Exploring chemistry and magnetism in adlayers at surfaces

The goal of my thesis was to understand, design and modify the properties of surfaces as a whole, as well as of surface-supported atoms and molecules. This way I discovered exciting differences and similarities between two- and three-dimensional systems, i.e. between the surface and the bulk, which came as a natural consequence in the pursuit of this aim. I have observed physicochemical phenomena that strictly require the specific characteristics of surfaces and also a number of effects that proceed in a very similar fashion when compared to the gas or the liquid phase. One thesis is, however, not enough to study all of the interesting phenomena in surface science, i.e. the field concerned with effects occurring when the dimensionality of the arrangement of atoms is decreased below three. Therefore I focused on exploring on-surface chemistry and magnetism – phenomena that are closely related, as they both depend on the interaction of atoms’ valence electrons with the surroundings.
The first example of how one can tune the properties of a surface is provided by adding a one-atom-thick layer of adsorbates – specifically O, N and Cl on Cu(001). During my work I discovered that this simple modification can drastically alter the reactivity of a surface, as studied using the self-metalation reaction of porphyrins, in which a metal atom is taken from the substrate and embedded in the molecule. Interestingly, this approach also allowed studying the interactions between the molecules, visualised in the formation of molecular self-assembled islands and clusters.
In the second studied system I investigated a surface covered by only single ad-atoms, not by a full layer of adsorbates. In this project I was interested in the influence of a substrate on the magnetic properties of single transition metal atoms. Isolated single atoms, due to their spherical symmetry, cannot possess any magnetic anisotropy – i.e. directional dependence of magnetic properties. The interaction with a surface can, however, induce such directional dependence, which in the case of Cr atoms deposited on a Bi substrate is found to reach the theoretically possible limit. It is, to the best of my knowledge, the first observation of such a giant magnetic anisotropy on a non-insulating substrate.
Due to the very limited nature of the periodic table of elements, it is desirable to change the properties of paramagnetic atoms even before depositing them on a surface. Due to the vast possibilities given by organic chemistry, inserting an atom in easily modifiable molecule is a simple way to achieve that. In this thesis I show that such an organic ‘cage’ around an atom can additionally modify the magnetic interaction between the paramagnetic ion and the underlying substrate. I was able to tune the molecule-substrate magnetic exchange coupling energy by using molecules with different functional groups.
Interestingly, molecule-surface magnetic interactions can also be used to study molecular motion. I also discuss the use of X-ray Magnetic Circular Dichroism for detecting out-of-plane molecular rearrangement in a model case, in which two phthalocyanines, MnPc and FePc, showed different adsorption energies with the former being able to push the latter away from the substrate.
During this thesis I also developed a method for creating a supramolecular chessboard-like arrangement built from two different molecules, namely MnPc and FeFPc. This approach has been successfully used by me and my colleagues in many projects that strictly required a surface-supported, alternating arrangement of molecules. Fascinating properties of this low-dimensional magnetic layer were controlled by chemical ligation as well as by the choice of the underlying substrate – Au(111), Ag(111) or ferromagnetic O-covered Co(001). Those different supports enabled studying different magnetic coupling interactions that are strong on ferromagnetic supports, while weak on diamagnetic.
This thesis expands the range of tuneable surface properties. This was achieved by the use of on-surface supramolecular engineering, an approach combining the design and modifications of molecules and surfaces, as well as the interactions between them.