STM in liquids. A scanning tunneling microscopy exploration of the liquid-solid interface.

This thesis reports of a series of atomic scale studies of the liquid-solid interface, carried out with a home-built liquid-cell Scanning Tunnelling Microscope (STM). The home-built liquid-cell STM is described in detail, and numerical simulations are performed to show that surfaces immersed in the liquid-cell STM can be equally clean and well-defined as surfaces in ultra-high vacuum. First a model system is studied: the gold-alkane interface (Au(111)-n-tetradecane). It is shown that alkanes from the liquid phase can form highly ordered monolayers at this interface, independent of the crystal structure of the gold surface. Next, by introducing low concentrations of porphyrins, it is demonstrated that highly ordered monolayers of porphyrins can be grown at the gold-alkane interface. Long double rows of porphyrins can be grown, templated by the initially present alkane monolayer. By performing Scanning Tunnelling Spectroscopy measurements, the Shockly-like electronic surface state, present on a clean gold surface in ultra-high vacuum, is shown to be present at the gold-alkane interface as well. The liquid-cell STM can be used as a nano-scale milling tool, which is shown by etching holes in a bismuth surface immersed in liquid alkanes. Finally, the liquid-cell STM is used to study a multi-step catalytic reaction at the liquid-solid interface, in real-time. The STM's unique ability to study each step of a chemical reaction on the single molecule level, yielded evidence that two neighbouring manganese porphyrins, adsorbed to a gold surface, can induce the homolytic dissociation of molecular oxygen, each binding one oxygen atom, thus forming reactive manganese-oxygen species which can act as a heterogeneous catalyst for epoxidation reactions. The activity of the catalyst was verified by performing an epoxidation reaction in the liquid-cell STM, while using the STM to continuously monitor the distribution of activated and inactive catalysts on the interface