Interfacial Dipole Formation and Surface-Electron Confinement in Low-Coverage Self-Assembled Thiol Layers: Thiophenol and p‑Fluorothiophenol on Cu(111)

Model systems of organic self-assembled monolayers are important in achieving full atomic-scale understanding of molecular-electronic interfaces as well as the details of their charge transfer physics. Here we use two-photon photoemission to measure the evolving unoccupied and occupied interfacial electronic structure of two thiolate species, thiophenol and p-fluorothiophenol, adsorbed on Cu(111) as a function of molecular coverage. Our measurements focus on the role of adsorbates in shifting surface polarization and effecting surface electron confinement. As the coverage of each molecule increases, their photoemission-measured work functions exhibit nearly identical behavior up to 0.4–0.5 ML, at which point their behavior diverges; this behavior can be fit to an interfacial bond model for the surface dipole. In addition, our results show the emergence of an interfacial electronic state 0.1–0.2 eV below the Fermi level. This electronic state is attributed to quantum-mechanical-confinement shifting of the Cu(111) surface state by the molecular adsorbates.