Inverse Phase Transitions in Two Dimensions

相变在我们的日常生活中无处不在，它们支配着自然过程，是工业材料加工中不可或缺的一部分。通常，固体材料在加热时融化，而液晶材料在冷却时结晶。早在1903年，当时的哥廷根教授古斯塔夫·塔曼就推测了相反的过程，即材料在冷却时融化。即便如此，古斯塔夫·塔曼本人也没有为这些材料找到证据，他的想法后来得到了证实。然而，到目前为止，逆转变的例子仅限于少数几个系统。有趣的是，吸附在表面上的分子已经显示出反向跃迁。我们最近证明了四醋酸钼吸附在铜(111)表面上，在室温下呈现出从有序结构到低温流动相的反向转变。理解这种逆转变的关键在于分子采用不同的吸附几何构型，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，对于这种二维系统中的逆相变的许多细节还知之甚少。例如，到目前为止，官能团在分子中的确切作用尚不清楚。在这个项目中，我们想要阐明表面吸附分子的二维系统中逆相变背后的物理化学原理。从已知的四醋酸钼在铜上的体系出发，我们希望在理解反向跃迁背后的基本驱动力的基础上，进一步识别二维体系。为此，我们希望在超高真空中使用扫描力显微镜研究各种分子在表面上的结构形成随覆盖率和温度的变化。该项目的目标将是获得关于哪些系统将显示反向过渡的预测能力。此外，我们的目标是通过调整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.