Tuning the performance of magnetic, semiconductor, and multifunctional hybrid nanostructures
- Autores: Mariona Escoda Torroella
- Directores de la Tesis: Amílcar Labarta Rodríguez (dir. tes.), Xavier Batlle i Gelabert (codir. tes.)
- Lectura: En la Universitat de Barcelona ( España ) en 2022
- Idioma: inglés
- Tribunal Calificador de la Tesis: María del Puerto Morales Herrero (presid.), Marta Estrader Bofarull (secret.), Victor Franco Puntes (voc.)
- Programa de doctorado: Programa de Doctorado en Física por la Universidad de Barcelona
- Materias:
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- Resumen
Over the last twenty years, the interest in nanomaterials has increased thanks to their potential applications in biomedicine, energy storage, and environmental remediation, among others. The main feature of these types of materials is the emergence of a new phenomenology due to their nanometric sizes such as superparamagnetism and plasmonics. However, one of the challenges is to unravel the reaction mechanisms of their synthesis by chemical methods. This thesis is focused on the synthesis and characterization of nanoparticles to optimize their properties as well as to improve the understanding of the correlation between their structural features and their magnetic, semiconductor, plasmonic properties.
Regarding magnetic iron oxide nanoparticles, two types of systems were studied. First, single-core nanoparticles were prepared by thermal decomposition of a metalorganic precursor. The study focused on the effect of the amount of 1,2-hexadecanediol and 1-octadecene used as a stabilizing agent and solvent, respectively. Through structural characterization, it was determined that above a certain threshold, the nanoparticles are small and single-crystalline. Consequently, their magnetic properties show the typical features of superparamagnetic nanoparticles (i.e., high saturation magnetization, low remnant magnetization, and low coercive fields). On the contrary, using low amounts of these reagents, the nanoparticles become larger and show crystalline defects and the coexistence of Fe3O4 and FeO. These nanoparticles show a poorer magnetic response and exchange bias due to the coexistence of the ferrimagnetic phase (Fe3O4) and the antiferromagnetic one (FeO). In addition, the structural properties of multi-core maghemite nanoparticles were thoroughly studied to understand their outstanding magnetic properties. These nanostructures composed of small cores arranged in large aggregates with a nanoflower shape show a magnetic behavior similar to effective superparamagnetism. This study showed that the crystalline texture is closely related to the collective magnetic behavior.
Then, we also monitored the synthesis of Bi2S3 nanoparticles by hot injection of a sulfur precursor. In this case, the reaction temperature and time were tuned to obtain different sizes and morphology, giving rise to nanodots, nanoneedles, and nanorods. The dimension confinement of these nanostructures led to tunable bandgap energy, and consequently, variable optical response. This fact may allow the optimization of the synthesis conditions to improve the absorption in the near-infrared region, which is useful for biomedical applications. In addition, to achieve multifunctional materials, Bi2S3 nanostructures were combined with gold by a double hot-injection method. The synthesis of these hybrid nanomaterials was achieved successfully as Bi2S3 nanorods decorated with gold nanoparticles were obtained. Although we observed quenching of the plasmonic response when Bi2S3 and gold were combined, the absorption along all the spectrum was increased. This result may be interesting to achieve the combination of photothermal properties with the intrinsic properties of Bi2S3 and Au as contrast agents for computed tomography.
All in all, we demonstrated that the thorough study of the synthesis and characterization of iron oxide and Bi2S3 nanostructures allowed a better understanding of their reaction mechanism and the correlation of their structural features with their macroscopic properties. In addition, they enable the optimization of the synthesis procedures to achieve more sustainable synthesis without compromising their outstanding properties.
