Modifying the Surface of a Rashba-Split Pb-Ag Alloy Using Tailored Metal-Organic Bonds

Hybridization-related modifications of the first metal layer of a metal-organic interface are difficult to access experimentally and have been largely neglected so far. Here, we study the influence of specific chemical bonds (as formed by the organic molecules CuPc and PTCDA) on a Pb-Ag surface alloy. We find that delocalized van der Waals or weak chemical $\pi$-type bonds are not strong enough to alter the alloy, while localized $\sigma$-type bonds lead to a vertical displacement of the Pb surface atoms and to changes in the alloy’s surface band structure. Our results provide an exciting platform for tuning the Rashba-type spin texture of surface alloys using organic molecules.

Figure 1

Model of the vertical adsorption geometry of the buckling of the pristine Pb-Ag surface alloy (a), as well as of CuPc (b) and PTCDA (c) adsorption on the Pb-Ag surface alloy as determined by NIXSW. For CuPc only one or two atoms are shown for each species while both structurally inequivalent PTCDA molecules (A, B) are shown in a side view on its shorter side.

Figure 2

Constant binding energy maps of the Fermi surface of the pristine Pb-Ag surface alloy (a), of the HOMO regions of CuPc (b) and PTCDA (c),(d). The left halves of (b), (c), and (d) show the emission of the substrate bands underneath the molecular film; the right halves show the band structure of the pristine Pb-Ag surface alloy (b), (c) and of a Pb monolayer film on Ag(111) (d), respectively. The insets of panels (c) and (d) show magnified momentum space regions around the symmetry equivalent $-\overline{M}$ points highlighted with identical color in the corresponding CBE maps. The momentum space curvature of both reference systems is modeled by $B$-spline functions (red dotted lines in lower insets) that are superimposed onto the momentum space curvature of the PTCDA covered Pb-Ag surface alloy (red dotted lines, upper insets).

Figure 3

UPS spectra of a monolayer film CuPc (lower spectrum) and PTCDA (upper spectrum) adsorbed on a Pb-Ag surface alloy. As reference, the UPS spectrum of the pristine Pb-Ag is shown in the middle. The left column shows constant binding energy maps of the momentum resolved photoemission intensity record at the HOMO binding energy ($E_B=1.5\mathrm{eV}$); the right column at the Fermi energy. In all CBE maps containing molecular features, the left half of the CBE maps show the theoretical calculations of the momentum space distribution of the corresponding molecular orbitals calculated for free molecules; the right half shows the experimental data. All data are obtained at room temperature using a momentum microscope and illumination by a helium discharge light source ($\hslash \omega =21.22\mathrm{eV}$).