Figure 1

Hard sphere models of the lateral and vertical molecular geometry of PTCDA on (a) Ag(111), (b) Ag(100), and (c) Ag(110):[12, 13, 15, 51, 52] Top views (top), side views along the long (middle) and along the short molecular axis (bottom). Ag atoms are shown in different shades of blue to indicate the vertical deviation Δ$d$ from their position in the relaxed uncovered surface (dashed line, $d_0$); H atoms are shown in white, C${}_{\mathrm{funct}}$/C${}_{\mathrm{peryl}}$ in light gray/gray, and O${}_{\mathrm{carb}}$/O${}_{\mathrm{anhyd}}$ in red/blue. In the side views, $d$ denotes the vertical adsorption height; the vertical scale is enlarged by a factor of 4 and referenced against the Bragg plane (dashed line, $d_{\mathrm{B}}$). The adsorption geometry of PTCDA is taken from the NIXSW experiments and the surface buckling from DFT calculations. O-Ag interactions are illustrated by black dotted lines. For (a) Ag(111), the side views refer to the molecule A.[58] For clarity, we show only the molecules within one unit cell of the periodic structure.Reuse & Permissions

Figure 2

Normalized x-ray photoemission spectra ($◯$) for (a,c) PTCDA/Ag(100) and (b,d) PTCDA/Ag(110). The former were taken at an emission angle of 2° and a photon energy of 3.024 keV, the latter at 45° emission and 4.280 keV. All spectra are corrected with a linear or Shirley background function. (a) and (b) XP spectra of the C1$s$ level. Green: C${}_{\mathrm{C}-\mathrm{C}}$, brown: C${}_{\mathrm{C}-\mathrm{H}}$, purple: C${}_{\mathrm{C}-\mathrm{CO}}$, gray: combined satellite corresponding to C${}_{\mathrm{C}-\mathrm{C}}$, C${}_{\mathrm{C}-\mathrm{H}}$, and C${}_{\mathrm{C}-\mathrm{CO}}$, light gray: C${}_{\mathrm{funct}}$, black: unassigned satellites. Note that for the evaluation of the NIXSW data all C atoms of the perylene core are combined to give the C${}_{\mathrm{peryl}}$ signal. (c) and (d) XP spectra of the O1$s$ level. Red: O${}_{\mathrm{carb}}$, blue: O${}_{\mathrm{anhyd}}$, black: unassigned satellite. The main photoemission peaks are drawn as full lines, the (corresponding) satellite peaks as dashed lines, in respective colors. In addition, the resulting sum of all components is shown as a black line.Reuse & Permissions

Figure 3

Exemplary normal incidence x-ray standing waves photoemission yield curves for (a,c) PTCDA/Ag(100) and (b,d) PTCDA/Ag(110). The energy scales refer to the nominal Bragg energies (see Table ). Experimental data points are shown as circles, while fit results are given as full lines. The sophisticated XP fit models allow a differential analysis of the NIXSW data: (a) and (b) C${}_{\mathrm{peryl}}$ (gray), C${}_{\mathrm{funct}}$ (light gray), (c) and (d) O${}_{\mathrm{carb}}$ (red), O${}_{\mathrm{anhyd}}$ (blue). In addition, the summed data of the individual components are shown in black. The measured reflectivity curves (Darwin–Prins curves, I${}_{\mathrm{refl}}$) are shown as dashed lines. Note that the scaling of the $x$ and $y$ axes is not identical for (a,c) PTCDA/Ag(100) and (b,d) PTCDA/Ag(110). Also, curves have been vertically offset for clarity. The lower signal-to-noise of the data for the Ag(110) surface is due to the smaller photoemission cross sections at the higher Bragg energy, a smaller transmission of the analyzer (see above), and a different data acquisition geometry.Reuse & Permissions

Figure 4

(a) Averaged vertical heights $d$ (solid lines, ) and bond lengths $b$ to neighboring surface atoms (dotted lines, ) for PTCDA adsorption on the low index Ag surfaces. Gray: C${}_{\mathrm{peryl}}$, light gray: C${}_{\mathrm{funct}}$, red: O${}_{\mathrm{carb}}$, blue: O${}_{\mathrm{anhyd}}$. Error bars are omitted for clarity; lines are guides to the eye. The calculated adsorption energy $E_{\mathrm{ads}}$ per molecule is also indicated (black solid line, ■). (b) Valence bond structure (top) and ball-and-stick model (bottom) of PTCDA. (c) Schematic binding scenario for PTCDA on Ag surfaces: Contributions of C${}_{\mathrm{peryl}}$ (gray) and O${}_{\mathrm{carb}}$ (red) to the potential energy $E$ as a function of the adsorption height $d$. Realized values for $d$ in the molecule are marked by dashed lines.Reuse & Permissions

Figure 5

Total electron density ρ(r) and electron density difference Δρ(r) for PTCDA on (a) Ag(111), (b) Ag(100), and (c) Ag(110). First and second rows: side views along the long molecular axis; third row: top views; fourth row: side views along the short molecular axis. The horizontal cuts are made 0.8 Å below the perylene core of the PTCDA molecule. Light gray: Ag, white: H, gray: C${}_{\mathrm{peryl}}$, light gray: C${}_{\mathrm{funct}}$, red: O${}_{\mathrm{carb}}$, blue: O${}_{\mathrm{anhyd}}$. (d) Lowest unoccupied molecular orbital (LUMO) of gas-phase PTCDA.[21] The black and white arrows indicate the positions of the vertical and horizontal cuts. Note that in (a) the PTCDA molecule is rotated against the $\left[\overline{1}10\right]$ substrate direction by 2°.Reuse & Permissions