Structure analysis of PTCDA/Ag(100) by low-energy electron diffraction and density functional theory

The adsorption geometry of the planar 3, 4, 9, 10-perylene-tetracarboxylic-dianhydride (PTCDA) moleculein the commensurate c(8 × 8) structure on Ag(100) was determined from the analysis of the intensities in low-energy electron diffraction (LEED-IV). Using data from different angles of incidence and optimized computercode, we were able to overcome earlier challenges given by the limitations of the experimental data set and thecalculation times required for the large unit cell with many atoms. Testing of different structures confirmed theon-top adsorption site for the center of the perylene core. The final Pendry R factor of RP = 0.180 for the on-topposition is significantly lower than the one for the fourfold hollow position (minimum RP = 0.369) that is henceexcluded. The molecule shows archlike deformation with a downshift of the terminal carboxylic groups. Boththe molecular structure and the adsorption height are in very good agreement with results from an earlier normalincidence x-ray standing wave (NIXSW) experiment and new density functional theory (DFT) calculations,which we performed in parallel for 0 K and in addition for 300 K. The LEED-IV analysis demonstrates that thePTCDA induces a relaxation (−0.08 versus −0.04 Å of the clean surface) and buckling (0.33 Å) of the topmostAg layer. Special attention was given to the Ag atom below the central ring of the PTCDA. The IV analysiswas rather insensitive to its vertical position, and a small R factor, close to the minimal, was also obtainedwhen this Ag atom was moved upward (RP = 0.185) or even an Ag vacancy site (RP = 0.171) was assumed.However, these structures could be excluded on the basis of DFT calculations. The vacancy structure has afree adsorption energy that is 0.18 eV larger compared to the favored geometry where this central Ag atom ispushed downward, partly due to the energy cost for the vacancy formation. The discussion of adsorbate-inducedformation of vacancy sites is important because it was reported for C60 on Ag(111). The up- and downwarddisplacements of the first-layer Ag atoms support the understanding of the chemical bond of the PTCDA tothe Ag substrate and reveal how the originally planar π system is locally distorted. Our analysis proves thatLEED-IV is a powerful technique for surface crystallography of large organic adsorbates.