Inverse Phase Transitions in Two Dimensions

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Phase transitions are omnipresent in our everyday life, they govern natural processes and constitute an integral part in industrial material processing. Usually, a solid material melts upon heating and a liquid crystallizes upon cooling. Already in 1903, Gustav Tammann, at that time professor in Göttingen, speculated about the inverse process, namely melting of a material upon cooling. Even so Gustav Tammann himself did not found evidence for such materials, his ideas later received confirmation. So far, however, examples of inverse transitions are limited to few systems. Interestingly, molecules adsorbed onto surfaces have been shown to exhibit inverse transitions. We could recently demonstrate that dimolybdenum tetraacetate adsorbed on Cu(111) exhibits an inverse transition from an ordered structure at room temperature to a mobile phase at low temperatures. The key for understanding this inverse transition lies in the fact that the molecule adopts different adsorption geometries with different binding strengths and internal degrees of freedom. In particular, the ordered structure must exhibit a high degree of internal freedom.However, many details of inverse phase transitions in such two-dimensional systems are poorly understood. For example, the precise role of the functional groups at the molecule remains unclear so far. Within this project, we want to elucidate the physico-chemical principles behind inverse phase transitions in two-dimensional systems of adsorbed molecules on surfaces. Starting from the known system of dimolybdenum tetraacetate on copper, we want to identify further two-dimensional systems based on understanding the basic driving forces behind inverse transitions. To this end, we want to study the structure formation of various molecules on surfaces as a function of coverage and temperature using scanning force microscopy in ultrahigh vacuum. Goal of the project will be to gain predictive power as to which systems will show an inverse transition. Moreover, we aim for tailoring the system in a way that the transition temperature can be tuned by adjusting the binding strengths and internal degrees of freedom of the molecules.