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，并构成了工业材料加工中不可或缺的一部分。通常，固体材料在加热时熔化，液体在冷却时结晶。早在1903年，古斯塔夫·塔曼，当时在哥廷根教授，推测逆过程，即熔化的材料冷却。即便如此，古斯塔夫·塔曼本人也没有找到这些材料的证据，他的想法后来得到了证实。然而，到目前为止，逆跃迁的例子仅限于少数系统。有趣的是，吸附在表面上的分子已被证明表现出逆跃迁。我们最近发现，吸附在Cu（111）上的四乙酸二钼在室温下呈现出有序结构，在低温下呈现出移动的相。理解这一逆跃迁的关键在于分子采用具有不同结合强度和内部自由度的不同吸附几何构型。特别是，有序结构必须表现出高度的内部自由度，然而，在这种二维系统中的逆相变的许多细节是知之甚少。例如，到目前为止，分子中官能团的确切作用仍然不清楚。在这个项目中，我们要阐明的物理化学原理背后的逆相变在二维系统的吸附分子的表面。从已知的铜上的四乙酸二钼系统开始，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识别二维系统。为此，我们想研究的结构形成的各种分子表面上的覆盖和温度的函数，使用扫描力显微镜在真空中。该项目的目标将是获得预测能力，以确定哪些系统将显示反向过渡。此外，我们的目标是定制的方式，可以通过调整分子的结合强度和内部自由度的转变温度的系统。

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.