed in the lateral dimensions. Conduction through these molecular SAMs is remarkably high given that the length of the "insulating" alkyl chains attached to the TTF unit is on the order of 15-20 Angstrom, a situation that arises in part from the tilted orientation of the molecules with respect to the surface and the loosely packed structure of the alkyl chains. Currents rivaling those in conjugated candidate "molecular wires" such as carotene dithiols and oligo-phenylene-ethynylenes are observed. The resistance of a single molecule in a SAM, determined in a two-terminal geometry using c-AFM, is estimated to be 25.7 +/- 0.3 GOmega.