| Rencontres "Géométrie et Matériaux", 12-13 décembre 2019, Paris (Jussieu) |
Ces rencontres font suite aux premières recontres éponymes organisées les 13-14 juin derniers au LPCNO à Toulouse (voir ici) Il s'agit toujours d'explorer les contacts interdisciplinaires existants ou potentiels autour de la synthèse de nouveaux matériaux (en particulier nanostructurés, mais pas exclusivement). Ceci à travers des exposés offrant une synthèse d'un domaine spécialisé qui doit être accessible à une audience venue de domaines aussi divers que mathématiques, physique et chimie et des temps de discussion.
Cet évènement est financé par le projet 80-Prime CNRS INS2I-INC "Assemblage de Supercristaux par Approche Prédictive" (Thomas Fernique, LIPN, Simon Tricard, LPCNO). Voyage et/ou séjour peuvent être financés - faire la demande avant début novembre en joignant une courte lettre de motivation à votre inscription (ci-dessous).
L'inscription est gratuite (déjeuners et dîner compris) mais obligatoire pour des raisons de logistique. Merci d'envoyer un mail en précisant votre laboratoire de recherche (ainsi que le nom de votre encadrant si vous êtes doctorant ou stagiaire).
Les rencontres auront lieu sur le campus de Jussieu, salle 5-23, 5ème étage tour 12-13 (salle de séminaire du LPTMC) Le repas du jeudi soir sera un buffet sur un parquet de Penrose
| Jeudi 12 décembre | Vendredi 13 décembre | |
|---|---|---|
| 09h00 | café | café |
| 09h30 | CANCELLED | S. MARIN-AGUILAR |
| 11h00 | I. GALANOV | B. PANSU |
| Déjeuner | Déjeuner | |
| 14h00 | A. COURTY | E. FAYEN |
| 16h00 | P. KALUGIN | C. CHINAUX-CHAIX |
| 17h00 | Th. FERNIQUE | |
| 19h00 | Dîner |
When Michael Faraday tried to make thin gold foils in 1856, he accidentally created a liquid suspension of small gold particles with a ruby-red color. The surface of the particles in this “colloid” are charged and strongly repel each other, so that they keep a distance – a distance that is large enough for the exact shape of the particles to play a minor role in the properties of the colloid. This repulsion has kept the dispersion perfectly stable for more than 150 years (it still sits in the basement of The Royal Institution).
Now remove the electrical charge and imagine two inorganic cores with diameters below 10 nm that are covered with organic molecules. Simple alkyl chains with a sulfur end group can bind to the gold and form a “wax” shell on the metal. The resulting particles form colloids in nonpolar solvents, but the particles in such colloids will come much closer than those of Faraday. Consequently, their shape and the molecular arrangement of the organic shell has greater effects on their colloidal properties.
In this talk, I will discuss how the stability of nonpolar colloids depends on shape and structure. Scattering data and detailed molecular modelling with explicit consideration of the solvents have led us to a much better understanding of the underlying mechanisms, but several experimental results remain surprising. This leads to interesting fundamental questions that are tightly connected to the preparation of materials from particles, as I will show.
Aperiodic tilings are sort of jigsaw puzzles often used to model quasicrystals. We are here interested in modeling the growth (or self-assembly) of quasicrystals. Namely, we describe a simple local algorithm which appears to grow an infinite family of aperiodic tilings. Starting from a "seed" (a finite set of tiles), tiles are added one by one at randomly chosen sites. A tile is added at only if there is only one way to do this so that no "forbidden local configuration" is created. The point is that since there is no choice on how to add the tile, we cannot make a bad decision. This algorithm appears to rapidly grow large round-shaped patterns, up to a proportion of missing tiles which can be made arbitrarily small by taking a large enough seed.
Chemical synthesis methods of colloidal nanocrystals (NCs) currently allow fine control over their composition, size, crystallinity and shape. The strategies for organizing into ordered superlattices have closely followed the progress of the synthesis. The nature of the solvent, the crystallinity of the NCs, the nature of the ligands or the deposition mode (nature of the substrate, temperature...) are shown as key parameters, controlling the organization of the NCs in the form of colloidal super-crystals or ordered 3D films. Binary super-crystals consisting of the periodic arrangement of nanocrystals of different nature and/or size have also been obtained and allow the emergence of new properties in optics, magnetism or catalysis. As a result, examples of nanocrystal-based materials whose functionality derives from their mesoscale structure are multiplying and follow the progress made in synthesis and assembly.
Flat-branched semi-simplicial (FBS) complexes appear naturally as a way to describe the local order in structures of finite local complexity. Surprisingly, these simple geometric objects contain a wealth of information about the global order as well! In particular, their geometry may encode fault-tolerant matching rules for assembly of quasiperiodic patterns and predict precise values of density for different local configurations. For a larger class of structures, exhibiting pure-point diffraction, FBS-complexes provide constraints on partial diffraction amplitudes, even in the presence of disorder.
L’objectif de cet exposé est de présenter le contexte dans lequel s’inscrit la thèse appartenant au projet 80-Prime CNRS INS2I-INC "Assemblage de Supercristaux par Approche Prédictive". Le but de cette dernière est de synthétiser par auto-assemblages des matériaux nanostructurés en se basant sur des modèles théoriques permettant de déterminer les empilements. Nous parlerons donc des supercristaux de façon générale ainsi que du lien entre les réseaux observés dans la littérature et la théorie, puis des premiers axes vers lesquels tend cette étude.
When liquids avoid crystallization at low temperatures or high densities, the systems undergo to an extreme slowing down on dynamics. Determination of which physical phenomena drive this slowing down remains an area of active research. Recent studies have highlighted the emergence of long-lived local structures related to the arrest as an important point. This is the case of hard-sphere and colloidal systems where the particles are arrested by cages formed by their neighbours as the temperature of the supercooled liquid decreases. Here, I will discuss the tools for changing the local structure of the cages. In particular, I will show that by adding short-range directional interactions, we can induce changes in the local structure and that reinforcing icosahedral geometry the system presents slower dynamics [5]. Additionally, I will show the importance of simpler dense structures such as tetrahedral clusters in hard-sphere systems, and their correlation with dynamics.
L’objectif de cet exposé est d’offrir une présentation générale des différentes approches expérimentales utilisées dans l’étude des auto-assemblages de nanoparticules. Il est d’abord nécessaire de les caractériser en régime dilué en déterminant leur forme, leur taille moyenne, leur polydispersité, leur cristallinité, la densité de ligands qui les recouvrent ou la charge qu’elles portent en solution aqueuse…. Quand on augmente leur concentration, il est possible d’avoir accès à des informations sur les interactions de paires entre particules notamment par diffusion de rayonnement (RX). Il existe ensuite différentes techniques pour les auto-assembler basées sur diverses approches physico-chimiques qui permettent de contrôler plus ou moins le mécanisme et la cinétique d’assemblage. L’étude de la structure de ces auto assemblages fait intervenir soit des méthodes locales (microscopie électronique) soit des méthodes plus globales comme la diffusion de rayonnement (RX).
Complex phases, such as Laves phases and two-dimensional dodecagonal quasicrystals, have been observed in recent self-assembly experiments of binary nanoparticle mixtures. One of the simplest ways to model those experiments is to consider binary mixtures of large and small hard disks, in 2D. In the infinite pressure limit, the stability analysis of candidate phases reduces to finding the best packing structure for a given size ratio and number ratio of the large and small disks. In this presentation, I will present so-called Floppy-Box Monte Carlo simulations, a systematic technique to generate candidate crystal structures, used to build an infinite pressure phase diagram of binary mixtures of hard disks. The resulting phase diagram of this simple system exhibits a surprising number of different phases, including random tilings and quasicrystals.
Je prouverai de façon élémentaire que pour disposer des disques disjoints de même taille sur le plan de façon à maximiser la proportion du plan couverte, la meilleur façon est de faire un empilement hexagonal compact, c'est-à-dire de les centrer sur une grille triangulaire. Je discuterai des généralisations en dimensions supérieures (notamment 3, 8 et 24) et avec plus de tailles (deux disques, 2 sphères...).