Scanning tunneling spectroscopy results are presented on oriented monolayer films of conjugated phenyl-based molecules self-assembled on gold. Three related molecules of different lengths are investigated. The molecules do not show resonances in electron transmission at least up to 1.5 V. We model the experiments successfully by the molecules acting as a potential barrier to electron transmission. The potential barrier is trapezoidal when the molecule has a permanent dipole moment (normal to the gold surface), in agreement with the asymmetric $I\left(V\right)\mathrm{}$ curves. For 2 V bias and larger, the film is modified by the measurement, which prevents measurements of possible resonances at higher energies.

• Received 6 December 1999

DOI:https://doi.org/10.1103/PhysRevB.62.10459