Analysis of Adsorbates and Interfacial Forces at Metal Oxide Interfaces at Defined Environmental Conditions

This work addresses the investigation of single particle contact forces and molecular forces on the basis of a detailed understanding of the adsorbate-particle interfacial structure. Oxide surfaces are typically hydroxylated under ambient conditions and exhibit adsorbate layers consisting of mono- and multilayers of water. Moreover, most often organic molecules spontaneously adsorb onto these high-energy surfaces leading thus to a reduction in the surface energy. Such organic adsorbates play a major role for the contact forces at small distances. TiO₂ and Al₂O₃ oxide surfaces served as reference oxide materials for the fundamental studies. AFM-based nanoshaving allowed for the analysis of such omnipresent molecular layers. Complementary to this approach, well-defined surfaces with controlled adsorbate chemistry, such as those obtained upon ultrahigh vacuum conditions, provided the basis for a detailed understanding of the measured contact forces. Thereby, the enlightenment of the role played by the molecular surface chemistry on contact forces between particles was carried out by means of the spectroscopic and microscopic analysis of controlled model adsorbates. Such monomolecular adsorbates were formed onto single-crystalline oxide surfaces under conditions of ultra-high vacuum and defined atmospheres. The spectroscopic surface analysis of the adsorbate structure was combined with AFM-based contact force-distance curve measurements to achieve a reliable correlation between measured forces and the given interface chemistry. In addition, single molecule force studies and PM-IRRAS spectroscopy in the presence of high water activities promote the understanding of molecular adsorbates on oxide surfaces under ambient conditions.