Tuning the cooperativity in spin-state switching of molecules in contact to surfaces

由于可以通过外部手段打开和关闭其分子磁矩，自旋交叉（SCO）分子是基于分子的自旋电子学中可编程构建块的非常有趣的候选者。然而，为了达到这个目的，分子需要被固定和接触，这意味着它们是由固体表面支撑的。相应地，表面上的自旋交叉分子的单和亚单分子层已经通过气相沉积等方法制备，近年来，我们的团队和其他人可以在这些系统中检测到热和光诱导的自旋态开关。然而，通常伴随着SCO分子在固体中的自旋转变的协同性在大多数情况下是不存在的。为了提高自旋交叉分子在表面上的开关性能，重建这些系统的协同性是非常必要的。这一目标涉及到关于协同性有效的最终尺度限制的一般问题，在目前的建议中，通过将单核自旋交叉分子逐步组装成二聚体、三聚体和（最终）无限链以及这些系统在表面上的沉积和表征来解决。对于多聚体，会产生分子内的协同作用，这是对表面吸附分子的研究。链状聚合物自旋交叉化合物在某种程度上已知在固态中表现出高度合作的自旋转变，我们想研究当在表面沉积这些聚合物的单层时是否可以保留这些特性。为此，一系列链状自旋交叉化合物将被合成并研究其在表面上的沉积能力。不同的技术，如在真空中从固体或溶液中沉积，以及在环境压力下从溶液中浸镀或旋转涂层，可能随后采用表面耦合策略，研究了产生这种系统的方法。此外，将探讨水平和垂直沉积几何形状。用原子力显微镜和扫描隧道显微镜检查了表面上存在的（可能有序的）聚合物自旋交叉化合物单层。利用x射线吸收和x射线光电子能谱研究了这些体系在热和光致自旋跃迁中的协同性。从热自旋跃迁的陡度以及对低温下光诱导自旋开关后的热反弛豫行为的详细分析来评估协同性。我们期望获得关于在固体表面上支撑的分子的协同自旋开关的因素的全面知识。

Due to the possibility to switch their molecular magnetic moment on and off by external means, spin-crossover (SCO) molecules are highly interesting candidates for programmable building blocks in molecule-based spin electronics. For that purpose, however, the molecules need to be immobilized and contacted, which implies that they are supported by a solid surface. Correspondingly, mono- and submonolayers of spin-crossover molecules on surfaces have been prepared by, e.g., deposition from the gas phase, and thermal and light-induced spin-state switching could be detected for these systems in recent years by our groups and others. However, the cooperativity, which usually accompanies spin transitions of SCO molecules in solids, is in most cases absent. In order to improve the switching properties of spin-crossover molecules on surfaces, it would be highly desirable to re-establish cooperativity for these systems. This goal, which relates to the general question regarding the ultimate scale limit at which cooperativity becomes effective, is addressed in the present proposal by the stepwise assembly of mononuclear spin-crossover molecules to dimers, trimers, and (ultimately) infinite chains and the deposition as well as characterization of these systems on surfaces. For the multimers, intramolecular cooperativity will emerge, which is studied for the surface-adsorbed molecules. Chain-like, polymeric spin-crossover compounds are in part known to exhibit highly cooperative spin transitions in the solid state, and we want to investigate whether these properties can be retained when depositing monolayers of these polymers on surfaces. To this end, a range of chain-like spin-crossover compounds will be synthesized and investigated regarding their deposition capability on surfaces. Different techniques such as deposition from solids or solution in vacuum as well as by dip or spin coating from solution under ambient pressure, possibly followed by on-surface coupling strategies, are examined to generate such systems. Moreover, horizontal as well as vertical deposition geometries will be explored. The presence of (potentially ordered) monolayers of polymeric spin-crossover compounds on surfaces is examined by atomic force microscopy and scanning tunneling microscopy. The cooperativity in the thermal and light-induced spin transitions of these systems is investigated by x-ray absorption and x-ray photoelectron spectroscopy. The cooperativity is evaluated from the steepness of the thermal spin transition as well as from the detailed analysis of the thermal back-relaxation behavior after an optically induced spin switching at low temperatures. We expect to gain comprehensive knowledge about the factors establishing cooperative spin switching of molecules supported on solid surfaces.