Inverse Phase Transitions in Two Dimensions

相变在我们的日常生活中无处不在，它们控制着自然过程，构成了工业材料加工中不可或缺的一部分。通常，固体材料加热时熔化，液体冷却时结晶。早在1903年，古斯塔夫·塔曼（Gustav Tammann），当时的Göttingen教授，就推测了相反的过程，即材料在冷却时熔化。尽管古斯塔夫·塔曼本人并没有发现这些材料的证据，但他的想法后来得到了证实。然而，到目前为止，逆转换的例子仅限于少数系统。有趣的是，吸附在表面上的分子表现出逆转变。我们最近可以证明，吸附在Cu（111）上的四乙酸二钼在室温下表现出从有序结构到低温下流动相的反向转变。理解这种逆转变的关键在于分子采用不同的吸附几何形状，具有不同的结合强度和内部自由度。特别是，有序结构必须表现出高度的内部自由度。然而，在这样的二维系统中，许多逆相变的细节还知之甚少。例如，到目前为止，分子中官能团的确切作用仍不清楚。在这个项目中，我们想要阐明在表面上吸附分子的二维系统中逆相变背后的物理化学原理。从已知的四乙酸二钼在铜上的体系开始，我们希望在理解逆跃迁背后的基本驱动力的基础上，进一步确定二维体系。为此，我们希望利用扫描力显微镜在超高真空条件下研究各种分子在表面上的结构形成与覆盖和温度的关系。该项目的目标将是获得预测能力，哪些系统将显示一个反向过渡。此外，我们的目标是通过调节分子的结合强度和内部自由度来调整系统的转变温度。

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.