Adsorption Dynamics and Bonding Analysis of Organic Molecules on Silicon(001) Surfaces
In this thesis, the adsorption of ethylene, tetrahydrofuran (THF), cyclooctyne and 5-Ethoxymethyl-5-methylcyclooctyne (EMC) on Si(001) surfaces is studied using computational methods. While ethylene and THF act as model systems that allow to understand how unsaturated carbon-carbon bonds and ether g...
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|In this thesis, the adsorption of ethylene, tetrahydrofuran (THF), cyclooctyne and 5-Ethoxymethyl-5-methylcyclooctyne (EMC) on Si(001) surfaces is studied using computational methods. While ethylene and THF act as model systems that allow to understand how unsaturated carbon-carbon bonds and ether groups interact with these surfaces, cyclooctyne and EMC are potential candidates for the formation of organic/semiconductor interfaces and therefore more application-oriented. The thesis is focusing on two aspects of adsorption: Bonding analysis and adsorption dynamics. In bonding analysis, periodic Energy Decomposition Analysis (pEDA), which allows to understand the formation of chemical bonds betwen molecule and surface, was applied. The reaction dynamics was simulated using two approaches: Statistical thermodynamics, which can be applied if thermodynamic equilibrium is achieved, and explicitly calculating the evolution of the system over time using ab initio molecular dynamics (AIMD).
For ethylene, the results show that a dative bond between the carbon-carbon double bond and an empty orbital at a surface atom forms in the weakly bound intermediate state. In contrast to physisorbed intermediates on metal surfaces, this state is not mobile. Additionally, the influence of surface pre-coverage by atoms and molecules on the reactivity of ethylene is investigated in a second study.
The study of THF reveals that the adsorption mirrors the acid-catalyzed cleavage of ethers in solution and that the mechanism is equivalent to a concerted nucleophilic substitution.
For cyclooctyne, it is explained why the formation of two molecule-surface bonds stabilizes the system far more than the formation of four such bonds. Ring strain and enhanced dispersion interactions due to the size of the molecule lead to additional stabilization in comparison to linear alkynes like acetylene. In contrast to alkenes, cyclooctyne can adsorb either directly into the final state or via a short lived transient state. However, the lifetime of this transient state is so low that isolation at usual experimental conditions is not possible.
The conclusive study of EMC shows that the molecule bonds selectively via the strained triple bond and therefore confirms its suitability as a building block for organic/semiconductor interfaces. The ether group does not affect the reactivity and adsorption dynamics of the triple bond and cyclooctyne results can be transferred to this part of the molecule. The reactivity of the ether group is influenced by the sterically demanding residue, however, adsorption of this group is highly unlikely.
Overall, the studies in this thesis show that the application of chemical concepts and methods can bring in valuable contributions to the field of surface science. The pEDA in particular allows to describe the bonding between molecule and surface both qualitatively and quantitatively, and therefore enables an understanding of the relative energies between different adsorption structures. Furthermore, the investigation of the dynamics allows to predict how the system evolves on different time scales and which structures form preferably. The approaches presented in this thesis can most likely be transferred to other systems as well (e.g. adsorption on metal surfaces) and allow to deliver new insight into different fields of research in surface science and material science.