The influence of functional groups on the molecular self-assembly on calcite
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Abstract
In this thesis, I report on the molecular self-assembly of benzoic acid derivatives on calcite (10.4) at room temperature studied by noncontact-atomic force microscopy (nc-AFM) in ultra-high vacuum. The results of this thesis reveal the manifold influences of slight adjustments at the molecular building block on the structure formation on an insulating substrate. A comparatively simple molecule-substrate system is presented, which, nevertheless, allows tuning the molecular self-assembly from mobile or metastable arrangements over stable two-dimensional patterns to wire-like structures with and without mesoscopic order.
As starting molecule, I chose dihydroxybenzoic acid (DHBA).
The different substitution positions of the hydroxyl groups for the six constitutional isomers of DHBA are found to result in six different structures on the surface. I discuss how the substitution positions affect the structure formation in three distinct aspects, namely the deprotonation tendency of the DHBA molecule, the intermolecular interaction and the molecule-surface matching. Positioning the hydroxyl groups closely to the carboxyl group favors deprotonation and, thereby, influences the type of molecule-substrate interaction. The distribution of the hydroxyl groups around the benzene core determines how many opportunities for intermolecular interaction are available. The specific position of each functional group is crucial for the match with the calcite surface.
The influence of the number of functional groups at the molecule is analyzed by comparing the assembly of 3,5-DHBA and 3-HBA. Both molecules form extended stripes with the same adsorption pattern in nc-AFM images. Removing one hydroxyl group, however, changes (a) the mobility of the molecules and (b) introduces a mesoscopic ordering: 3-HBA stripes arrange equidistantly with coverage-dependent distance.
The influence of the type of the hydrogen donating groups on the hydrogen-bonded networks is investigated by switching between hydroxyl and amino substituted molecules. Hydroxyl and amino groups at the benzene core are found to participate in a similar way to the structure formation. A greater variation is achieved by switching between benzoic acids and benzamides, in which case I observe a transition from one-dimensional to two-dimensional growth. In all cases, the hydrogen-bonded networks are more stable with oxygen containing groups than with nitrogen containing groups, following the trend of stronger electronegativity. Quite plausible, the influence of changing a functional group is larger with smaller number of interacting groups.
Finally, I explore 2,5-DHBA on calcite (10.4) as a model system to study the long-range distance dependence of the Kelvin probe force microscopy signal caused by monopole (deprotonated 2,5-DHBA) and dipole (protonated 2,5-DHBA) arrangements on an insulating substrate. Two-dimensional maps of Kelvin probe force curves reveal a long-range vertical and lateral contrast which is distinctly different above the different molecular islands. I further demonstrate how subtraction of the calcite background effectively cancels the effect of the probe tip. Thus, background subtraction allows studying the signal caused by the monopole and dipole arrangements adsorbed on calcite quantitatively.