Vibrational and Structural Properties of PTCDA on Ag(110)

The structure and vibrational properties of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) molecular layers have been investigated on the nonhexagonal Ag(110) substrate. Based on their distinct and unambiguous vibrational signatures, the existing brick wall (BW) and herringbone (HB) phases have been identified and discriminated using infrared absorption spectroscopy. Their particular long-range ordering, e.g., their authenticity, has been verified using spot-profile analysis low energy electron diffraction (SPA-LEED). At variance with earlier studies, we provide conclusive evidence for a commensurate HB structure. Due to rather strong molecule–metal and intermolecular interactions, the transition between the BW and the more dense HB phase proceeds only partially, and we have developed recipes to produce uniform layers comprising either of the two long-range ordered phases. Comparison of PTCDA adsorption on Ag(110), Ag(111), and Au(111) substrates reveals different scenarios regarding interfacial dynamic charge transfer (IDCT) associated with the excitation of vibrational modes. Specifically, the effect of strong, medium strong, and weak interactions with the substrate, in conjunction with chemical bond formation, has been examined regarding the extent of IDCT and the asymmetries of the vibrational line shapes. Our findings are supported by reports in the literature analyzing the location of the former LUMO with respect to the (substrate) Fermi energy as well as the vertical bonding distances and molecular distortions of the PTCDA molecular plane.