If the rich functionality of organic molecules is to be exploited in devices such as light-emitting diodes or field-effect transistors, interface properties of organic materials with various (metallic and insulating) substrates must be tailored carefully. In many cases, this calls for well-ordered interfaces. Organic epitaxy-that is, the growth of molecular films with a commensurate structural relationship to their crystalline substrates--relies on successful recognition of preferred epitaxial sites. For some large pi-conjugated molecules ('molecular platelets') this works surprisingly well, even if the substrate exhibits no template structure into which the molecules can lock. Here we present an explanation for site recognition in non-templated organic epitaxy, and thus resolve a long-standing puzzle. We propose that this form of site recognition relies on the existence of a local molecular reaction centre in the extended pi-electron system of the molecule. Its activity can be controlled by appropriate side groups and--in a certain regime--may also be probed by molecularly sensitized scanning tunnelling microscopy. Our results open the possibility of engineering epitaxial interfaces, as well as other interfacial nanostructures for which specific site recognition is essential.
The near-surface structure of the room-temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide has been investigated as a function of temperature between 100 and 620 K. We used a combination of photoelectron spectroscopies (XPS and UPS), metastable induced electron spectroscopy (MIES), and high-resolution electron energy loss spectroscopy (HREELS). The valence band and HREELS spectra are interpreted on the basis of density functional theory (DFT) calculations. At room temperature, the most pronounced structures in the HREELS, UPS, and MIES spectra are related to the CF3 group in the anion. Spectral changes observed at 100 K are interpreted as a change of the molecular orientation at the outermost surface, when the temperature is lowered. At elevated temperatures, early volatilization, starting at 350 K, is observed under reduced pressure.
We analyze the electronic interaction of PTCDA, a ribbon-shaped functionalized polycyclic molecule commonly used as model system, with various silver single crystal surfaces, using high resolution electron energy loss spectroscopy as a probe. Special emphasis is placed on the coupling of hybrid electronic states to intramolecular vibrations. Under certain circumstances, this coupling leads to an extremely strong activation of otherwise forbidden vibrational modes. Apart from a strong intramolecular electron-phonon interaction, the formation of a metallic ͑i.e., partially filled͒ molecular band is a prerequisite for this effect, which is reported here both for the PTCDA/Ag͑111͒ interface and for ultrathin potassium-doped PTCDA films on Ag͑110͒ while undoped PTCDA/Ag͑110͒ does not show this behavior. In the second part of the paper, we formulate and apply a theoretical model for the interfacial electron-phonon coupling, with the goal to understand the observed Fano line shapes in the case of PTCDA/Ag͑111͒. This model allows us to accurately describe the observed line shape and trace it back to a nonadiabatic interaction between electronic states and molecular vibrations, secondly to rationalize why the K/PTCDA/Ag͑110͒ system exhibits adiabatic behavior, and thirdly to extract values for the intramolecular electron-phonon coupling in PTCDA which are in good agreement with other data. The implications of our experimental observations and the theoretical analysis are discussed.
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