Probing the adsorption of fluorine on aluminum (111) surfaces by low-energy ion scattering and density functional calculations

摘要

Low-energy ion scattering (LEIS) measurements were performed to probe the adsorption of fluorine to the aluminum (111) surface. When well-ordered surfaces were exposed to fluorine using a XeF_2 source, LEIS signals characteristic of surface ordering disappear, indicating that fluorine covers the surface in a disordered fashion. At submonolayer coverages, a slight enhancement along the [211] direction is observed, suggesting that sites with 3-fold symmetry may be energeticaly somewhat more stable than the on-top sites. These observations are supported by density functional calculations for a 7-layer periodic-slab structure. The calculations indicate that the binding energies of fluorine to the three different sites are very similar. Although the on-top site is found to be the least stable, its energy is predicted to lie only ～ 3.5 kcal mol~(-1) above the fcc site (predicted to be the lowest-energy site). The Al-F binding energy to the on-top site is predicted to be 107 kcal mol~(-1), which is considerably smaller than the Al-F dissociation energy in gas-phase AlF_3.