Segregation within bi-metallic nanoparticles at the atomic scale through a combined theoretical and experimental approach

Abstract

The analysis of nanoparticles (NPs) on a nanometric scale for applications in real-life conditions remains a considerable challenge at the present time. In this context, the use of bi-metallic NPs is strongly envisaged in the field of catalysis, with the function of promoting and accelerating the kinetics of surface chemical reactions. It is therefore essential to describe the structure and chemical composition of the surfaces, which interact directly with the surrounding medium in which the NPs are immersed. In the context of this thesis, we have developed a combined theoretical and experimental approach at the atomic scale, with the aim of studying two types of alloy in particular: Gold-Copper (Aux-Cu1-x) and Nickel-Aluminium (Nix-Al1-x).
Using laser synthesis of 5 nm facetted octahedral Aux-Cu1-x NPs and aberration-corrected electron microscopy observations in probe mode, we developed a method for analyzing the chemical composition of each atomic plane. In this way, we have demonstrated a strong segregation effect of gold on the surface, as well as different concentration profiles within the NPs depending on the chemical order (ordered or disordered). In the case of an ordered Au0.5Cu0.5 composition of L10 phase, we have characterized a structure rarely observed until now, corresponding to the presence of the three possible variants of L10 phase within the same particle. In parallel, atomic-scale simulations have enabled more precise analyses to be carried out, considering infinite plane stacks and NPs of different sizes and compositions.
The excellent agreement between simulations and experimental analyses strengthens the relevance of our results and demonstrates the importance of this dual approach, which we subsequently applied to the study of the surface properties of Nix-Al1-x-type NPs. First, we optimized the synthesis parameters to obtain NPs with defined sizes and compositions. Experimental surface analyses coupled with atomistic simulations enabled us to observe a hitherto unseen phenomenon. Indeed, an almost complete segregation of the aluminum appears until the formation of NPs adopting a core (Nickel) – shell (Aluminum) structure, for all the concentrations studied, thus preventing any alloy formation. This is all the more surprising given that, in the bulk state and for a composition of 50% nickel and 50% aluminium, the ordered B2 phase, known for its stability and resistance to corrosion, appears. These striking structural differences between the nanometric and macroscopic scales once again demonstrate the unique physics that exist in the world of the infinitely small.

PhD Candidate :
Grégoire Breyton

Jury :

Dr. Christine Goyhenex – IPCMS – Referee
Pr. Claude Henry – CINaM – Referee
Dr. Pascale Bayle – CEA/Grenoble – Reviewer
Dr. Geoffroy Prévot – INSP – Reviewer
Dr. Hakim Amara – LEM – PhD co-supervisor
Pr. Christian Ricolleau – MPQ – PhD supervisor

Friday 15th December 2023 at 14h00
Pierre-Gilles de Genes Amphitheater, Paris Cité University, Paris

Impact of Mechanical Loading on Deformation and Electronic Properties of Metallic Nanoparticles

Abstract

Metallic nanoparticles (NP) possess unique properties, distinct from bulk materials, offering potential application in mechanics, catalysis and optics. This thesis examines how NPs’ mechanical properties, influenced by shape, size, and composition, affect their electronic properties. Using Molecular Dynamics and Finite Element simulations, we demonstrate shape’s significant effect on the effective elastic response. Our findings highlight that plasticity is controlled by both shape and size with a universal size effect for face-centered-cubic crystalline NPs. In alloyed structures, both strengthening and softening mechanisms are observed, indicating local order’s influence on elasticity and plasticity. Finally, through tight-binding and ab initio calculations, we reveal that plastic deformation creates new reactive NP surface sites.

PhD candidate:
Matteo Erbi’

Jury:

Pr. Riccardo Ferrando – University of Genoa (Italy)- Referee
Dr. Julien Godet – University of Poitiers – Referee
Pr. Francesco Montalenti – University of Milan-Bicocca (Italy) – Reviewer
Dr. Christine Mottet – CINaM – Reviewer
Dr. Fabio Pietrucci – Sorbonne University – Reviewer
Dr. Barbaru Putz- Empa (Suisse) – Reviewer
Dr. Riccardo Gatti – LEM – PhD co-supervisor
Dr. Hakim Amara – LEM – PhD supervisor

Friday 24th November 2023 at 2 pm
Contensou Room, ONERA, 29 Avenue de la Division Leclerc,92320, Chatillôn

 

Synthèse par CVD de films de nitrure de bore aux propriétés optimisées pour dispositifs en optoélectronique

Résumé

Dans la famille des matériaux bidimensionnels (2D), le nitrure de bore a été identifié comme un matériau stratégique. Ce semi-conducteur à grand gap (>6eV), atomiquement plan, résistant chimiquement et thermiquement, peut jouer plusieurs rôles dans les hétérostructures de matériaux 2D : substrat de graphène pour préserver la mobilité exceptionnelle de ses porteurs de charge ou couche encapsulante pour protéger d’autres matériaux 2D sensibles à leur environnement ou exalter leurs propriétés. Des démonstrateurs de principe ont été réalisés avec des monocristaux de BN. Les dimensions latérales et l’homogénéité en épaisseur du BN sont limitées par la dimension initiale millimétriques des cristaux et leur mise en oeuvre par exfoliation mécanique. Cette technique est donc difficilement industrialisable. Il est nécessaire de développer des synthèses de films de BN de dimensions, structure et qualité contrôlées pour permettre une montée en échelle. Dans cette thèse en partenariat avec la PME Annealsys, nous avons choisi de développer la synthèse de films de BN sur nickel par dépôt chimique en phase vapeur à basse pression (LPCVD). Dans un premier temps, nous avons transposé sur le bâti de l’équipementier Annealsys le procédé de synthèse de BN sur des substrats de nickel polycristallin à partir de borazine déjà maitrisé par l’équipe. Nous avons confirmé que la morphologie et la qualité du BN dépend de l’orientation cristallographique du nickel sous-jacent et que l’orientation (111) du nickel est la plus favorable pour la synthèse de film continu de BN. Nous avons donc ensuite travaillé avec des substrats monocristallins de Ni(111) /YSZ/Si(111). Nous avons porté une attention particulière à la préparation de ces substrats spécifiques et développé un traitement de stabilisation in-situ dans le bâti de dépôt, compatible avec un procédé industriel. La structure et la qualité des films de BN synthétisés, i.e. épaisseur, rugosité, séquence d’empilement, cristallinité et taille de domaines, ont été caractérisées de l’échelle atomique à l’échelle millimétrique par un panel de techniques de microscopies et spectroscopies (AFM, MEB, Raman, MET. . .). Nous avons mis en place une méthodologie de caractérisation statistique à l’échelle centimétrique, indispensable à la vérification de l’homogénéité des films de BN, prérequis pour la fabrication de dispositifs performants. Nous avons fait varier des paramètres de synthèse clés tels que la quantité de gaz précurseur ou l’épaisseur du substrat de nickel et étudié leur impact sur les films de BN. Les résultats sont discutés d’un point de vue mécanisme de croissance.

 

Candidate:
Laure Tailpied

Jury:
Pr. Luc Imhoff – Université de Bourogne- Rapporteur
Dr. Laëticia MARTY – Université Grenoble Alpes – Rapporteur
Dr. Berangère Toury – Université Lyon 1 – Examinatrice
Pr. Franck Vidal – Sorbonne Université – Examinateur
Dr. Jean-Manuel Decams – Annealsys – Invité
Dr. Amandine Andrieux-Ledier – ONERA – Encadrante
Dr. Annick Loiseau – ONERA – Directrice de thèse

Mercredi 25 avril 2023 à 14h00
Salle Contensou, ONERA, 29 Avenue de la Division Leclerc,92320, Chatillôn

Optical properties of black phosphorus: from bulk crystal to atomic layers

Abstract

Black phosphorus is a small gap semiconductor (about 0.3 eV) that has recently joined the family of two-dimensional materials. Due to its modulable band gap from mid-infrared to visible depending on the thickness, its strong anisotropy in the atomic plane as well as the high mobility of charge carriers it is promised to a high application potential in the field of optoelectronics. The objective of this thesis was to study the optical properties of the black phosphorus crystal and its atomic layers.


After a description of the different instrumental developments realized during this thesis, the methods of
fabrication of the samples are discussed. Two points have to be mastered: The elaboration of thin layers and their protection from environmental conditions to avoid their oxidation. In a first part, several methods known as “Top-Down”(mechanical exfoliation, gold assisted exfoliation, ion etching) are compared on the basis of the quality, the size, the thickness of the obtained samples as well as the ease of the operating mode execution. In a second part, two methods of thin film protection are presented: alumina passivation (by ALD or aluminum evaporation) and encapsulation of BP flakes into hBN flakes (hBN / BP / hBN heterostructures).


The strong anisotropy of black phosphorus makes the identification of the orientation of the crystallographic axes a key point in the study of the material. For this purpose, a procedure has been proposed using polarized Raman spectroscopy. It has been confronted and validated by different experimental means (TEM observations, EBSD) and theoretical means (modeling of the Raman intensity in thin films). The vibrational properties have also been studied as a function of the number of atomic layers. Several effects have been noticed at high (> 100 cm-1) and low (< 100 cm-1) frequencies and are attributed to dimensionality reduction and resonance phenomena. Thanks to the peculiar excitation conditions used in this study, a large number of modes related to inter-plane vibrations are for the first time identified and have been shown to be accurate indicators of crystallite thickness. The photoluminescence of the bulk crystal is for the first time studied at room and cryogenic temperatures.

Several band-edge emission components have been identified as excitonic, including a fine line due to the free exciton. The analysis of their behavior as a function of temperature as well as a calculation of the binding energy of the free exciton taking into account the anisotropy of the medium have made it possible to establish a new reference value for the black phosphorus gap at 0.287 eV at 2 K. The photoluminescence study of the exfoliated crystals revealed the disappearance of the fine line of luminescence in favor of a wide band. This change is attributed to the density of defects introduced by the mechanical exfoliation as evidenced by a broadening of Raman bands. The photoluminescence band was followed as a function of the thickness of the exfoliated layers down to 8 atomic layers. Below a threshold thickness evaluated at 25 nm, a shift of the band towards high energies is highlighted, and is very well described by a quantum confinement model. No significant difference is observed between the alumina passivated and hBN encapsulated samples, which indicates that the dielectric effects are not predominant in the thickness range studied.

Phd Candidate:
Etienne Carré

Jury:
Christophe TESTELIN – Directeur de recherche, CNRS, Sorbonne Université – Rapporteur
Laëticia MARTY – Chargée de recherche, CNRS,Université Grenoble Alpes – Rapporteur
Bruno MASENELLI – Professeur des universités, INSA Lyon – Examinateur
Aurélie PIERRET – Ingénieure de recherche, CNRS, École Normale Supérieure Paris – Examinatrice
Pierre SENEOR – Professeur des universités, CNRS, Université Paris Saclay – Examinateur
Annick LOISEAU – Directrice de recherche, ONERA, Sorbonne Université – Directrice de thèse
Julien BARJON – Professeur des universités, UVSQ – Directeur de thèse
Ingrid STENGER – Maîtresse de conférences, UVSQ – Encadrante

 

Thursday 23th June 2022, 14h30
Salle Contensou, ONERA, 29 Avenue de la Division Leclerc,92320, Chatillôn

Modelling of the propagation of a short crack in ductile material coupling phase-field method and dislocation dynamics

Abstract

The propagation of short cracks in FCC metals is strongly influenced by microstructures, in particular associated with the linear defects of the crystals, i.e. dislocations.

In this work, a new coupling between two methods at the mesoscale is proposed to investigate the interaction of moving cracks with three-dimensional dislocation microstructures. First, crack propagation is predicted by a phase field model. In this approach, cracks are described by some continuous damage field that evolves so as to minimize the total free energy, including stored elastic energy and surface energy associated with the crack. Second, dislocation microstructures are handled by a Dislocation Dynamics (DD) model that describes plastic deformation by the movement of dislocations under external loading.

To couple both models, the DCM (Discrete-Continuous Model) approach is used, where dislocations are described by continuous fields (eigenstrain or Nye tensor) in an elastic solver. Fast Fourier Transform (FFT) based solvers are used for their computational efficiency. Particular discretization schemes have been adopted to minimize the smoothing of dislocation cores, usually performed in MDC approaches. The different schemes are carefully analyzed with respect to the quality of the predicted fields. In addition, the resulting model is implemented using efficient parallelization solutions.

Thanks to this new coupling, we have been able to study the elastic shielding on crack propagation according to the nature of the slip systems and the dislocations density. We have also been able to investigate phenomena and ingredients rarely accounted for, such as dislocation cross slips close to the crack front or the influence of the number of sources. This mesoscale method constitutes a breakthrough for the thorough analysis of physical mechanisms controlling the early stages of fracture in metallic materials.

Keywords : Crack, Plasticity, Multiphysics modelling, Dislocation Dynamics, Phase Field

Phd Candidate:
Luis Eon

Jury:
Stéphane Berbenni – Directeur de Recherche CNRS, LEM3, Metz – Rapporteur
Samuel Forest –  Directeur de Recherche CNRS, CDM, Evry  – Rapporteur
Véronique Doquet – Directrice de Recherche CNRS, LMS, Palaiseau  – Examinatrice
Lionel Gélébart – Ingénieur-chercheur HdR,  CEA/DEN, Gif-sur-Yvette – Examinateur
Rénald Brenner – Directeur de Recherche CNRS, D’Alembert, Paris – Examinateur
Yoann Guilhem – Maître de conférences, LMPS, Gif-sur-Yvette – Examinateur
Riccardo Gatti – Chargé de Recherche CNRS, LEM, Châtillon – Encadrant de thèse
Benoît Appolaire – Professeur des Universités, IJL, Nancy – Directeur de thèse

 

Tuesday 14th June 2022, 10h00
Salle Contensou, ONERA, 29 Avenue de la Division Leclerc,92320, Chatillôn

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