Spin crossover phenomena: towards molecule-based memories and switches
- Autores: María del Pilar Maldonado Illescas
- Directores de la Tesis: José Ramón Galán Mascarós (dir. tes.)
- Lectura: En la Universitat Rovira i Virgili ( España ) en 2017
- Idioma: español
- Tribunal Calificador de la Tesis: José Antonio Real (presid.), José Sánchez Costa (secret.), Corine Mathoniere (voc.)
- Programa de doctorado: Programa de Doctorado en Ciencia y Tecnología Química por la Universidad Rovira i Virgili
- Materias:
- Enlaces
- Tesis en acceso abierto en: hdl.handle.net TDX
- Resumen
Nowadays, the idea that molecules can be used as a component of electronic devices is bringing maximum attention from science and technology, since molecules represent the miniaturization limit. As the smallest possible and addressable parts. To store information in a single molecule, it needs to possess a low-lying metastable state, accessible through external stimuli. The concept of bistability between these two electronic states of a molecule passing from one state (OFF) to another state (ON) under the influence of an external stimulus could be at the basis of novel molecule-based electronics.
Among bistable molecules, those exhibiting spin crossover phenomena are some of the most attractive candidates. They are a paradigmatic example of bistable materials where the magnetic state can be changed from a low-spin (LS) configuration into a low-lying meta-stable high-spin (HS) configuration through a variety of external stimuli, electronic state can be switched between the ground state and a low-lying meta-stable configuration under thermal control, pressure or light irradiation. Remarkably, the metastable state can be populated fast, showing extremely long lifetimes (over days at room temperature), as required to execute logic operations. Furthermore, this bistability is preserved at the nanoscale in sub-monolayer arrays, contrary to what occurs in classical magnetic media, and it is expected to be preserved at the single molecule level. How ever, the molecular limit has not been reached yet.
In this thesis, we present our efforts into developing molecular Iron (II) complexes with 1,2,4-triazole derivatives, to study the dynamics and features of this materials In chapter 1, a brief introduction is presented to introduce the broader goal of our research.
In Chapter 2, we present the synthesis of a new anionic spin crossover compound with room temperature bistability. This trimer (compound 1), formed by three Iron (II) where the central iron center exhibits bistability. Moreover, We were able to tune the bistable dynamiocs by cation exchange (compound 2). These molecular materials hold the record for highest and widest hysteresis, thanks to extremely slow transitions. We associate this behavior to a single-molecule origin. On a second research line, we have been exploiting the bistability of spin crossover compounds to use them as triggers to switch the transport properties of organic polymers in hybrid composite materials.
In chapter 3, we present the synergy observed in composite materials combining a commercial conducting polymer, PEDOT:PSS and compound 1. This combination yields a thin film where magnetic and conducting properties affect each other. It is particularly relevant to highlight the present of even slower dynamics for the spin transition in these materials, supporting once and again the molecular origin of this striking features.
In chapter 4, we present the results of composite films formed by an Iron (II) 1,2,4-triazole polymer and the organic conductor Polypyrrole (PP). In this case, the better mechanical properties of PP (when compared with PEDOT:PSS), allowed us to study the effect of processing at different pressures. This has a profound effect upon the synergy and bistability. This strategy yielded bistable switches with a difference of two orders of magnetide between the resistivity of each state (ON and OFF).
Finally, in chapter 5, we explored the thermoelectric properties of these Iron (II) 1,2,4-triazole/PP composite materials. Polypyrrole has been reported as moderate thermothermoelectric materials, and we have observed a significant enhancement of these properties when the spin crossover materials is embedded in the film.
All compounds were characterized by the usual techniques such as elemental analysis, X-ray diffraction, FT-IR spectroscopy, NMR spectroscopy, thermogravimetric analysis, Scanning electron microscopy, although other techniques more specific were employed. The most remarkable is the magnetic measurements performed by a susceptometer. Moreover, electrical measurements were carried out employing a sourcemeter and a nanovoltmeter. Finally, thermoelectric measurements were performed using a nanovoltmeter and two thermal controlled peltiers.
As a conclusion, we accomplished successfully the preparation and characterization of spin-crossover material/semiconducting organic polymer hybrid materials. Our results show that the spin transition of the spin crossover compound has a profound influence on the conducting properties of the hybrid materials, with appearance of a temperature-dependent conductivity transition between two different states with memory effect. The bistability of these films can be easily tuned in temperature and magnitude by modifying the preparation procedure. The conductivity behavior can be described by the pressure dependence density of states at the energy band structure in the semiconducting polymer and variable range-hopping model. On the other hand, triazole ligands have been widely used for the synthesis of one-dimensional Iron (II) spin crossover compounds as they provide an ideal environment to promote spin transition. We isolated succesfully a polyanionic spin crossover trimer which exhibit a large thermal hysteresis in magnetic measurements and with a record TTIESST ever observed in this kind of compounds. Therefore, a cation exchange can be performed for this trimer achieving a magnetic behavior more abrupt and with a dynamic faster than the other one.
