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

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.

Substrate-mediated band-dispersion of adsorbate molecular states

“…It is smaller than 100 meV for the distances considered here 36 and is not relevant for the present discussion of the band dispersion. Such a value is very similar to the experimentally observed relation between distance and LUMO position for PTCDA/ Ag(111) 33,34 . Note that the temperature decrease from 300 to This effect has been considered in our DFT calculations and Fig.…”

“…The strength of the molecule-substrate interaction, and thereby the magnitude of the substrate-enhanced band width of the LUMO, clearly depends on the vertical distance between the molecule and the substrate. In view of several investigations on very similar systems, such as PTCDA/Ag(111) 33,34 and NTCDA/Ag(111) 35 , a reduction of this vertical bonding distance can be achieved by lowering the substrate temperature and is accompanied by an energy shift of the LUMO towards larger binding energies. Figure 4 shows energy distribution curves of PTCDA/Ag(110) recorded in the (1 10) direction at k ¼ 1.45 Å À 1 and thus at the binding energy minimum of the LUMO measured at sample temperatures of 300 K (top), 45 K (middle) and 10 K (bottom).…”

Many-body dispersion effects in the binding of adsorbates on metal surfaces

“…On the experimental side, the adsorption geometry and the individual atomic adsorption heights of PTCDA on Ag(111) are known from x-ray standing wave (XSW) measurements 80 . The adsorption energy, typically extracted from TPD measurements, however, is not experimentally known due to the molecule being destroyed upon thermal desorption 84 86 .…”

Role of functional groups in surface bonding of planarπ-conjugated molecules

“…The photon flux is estimated at about 40 photons per molecule and second. 15,24 For experimental details concerning the Ag(111) surface, we refer to Ref. 15.…”

“…14 It has already been demonstrated 1,15 that the vertical adsorption heights of O carb and O anhyd are different for PTCDA/Ag(111) and that the molecule exhibits a saddle-like adsorption conformation [see Fig. 1(a)].…”