Scanning tunneling spectroscopy on pi-conjugated phenyl-based oligomers: Asimple physical model

The current-voltage (I-V) characteristics obtained by scanning tunneling sp ectroscopy (STS) on uncoated gold substrates and gold substrates coated wit h self-assembled monolayers of aromatic thiols are shown to be in good quan titative agreement with the Landauer-Buttiker approach to the heterojunctio n conductance. The self-assembled monolayers consist of one and three chain ed phenyl rings, forming an azomethine oligomer; both monomer and trimer ar e chemisorbed on gold via sulphur while terminated from the opposite side b y an amine head group. It is found that if the electrostatic energy of the applied potential is ramped within the HOMO-LUMO energy gap (highest occupi ed molecular orbital-lowest unoccupied molecular orbital) the particular fo rm of the apparent I-V relations reflect an interplay of (i) the WKB factor that accounts for the through-air tunneling between the STM tip and the co ated/uncoated substrate, (ii) resonancelike structure of the tip-apex local density of states (LDOS), and (iii) the relative position and intensity of the HOMO and LUMO level peaks in the molecular LDOS. The latter two factor s give rise to asymmetry of I-V characteristics. The tunneling spectroscopy measurements and their theoretical evaluation indicate that the STM tip pr obes a molecular carbon atom rather than the uppermost nitrogen, possibly d ue to plowing through the molecules. The findings of the work are supported by a close similarity of measured and calculated I(V) curves, which is obt ained on different but genetically related samples, at different setpoints of the tunnel current, and with essentially one and the same set of paramet ers (inferred from experiments and semi-empirical self-consistent modeling of the molecular electronic structure).