Scanning tunneling spectroscopy on π-conjugated phenyl-based oligomers:: A simple physical model

The current-voltage (I-V) characteristics obtained by scanning tunneling spectroscopy (STS) on uncoated gold substrates and gold substrates coated with self-assembled monolayers of aromatic thiols are shown to be in good quantitative agreement with the Landauer-Buttiker approach to the heterojunction conductance. The self-assembled monolayers consist of one and three chained phenyl rings, forming an azomethine oligomer; both monomer and trimer are chemisorbed on gold via sulphur while terminated from the opposite side by 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 occupied molecular orbital-lowest unoccupied molecular orbital) the particular form 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 coated/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 factors give rise to asymmetry of I-V characteristics. The tunneling spectroscopy measurements and their theoretical evaluation indicate that the STM tip probes a molecular carbon atom rather than the uppermost nitrogen, possibly due 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 obtained on different but genetically related samples, at different setpoints of the tunnel current, and with essentially one and the same set of parameters (inferred from experiments and semi-empirical self-consistent modeling of the molecular electronic structure).