Computational investigations of molecular assemblies on the Au (111) surface
Calderón, Camilo Emiliano
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The interaction potentials of small organic molecules with metallic surfaces is of key interest in the field of molecular rotors. A study is presented here where the motion of dimethyl sulfide upon the Au (111) surface was determined to be mainly precessional. This result calls into question force fields used in simulating the dynamics of molecules at metal surfaces. Additionally, the interaction of styrene with the Au (111) surface was studied, motivated by reports of ferroelectric transitions at 5 K present in assemblies of this molecule on the same surface. It is found that this molecule rotates about the ethylene moiety, and that there is essentially no enhancement of the lateral components of its dipole moment upon adsorption, lending computational support for the experimental observations. The electrodeposition of Ag on Au( 111) from chloride-containing electrolytes has resulted in surfaces whose atomic scale structure shows extraordinary resilience when exposed to ambient conditions and elevated temperatures. The plane-wave DFT approach used here suggested that this stability is due to the presence of a silver monolayer on the Au (111) surface, capped with a highly mobile CI- adlayer. The interatomic spacings found at the optimum coverage, 5/9 ML, produce a steric barrier that prevents attack by atmospheric oxidizers; at the same time, the high mobility of the chloride adlayer was found to be due to the presence of very low barriers to CI- adatom translocation. A related system, the expanded and metastable Au(111)-(4x4)-Ag structure, was investigated due to its ubiquity in the literature. The DFT calculations indicate that it is very likely that the surface is actually that of Ag/Au(111)-(4x4)-Cl, given the prevalence of chloride contaminants in typical laboratory environments. The last aspect of this thesis involves the development of a method for improving the convergence of free energy perturbation calculations. The adaptive force matching method is used for the development of a high quality force field which is then used for sampling conformational space. The static dielectric constant of ice-Ih was calculated with this method, and it was found that the PW91 functional underestimates this quantity by about 20%.
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