First-principles investigation of a symmetry incompatible adsorbate-substrate system: PF3 on Cu(001)

Abstract

The structural relaxation of the adsorption of PF3 on Cu(001), resulting in a c(4 × 2) overlayer, was studied using first-principles Density Functional Theory (DFT) calculations in the generalized gradient approximation (GGA). Unlike previous experimental results, we found no tilting of the molecules on the surface, with the C3v axis of PF3 perfectly perpendicular to the surface. Then using Density Functional Perturbation Theory (DFPT), we calculated the phonon spectra of the Cu (001): PF3 c(4 × 2) at two high symmetry points in the surface Brillouin zone, Γ¯ and M¯. By plotting the projected density of states, we found modes localized on the surface and determined how they changed upon adsorption. Our results indicate considerable mixing with bulk phonon modes at certain back-folded q-points of the bare-surface Brillouin zone. These modes have dominant vertical vibration on the topmost layer. Modes with surface covered atoms that move with the molecule as a unit can undergo softening up to 1.5 meV. However, modes with dominant motion on the second layer remain unperturbed. In addition, we studied molecular external modes (i.e. modes that arise from free motions of the PF3 molecule) that are in the range of bulk and surface modes of Cu. Depending on the symmetry of the underlying structure, PF3 vibrations can remain isolated and induce a localized surface mode or mix with bulk modes and become a resonance.