Normal-incidence x-ray standing-wave determination of the adsorption geometry of PTCDA on Ag(111): Comparison of the ordered room-temperature and disordered low-temperature phases

Normal incidence x-ray standing wave (NIXSW) experiments have been performed for monolayers of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) adsorbed on the Ag(111) surface. Two phases were analyzed: the low-temperature phase (LT phase), which is disordered and obtained for deposition at substrate temperatures below 150 K, and the ordered phase, which is obtained for deposition at room temperature (RT phase). From the NIXSW analysis the vertical bonding distances to the Ag surface were obtained for the averaged carbon atoms and the two types of chemically different oxygen atoms in the terminal anhydride groups. For the LT phase, we find about 2% $\left(0.05\text{Å}\right)$ and 8% $\left(0.21\text{Å}\right)$ smaller averaged bonding distances for the C and O atoms, respectively, compared to the RT phase. In both phases, the planar geometry of the free molecule is distorted; in particular, the carboxylic O atoms are closer to the surface by $0.20\text{Å}$ (RT) and $0.31\text{Å}$ (LT) with respect to the averaged C distance. The difference between the vertical bonding distances of the carboxylic and anhydride O atoms is found to be 0.32 (RT) and $0.33\text{Å}$ (LT). These structural parameters of the two phases are compared to those of PTCDA monolayers adsorbed on Au(111) and Cu(111) surfaces and are discussed in the frame of current bonding models.

Figure 1

(Color online) Photoemission (PE) spectra of the PTCDA $\text{O}1s$ level for (a) the RT phase and (b) the LT phase taken at a pass energy of 23 eV. The two spectra at the top were taken at off-Bragg photon energies and were used for the determination of the peak components (see text). The three spectra below were taken at photon energies close to the Bragg energy [$E_{\text{Bragg}}^{\text{exp}}\left(\text{RT}\text{phase}\right)=2633\text{eV}$ and $E_{\text{Bragg}}^{\text{exp}}$ $\left(\text{LT}\text{phase}\text{at}100\text{K}\right)=2635\text{eV}$] and were used for the NIXSW analysis. $\Delta E=E_{\text{photon}}-E_{\text{Bragg}}$. The solid lines are fits to the experimental data on the basis of the indicated peak components. The peak components from the carboxylic oxygen are indicated in red, those of the anhydride oxygen in dark blue. A detailed description of all components is given in Table . Experimental Bragg energies given here differ from nominal values due to $\sim 3°$ off-normal incidence. Note that the binding energy scale in (a) has been slightly shifted with respect to that given in Ref. 6 due to an improved calibration.

Figure 2

Adsorption of water at low temperatures. $\text{O}1s$ photoemission spectra of the LT phase of PTCDA on Ag(111) for different times of exposure to the x-ray beam taken at a pass energy of 23 eV. (a) Fresh spot of a sample being held at low temperature for 11 h. (b) Same spot after 6 h exposure to the x-ray beam. The change in the peak shape is due to photon-induced desorption of water. The photon energy was about 17 eV below the Bragg condition.

Figure 3

Exemplary photoemission yield curves derived from $\text{C}1s$ spectra (measured at a pass energy of 47 eV) as a function of photon energy for (a) the RT phase and (b) the LT phase (symbols) and fits (solid lines) with respective coherent fractions $f_{\text{C}}$. Photon energies are measured relative to the respective Bragg energies.

Figure 4

(Color online) Exemplary set of photoemission yield curves derived from $\text{O}1s$ spectra (measured at a pass energy of 23 eV) as a function of photon energy for (a) the RT phase and (b) the LT phase (symbols) and fits (solid lines) with respective coherent fractions $f_{\text{C}}$. Photon energies are measured relative to the respective Bragg energies. From top to bottom: $\text{O}1s$ total intensity, intensity related to the carboxylic oxygen (red), intensity related to the anhydride oxygen (blue), reflectivity and the “spurious peak,” respectively. The data points were taken on a nonequidistant grid for optimized data statistics. Fits were done according to Eq. (1) with a least-square fitting routine with equal weights of all data points. Photon energies are measured relative to the respective Bragg energies.

Figure 5

(Color online) Argand diagrams of the averaged structural parameters determined for the two adsorption states of Table : (a) RT phase and (b) LT phase. For further details see text.

Figure 6

(Color online) Hard sphere models of PTCDA/Ag(111) for the RT state and LT state, respectively. For comparison, models for PTCDA on Au(111) (Ref. 20) and Cu(111) (Ref. 21) are given. Note that on Au(111), the O positions could not be determined due to experimental reasons. The vertical length scale is expanded by a factor of 3. Red/blue circles: carboxylic/anhydride O atoms; light/dark gray circles: C atoms in anhydride groups/perylene core. The molecule is viewed along its long axis. The inset illustrates the structure of the PTCDA molecule.

Figure 7

(Color online) Vertical bonding distances $d_{\text{C}}^{\text{vdW}}$ of PTCDA corrected by the van der Waals radii $\left(r_{\text{vdW}}\right)$ of the surface atoms versus workfunctions $\varphi$ on different (111) coinage metal surfaces $\left(d_{\text{C}}^{\text{vdW}}=d_{\text{C}}-r_{\text{vdW}}\right)$. The correction by $r_{\text{vdW}}$ accounts for differences in the atomic sizes (Ref. 51). For the $d_{\text{C}}$ values of PTCDA/Cu(111) and PTCDA/Au(111) see Refs. 21, 20, respectively. Work functions of the clean metals are from Ref. 53. The dashed line represents a linear fit to the data points. The empirically found correlation between $d_{\text{C}}^{\text{vdW}}$ and $\varphi$ is given. The inset illustrates schematically the energetic lowering and occupation by electrons of the LUMO and is discussed in the text.