Energy level alignment and site-selective adsorption of large organic molecules on noble metal surfaces

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.