The Adsorption Geometry of PTCDA on Ag(111)

Abstract

The bonding lengths of a large pi-conjugated molecule which was adsorbed on a metal surface were determined for the first molecular layer. The system consisting of the organic molecules 3,4,9,10-Perylenetetracarboxylic dianhydride (PTCDA) grown on a silver (111) surface was investigated. PTCDA on Ag(111) appears in two phases in the first layer: The commensurate long range ordered monolayer at room temperature (RT phase) is a stable phase, whereas the disordered phase which is grown at temperatures below 160 K (LT phase) is a metastable phase. The bonding distance of the molecules from the surface is an indication for the bonding strength. Distortions of the molecules from the planar geometry give additional information on the bonding mechanism. Using NIXSW, the vertical distance of the molecules was investigated by the core-level C1s transition. Since carbon is the main element of the molecules, its vertical distance corresponds to the averaged molecular distance. Furthermore, the distances of the oxygen atoms of the molecules were determined by using the O1s transition. In the molecule, two types of chemically different oxygen atoms exist: the four outer carboxylic oxygen atoms and the two inner anhydride oxygen atoms. For the first time, this chemical shift of one atom sort within a molecule was utilized for a separation of the photoemission spectra which were taken in a standing wave experiment. Within this work, different vertical positions for atoms of the same element could be identified. For the RT phase an average molecular bonding distance of 2.86 Å was measured. For the LT phase the corresponding value is 2.80 Å. Thus, the molecules in the LT phase are 0.06 Å closer to the Ag surface than the molecules in the RT phase, this result clearly is significant. In the LT phase, a stronger intramolecular distortion was observed, the oxygen atoms lie 0.14 Å below the carbon core, whereas the molecules in the RT phase do not exhibit such a strong distortion, the result for the (total) oxygen atoms is the same as for the carbon atoms, within the errors. By analyzing the separated oxygen types the surprising result was achieved that these chemically different oxygen atoms occupy different vertical positions on the surface. For both phases, it was found that the anhydride oxygen atoms are clearly above the carboxylic oxygen atoms. For the RT phase this internal distortion is 0.32 Å. For the LT phase, this difference corresponds to 0.33 Å. The average vertical distance of the carboxylic oxygen atoms in the RT phase is 0.20 Å below the carbon core and 2.66 Å above the surface. For the LT phase, however, this molecular distortion is larger, the carboxylic oxygen atoms lie 0.31 Å below the carbon core and the average vertical distance to the surface is only 2.50 Å which is 6% smaller than for the RT phase.