New Journal of Physics, Vol. 18, Nr. 11, S. 113022. (2016)
For exploiting the full potential of organic materials for future organic electronic devices it is of crucial importance to understand structural and electronic properties of metal-organic interfaces and adsorbate systems, in particular electronic interactions and growth mechanisms. Phthalocyanine molecules represent one class of materials which are very frequently discussed in this context. They feature an appealing tunability in terms of structural, electronic and magnetic properties, simply by exchanging the central (metal) atom or group of atoms. Here we present a comprehensive study of one of the model systems in this field, TiOPc on Ag(111). We discuss structure formation and growth from submonolayer to multilayer films, based on results obtained by electron diffraction, scanning tunneling microscopy, electron energy loss spectroscopy, x-ray standing waves, photoelectron spectroscopy and pair potential calculations. Similar to related metal-phthalocyanine adsorbate systems we find three distinct phases in the submonolayer regime, a disordered gas-like 'g-phase', a commensurate 'c2-phase' at low temperature, and a 'p.o.l.-phase' consisting of a series of point-on-line structures with continuously shrinking unit cells. For the latter a uniform TiO-up configuration (Ti–O group pointing towards vacuum) was found. Hence, the first-layer molecules form a strong dipole layer, the dipole moment of which is compensated by molecules adsorbing in the second layer at hollow-sites in TiO-down geometry (Ti-O group pointing towards the surface). The Coulomb interaction between the dipole moments in the first and second layer stabilizes this bilayer structure and causes a bilayer-by-bilayer growth mode of molecular films above a thickness of 2 ML. We report the structural properties (vertical adsorption heights, inter-layer distances, inplane orientations and positions) of the molecules in all phases in detail, and discuss the effect of inelastic dynamical charge transfer. Our results contribute to a comprehensive understanding of this interesting adsorbate system and, in comparison with earlier studies on CuPc, H2Pc and SnPc on Ag(111), we shine new light on the interesting interplay of molecule-molecule and molecule-substrate interactions.