Synthesis, deposition and characterization of magnetic molecules on surfaces

Abstract

[eng] This doctoral thesis is dedicated to studying the synthesis and deposition of compounds with interesting magnetic properties on surfaces from an experimental perspective. The compounds studied are coordination compounds with properties ranging from single-molecule magnets (SMMs) to compounds with interesting properties for quantum computing (molecular qubits). These compounds can be simple molecules or more complex structures like metal-organic frameworks (MOFs). Through rigorous and numerous characterization techniques, the obtained systems have been studied in detail, with the aim of revealing how the compounds are affected during the deposition process and once they are in contact with the substrate. The thesis begins with a general introduction, reviewing the concept of using molecules as fundamental components for new devices, focusing on the phenomenon of SMMs and molecular qubits, as well as bidimensional MOFs. The introduction also covers the different experimental methods for depositing compounds on surfaces, as well as the most relevant techniques for characterizing the obtained systems. The first experimental chapter discusses the functionalization of substrates through the reaction known as silanization. This procedure allows obtaining different substrates with functional groups, such as carboxylate or terminal pyridine groups. This chapter presents a versatile method for different silanes and various substrates, such as silicon Si(100) or aluminum oxide in the form of single crystal (Al2O3(0001)) and microparticles. In Chapter 3, one of the substrates obtained in Chapter 2 is used to perform surface chemistry and deposit two compounds formed by lanthanides. The substrate is silicon with terminal carboxylate groups. These carboxylate groups aim to coordinate with the metals of the compounds, resulting in chemisorption. One of the compounds has dysprosium as the metal, while the other contains europium. The two compounds are analogous, as is their deposition. The dysprosium compound is an SMM, while the europium has very interesting luminescent characteristics that allow studying the deposition through these properties. The characterization performed allows us to conclude that the compounds remain intact on the surface and maintain their magnetic (in the case of dysprosium) and luminescent (in the case of europium) properties. Additionally, a percentage of the molecules form ordered structures on the surface, as observed through synchrotron radiation techniques. In Chapter 4, two molecular qubits containing copper and nickel, respectively, are deposited on silicon and aluminum oxide substrates with terminal pyridines. The compounds in their crystalline structure (in the bulk) have axial pyridines. Thus, through a ligand exchange reaction, these qubits could anchor to the surface. The compounds are deposited on the substrates and analyzed using numerous characterization techniques, which suggest that the compounds remain intact on the surface, forming very thin layers across the substrates. In Chapter 5, the deposition and growth of bidimensional MOFs with lanthanides on functionalized silicon and aluminum oxide substrates are investigated. The substrates obtained in previous chapters are used. The synthesis of different MOFs with dysprosium (SMM) and gadolinium (magnetocaloric effect and molecular qudit) as central metals is detailed. The characterization of the deposited MOFs is carried out using techniques such as atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), which confirm that the MOFs have been correctly deposited and maintain their structural and magnetic properties. Additionally, X-ray magnetic circular dichroism (XMCD) techniques are used to demonstrate that a significant portion of the MOFs are oriented. Finally, the thesis concludes with a summary and general conclusions, along with comments on future steps to be taken.

[cat] Aquesta tesi doctoral estudia la síntesi i deposició de compostos amb propietats magnètiques en superfícies, incloent-hi imants d’una sola molècula (SMM) i qubits moleculars, que poden ser molècules simples o estructures complexes com els "metal-organic frameworks" (MOFs). Mitjançant diverses tècniques de caracterització, s'analitza com aquests compostos es veuen afectats durant el procés de deposició i en contacte amb el substrat. La tesi comença revisant l'ús de molècules en nous dispositius, centrant-se en SMM, qubits moleculars i MOFs bidimensionals, així com els diferents mètodes de deposició de compostos en superfícies i les tècniques de caracterització més rellevants. Per obtenir substrats funcionals, s'utilitza la silanització per obtenir substrats amb grups carboxilat o piridines terminals. Aquest mètode es demostra versàtil per a diversos silans i substrats com el silici Si(100) i l'òxid d'alumini (Al2O3(0001)). A continuació, es dipositen compostos de lantànids, com el disprosi (SMM) i l’europi (luminescent), sobre substrats de silici amb grups carboxilat terminals. La caracterització mostra que aquests compostos mantenen les seves propietats magnètiques i luminescents en la superfície, formant estructures ordenades. Es dipositen qubits moleculars de coure i níquel sobre substrats de silici i òxid d’alumini amb piridines terminals, mitjançant una reacció d’intercanvi de lligand. Les tècniques de caracterització indiquen que els compostos romanen intactes, formant capes fines sobre els substrats. Finalment s'investiga la deposició i creixement de MOFs bidimensionals amb disprosi (SMM) i gadolini (efecte magnetocalòric i qudit molecular) sobre substrats funcionalitzats. La caracterització confirma que els MOFs es dipositen correctament i mantenen les seves propietats estructurals i magnètiques, amb un petit percentatge mostrant una orientació significativa.