Exploring the multifunctionality of a subphthalocyanine molecule: from surface phenomena and fractal growth to integration in spintronic devices
- Autores: Safaa Elidrissi Moubtassim
- Directores de la Tesis: Eugenio Coronado (dir. tes.)
- Lectura: En la Universitat de València ( España ) en 2025
- Idioma: español
- Número de páginas: 200
- Tribunal Calificador de la Tesis: Valentin Alek Dediu (presid.), Rosa Córdoba Castillo (secret.), Giovanni Bottari (voc.)
- Programa de doctorado: Programa de Doctorado en Nanociencia y Nanotecnología por la Universidad de Alicante; la Universidad de Castilla-La Mancha; la Universidad de La Laguna; la Universidad Jaume I de Castellón y la Universitat de València (Estudi General)
- Materias:
- Enlaces
- Tesis en acceso abierto en: TESEO
- Resumen
The present dissertation, entitled "Exploring the Multifunctionality of a Subphthalocyanine Molecule: Surface Phenomena, Fractal Growth, and Integration into Spintronic Devices", provides an analysis of boron subphthalocyanine chloride (BsubPc-Cl) on surfaces, within devices and self-aggregation.
The first part of this investigation delves into boron subphthalocyanine chloride (BsubPc-Cl) behaviour when adsorbed onto surfaces, exploring its morphology, spectral characteristics, and physical properties. By analysing how BsubPc-Cl adheres to and interacts with different substrates, the study sheds light on the critical factors vital for their applications in subsequent spintronic devices. Morphological studies reveal that BsubPc-Cl forms uniform, amorphous films with exceptionally smooth surfaces. The films exhibit homogeneous distribution and minimal defects. Chemical integrity analyses using spectroscopic techniques (Raman and Infrared spectroscopies) confirm the preservation of molecular properties during deposition, ensuring reliability for device integration. The photoluminescent study further demonstrates the photophysical properties of BsubPc-Cl films, including distinct photoluminescence characteristics influenced by film thickness and temperature; the marked increase in photoluminescence intensity at lower temperatures is attributed to reduced non-radiative decay processes. MOKE measurements illustrated that the two ferromagnetic (FM) electrodes, NiFe and Co, exhibit distinct hysteresis loops, a critical characteristic for the devices to be elaborated upon in the following chapter. Finally, the electronic structure of BsubPc-Cl on two FM surfaces, Fe and Co, was investigated through photoemission spectroscopy, providing valuable information on energy level shifts and underscoring the significance of substrate choice in determining molecular electronic coupling, which is crucial for optimising device performance. Together, these results provide a foundation for integrating BsubPc-Cl into advanced technologies and set the stage for further exploration in subsequent chapters.
The integration of BsubPc-Cl into spintronic devices is a central theme of this project. The starting point was the inquiry concerning how this molecule can enhance the functionality of a spintronic system. In particular, the integration of BsubPc-Cl into NiFe/BsubPc-Cl/Co junctions was studied. Spintronic junctions were characterised by analysing current-voltage (I-V) characteristics and magnetoresistance (MR) behaviour. Variability in electrical resistance underscores the importance of optimising fabrication processes. In the 5-10 nm molecular thickness regime, tunnelling was found to be the primary transport mechanism, while observed MR effects are linked to spin-dependent tunnelling probabilities and interfacial magnetic properties. Devices with thicknesses of 5-7 nm exhibited negative MR under positive bias (Co positively biased relative to NiFe) and positive MR under negative bias. In contrast, thicker junctions (7-10 nm) displayed negative MR under positive bias, with no MR under negative bias. The pronounced MR bias asymmetry aligns with the negatively polarised d-band of cobalt.
A groundbreaking contribution of this work lies in the discovery and characterisation of self-assembled dendritic structures of BsubPc-Cl with fractal morphologies, highlighting the relationship between molecular geometry, aggregation phenomena, and their optoelectronic properties. Thermal annealing was found to accelerate the transformation of uniform BsubPc-Cl films into branched structures, with morphology and complexity evolving with film thickness. Morphological analysis using optical and atomic force microscopy reveals the hierarchical nature of the dendrites, where primary branches give rise to thinner secondary structures consistent with fractal growth principles. These dendrites exhibit anisotropic optical properties and a high degree of spatial order, driven by competing forces such as surface mobility and dipole-dipole interactions. Fractal analysis of the dendritic formations demonstrates a progressive increase in fractal dimension with film thickness, indicating denser and more complex spatial occupation. This behaviour is attributed to the aggregation of BsubPc-Cl molecules into J-aggregates, which enhance intermolecular interactions and lead to ordered growth. Conductive atomic force microscopy (c-AFM) further reveals that the dendrites possess superior electrical conductivity compared to amorphous films, highlighting the role of crystalline order in facilitating charge transport. The chapter further explores the optoelectronic implications of these dendritic structures, particularly their aggregation-induced emission enhancement (AIEE). Photoluminescence studies show that dendritic formations suppress nonradiative decay pathways, leading to increased radiative recombination and enhanced light emission. The redshift in photoluminescence with increasing film thickness underscores the effect of molecular packing and exciton delocalisation in these ordered structures. This unique combination of fractal morphology, enhanced conductivity, and photoluminescent properties highlights the potential of BsubPc-Cl dendrites for applications in optoelectronics and molecular photonics.
