Atomic Structure and Work Function Modulations in Two-Dimensional Ultrathin CuI Films on Cu(111) from First-Principles Calculations

In electrochemical systems, upon applying an electrode potential, complicated surface reconstructions between halogen atoms (iodide anion) and the metal substrate (copper facet) have been observed from the ordered halide adlayers to ultrathin metal halide films. Although the global geometry of the ultrathin CuI film on Cu(111) was proposed, the local geometry is still not well-characterized, which is necessary to further explore its surface electronic structure. Thus, we performed van der Waals-corrected density functional theory calculations to examine the early stages of CuI ultrathin film formation on Cu(111) within the framework of ab initio (electrochemical) thermodynamics and report detailed surface atomic structures of the prepared ultrathin CuI films with their associated surface thermodynamics and simulated scanning tunneling microscopy images. Here, we find that due to the unique atomic arrangements in the ultrathin CuI film, the surface work function is uniquely influenced by pronounced charge transfer effects rather than polarization alone. These surface electronic effects are captured by analyzing the electronic charge density differences at the interfacial CuI layers. Finally, these results suggest that the surface work function is modulated by a competition between charge transfer and polarization, where the local surface structure determines their relative contributions.