In this thesis I present a complete and detailed guide for the development process and fabrication of efficient single-electron transistors (SETs) and a better single-molecule magnets (SMMs) deposition yield. Starting from a commercial Si/SiO2 wafer I show the steps for the deposition of different layers to fabricate a SET as well as the improvements achieved in those for a completely functional SET device. The development process is based on a combination of optical lithography and e-beam lithography with metal deposition in ultra-high vacuum. The improvements involve a better conductance in the Al gate component, with a controlled formation of the superficial oxide layer and a faster feedback electromigration-induced breaking of Au nanowires for the creation of nanogaps at room temperature. The gate component is improved by increasing its thickness and exposing it to plasma oxidation for the complete oxidation of its surface. The nanowire breaking is realized at room temperature to make use of the surface tension of Au, which, after a previous feedback procedure, eventually opens the final gap in the nanowire. Finally, I demonstrate a new technique that allows increasing the yield of having a SMM connected in the nanowire gap. This new technique is based on monitoring the resistance of the broken nanowires during the SMM deposition from a controlled liquid solution at room temperature. When the resistance ( > GΩ for open gaps) drops to values below Mega-ohms (characteristic resistance of a molecule bridging the gap) for a number of nanowires in the chip, the device is then ready for low temperature measurements.

New 1,2,3,5-dithiadiazolyl (DTDA), 1,2,3,5-diselenadiazolyl (DSDA) radicals and their resulting metal complexes were synthesized and characterized. The overarching theme of this thesis is the utility of intermolecular interactions for facilitating previously unseen magnetic behaviours in thiazyl radical-metal complexes. This thesis contains the first examples of thiazyl radical metal complexes acting as molecule based magnets. The 4-benzoxazol-2′-yl-1,2,3,5-dithiadiazolyl (boaDTDA) radical and its selenium analogue 4-benzoxazol-2′-yl-1,2,3,5-diselenadiazolyl (boaDSDA) were coordinated to several paramagnetic metal ions including transition metal ions Mn(II), Co(II) and Ni(II). The Ni(hfac)2(boaDTDA) and Ni(hfac)2(boaDSDA) complexes are isomorphous and both demonstrate step like π-stacking leading to additional ferromagnetic (FM) intermolecular interactions. The Mn(hfac)2(boaDTDA) (hfac = 1,1,1,5,5,5-hexafluoroacetylacetonato) complex was the first DTDA metal complex to conclusively show that intermolecular S(DTDA)…O(hfac) contacts can lead to intermolecular anti-ferromagnetic (AF) interactions which, in turn, can lead to a large spin ground state. Based on the magnetic properties of the Mn(hfac)2(boaDTDA) complex, a new DTDA biradical ligand, 4,6-bisDTDApyrimidine (bisDTDApym), was developed and coordinated to Mn(hfac)2. The resulting dinuclear Mn(II) complex, [Mn(hfac)2]2(bisDTDApym), is arranged in the solid state by short S(DTDA)…O(hfac) interactions forming two dimensional ferrimagnetic sheets. These ferrimagnetic sheets AF couple to one another, giving rise to AF ordering below 4.5 K. The [Mn(hfac)2]2(bisDTDApym) is the first thiazyl metal complex to magnetically order and is a unique example of a molecular coordination complex which magnetically orders. This thesis also presents the synthesis and characterization of precursor materials of the form Ln(hfac)3(DME) (DME = dimethoxyethane) for coordination reactions to thiazyl radical ligands. The Dy(hfac)3(boaDTDA) and Dy(hfac)3(pyDTDA) (pyDTDA = 4-(2′-pyridyl)-1,2,3,5-DTDA) complexes demonstrate single molecule magnetism with energy barriers of 100 K and 70 K respectively. Ten-coordinate Ln(hfac)3(pyDTDA)2 (Ln = La, Ce, Pr) complexes demonstrate phase transition behaviour between dimerized and undimerized phases and were studied by X-ray crystallography and magnetometry. The aforementioned compounds are some of the over 50 new compounds which have been synthesized and fully characterized in this thesis.

Considerable efforts are being devoted to designing enhanced molecular magnetic materials, in particular single molecule magnets (SMMs) that can meet the requirements for future technologies such as quantum computing and spintronics. A current trend in the field is enhancing the global anisotropy in metal complexes using single-ion anisotropy. The work in this dissertation is devoted to the synthesis and characterization of new building blocks of the highly anisotropic early transition metal ion V(III) with the aim of incorporating them into heterometallic molecular materials. The results underscore the importance of tuning the local coordination environments of metal ions in order to ensure enhanced single ion anisotropy. A family of mononuclear axially distorted vanadium (III) compounds, A[L_(3)VX_(3)] (3-9) (X = F, Cl or Br, A^(+) = Et_(4)N^(+), nBu_(4)N^(+) or PPN^(+) , L_(3) = Tp or Tp* (Tp = tris(-1-pyrazolyl)borohydride), Tp* = tris(3,5-dimethyl-1-pyrazolyl)borohydride)), and [Tp*V(DMF)_(3)](PF_(6))_(2) were studied. Replacement of the Tp ligand in 3 with the stronger π-donor Tp* results in a near doubling of the magnitude of the axial zero-field splitting parameter D_(z) (D_(z) = -16.0 cm^(-1) in 3, and -30.0 cm^(-1) in 4) as determined by magnetic measurements. Such findings support the idea that controlling the axial crystal field distortion is an excellent way to enhance single-ion anisotropy. High Field-High Frequency EPR measurements on 4 revealed an even higher D value, -40.0 cm^(-1). Interestingly, compound 4 exhibits evidence for an out-of-phase ac signal under dc field. In another effort, a new series of vanadium cyanide building blocks, PPN[V(acac)_(2)(CN)_(2)]∙PPNCl (13) (acac = acetylacetonate), A[V(L)(CN)_(2)] (A^(+) = Et_(4)N^(+), L = N,N'-Ethylenebis(salicylimine) (14), A = PPN^(+), L = N,N'-Ethylenebis(salicylimine) (15), L = N,N'-Phenylenebis(salicylimine) (16), and L = N,N'-Ethylenebis(2-methoxysalicylimine) (17)) were synthesized. Magnetic studies revealed moderate Dz values (-10.0, 5.89, 3.7, 4.05 and 4.36 cm^(-1) for 13-17 respectively). The first family of cyanide-bridged lanthanide containing molecules with a trigonal bipyramidal (TBP) geometry, (Et_(4)N)_(2)[(Re(triphos)(CN)_(3))_(2)(Ln(NO_(3))_(3))_(3)]-∙4CH_(3)CN (19-27 with Ln = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy and Ho) were prepared using the [(triphos)Re(CN)_(3)]^(-) building block, results that add valuable information to our database of compounds with a TBP geometry. Magnetic studies revealed diverse magnetic responses including slow relaxation of the magnetization at zero field for 25 and 26 , an indication of SMM behavior.

No description available.

In order for molecular magnetic materials to become applicable, they must retain their magnetisation at reasonable temperatures, which can be achieved with high energy barriers for spin reversal and high blocking temperatures. In the field of Single-Molecule Magnets (SMMs), over the last decade, the main focus has shifted from large spin complexes to highly anisotropic systems which have displayed record energy barriers. There are two main methods of increasing magnetic anisotropy in a complex: i) Choosing a metal ion that boasts high magnetic anisotropy then coupling two such ions through magnetic interactions to induce large global anisotropy, and ii) maintain a low spin or use a mononuclear complex while minimising quantum tunnelling of the magnetisation by controlling the geometric features of the metal ion. Both strategies are equally valid and have been explored in this thesis using dinuclear lanthanide as well as mononuclear 3d complexes. In the pursuit of high-barrier SMMs via alignment of anisotropy axes, two dinuclear, quadruple-stranded helicates and one mesocate were isolated and are described in detail herein, both structurally and magnetically. Furthermore, theoretical calculations have been performed to determine the energies of Kramers doublets on each DyIII centre to derive magneto-structural correlations. To induce magnetic interactions between DyIII ions, a centrosymmetric dinuclear SMM was synthesised. Investigation of the crucial DyIII…DyIII interaction as well as its effect on the quantum tunnelling of the magnetisation has been carried out using ab initio calculations and magnetic dilution studies. Using the same system, a method of greatly enhancing the energy barriers in SMMs has been developed. It involves modifying the coordinating ligands to include electron withdrawing groups in order to yield more anisotropic metal ions. The energy barrier for spin reversal has been increased 7-fold in one case. While lanthanide chemistry has proven to be quite versatile and promising, a new branch of nanomagnets is currently being pursued: mononuclear 3d complexes as SMMs. The advantages of 3d metals include high anisotropy per ion, low spin (as anisotropy decreases with increasing spin), well-understood electronic structures and clear correlations between geometry and magnetic anisotropy. The structural and magnetic properties of three complexes based on CoII and terpyridine ligands as well as a seven-coordinate CoII complex with positive anisotropy are discussed at length. The unique slow relaxation dynamics and spin crossover behaviour has been followed using DFT and ab initio calculations, as well as EPR and magnetic dilution studies. Overall, this thesis describes the efforts taken to synthesise high-barrier nanomagnets through understanding the origins and mechanisms of slow magnetic relaxation in both lanthanide and 3d metal complexes.

Molecular Nanomagnets have been proposed as plausible candidates in a variety of futuristic applications. Thorough understanding of the magnetic properties of these systems is therefore necessary to develop devices that include such units. The aim of this thesis is to synthesise and structurally and magnetically characterise a range of systems that could be used as elementary units in three proposed applications such as: data storage devices, magnetic refrigerants and qubits for quantum computing. A series of mixed 3d/4f metal complexes were synthesised through solvothermal reactions and characterised by X-ray single crystal diffraction and SQUID magnetometry. Through indirect methods it was possible to obtain high magnetic entropy change for some systems. It was also possible to obtain some insight into the magnetic interactions within the systems through modelling the magnetic data. The role of the 4f-4f and 3d-4f interactions in two sets of molecules is also described. The first study is in an asymmetric dysprosium dimer, where through a range of experimental techniques and advanced theoretical methods, such ab-initio calculations we are able to explain the role of the intramolecular interactions and their effect on the SMM properties of this system. Similarly, insight into the role of the 3d-4f interactions is achieved through the observation of the magnetic behaviour of a family of 27 tetranuclear systems, though SQUID data and ab-initio calculations. Finally, chemical functionalization of a well-proposed qubits, namely {Cr7Ni} and subsequent reaction with a redox active metal ion, CoII/III, two {Cr7Ni} systems are linked. The magnitude of the exchange interaction between the {Cr7Ni}-CoII-{Cr7Ni} was determined through Electron Paramagnetic Resonance. Furthermore, by chemical oxidation/reduction of the cobalt between paramagnetic and diamagneticstates, i.e. CoII and CoIII respectively, we demonstrate that the interaction can be switched ON/OFF. This characteristic makes of these systems candidates to function as a SWAP gate.

Les molécules-aimants sont des complexes moléculaires contenant des ions des métaux de transition ou des lanthanides capables de présenter le phénomène de blocage de l’aimantation en dessous d’une température de blocage Tb. Ce blocage est dû à la présence d’une barrière d’énergie de réorientation de leur aimantation à cause de la présence d’une anisotropie magnétique uniaxiale qui conduit à la présence de deux états stables de l’aimantation.Ces deux états stables sont adressables avec un champ magnétique extérieur. Il est donc,théoriquement, envisageable d’utiliser ces molécules comme unités de base pour le stockage « classique » de l’information.Néanmoins, à cause de la nature quantique des molécules, une relaxation entre les deux états de l’aimantation a lieu à basse température par effet tunnel à travers la barrière d’énergie. Cet effet tunnel a plusieurs causes dont une correspondant à une légère déviation de l’anisotropie magnétique de la situation strictement axiale. Cet effet annule le caractère bistable (classique) des molécules les rendant inutilisables comme bits classiques pour le stockage de l’information. Mais, la présence de l’effet tunnel conduit à une situation particulière à basse température où deux niveaux sont présents séparés par une énergie liée au caractère non axiale (rhombique) de l’aimantation (cas où le spin est entier). Un système à deux niveaux est appelé bit quantique(qubit) et constitue l’unité de base pour la construction d’ordinateurs quantiques si plusieurs conditions sont réunies.Ainsi, pour concevoir des bits classiques ou quantiques, il est indispensable comprendre au niveau microscopique la nature de l’anisotropie magnétique et les facteurs qui l’influencent.Ce travail de thèse est consacré à l’étude théorique de la nature de l’anisotropie magnétique dans des complexes mononucléaires et binucléaires de Ni(II) (S = 1)et de Co(II) (S = 3/2). Des calculs de type ab initio, basés sur la théorie de la fonction d’onde,qui permettent d’extraire les paramètres de l’hamiltonien de spin de l’anisotropie magnétique ont été effectués. Des calculs sur des objets modèles et molécules réelles qui permettent de séparer l’effet des différents paramètres structuraux et électroniques des ligands sur la nature et l’amplitude de l’anisotropie magnétique ont aussi été réalisés.La comparaison entre les calculs sur des complexes modèles et sur des complexes réels permet de rationaliser les propriétés magnétiques des complexes réels et surtout de proposer des stratégies pour la synthèse de nouveaux complexes avec les propriétés souhaitées. L’étude de complexes binucléaires qui peuvent être considérés comme la première étape pour la conception de porte logique quantique a été réalisée. Les calculs sur les complexes binucléaires sont réalisés en fragmentant les molécules en deux espèces mononucléaires. Pour les complexes binucléaires de Ni(II) et Co(II), des calculs de type Density Functional Theory (DFT) pour évaluer l’amplitude et la nature de l’interaction d’échange ont été menés. Pour étudier l’influence d’une perturbation extérieure sur les propriétés magnétiques, l’influence d’un champ électrique placé parallèle et perpendiculaire à l’axe de facile aimantation d’un complexe de Ni(II) a été étudiée. Le champ électrique peut influencer les propriétés d’anisotropie de manière importante ouvrant la possibilité à la manipulation des molécules par cette perturbation. / Single molecule magnets are molecular complexes containing transition metal or lanthanides ions which are able to block their magnetization below a certain blocking temperature Tb. This blocking is caused by an energy barrier separating the two orientations of magnetization leading to two stable magnetization states. These two states can be controlled by an external magnetic field.Therefore, it is theoretically possible to use these molecules as bits which are able to store“classical” information. However, due to the quantum nature of these molecules, the relaxation of magnetization can exist even at low temperatures. This phenomenon is called the quantum tunneling effect and prevents the bistable (classical) behavior of the magnetic properties, as well as their use as classical bits for data strorage.Yet, the quantum tunneling of the magnetization also leads to a particular situation at a low temperature where two levels are separated by an energy related to the non-axial character(rhombic) of the magnetization (when the spinis an integer). Such two-levels system could be used as a quantum bit (qbit) which is the basic unit for quantum information processing. Thus,the design of classical or quantum bits require a precise understanding of magnetic properties and their origin at a microscopic level.The Ph.D work was devoted to the theoretical study of the magnetic anisotropy in mononuclear and binuclear Ni(II) (S=1) and Co(II) (S=3/2) complexes. Ab initio calculations based on the wave function theory were carried out and the spin Hamiltonian parameters were extracted. Model complexes were used to investigate the structural and electronic parameters causing magnetic anisotropy.Calculations were, also, performed on complexes synthesized in the laboratory.Comparison between real and model complexes allowed rationalizing the magnetic properties and imagining new synthesis strategies leading to the desired magnetic properties. Binuclear complexes that can be considered as double qbits and used to build quantum logic gates were also investigated. The calculations were performed by fragmenting the binuclear complexes into two mononuclear units in order to study the local anisotropy of each metal ion.The exchange interaction was investigated using Density Functional theory (DFT). In order to study the influence of an external perturbation on magnetic properties, the magnetic properties of a mononuclear Co(II) complex under an external electric field applied parallel or perpendicular to the axis of easy magnetization were calculated. The application of an electric field can lead to important modifications of magnetic properties. Thereby, offering the possibility to the manipulation of these molecules by external electric fields.

The controlled assembly of molecular building blocks is an emerging strategy that allows for the preparation of materials with tailor-made properties. This involves the precise combination of molecular subunits that interact with one another via specifically designed reactive sites. Such a strategy has produced materials exhibiting remarkable properties, including those based on metal-organic frameworks and single-molecule magnets. The present Thesis aims to highlight how such metal-organic assemblies can be engineered at the molecular level to promote certain desired functionalities. Specifically, Chapter 2 will focus on the confinement effects of a crystalline sponge on a ferrocene-based guest molecule that is nanostructured within the porous cavities of a host material. In doing so, we evaluate how one can exert some level of control over the binding sites of the guest molecule, through the addition of electron-withdrawing groups, as well as tuning the physical properties of the guest itself through molecular encapsulation. Notably, we demonstrate a distinct change in the dynamic rotational motion of the ferrocene molecules once confined within the crystalline sponge. In Chapter 3, we investigate the generation of slow relaxation of the magnetization from a Co(II)-based metal-organic framework. We compare this to a closely related 2D Co(II) sheet network, and how slight changes in the crystal field, probed through computational methods, can impact the magnetic behaviour. This type of study may be particularly beneficial in the optimization of single-ion magnets, by sequestering metal centres in select chemical environments, and minimizing molecular vibrations that may offer alternative magnetic relaxation pathways. We extend these principles in Chapter 4, through the use of a nitrogen-rich ligand that acts as a scaffold for Ln(III) ions, thereby yielding 0D and 1D architectures. The coordination chemistry of Ln(III) ions with N-donor ligands remains scarce, especially when evaluated from a magnetic perspective, and therefore, we sought to determine the magnetic behaviour of such compounds. The monomeric unit displays clear single-molecule magnet behaviour with an energetic barrier for the reversal of the magnetization, while the 1D chain displays weaker magnetic characteristics. Nevertheless, such compounds incorporating nitrogen-rich ligands offer much promise in the design of environmentally-friendly energetic materials. In Chapter 5, we take a look at different two different systems that involve the formation of radical species. On one hand, we can promote enhanced magnetic communication between Ln(III) ions, which is typically quite challenging to achieve given the buried nature of the 4f orbitals, and on the other hand, we rely on a redox-active ligand to design stimuli-responsive metal-organic assemblies. The latter case provides access to “smart” molecular materials that can respond to changes in their environment. Here, a multi-stimuli responsive nanobarrel was studied, which displayed sensitivity to ultraviolet radiation, heat and chemical reduction. Lastly, Chapter 6 provides a new method for the systematic generation of cationic frameworks, termed Asymmetric Ligand Exchange (ALE). This strategy focuses on the replacement of linear dicarboxylates with asymmetric linkers that features one less negative charge, in order to tune the ionicity of porous frameworks. This allows for the retention of the structural topology and chemical reactivity of the original framework, representing distinct advantages over other similar strategies. Methods to retain permanent porosity in such cationic frameworks are also proposed. Altogether, these studies highlight how the directed assembly of ordered networks can generate varied properties of high scientific interest.

The prominent features of single molecule magnets (SMMs), such as the quantum tunneling of the magnetization (QTM), are conventionally understood through the giant spin approximation (GSA) which considers the molecule as a single rigid spin. This model often requires the inclusion of high order anisotropy terms in the Hamiltonian, a manifestation of admixing of low lying excited states that can be more naturally understood by employing a multi-spin (MS) description i.e. considering the individual spins and the interactions between ions within the molecule. However, solving the MS Hamiltonian for high nuclearity molecules is not feasible due to the enormous dimensions of the associated Hilbert space that put it beyond the capability of existing computational resources. In contrast, low nuclearity systems permit the complete diagonalization of the MS Hamiltonian required to sample the effect of internal degrees of freedom, such as exchange interactions and single ion anisotropies, on the QTM. This dissertation focuses on the study of low nuclearity SMMs in view of understanding these subtle quantum effects. To accomplish this, we have developed a series of magnetic characterization techniques, such as integrated microchip sensors resulting from the combination of two dimensional electron gas (2DEG) Hall-Effect magnetometers and microstrip resonators, capable of performing measurements of magnetization and EPR spectroscopy simultaneously. The thesis bases on a comparative study of two low nuclearity SMMs with identical magnetic cores (Mn4 dicubane) but differing ligands. Notably, one of these SMMs lacked solvent molecules for crystallization; a characteristic that gives rise to extremely sharp resonances in the magnetization loops and whose basic QTM behavior can be well explained with the GSA. On the contrary, the second SMM exhibited mixed energy levels, making a MS description necessary to explain the observations. We have also examined the role of internal degrees of freedom on more subtle QTM phenomena, leading to the explanation of asymmetric Berry-phase interference patterns observed in a Mn4 SMM in terms of a competition between different intermolecular magnetic interactions, i.e. non-collinear zero-field splitting tensors and intramolecular dipolar iii interactions, resulting in astonishing manifestations of the structural molecular symmetry on the quantum dynamics of the molecular spin.

Les nanotubes de carbone (CNT) à basse température se comportent comme des points quantiques pour lesquelles les niveaux électroniques deviennent quantifiés. Le transport électronique à travers une jonction-CNT est caractérisé par le phénomène de blocage de Coulomb, dont les spécificités dépendent du couplage entre le nanotube et les électrodes métalliques. Le blocage de Coulomb est extrêmement sensible au moindre changement électrostatique, faisant des jonctions-CNT de précis électromètres. Par exemple, si l'on couple un système magnétique à un nanotube, le transport électronique sera influencé par l'état de spin du système magnétique (effet magnéto-Coulomb). Ce projet de thèse présente des mesures de transport électrique sur un système hybride se composant d'un nanotube de carbone rempli de nanoparticules magnétiques (Fe). Ces mesures, réalisées à très basses températures (40 mK), ont permis de mettre en évidence le comportement hystérétique de la conductance en fonction du champ magnétique, et en particulier la présence de saut de conductance à champ magnétique fini. Nous expliquons ces résultats en termes d'effet magnéto-Coulomb : le renversement d'aimantation des particules de fer à champ magnétique fini provoquant une variation de charge effective due à l'effet Zeeman. Ces mesures sont une étape vers l'étude de l'anisotropie magnétique de nanoparticules individuelles. / Carbon Nanotubes at low temperature behave as Quantum Dots for which charging processes become quantized, giving rise to Coulomb Blockade depending upon the coupling to the leads. Any small change in the electrostatic environment (tuned by the gate electrode) can induce shift of the stability diagram (so called Coulomb Diamonds) of the device, leading to conductivity variation of the Quantum Dot. A carbon nanotube can therefore be a very accurate electrometer. For example, if a magnetic system is electronically coupled to a nanotube, its electron conduction may be influenced by the spin state of the magnetic system (magneto- Coulomb effect). In this thesis, we report on the electrical transport measurements of such hybrid systems where a carbon nanotube is filled with magnetic nanoparticles such as Iron(Fe). We find that low-temperature (~40mK) current-voltage measurements of such devices can show a hysteretic behaviour in conductance with sharp jumps at certain magnetic fields. We explain the results in terms of the magneto-Coulomb effect where the spin flip of the iron island at non-zero magnetic field causes an effective charge variation in the Nanotube due to the Zeeman energy. Our studies are a step forward towards the study of the magnetic anisotropy of individual nanoparticles. We believe our findings have important implications for sensitive magnetic detectors to study the magnetization reversal of individual magnetic nanoparticle or molecule, even weakly coupled to a carbon nanotube.