Substrate-induced effects in thin films of a potential magnet composed of metal-free organic radicals deposited on Si(111)

Synthesis and Thin Films of Thermally Robust Quartet (S = 3/2) Ground State Triradical

High-spin (S = 3/2) organic triradicals may offer enhanced properties with respect to several emerging technologies, but those synthesized to date typically exhibit small doublet quartet energy gaps and/or possess limited thermal stability and processability. We report a quartet ground state triradical 3, synthesized by a Pd(0)-catalyzed radical-radical cross-coupling reaction, which possesses two doublet-quartet energy gaps, ΔE_DQ ≈ 0.2-0.3 kcal mol^-1 and ΔE_DQ2 ≈ 1.2-1.8 kcal mol^-1. The triradical has a 70+% population of the quartet ground state at room temperature and good thermal stability with onset of decomposition at >160 °C under an inert atmosphere. Magnetic properties of 3 are characterized by SQUID magnetometry in polystyrene glass and by quantitative EPR spectroscopy. Triradical 3 is evaporated under ultrahigh vacuum to form thin films of intact triradicals on silicon substrate, as confirmed by high-resolution X-ray photoelectron spectroscopy. AFM and SEM images of the ∼1 nm thick films indicate that the triradical molecules form islands on the substrate. The films are stable under ultrahigh vacuum for at least 17 h but show onset of decomposition after 4 h at ambient conditions. The drop-cast films are less prone to degradation in air and have a longer lifetime.

Stability of radical-functionalized gold surfaces by self-assembly and on-surface chemistry

We have investigated the radical functionalization of gold surfaces with a derivative of the perchlorotriphenylmethyl (PTM) radical using two methods: by chemisorption from the radical solution and by on-surface chemical derivation from a precursor. We have investigated the obtained self-assembled monolayers by photon-energy dependent X-ray photoelectron spectroscopy. Our results show that the molecules were successfully anchored on the surfaces. We have used a robust method that can be applied to a variety of materials to assess the stability of the functionalized interface. The monolayers are characterized by air and X-ray beam stability unprecedented for films of organic radicals. Over very long X-ray beam exposure we observed a dynamic nature of the radical-Au complex. The results clearly indicate that (mono)layers of PTM radical derivatives have the necessary stability to withstand device applications.

Nanoscale Studies of Organic Radicals: Surface, Interface, and Spinterface

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 Al₂O₃(112̅0) single crystals, SiO₂, and ideal TiO₂(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.

Electronic structure and stability of fluorophore-nitroxide radicals from ultrahigh vacuum to air exposure

Thin film processes of organic radicals remain widely unknown, although these materials may have a significant technological potential. In aiming at their use in applications, we explore the electronic structure of thin films of a nitronyl nitroxide radical attached to a fluorophore core. According to our findings, this molecule maintains its radical function and, consequently, its sensing capabilities in the thin films. The films are characterized by a high structural degree of the molecular arrangement, coupled to strong vacuum and air stability that make this fluorophore-nitroxide radical an extremely promising candidate for application in electronics. Our work also identifies a quantitative correlation between the results obtained by the simultaneous use of X-ray photoemission and electron paramagnetic resonance spectroscopy. This result can be used as a standard diagnostic tool in order to link the (in situ-measured) electronic structure with classical ex situ paramagnetic investigations.