Structural investigation of the adsorption of SnPc on Ag(111) using normal-incidence x-ray standing waves

The bonding of tin(II)-phthalocyanine (SnPc) on Ag(111) was studied using the normal incidence x-ray standing wave technique. For an incommensurate monolayer structure at room temperature, it was found that the SnPc molecules adsorb in a “Sn-down” geometry, i.e., the Sn atoms lie below the molecular plane. A significant bending of the benzene rings toward the surface indicates that these rings contribute to a chemisorptive bonding of the molecule to the surface. This effect is found to be even enhanced for another phase, a commensurate submonolayer structure that is only stable at low temperature. In this phase, the molecules are located significantly closer to the surface in a mixed “Sn-up”-“Sn-down” configuration. Nondipolar contributions to the photoelectron yield were also investigated and taken into account.

Figure 1

Schematic picture of the free SnPc molecule.

Figure 2

Typical photoelectron spectra of the C1s (left) and Sn3d (right) of crystal A with a monolayer SnPc at room temperature.

Figure 3

Typical photoelectron spectra of the N1s region: the clean silver crystal with the Ag3d plasmons (left) and with a monolayer SnPc at room temperature (right).

Figure 4

(Color online) XSW scans of the incoherent multilayer. The C1s and N1s PES yields and the reflectivity curves are shown.

Figure 5

(Color online) XSW scans of the incommensurate monolayer structure on crystal A. C1s, N1s, and Sn3d PES yields and reflectivity curves are shown.

Figure 6

(Color online) Argand diagram of the XSW results obtained from the monolayer structure at room temperature. Blue (dark) symbols represent the measurements on crystal A, green (light) symbols correspond to crystal B. Open symbols are results from the (111) Bragg reflection, full symbols from the $\left(\overline{1}11\right)$ Bragg reflection.

Figure 7

Schematic models of the adsorption geometries for (a, b) the incommensurate RT structure and (c) the commensurate LT structure. Note that (a, b) shows two possible, alternative scenarios, while (c) shows two different geometries which occur in the same structure. Numbers represent the height of the corresponding atomic species above the uppermost bulk atomic plane in angstroms. For the RT structure, the values correspond to the average of the obtained values for the crystals A and B, for the LT structure the results for crystal A are shown.

Figure 8

(Color online) Argand diagram of the XSW results obtained from the commensurate submonolayer at $150\mathrm{K}$. Blue (dark) symbols represent measurements on crystal A, green (light) symbols correspond to crystal B.

Figure 9

(Color online) Argand diagram of the XSW results of the Sn atoms obtained from the commensurate submonolayer structure at $150\mathrm{K}$. Blue (dark) symbols represent measurements on crystal A, green (light) symbols correspond to crystal B. The arrows indicate the positions of the Sn-up and Sn-down atoms.