(217f) Chemically Titrating Nucleation Sites On Oxide Surfaces with Fluorescent Probes

Metal and semiconductor particle growth and metal film growth on oxide surfaces, which are integral to many catalytic and microelectronic applications, are likely associated with defect sites that could include strained siloxane bridges or surface oxygen vacancies.  Planar surfaces are ideal for studying such processes.  However, identification of chemically distinct surface sites, which may play an important role in the early stages of nucleation, on low surface area oxides is challenging.  We have developed a method in which SiO₂ surfaces are prepared in an ultra-high vacuum environment and specific surface sites are subsequently titrated with fluorescent probe molecules based around derivatives of perylene, such as perylene-3-methanol and perylene-3-methanamine.  This strategy is widely applicable to the study of surface chemistry of many oxide systems.  In situ laser-induced fluorescence measurements provide information on the arrangement of probe molecules on the surface, based on both the position and intensity of the emission peaks.  Ex situ chemical removal of the probe molecules in solution enables the direct determination of surface site density.  Through the use of a high quantum yield fluorophore, a detection limit below 0.001 monolayers is achieved.  Specificity is achieved by selecting appropriate functional groups on the probe molecule, such as alcohol, amine, or vinyl groups, to react with the anticipated sites.  Using this approach, the influence of different processing steps on the densities of surface hydroxyl groups and defect sites on SiO₂ can be explored.  Correlations between the densities of various surface sites and nuclei densities during the early stages of film growth will be reported.