Structural and vibrational properties of the TiOPc monolayer on Ag(111)

Abstract

The evolution of titanyl-phthalocyanine (TiOPc) thin films on Ag(111) has been investigated using IRAS (infrared absorption spectroscopy), SPA-LEED (low energy electron diffraction) and STM (scanning tunneling microscopy). In the (sub)monolayer regime various phases are observed that can be assigned to a 2D gas, a commensurate and a point-on-line phase. In all three phases the nonplanar TiOPc molecule is adsorbed on Ag(111) in an oxygen-up configuration with the molecular π-conjugated backbone oriented parallel to the surface. The commensurate phase reveals a high packing density, containing two molecules at inequivalent adsorption sites within the unit cell. Both molecules assume different azimuthal orientations with respect to the Ag(111) substrate which is ascribed to the interplay between the occupation of preferred sites as well as distinct azimuthal orientations and a short ranging Pauli-Type repulsion to avoid overlapping of neighboring molecules. At full saturation of the monolayer the latter interaction leads to a further azimuthal reorientation that enables an even higher packing density. Thereby, the plain commensurate order is lost and a point-on-line phase is formed. DFT calculations have been used to study different adsorption geometries of TiOPc on Ag(111). The most stable configurations among those with pointing up oxygen atoms (bridge+, bridgex, topx) seem to correspond to those identified experimentally. The calculated dependence of the electronic structure and molecular dipole on the adsorption site and configuration is found to be rather small.