Characterization of organic molecules at metal/electrolyte interfaces

Abstract

The spontaneous self-assembly of organic molecules is considered a promising “bottom-up” approach in nanotechnology to create surface patterns, molecular sensors, and molecular logic devices with nanometer dimension, and complex organic structures. Porphyrins, for example, have attracted much attention of surface electrochemists within the last few decades due to the wide range of potential applications of this class of molecules, for instance, as catalysts in the field of “green energy”, as devices in state-of-the-art electronics, as sensors and collectors or emitters of light etc. to name only a few. However, in particular, larger organic molecules tend to be thermally labile, these molecules are better deposited from solution. Furthermore, if these molecules are deposited electrochemically in the form of ions (from their solvable salts) the electrochemical potential is a further very useful control parameter which allows to influence the self-assembly process. Thus, the electrochemical solid-liquid interfaces is a good choice in order to study the 2D self-organization process of organic molecules.<br /> It is well-know that the solid/liquid interface plays a fundamental role in a diverse range of phenomena encountered in biological, chemical and physical processes, for example in many electrochemical, electrocatalytic and biological reactions. In the case of a metal in contact with an electrolyte, a lot of interesting processes can be carried out on the metal surface, such as adsorption/desorption of species from the electrolyte like anions and cations, e.g. organic ions, surface reactions like corrosion/passivation, deposition/growth of new compounds, etching/plating etc.. Unraveling the atomic structure at a solid/liquid interface and of processes occurring at this interface with atomic/molecular resolution is, therefore, one of the major challenges of today's surface science to investigate and to understand the elementary steps of these processes with model systems in order to apply them effectively in our daily life.<br /> In this dissertation has been explored the electrochemical deposition route to study the self-assembly of Tetra(N-methyl-4-pyridyl)-porphyrin molecules (TMPyP) and Tetra(4-trimethylammoniophenyl) porphyrin molecules (TTMAPP) on an iodide-modified Au(100) electrode surface by means of in-situ Electrochemical Scanning Tunneling Microscopy (EC-STM) with submolecular resolution. This enables unprecedented insight into such self-assembly phenomena at solid-liquid interfaces in the presence of anions and organic molecules as a function of electrode potential. Results of the investigations of TMPyP and TTMAPP molecules on an iodide-modified Au(100) surface in both cases show a long-range periodic superstructures beyond the molecular arrangement with phase transitions which is dependent on the substrate structure and applied electrode potential. The resultant structures, imaged with submolecular resolution by in situ STM, are clearly correlated with the redox state of the molecules as indicated by cyclic voltammetry. As a result, detailed structure models are derived and are discussed in terms of the prevailing interactions.