Molecular arrangement and electrical conduction of self-assembled monolayers made from terphenyl thiols

We investigated molecular arrangement change of self-assembled monolayers (SAMs) made from [1,1′:4′,1″-terphenyl]-4-thiol (TP0) on Au(1 1 1) using scanning tunneling microscopy. In an ethanol solvent, lying down phase of TP0 molecules where the conjugated rings are expected to contact directly the Au surface, were observed at the initial stage of TP0 SAM growth (at 1 min immersion). After 24 h immersion, a molecular lattice, which is similar to (√3×√3)R30° structure (perpendicular arrangement) was observed. We further investigated the molecular arrangement dependence of electrical conduction of the TP0 SAMs using conductive atomic force microscopy technique. The resistances of the TP0 SAMs were ≈104 Ω (at parallel arrangement) and ≈108 Ω (at perpendicular arrangement). The resistance of the lying down TP0 SAM was lower than that of benzylmercaptane SAM where the thickness was identical to lying down TP0 SAM. The result indicated that the direct contact of the conjugated ring to the Au surface could further reduces the tunneling resistance.