Visualization of nucleation and growth of supramolecular networks on Cu(001) and Au(111)

Abstract

This thesis describes an in-situ study of the nucleation and growth behavior of two-dimensional supramolecular networks formed by 4,4'-biphenyldicarboxylic acid (BDA) molecules on metal crystal surfaces. The Cu(001) and the Au(111) surfaces were used as templates. The supramolecular networks offer great promise for the use as flexible templates for the fabrication of novel nanoscale structures. On both surfaces, the flat-lying BDA molecules form large two-dimensional islands in which the molecules organize in highly ordered networks. The nucleation and growth of the supramolecular networks was studied with Low Energy Electron Microscopy (LEEM). LEEM allows observing the nucleation and growth of the supramolecular networks in real time with a large field of view and at variable temperature. Simultaneously, the density of adsorbed molecules in a disordered, 2D gas like phase can be measured. This allowed to obtain a fundamental understanding of the thermodynamic driving forces behind the formation of the networks and the influence of the substrate and temperature on the growth. For example, by analyzing the 2D phase diagram of BDA on Cu(001), the 2D cohesive energy (0.35 eV) of the molecules in the networks was found. Another example is the observation of huge size fluctuations during nucleation of BDA domains on Cu(001), with critical nucleus sizes exceeding several hundred molecules. By optimizing the growth conditions (i.e., temperature and growth rate), single crystalline domains exceeding lateral dimensions of several micrometers could be grown. The influence of the substrate morphology on the growth of these large domains was studied. While the carboxylic acid groups of the molecules are deprotonated on Cu(001), they remain protonated on Au(111) at room temperature. Upon annealing the substrate, we can observe the (partial) deprotonation of the molecules, which results in a small rearrangement of the molecular structure, but a spectacular change of the shape of the 2D domains. The obtained results provide new insights in the formation processes and their decisive steps and also hints on how to improve the quality of the molecular networks constituted by benzoic acids on single crystalline surfaces.