enlarge the image: Concept of research, Figure: Ralf Tonner-Zech
Concept of research, Figure: Ralf Tonner-Zech

The unique feature of this approach is the local, chemical view on elementary processes at surfaces, interfaces or in bulk materials with the aid of electronic structure theory in contrast to the band-like views inspired by physics, which is often taken in the field. The view on electronic properties is taken quantitatively with several quantum chemical methods for computation and analysis, going beyond the mostly qualitative approaches. An important aspect is to generalize findings from specific systems investigated to derive models and trends, e.g. for typical reaction channels or bonding features. Model systems as well as actually experimentally realized systems are investigated with this approach.

Our Research Projects

Chemical bonding is a powerful heuristic concept mostly used in molecular chemistry. Although some concepts have been transferred to surface and material sciences, the predictive power of chemical bonding investigations has not been explored to a large extent. Here, we extended a method which is well-known from molecular chemistry to extended systems, enabling for example the analysis of surface-adsorbate complexes (pEDA).[Paper 54] This method allows quantitative analysis of chemical bonding, understanding of reactivity and prediction of trends. In combination with other methods of bonding analysis and solid state theory, a comprehensive picture for extended systems is reached.

The method can be used for different substrates (insulators, semiconductors, metals) and all molecular adsorbates. [Paper 54, 97]

enlarge the image: peda, Grafik: Ralf Tonner-Zech
peda, Grafik: Ralf Tonner-Zech

Metal organic vapour phase epitaxy (MOVPE) is a powerful experimental method to produce thin films of materials. The elementary reaction steps in this process are far from being understood and progress is mainly obtained by experience and trial and error. We aim at a description of all relevant steps in MOVPE with a hierarchy of quantum chemical methods (density functional theory, ab initio molecular dynamics, kinetic Monte Carlo) to derive microscopic insights for the crucial nucleation phase of the deposition process. For the model material gallium phosphide, the gas phase processes are understood [Paper 48] and provided new insights regarding the importance of competing decomposition channels. A detailed understanding of the crucial mechanism of β-hydrogen elimination for group 15 precursor molecules [Paper 55] were used to predict modified precursors with tailored decomposition barriers [Paper 56] and to investigate decomposition channels for further compounds.[Paper 57, 64]

In collaboration with experimental investigations from physics and material sciences we could explain the highly unexpected formation of pyramidal structures in GaP. [Paper 62] Our prediction that tert-butylphosphine will arrive essentially intact at the surface could subsequently be confirmed experimentally. [Paper 69] The understanding for chemically driven growth processes can be easily transferred to other techniques like atomic layer deposition (ALD) and other material classes.

enlarge the image: Movep-Large, Figure: Ralf Tonner-Zech
Movep-Large, Figure: Ralf Tonner-Zech

Organic functionalization of semiconductor surfaces allows to tailor the properties of the substrate by using the vast possibilities of organic chemistry to modify the adsorbate molecules. This can enable applications like sensors integrated on surfaces. We intensely investigated functionalization of silicon surfaces with organic molecules.[Paper 97] Considering kinetic and thermodynamic properties of model adsorbates (e.g. ethene, ethyne, tetrahydrofurane) as well as adsorbates which can further be functionalized toward internal interfaces (cyclooctyne and derivatives). [Paper 79, 84]

The interation of amino acids with metal-oxide surfaces (TiO2) is relevant for questions of biocompatibility and provides another opportunity to study organic functionalization. [Paper 23]

enlarge the image: Organic functionalization of surfaces, Figure: Ralf Tonner-Zech
Organic functionalization of surfaces, Figure: Ralf Tonner-Zech

In the next step, these organic layers on surfaces can be extended to internal interfaces. This requires a theoretical description of chemical reactions directly at the surface and subsequently for the interface formation reactions as well as prediction of spectroscopic characteristics of the resulting interfaces.

Organic molecules interacting with metal substrates also form internal interfaces with interesting structural and electronic properties. The partial occupation of unoccupied molecular states upon adsorption leads to interesting effects like the interfacial dynamical charge transfer. [Paper 68] The spectroscopic (IR, 2PPE) and quantum chemical investigation of the resulting electronic states at the interface provide a valuable approach to learn more about the interface properties. [Paper 43, 60, 75] This also allows investigation of unusual molecule-surface interactions.[Paper 93]

As outlined above, an in-depth understanding of molecular systems is mandatory for the transfer of concepts to the material sciences. The gas phase reactivity in the MOVPE process is one focus here,[Paper 48, 57, 64, 86] together with further work on reaction mechanisms in organic and inorganic chemistry.[Paper 25, 35, 72] Bonding analysis of transition metal-carbene complexes are a great opportunity for the exploration of e.g. the interplay of steric and electronic effects. [Paper 58] More recently, these activities are extended toward light adsorption and photochemical reactions.[Paper 94, 95]

Molecular crystals provide the possibility to study the interplay between strong intra-molecular and weak inter-molecular forces. This can lead to subtle effects that might even show up in high-resolution experimental measurements. [Paper 38]

Molecular clusters are an intermediate between molecules and bulk systems and lend themselves toward further analysis, e.g. to understand ligand bonding or the analysis of cluster to bulk transitions. [Paper 50]