Diffusion and structure formation of molecules on Calcite(104)
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Abstract
The arrangement of molecules is of utmost importance for the properties of a material, as apparent, for example, in biominerals. Therefore, it is important to investigate how molecules arrange themselves, a process that is referred to as molecular self-assembly.
In this thesis, the self-assembly of several molecules on the insulating calcite(104) surface in ultra-high vacuum is examined using frequency-modulation atomic force microscopy (FM-AFM). The calcite(104) surface is of special interest, for instance due to its relevance for biomineralisation.
The main goal of this thesis is to understand molecular self-assembly at a fundamental level by studying the diffusion process of molecules. The ability of the molecules to move on the surface is essential for self-assembly, because immobile molecules cannot form ordered structures.
For the examination of the diffusion process, short image series of diffusing 2,5-dihydroxybenzoic acid molecules were acquired at low temperatures. A post-processing procedure to extract, among other things, the diffusion jump length and diffusion directions from these data is presented. Surprisingly, these molecules do not diffuse along the main symmetry directions of calcite(104), but along directions diagonal to the main symmetry directions instead.
In this context, the influence of different functional groups on the diffusion of terephthalic acid and two derivatives, namely aminoterephthalic acid and bromoterephthalic acid, is studied as well. The diffusion barrier is estimated in this case by identifying the temperature at which self-assembly takes place. It is found that the diffusion barrier is significantly higher for aminoterephthalic acid than for terephthalic acid and bromoterephthalic acid. For aminoterephthalic acid a detailed analysis of the diffusion and self-assembly process is shown in addition. This analysis includes direct imaging of all processes important for self-assembly: formation and dissociation of unstable clusters, nucleation and growth.
Another goal of this thesis is to learn more about the structure formation. The structures formed by molecules depend not only on temperature, but also on coverage.
Therefore, measurements with increasing coverage are performed in this thesis for terephthalic acid and 2,5-dihydroxybenzoic acid to characterise these systems. It is disclosed that 2,5-dihydroxybenzoic acid forms ordered structures only upon a certain coverage at room temperature. For terephthalic acid existing measurements are completed by identifying, for instance, a dense structure observed at high coverages as upright standing molecules.
In addition to the above-mentioned systems, the formation of the first water layer on a cooled calcite(104) surface is investigated with measurements at different coverages. It is discovered that water forms a (2x1) superstructure at low coverages and a (1x1) superstructure at a full monolayer coverage. These measurements represent the first direct imaging of water at sub-monolayer coverage on calcite(104) at molecular resolution.