The evolution of titanyl-phthalocyanine (TiOPc) thin films on Ag(111) has been investigated using IRAS, SPA-LEED and STM. 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 non-planar TiOPc molecule is adsorbed on Ag(111) in an oxygen-up configuration with the molecular -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 which is ascribed to preferred sites and azimuthal orientations with respect to the Ag(111) substrate and, to a lesser extent, to a minimization of repulsive Pauli interactions between adjacent molecules at short distances. At full saturation of the monolayer the latter interaction becomes dominant and the commensurate long range order is lost. 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 + , bridge x , top x ) 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. 2

The initial stages of copper(II)−phthalocyanine (CuPc) thin film growth on Ag(111) have been investigated using Fourier transform infrared absorption spectroscopy (FT-IRAS), spot profile analysis low-energy electron diffraction (SPA-LEED), and thermal desorption spectroscopy (TDS). Starting at (sub)monolayer coverages up to 5 monolayers (ML), a number of ordered overlayers are found. Vibrational spectroscopy shows characteristic spectroscopic signatures for the individual submonolayer phases as well as for the bilayer, trilayer, and multilayers. Highly asymmetric line shapes of the in-plane vibrational modes of submonolayer CuPc provide unequivocal evidence for interfacial dynamical charge transfer between the metal electronic states and CuPc molecular orbitals, indicative for a partially filled LUMO at the Fermi energy as well as the prevalence of severe nonadiabaticity in the electron−vibron coupling. Growth of the second and third CuPc layers proceeds in a layer-by-layer fashion (Frank van der Merwe growth). Higher layers deposited at 300 K, on the other hand, transform into 3D crystallites on top of the CuPc trilayer upon annealing. For CuPc/Ag(111) monolayers thermal desorption spectra reveal intact CuPc desorption for coverages above 0.9 ML. At lower coverages an alternative reaction path involving partial dissociation of the CuPc molecules is found, with 1 / 4 Pc as the desorbing stable product species. In this study IR absorption spectra have been obtained at an exceptionally high spectral resolution of 0.5 cm −1 , which allows a spectral discrimination of molecular species with unprecedented detail. Specifically, CuPc mono-, bi-, and trilayers as well as the bulk-like crystalline phase of CuPc have been discriminated based on clearly resolved, closely spaced vibrational bands.

The operation of organic optoelectronic devices relies notably on the bulk properties of compound molecular species, but even more so on the influence of interfaces thereof since it is at the interface where elemental electronic processes take place. Their identification and characterization thereby requires that these critical sections of a device are well defined and can be prepared with low defect density. In this context titanyl phthalocyanine (TiOPc) arises as an excellent candidate that reveals the formation of a stable bilayer structure with a characteristic "up-down" molecular arrangement that optimizes the dipole-dipole interaction within the bilayer. In our experimental study, long-range ordered TiOPc bilayers have been grown on Ag(111) surfaces and analyzed using infrared absorption spectroscopy and scanning tunneling microscopy. By monitoring the prominent Ti[double bond, length as m-dash]O stretching mode in IRAS and identifying local configurations in STM, a microscopic model for the growth of TiOPc bilayers on Ag(111) is suggested. We demonstrate that defect structures within these bilayers lead to characteristic vibrational signatures which react sensitively to the local environment of the molecules. Thermal desorption spectroscopy reveals a high thermal stability of the TiOPc bilayer up to 500 K, which is attributed to hydrogen bonds between oxygen of the titanyl unit and the hydrogen rim of phthalocyanines in the second layer, in addition to contributions arising from the oppositely oriented axial dipole moments and the ubiquitous van der Waals interactions.

The structural properties and thermal evolution of heterolayers comprising copper-phthalocyanine (CuPc) and 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) on Ag(111) were investigated using FT-IRAS, SPA-LEED, and TDS. The bilayer systems have been prepared at low temperatures (T ≃ 80 K) and annealed to successively higher T. The layered arrangement of CuPc deposited onto PTCDA/Ag(111) displays unusual thermal stability up to 450–500 K when intermixing sets in. For layers with a reversed stacking sequence, exchange processes of first- and second-layer molecules take place already at 200–300 K. PTCDA shows a stronger tendency to occupy first-layer sites, displacing CuPc from the direct contact with the metal substrate; continued annealing, however, leads to a depletion of PTCDA within the deposited organic film. This apparent paradoxon is resolved by comparing two related quantities, the adsorption energy per area (for parallel adsorbed molecular species) and the adsorption energy per molecule. As a consequence of the dissimilar “footprint” areas of CuPc and PTCDA, and the larger overall size of CuPc, the former quantity is larger for PTCDA, while the latter quantity is larger for CuPc. The experimental observations regarding layer exchange, intermixing, and modifications in thin film composition and structure are then readily explained.

Thin pseudomorphic as well as thick “Pt(111)-like” Pt/Ru(0001) bimetallic substrates have been used to study the oxidation of methanol (CD3OH) to CO2. Specifically, molecular (cold deposited) and ordered atomic (annealed) oxygen precovered Pt n /Ru(0001) layers with n = 1–15 ML have been studied. Thereby, the thickness of the Pt films primarily influences the adsorption probability of O2 and much less the energy barriers for methanol reactions. In particular, no indication of a strain effect could be detected, and the reactions of methanol on Pt n /Ru(0001) multilayer films closely follow the known reaction scheme on Pt(111) substrates: On O2/Pt n /Ru(0001), methanol is directly oxidized to formate (DCOO) and eventually desorbs as CO2 (300 K). The formate producing reaction is thereby promoted by dissociating O2 molecules (130 K) and, at higher temperatures (130–200 K), by disordered atomic oxygen. This is in contrast to methanol postadsorbed onto ordered atomic oxygen covered Pt n /Ru(0001) layers, which exhibit formaldehyde formation and decomposition to CO and H. Interestingly, during methanol dissociation at 160–200 K, a remarkable fraction of methanol molecules experiences an H ↔ D exchange of the hydroxyl group hydrogen, leading to the dominant desorption of CD3OD. Our observation that the relative fraction of desorbing CD3OH is exceptionally low implies a bold primary isotope effect, which we attribute to the dissimilar zero point energies associated with OH and OD vibrations of methanol.