Abstract

In recent two decades, there has been a large interest in organic molecules on metallic as well as insulating substrates. This interest is caused by the need to understand fundamental properties of large organic molecules on solid surfaces at the level that properties of smaller adsorbates, like carbon monoxide or oxygen molecule, are understood. In addition, theoretical and experimental studies in this field are driven by potential applications of organic materials as active components in light-emitting diodes (OLEDs) and fieldeffect transistors (FETs), as well as by on-going efforts to use single molecules as building blocks in nano-electronic and nano-mechanical devices. This Thesis deals with two aspects of large organic molecules on metal surfaces: local adsorption geometry and energy level alignment. Molecules bind to specific sites on metallic surfaces which correspond to the lowest total energy of the molecule-substrate system. It is of fundamental interest to understand the electronic causes of the interaction between the molecule and the surface. Ultimately, one would like to gain understanding of what causes molecule-substrate attraction and why this attraction is stronger for some particular geometries than for others. Another important aspect is the alignment of molecular levels with respect to the Fermi level of the metal. This level alignment governs the electron injection from the metal to the molecule (or vice versa) in electronic devices. At the beginning of the Thesis, we review our main theoretical tool, density functional theory (DFT), and present details of the plane-wave implementation of DFT. We introduce concepts which are useful in analyzing surface science systems, such as surface energy, work function, electron density difference, difference in density of states, etc. We present calculations of copper and silver bulk and surfaces to assess how density functional theory performs for noble metals. We then investigate a specific surface science system to demonstrate these concepts, namely, chlorine adsorbed on the Ag(111) surface at submonolayer coverages. We find that the adsorption energy of Cl on Ag(111) is about 2.9 eV and depends only weakly on coverage. The Ag-Cl bond is very strong and can be best described as ionic. Adsorption of Cl on the Ag(111) surface leads to electron charge transfer from the metal to the adsorbate. Each chlorine atom acquires about 0:2 additional electrons upon adsorption. Because of this charge transfer the work function of adsorbate-covered substrate increases. We find a very good agreement between theory and available experimental data. Small dependence of adsorption energy on coverage can be explained by lateral repulsion of adsorption-induced dipoles. Chapter 4 of the Thesis is devoted to site-selective adsorption of one specific molecule, 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTCDA), on the Ag(110) surface. We perform large-scale density functional calculations of several local adsorption sites and analyze the results in great detail. Calculations reveal that NTCDA prefers adsorption geometry in which the peripheral oxygen atoms lie directly above the silver atoms in the [1�10] atomic rows. This nicely agrees with available experimental data. From the analysis of DFT calculations we are able to understand why this happens. Firstly, NTCDA is a molecule with electron accepting properties. In the gas-phase molecule the oxygens of the side groups are negatively charged while the central naphthalene core is positively charged. When the molecule is adsorbed on the Ag(110) surface, about 0:4 electrons are transfered to the lowest occupied molecular orbital (LUMO). Silver atoms in the topmost atomic layer become positively charged and this causes electrostatic attraction between negatively charged oxygen atoms of NTCDA and positively charged silver atoms. This attraction is maximum when oxygens are just above the silver atoms in the [1�10] atomic rows. Thus, on the basis of DFT calculations, we have developed a model for site-selective adsorption of NTCDA on the Ag(110) surface. This model should also be applicable in case of adsorption of a related molecule, PTCDA, on the same surface. In Chapter 5 we analyze the energy level alignment of copper octaethylporphyrin (CuOEP) on three metal surfaces: Ag(001), Ag(111) and Cu(111). The experiments that this analysis is based on were performed in the Institute of Physics of University of Basel, in the NanoLab group. We first critically review and discuss different physical mechanisms that lead to a formation of the interface dipole at metal-organic interfaces. These different mechanisms are: charge transfer (as described by the so-called induced density of interface states (IDIS) model), polarization of the adsorbate near the metal surface, push-back effect, which is a consequence of the Pauli exclusion principle, permanent electrostatic dipoles at interfaces, and charge transfer caused by chemical interactions. Then we discuss in detail experimental results and evaluate the contribution of each mechanism to the total interface dipole. We conclude that the push-back effect is the most important for CuOEP/metal interfaces.

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