Surface-Induced Modification of Supported Late Transition Metal Complexes

University dissertation from Division of Synchrotron Radiation Research, Department of Physics, Faculty of Science, Lund University

Abstract: Popular Abstract in English In our current world, the exponential growth in demand for smaller and faster devices leads to the fact that we will eventually face the problem that traditional semiconductor technologies will reach their limitation in terms of size and speed. Humanity's consciousness to tackle the global ecological issues in the next decade, will demand the substitution of all non-reusable and thus polluting industrial catalysts to environmentally-friendly and reusable ones. These problems can only be resolved from targeted science programmes, aimed at addressing the sustainable and environmentally friendly development of our society. The development of molecule-based technology represents a potential contribution to this ecological vision. The field of molecule-based technology has developed in parallel with nanotechnology over the past decades. However, these systems can offer their own unique functional properties for prospective applications, compared to more traditional, hard condensed matter-based nanotechnologies. This is due to the small size, low cost, and structural perfection that molecules have to offer. The essence of their properties goes beyond classical physics, due to their quantum nature. This fact makes molecule systems as equally fascinating from a physics prospective as they are for their potential use in new device industries. In the scope of this thesis, I investigate the properties of supported transition metal complexes. What are transition metals and what makes them so special? Most of the elements only use valence electrons - i.e., electrons that participate in the creation of chemical bonds, from their outer electron orbitals to form bonds with other elements. Transition metals use the two outermost orbitals and, thus, have more valence electrons. This allows them to create bonds with many elements in a variety of shapes. All transition metals form stable compounds and depending on the amount of remaining valence electrons and their distribution in the outer orbitals can have different properties. The amount of borrowed or taken electrons by transition metal defines its unique properties, and is called oxidation number or oxidation state. Transition metals can have multiple oxidation states and knowing the oxidation state of the transition metal helps to determine the ability of the compound to react (exchange electrons and create bonds) with other species. Transition elements tend to form complexes, i.e. molecules in which a group of atoms cluster around a single metal atom. The complexes discussed in this thesis contain organic parts that create a framework around the transition metal atom, which looses some electron density to this framework to become an ion species. Organic refers a carbon-containing compound, where carbon atoms in the form of rings or long chains can be attached to other atoms, such as hydrogen, oxygen, and nitrogen. The organic part of the molecule determines the amount of the given or obtained electrons and thus defines the electron properties of the compound. The most active part is the transition metal ion, which often - although not always - is responsible for all the interesting interactions with surrounding matter. By varying the chemical environment of transition metal complexes, e.g. by placing them on different surfaces or adding different molecules, such as atmospheric gases or industrial products, one can change the density and distribution of the valence electrons in the metal centre of the molecule. In other words, the original properties of the complex and the electronic and magnetic properties of the complex are modified. The realisation that the electronic and magnetic properties of the transition metal organic compound can be tailored selectively has created a large diversity of possibile applications of these complexes. These include the creation of data storage devices, replacement of traditional semiconductor electronics, computer applications, gas sensing systems, as well as other applications. One of the transition metal complexes which still holds attention of many scientists and engineers since its discovery almost 100 years ago is the phthalocyanine compound. The structure of this molecule is chemically very stable, such that many substitute species with unique properties can be synthesised. Phthalocyanines have been the focus of active research owing to their biological tolerance, semiconducting properties, possibility of manipulation of the electronic and magnetic properties, and due to their high light absorptivity. In view of the high demand of this compound in many different fields a knowledge of phthalocyanines' thermal, electronic, and magnetic properties is essential. In this work the iron phthalocyanine (FePc) compound has been deposited on different surfaces as a model system to investigate the application of metal phthalocyanine molecules in electronic devices. This has been performed to study how the electronic and chemical properties can be tuned by depositing them on different types of supporting material. In my work I show that the FePc molecule may take different electron densities from the surface depending on the support. This electron density can be unevenly distributed in the molecule and is also accompanied by a distortion away from its perfectly planar molecular structure. In this molecular distortion, two opposing parts of the molecule curve slightly upwards, while the other orthogonal parts bend slightly downwards toward the surface, in a so called symmetry reduction or symmetry breaking configuration. In my work the spectroscopic fingerprint of this effect has been demonstrated for the first time. The presence of a slightly higher electron density at one atom of the molecule decreases the energy required to remove an electron from the deeper orbitals of the atom, which is known as its binding energy. The presence of different amount of electron density on different atoms leads to the creation of several different binding energies and, consequently, the identification of a broader overall spectral line for the molecule. In addition, the response to heating of the FePc molecules on the surface was also investigated. These studies were motivated by the preparation of molecular optoelectronic devices, where the evaporation of unwanted additional layers of molecules has been observed previously. Therefore, it is important to know how temperature influences the properties of the remaining single layer of molecules. My research shows that the increase of the temperature of the surface to the evaporation temperatures of FePc leads to irreversible processes during which most of the molecules merge together. There is also some evidence that indicates that the molecules shorten the bond distance with the surface after this heat treatment process. In this work FePc molecules are investigated not only from the scope of application in molecular electronics, but also from the perspective of gaining additional fundamental knowledge. The structural and electronic properties of the FePc molecules are very similar to that in active part of haem, the protein in the blood of all vertebrates responsible for activating, storage, and transfer of molecular oxygen. In this research, the FePc molecule is considered to mimic the active properties of haem. During my experiments I observed that the reactivity of the compound with respect to molecular oxygen changes depending on the nature of the surface the molecules are adsorbed on. My investigations can potentially help to create a better understanding of the mechanism of the biologically important haem-oxygen interaction. The final part of this thesis is aimed at understanding the role of transition metal complexes in catalysis, a work of huge importance to both industry and ecology. Catalysts function by speeding up selected chemical reactions and thus lead to the increased production of a desired product. Catalysts function by either triggering reactions, or by making reactions occur at faster times and with less of energy consumption. Heterogenisation of transition metal catalysts studied here is advantageous for several reasons. Firstly, because they are supported on a surface, they are not consumed during a reaction. Secondly, only a small amount of the material is required to act as a catalysis. Thirdly, they are reusable and therefore cheap and ecological to use. Since supported transition metal complexes show very promising results in catalytic science, I have also investigated the properties of organometallic complexes with palladium as an active centre. The ultimate achievement of this thesis is its contribution to the understanding of the precise mechanisms of molecular manipulation, adsorption, temperature modifications, reaction sites and catalytic selectivity. The work presented here provides a solid foundation towards improvement in the development of smart design of catalytic materials and single-molecules electronic devices with better performance and lower cost.