Interfacce ibride: adsorbimento di molecole aromatiche sui metalli

This thesis presents density functional theory (DFT) studies of the adsorption of aromatic molecules on metals. These hybrid interfaces are found at the electrical contacts in the organic electronic devices. Understanding the precise electronic structure including the energy alignment between the Fermi level of the metal and the HOMO/LUMO states of the molecule is essential as it influences the carrier injection across the interface, which in turn affects the overall performance of these devices. The topography of the first layer of adsorbates is very important as it drives the orientation of the subsequent growing layers. In this respect I have studied by DFT including van der Waals interactions the structural, electronic, and spectroscopic properties of pentacene adsorbed on the Al(001) surface in collaboration with experimentalists who measured the X-ray photoemission spectra (XPS), the near-edge X-ray absorption fine structure (NEXAFS), and the surface charge maps by scanning tunneling microscopy (STM). We find a major change of the molecular backbone resulting in a peculiar V- shape bending, due to the direct anchoring of the two central carbons atop the two Al atoms underneath. In the most stable adsorption configuration, pentacene is oriented with the long axis parallel to the substrate [110] direction, where such anchoring is favored by optimally matched interatomic distances which results in filling of the LUMO. Next I have investigated computationally K doping of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) adsorbed on Ag(111), during my secondment at the Graz University of Technology, Austria. In fact another key parameter to be tuned as far as the device performance is concerned is the number of charge carriers which can be improved by doping. Alkali metal atoms are frequently used for simple yet efficient n-type doping of organic semiconductors. The incorporation of dopants from the gas phase into molecular crystal structures needs to be controlled and well understood in order to optimize the electronic properties (charge carrier density and mobility) of the target material. We found that K doping induces distinct stoichiometry-dependent structural reordering processes, resulting in highly ordered and large KxPTCDA domains. The emerging structures were found to be stable for stoichiometries x = 2 and 4. These were analyzed by an experimental group that used low temperature STM, scanning tunneling hydrogen microscopy (STHM), and low-energy electron diffraction (LEED). The DFT calculations have proven essential for a correct interpretation of the experimental ST[H]M data. In this way we have determined the K atom positions, located in the vicinity of the oxygen atoms of the PTCDA molecules with sub-Ångstrom precision. Our calculations have shown that the K atoms eventually lose their electrons to PTCDA and the Ag substrate thereby filling the LUMO of the former and reducing the work function of the latter.