Temperature programmed studies of molecular assemblies on gold

Sammanfattning: This thesis investigates the properties of ultra-thin layers of organic molecules deposited at or adsorbed onto thin films of gold. The two main experimental techniques used in this thesis work are Temperature Programmed Desorption-Infrared Reflection Absorption Spectroscopy (TPD-IRAS), an excellent technique to probe the structure and orientation of molecular layers on metallic substrates, and Temperature Programmed Desorption-Mass Spectroscopy (TPD-MS), a very useful tool to study the energetics of molecular layers and adsorbates on solid surfaces. The two techniques are especially powerful when used simultaneously because they can be used to 1) follow molecular rearrangement phenomena occurring prior and during desorption; 2) find correlations between such rearrangement phenomena and the ultimate binding strength of the molecules/adsorbates to the surface.The TPD-IRAS and TPD-MS techniques have been used to study: I) the structure and desorption dynamics of DNA bases on gold; II) the phase behavior of oligo(ethylene glycol) (OEG)-terminated selfassembled monolayers (SAMs) on gold; and III) the nucleation, growth and structure of D2O-ice on SAMsat liquid nitrogen temperatures in ultra high vacuum.The first part investigates the orientation, structure and binding strength of DNA bases spontaneously adsorbed to gold. The four DNA bases interact very differently on and with the gold surface. Guanine and adenine interact strongly with the surface, and displays a series of complex structural transitions during the desorption event. Adenine form strong bonds with the gold surface, whereas cohesive interactions seem to dominate for guanine. Cytosine and thymine display a less complicated desorption behavior, and the corresponding desorption energies are lower/ much lower than those observed for adenine and guanine. These results are in qualitative agreement with recent studies of the activity of immobilized oligonucleotides on gold nanoparticles.The second part is a study of SAMs of thiolated molecules on gold exposing OEG-tails in different conformations, all trans and helical, toward the ambient. The temperature dependence is investigated using TPD-IRAS, and it is found that the helical OEGs undergo a reversible phase transition into the all trans oramorphous states at approximately 60 °C, depending on chemical groups used to attach the OEG tail tothe alkyl thiol portion.The third part focuses on ultra-thin ad-layers of D2O deposited onto OEG substrates at low temperature in UHV. Extensive simulations of RA spectra of the D2O-ice overlayers are performed using Maxwell Garnett effective medium theory to support the interpretation of the experimental data. These simulations reveal that the ice overlayers contain a significant and varying volume fraction of voids. Isothermal annealing ofthe ice overlayers shows that the kinetics of the amorphous to crystalline phase transition of ice, normally observed at about 140 K, is strongly dependent on the conformation of the OEG layer. The kinetics is fast on helical OEG SAMs most likely because of the existence of specific nucleation sites that governs the crystalline formation of ice.The binding strength and structure of D2O deposited to biomimetic phosphate SAMs with H+, Na+ and Ca2+ as counter-ions are also investigated. D2O is tightly bound to Ca2+- and Na+- phosphate SAMs, affecting several ad-layers of D2O. These results may have implication for the chemistry occurring at biomineral surfaces, and specifically for the role of water on the nucleation and growth of hydroxyapatite, the inorganic component in bone.