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Research

“Computational chemistry for Functional Materials”


The research idea of the group builds around the approach to use bonding and reactivity concepts from the world of molecular chemistry to solve material sciences challenges. Quantitative approaches based on electronic structure theory from quantum chemistry and solid state theory are the methodological focus. This approach enables a fundamental understanding of elementary structural and electronic phenomena via explanation of experimental observations (interpretation) and proposal of new experiments (prediction). Long-term goal is the development and improvement of materials with desired properties (functional materials) in a feedback loop with experimental groups in chemistry, physics and material sciences.

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.

Recent research topics

A) Chemical bonding in periodic systems

  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).[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.[54, 97]

Scheme of the pEDA



B) Inorganic functionalization of semiconductor surfaces

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[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[55] were used to predict modified precursors with tailored decomposition barriers[56] and to investigate decomposition channels for further compounds.[57, 64]

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


C) Organic functionalization of surfaces

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.[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).[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.[23]

 
Präkursor-Komplex
SN2-artige Ringöffnung an der Oberfläche

D) Internal interfaces

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.[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.[43, 60, 75] This also allows investigation of unusual molecule-surface interactions.[93]

E) Bonding and reactivity in molecular systems

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,[48, 57, 64, 86] together with further work on reaction mechanisms in organic and inorganic chemistry.[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.[58, 58] More recently, these activities are extended toward light adsorption and photochemical reactions.[94, 95]

E) Spectroscopy and bonding in solids

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.[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.[50]

Methods

The majority of the work uses density functional theory (DFT)-based methods. We rigorously test the different functional classes for the respective research questions (generalized gradient approximation; hybrid; range-separated hybrid, etc.). Another focus is the correct description of dispersion interactions, which is mostly carried out with an accurate, benchmarked semiempirical scheme (DFT-D3). Investigations of molecular systems further allow the use of advanced wavefunction-based methods like MP2 and CCSD(T).

Analysis of the electronic structure is carried out with a wide range of methods, e.g. molecular orbital analysis, partial charges, topological analysis of the electron density and as a core topic of the group: energy decomposition analysis in the conventional molecular implementation (EDA) as well as our extended version for extended systems (pEDA).

Recent activities complement the static calculations with ab initio molecular dynamics (AIMD) approaches, calculation of phonon dispersion and reaction kinetics at surfaces. For example, with AIMD we could derive important findings for the adsorption dynamics of complex adsorbates on surfaces and lifetime of intermediate states.[80, 85, 91]

In collaboration with the Koch group (Marburg), we combined ground-state DFT calculations with many-body semiconductor theory to explain and predict optical properties of multinary semiconductor materials.[83, 90]

Zuletzt aktualisiert: 18.03.2019 · rosenow4

 
 
 
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