Institute of Physical and Theoretical Chemistry , University of Tübingen , Auf der Morgenstelle 18 , 72076 Tübingen , Germany.
Acc Chem Res. 2018 Mar 20;51(3):753-760. doi: 10.1021/acs.accounts.7b00612. Epub 2018 Feb 21.
In the last decade technology has brought significant changes to our lives, including new habits and a new view on social relationships. These technological innovations are based on several factors, one of which is miniaturization. This was made possible also due to the discovery and synthesis of new materials with characteristics at the nanoscale that are designed for specific purposes. This "on purpose" approach, joined to the development of preparation and growth methods, has led to use of thin films rather than bulk materials in devices. Using thin films makes devices easier to produce, and using films for coating protects the devices and gives specific properties to surfaces. For several decades thin films, surfaces, and interfaces have been intensively investigated. Indeed, device performances rely on the optimized match of thin films of different natures, such as organic and inorganic semiconductors and metals for contacts. Surprisingly, in comparison, little attention has been devoted to the deposition of organic radicals on a substrate. This might be because these materials are considered not stable enough for evaporation. In this work, we demonstrate that it is possible to evaporate and deposit organic radicals onto well-defined surfaces under controlled conditions, without degradation. Using soft X-ray spectroscopies, performed also at synchrotrons, we investigate thin film processes, surfaces, and interfaces at the nanoscale, when organic radicals are deposited on metal and metal oxide surfaces. We suggest how to design organic radicals bearing in mind the thermodynamic factors that govern thin film stability, with the purpose of obtaining not only a chemically stable radical, but also stable thin films. We investigate the thermal and air stability of the deposited films, and we explore the influence of the surface/radical chemical bond and the role of surface defects on the magnetic moment at the interface. We find that organic radicals are physisorbed and keep their magnetic moment on inert and passivated surfaces such as Au(111) and AlO(112̅0) single crystals, SiO, and ideal TiO(110) single crystals, while defective sites such as oxygen vacancies or the presence of OH groups lead to chemisorption of the organic radicals on the surface with quenching of their magnetic moment. Our work shows that the use of X-ray based techniques represents a powerful approach to reveal the mechanisms governing complex interfaces, such as radical/metal and radical/metal-oxide, where it is important to describe both charge and spin behavior (spinterfaces). It also makes it possible to conceive new experiments to investigate the magnetic character of the thin films versus their structural properties, toward tuning the arrangement of the molecules in films. Controlling the molecular arrangement will give the opportunity to tune the mutual position and orientation of the molecules, that is, of the single magnetic moments in the films, "imprinting" their magnetic properties. A deep understanding of stable radical/inorganic spinterfaces may open the way to use radicals in solid state devices or as quantum bits with dedicated configurations, as proposed for other molecular quantum bits, and in spin-based electronics.
在过去的十年中，科技给我们的生活带来了重大变化，包括新的习惯和对社会关系的新看法。这些技术创新基于几个因素，其中之一是微型化。这也是由于发现和合成了具有纳米级特性的新材料，这些材料是为特定目的而设计的。这种“有目的”的方法，加上制备和生长方法的发展，导致了在器件中使用薄膜而不是块状材料。使用薄膜使器件更容易制造，而使用薄膜进行涂层可以保护器件并赋予表面特定的性质。几十年来，人们一直在对薄膜、表面和界面进行深入研究。事实上，器件的性能依赖于不同性质的薄膜的优化匹配，例如有机和无机半导体以及金属的接触。令人惊讶的是，相比之下，人们很少关注有机自由基在衬底上的沉积。这可能是因为这些材料被认为不够稳定，无法蒸发。在这项工作中，我们证明了在受控条件下可以蒸发和沉积有机自由基到定义明确的表面上，而不会发生降解。我们使用软 X 射线光谱学，包括同步加速器，在有机自由基沉积到金属和金属氧化物表面时，在纳米尺度上研究薄膜过程、表面和界面。我们提出了如何设计有机自由基，同时考虑到控制薄膜稳定性的热力学因素，目的是不仅获得化学稳定的自由基，而且获得稳定的薄膜。我们研究了沉积膜的热稳定性和空气稳定性，并探索了表面/自由基化学键的影响以及表面缺陷对界面处磁矩的作用。我们发现有机自由基是物理吸附的，并且在惰性和钝化表面（如 Au(111)和 AlO(112̅0)单晶、SiO 和理想的 TiO(110)单晶）上保持其磁矩，而缺陷位，如氧空位或存在 OH 基团会导致有机自由基在表面上发生化学吸附，并使磁矩猝灭。我们的工作表明，基于 X 射线的技术代表了一种强大的方法，可以揭示复杂界面（如自由基/金属和自由基/金属氧化物）的机制，在这些界面中，描述电荷和自旋行为（spinterfaces）很重要。它还使得可以设计新的实验来研究薄膜的磁性与其结构特性之间的关系，以调整薄膜中分子的排列。控制分子排列将有机会调整分子在薄膜中的相互位置和取向，即薄膜中单个磁矩的取向，从而“印记”它们的磁性。对稳定自由基/无机 spinterfaces 的深入了解可能为在固态器件中使用自由基或作为具有专用配置的量子位（如其他分子量子位所提出的）开辟道路，并为基于自旋的电子学开辟道路。
