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knowledge of the atomic-scale processes involved in the nucleation and growth at electrode surfaces and their dependence on electrolyte composition, electrode potential, and substrate structure is required.

Studies of mechanistic aspects of surface processes have benefited strongly from the advent of scanning tunneling microscopy (STM) and atomic force microscopy (AFM). By observing local changes in the atomic structure and the surface morphology the individual atomic processes could often be deduced in these experiments. From the rapidly increasing number of results obtained by these methods under well-defined conditions, in particular under ultrahigh vacuum (UHV), a detailed view on the microscopic aspects of phase transitions in adsorbed layers and of thin film growth is emerging [1], In a similar way, in-situ STM and AFM studies are well suited to unravel phase formation processes in an electrochemical environment on an atomic scale. The electrochemical interface offers several advantages over surfaces in vacuum for the study of these phenomena. First, most processes can be performed under or close to thermodynamic equilibrium and secondly they can be easily controlled by the applied potential. Due to this control, in-situ electrochemical STM experiments are ideally suited for direct, time-resolved observations of growth processes, as will be shown below.

In this paper recent results of our in-situ STM studies on the structure of bare and adsorbate-covered electrode surfaces are summarized. In particular, we discuss transitions between different phases on these surfaces, which often proceed via nucleation and growth processes. This includes structural transitions in the electrode surface layer, phase transitions in adsórbate layers, electrodeposition processes, and dynamical fluctuations at the metal-electrolyte interface under equilibrium. We show that in-situ STM provides a valuable tool for time-resolved, atomic-scale studies of such processes. For experimental details and for in-depth discussions the reader is referred to the original literature.

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