No abstract

No abstract

Normal incidence x-ray standing wave ͑NIXSW͒ experiments have been performed for monolayers of 3,4,9,10-perylene-tetracarboxylic-dianhydride ͑PTCDA͒ adsorbed on the Ag͑111͒ surface. Two phases were analyzed: the low-temperature phase ͑LT phase͒, 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 phase͒. 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% ͑0.05 Å͒ and 8% ͑0.21 Å͒ 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 Å ͑RT͒ and 0.31 Å ͑LT͒ 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 ͑RT͒ and 0.33 Å ͑LT͒. These structural parameters of the two phases are compared to those of PTCDA monolayers adsorbed on Au͑111͒ and Cu͑111͒ surfaces and are discussed in the frame of current bonding models.

The trends in the bonding mechanism of 3,4,9,10-perylenetetracarboxylic acid dianhydride (PTCDA) to the Ag(111), Ag(100), and Ag(110) surfaces were analyzed on the basis of data obtained from x-ray standing waves and dispersion-corrected density functional theory. Of importance are the attractive local O-Ag bonds on the anhydride groups. They are the shorter, the more open the surface is, and lead even to partly repulsive interactions between the perylene core and the surface. In parallel, there is an increasing charge donation from the Ag surface into the π system of the PTCDA. This synergism explains the out-of-plane distortion of the adsorbed PTCDA and the surface buckling.

The normal incidence x-ray standing wave (NIXSW) technique is used to determine the adsorption geometry of submonolayer 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) adsorbed on the Ag (110) surface. An accurate analysis of both C1s and O1s photoemission (PE) spectra allows the respective adsorption heights of carbon and oxygen atoms in different chemical environments within PTCDA to be distinguished. Due to the intricacy of the PE fitting models, a systematic error analysis of NIXSW structural parameters was developed and employed. Based on the adsorption geometry of PTCDA on Ag(110) a bonding mechanism is discussed.