N-HETEROAROMATIC ACID ADLAYER STRUCTURES UNDER HYDROGEN BONDING INFLUENCE:

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2007-02-20

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In this dissertation, I investigate the adsorption and growth of thin films of N-heteroaromatic acids and related molecules on Ag(111) surface. The N-heteroaromatic acids have ring-nitrogen and carboxylic groups. Hence, two primary hydrogen bond motifs are observed in their bulk crystal structures: head-to-tail hydrogen bond and tail-to-tail hydrogen bond. Furthermore, the high electron density of the aromatic rings increases the complexity of their electrostatic properties. The surface structure thus reflects the delicate balance between the intermolecular interactions and adsorbate-substrate interactions.

The surface adsorption model of isonicotinic acid (INA) is suggested based upon the STM and XPS study. The monolayer structure is stabilized by the head-to-tail hydrogen bonds along   direction and weak side link hydrogen bonds. 12 rotational/reflection domains c0-exist on the surface. Using reflection absorption infrared spectroscopy (RAIRS), the layer-dependent orientation of the isonicotinic acid is determined. In the first monolayer, INA molecules are qseudo-parallel to the metal surface. The second and third layers possesses a combined twist (  ~ 20°) and tilt ( ~ 10°) orientation. This incline angle is contributed to the slight distortion ( 0.2%) along side link hydrogen bond direction upon the registration to the substrate. 

With two additional aromatic rings, 9-acridine carboxylic acid (ACA) possesses a greater quadrupole. The surface structure is thus affected. The main interaction is still head-to-tail hydrogen bonding. However, electrostatic interactions are the main interactions to connect the head-to-tail chains. RAIR spectra reveal that at low coverage, the mobile species are head-to-tail linked. A proposed orientation of the monolayer films are given based upon the characteristic vibrational frequencies. The vibrational spectra of Anthroic acid (AA) is also done for the comparison reason. The orientation difference between the ACA monolayer and the AA monolayer are contribute to their electronic properties.

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