Spintronique moléculaire : des semi-conducteurs organiques aux monocouches auto-assemblées

This thesis targets the field of molecular spintronics and more particularly the new spin polarization tailoring opportunities, unachievable with inorganic materials, which arise from the ferromagnetic metal/molecule hybridization at the interface.: the new concept of Spinterface.In a first part we investigate Self-Assembled Monolayers (SAMs) based magnetic tunnel nanojunctions. This system appears to be a highly promising candidate to engineer the properties of spintronics devices at the molecular level since SAMs are the equivalent of a molecular LEGO building unit. We present the functionalization of the half-metallic manganite (La,Sr)MnO3 (LSMO) with alkyl phosphonic acids SAMs and the fabrication of LSMO/SAMs/Co magnetic tunnel nanojunctions with an area of few 10 nm2. MR of 30% to 50% is observed in most of the devices, while we report even up to 250% tunnel magnetoresistance (TMR) at low temperature. The most striking point is the robustness of the signal with bias voltage with still 20% TMR observed in the volt range. The influence of the molecular chain length is also investigated and represents a first step towards achieving molecular tailoring.In a second part we develop organic spintronics devices relying on high Curie temperature metallic ferromagnetic electrodes and standard organic semiconductor such as Co/Alq3/Co organic spin valves (OSVs). Junctions have a large area (section of 50 or 100 µm) and are fabricated in-situ by shadow mask. Magnetoresistance (MR) effects at room temperature are investigated with -4% MR observed in Co/Alq3/Co OSVs and +8% MR in Co/MgO/Alq3/Co OSVs. The role of the two interfaces on the spin polarization properties of the devices is also investigated. A stronger spin-dependent hybridization is found to occur at the bottom Co/Alq3 interface inverting the spin polarization on the first molecular layer. The observation of spin polarization inversion at room temperature demonstrates that spinterface effects can strive up to room temperature.Cette thèse s’inscrit dans le domaine de la spintronique moléculaire. Elle s’intéresse plus précisément aux nouvelles opportunités de façonnage de la polarisation de spin qui découlent de l'hybridation métal ferromagnétique/molécule à l'interface : le nouveau concept de « spinterface ».Dans une première partie nous présentons l’étude de nanojonctions tunnel magnétiques à base de monocouches auto-assemblées (SAMs). Ce système est un des plus prometteur dans l’optique de moduler les propriétés des dispositifs de spintronique par ingénierie chimique, tel un LEGO moléculaire. Nous y présentons la fonctionnalisation de la manganite demi-métallique (La,Sr)MnO3 (LSMO) avec des SAMs d’acides alkylphosphoniques et la fabrication de nanojonctions LSMO/SAMs/Co avec une surface de quelque 10 nm2. Une magnétorésistance de 30% à 50% est observée dans la majorité des dispositifs avec une magnétorésistance tunnel (TMR) jusqu'à 250 % à basse température. Un point remarquable est aussi le comportement très robuste du signal avec la tension: environ 20% de TMR est encore observée au-dessus d’une tension de 1 V. L'influence de la longueur de la chaîne moléculaire a été aussi étudiée et représente un premier pas vers la modulation des dispositifs au niveau moléculaire. Dans une deuxième partie nous présentons l’étude des dispositifs organiques à base de métaux ferromagnétiques à haute TC (température de Curie) et semi-conducteurs organiques. Nous avons réalisé des vannes de spin de Co/Alq3/Co avec des sections de 50 ou 100 µm et fabriquées in-situ par « shadow mask ». Des mesures à température ambiante ont permis d’observer -4% de magnétorésistance (MR) dans une vanne de spin Co/Alq3/Co et +8% MR dans une vanne de spin de Co/MgO/Alq3/Co. Le rôle des deux interfaces sur les propriétés de polarisation de spin des dispositifs est aussi étudié et détaillé. Une forte hybridation métal/molécule dépendant du spin à l'interface inferieure de Co/Alq3, présentant un effet de spinterface (inversion de la polarisation en spin), est observée. Ces études montrent que les effets de spinterface, comme l’inversion de la polarisation de spin, peuvent persister dans un dispositif jusqu’à température ambiante

Raman Spectra of ZrS2 and ZrSe2 from Bulk to Atomically Thin Layers

In the race towards two-dimensional electronic and optoelectronic devices, semiconducting transition metal dichalcogenides (TMDCs) from group VIB have been intensively studied in recent years due to the indirect to direct band-gap transition from bulk to the monolayer. However, new materials still need to be explored. For example, semiconducting TMDCs from group IVB have been predicted to have larger mobilities than their counterparts from group VIB in the monolayer limit. In this work we report the mechanical exfoliation of ZrX2 (X = S, Se) from bulk down to the monolayer and we study the dimensionality dependence of the Raman spectra in ambient conditions. We observe Raman signal from bulk to few layers and no shift in the peak positions is found when decreasing the dimensionality. While a Raman signal can be observed from bulk to a bilayer for ZrS2, we could only detect signal down to five layers for flakes of ZrSe2. These results show the possibility of obtaining atomically thin layers of ZrX2 by mechanical exfoliation and represent one of the first steps towards the investigation of the properties of these materials, still unexplored in the two-dimensional limit

Origin of the Chemiresistive Response of Ultrathin Films of Conductive Metal-Organic Frameworks

Conductive metal-organic frameworks are opening new perspectives for the use of these porous materials for applications traditionally limited to more classical inorganic materials, such as their integration into electronic devices. This has enabled the development of chemiresistive sensors capable of transducing the presence of specific guests into an electrical response with good selectivity and sensitivity. By combining experimental data with computational modelling, a possible origin for the underlying mechanism of this phenomenon in ultrathin films (ca. 30 nm) of Cu‐CAT‐1 is described

MUSE Analysis of Gas around Galaxies (MAGG) -- V: Linking ionized gas traced by CIV and SiIV absorbers to Lyα{\alpha} emitting galaxies at z≈3.0−4.5z\approx 3.0-4.5

We use 28 quasar fields with high-resolution (HIRES and UVES) spectroscopy from the MUSE Analysis of Gas Around Galaxies survey to study the connection between Ly${\alpha}$ emitters (LAEs) and metal-enriched ionized gas traced by CIV in absorption at redshift $z\approx3-4$. In a sample of 220 CIV absorbers, we identify 143 LAEs connected to CIV gas within a line-of-sight separation ${\rm \pm 500\,km\,s^{-1}}$, equal to a detection rate of $36\pm 5$ per cent once we account for multiple LAEs connected to the same CIV absorber. The luminosity function of LAEs associated with CIV absorbers shows a $\approx 2.4$ higher normalization factor compared to the field. CIV with higher equivalent width and velocity width are associated with brighter LAEs or multiple galaxies, while weaker systems are less often identified near LAEs. The covering fraction in groups is up to $\approx 3$ times larger than for isolated galaxies. Compared to the correlation between optically-thick HI absorbers and LAEs, CIV systems are twice less likely to be found near LAEs especially at lower equivalent width. Similar results are found using SiIV as tracer of ionized gas. We propose three components to model the gas environment of LAEs: i) the circumgalactic medium of galaxies, accounting for the strongest correlations between absorption and emission; ii) overdense gas filaments connecting galaxies, driving the excess of LAEs at a few times the virial radius and the modulation of the luminosity and cross-correlation functions for strong absorbers; iii) an enriched and more diffuse medium, accounting for weaker CIV absorbers farther from galaxies.Comment: 28 pages, 21 figures, 10 tables. Submitted to MNRAS after accounting for reviewer's comment

Direct visualization of pyrrole reactivity by confined oxidation in a Cyclodextrin Metal‐Organic Framework

Metal-organic frameworks can be used as porous templates to exert control over polymerization reactions. Shown here are the possibilities offered by these crystalline, porous nanoreactors to capture highly‐reactive intermediates for a better understanding of the mechanism of polymerization reactions. By using a cyclodextrin framework the polymerization of pyrrole is restricted, capturing the formation of terpyrrole cationic intermediates. Single‐crystal X‐ray diffraction is used to provide definite information on the supramolecular interactions that induce the formation and stabilization of a conductive array of cationic complexes

Resolving the physics of Quasar Lyα\alpha Nebulae (RePhyNe): I. Constraining Quasar host halo masses through Circumgalactic Medium kinematics

Ly$\alpha$ nebulae ubiquitously found around z>2 quasars can supply unique constraints on the properties of the Circumgalactic Medium, such as its density distribution, provided the quasar halo mass is known. We present a new method to constrain quasar halo masses based on the line-of-sight velocity dispersion maps of Ly$\alpha$ nebulae. By using MUSE-like mock observations obtained from cosmological hydrodynamic simulations under the assumption of maximal quasar fluorescence, we show that the velocity dispersion radial profiles of Ly$\alpha$-emitting gas are strongly determined by gravity and that they are thus self-similar with respect to halo mass when rescaled by the virial radius. Through simple analytical arguments and by exploiting the kinematics of HeII1640\.A emission for a set of observed nebulae, we show that Ly$\alpha$ radiative transfer effects plausibly do not change the shape of the velocity dispersion profiles but only their normalisation without breaking their self-similarity. Taking advantage of these results, we define the variable $\eta^{140-200}_{40-100}$ as the ratio of the median velocity dispersion in two specifically selected annuli and derive an analytical relation between $\eta^{140-200}_{40-100}$ and the halo mass which can be directly applied to observations. We apply our method to 37 observed quasar Ly$\alpha$ nebulae at 3<z<4.7 and find that their associated quasars are typically hosted by ~$10^{12.16 \pm 0.14}$ M$_{\odot}$ haloes independent of redshift within the explored range. This measurement, which is completely independent of clustering methods, is consistent with the lowest mass estimates based on quasar auto-correlation clustering at z~3 and with quasar-galaxies cross-correlation results.Comment: 23 pages, 13 figures, 2 tables. Accepted for publication in MNRA

A perpendicular graphene/ferromagnet electrode for spintronics

We report on the large-scale integration of graphene layers over a FePd perpendicular magnetic anisotropy (PMA) platform, targeting further downscaling of spin circuits. An L10 FePd ordered alloy showing both high magneto-crystalline anisotropy and a low magnetic damping constant, is deposited by magnetron sputtering. The graphene layer is then grown on top of it by large-scale chemical vapor deposition. A step-by-step study, including structural and magnetic analyses by x-ray diffraction and Kerr microscopy, shows that the measured FePd properties are preserved after the graphene deposition process. This scheme provides a graphene protected perpendicular spin electrode showing resistance to oxidation, atomic flatness, stable crystallinity, and perpendicular magnetic properties. This, in turn, opens the way to the generalization of hybrid 2D-materials on optimized PMA platforms, sustaining the development of spintronics circuits based on perpendicular spin-sources as required, for instance, for perpendicular-magnetic random-access memory schemes

WS2 2D Semiconductor Down to Monolayers by Pulsed-Laser Deposition for Large-Scale Integration in Electronics and Spintronics Circuits

We report on the achievement of a large-scale tungsten disulfide (WS2) 2D semiconducting platform derived by pulsed-laser deposition (PLD) on both insulating substrates (SrTiO3), as required for in-plane semiconductor circuit definition, and ferromagnetic spin sources (Ni), as required for spintronics applications. We show thickness and phase control, with highly homogeneous wafer-scale monolayers observed under certain conditions, as demonstrated by X-ray photoelectron spectroscopy and Raman spectroscopy mappings. Interestingly, growth appears to be dependent on the substrate selection, with a dramatically increased growth rate on Ni substrates. We show that this 2D-semiconductor integration protocol preserves the interface integrity. Illustratively, the WS2/Ni electrode is shown to be resistant to oxidation (even after extended exposure to ambient conditions) and to present tunneling characteristics once integrated into a complete vertical device. Overall, these experiments show that the presented PLD approach used here for WS2 growth is versatile and has a strong potential to accelerate the integration and evaluation of large-scale 2D-semiconductor platforms in electronics and spintronics circuits

A Crucial Role for the Protein Quality Control System in Motor Neuron Diseases.

Motor neuron diseases (MNDs) are fatal diseases characterized by loss of motor neurons in the brain cortex, in the bulbar region, and/or in the anterior horns of the spinal cord. While generally sporadic, inherited forms linked to mutant genes encoding altered RNA/protein products have also been described. Several different mechanisms have been found altered or dysfunctional in MNDs, like the protein quality control (PQC) system. In this review, we will discuss how the PQC system is affected in two MNDs-spinal and bulbar muscular atrophy (SBMA) and amyotrophic lateral sclerosis (ALS)-and how this affects the clearance of aberrantly folded proteins, which accumulate in motor neurons, inducing dysfunctions and their death. In addition, we will discuss how the PQC system can be targeted to restore proper cell function, enhancing the survival of affected cells in MNDs