Wetting dynamics at the nanoscale
Wetting dynamics describes the processes involved when a liquid spreads on a solid surface. It's an ubiquitous phenomenon in nature, for example when dew beads up on a leaf, as well as in many processes of industrial interest, from the spreading of paint on a wall to the development of high-performance coating processes in nanotechnology. Today, wetting dynamics is relatively well understood in the case of perfectly smooth, homogeneous model solid surfaces, but not in the case of real surfaces featuring roughness and/or chemical heterogeneity, for which fine modeling of the mechanisms remains a major challenge. The main goal of this thesis is to understand how nanometric roughness influences wetting dynamics.
This project is based on an interdisciplinary approach combining physics and surface chemistry. The PhD student will conduct systematic model experiments, combined with multi-scale visualization and characterization tools (optical microscopy, AFM, X-ray and neutron reflectivity, etc.).
Thanks to the complementary nature of the experimental approaches, this thesis will provide a better understanding of the fundamental mechanisms of energy dissipation at the contact line, from the nanometric to the millimetric scale.
Understanding the signals emitted by moving liquids
Elasticity is one of the oldest physical properties of condensed matter. It is expressed by a constant of proportionality G between the applied stress (s) and the deformation (?): s = G.? (Hooke's law). The absence of resistance to shear deformation (G' = 0) indicates liquid-like behavior (Maxwell model). Long considered specific to solids, shear elasticity has recently been identified in liquids at the submillimeter scale [1].
The identification of liquid shear elasticity (non-zero G') is a promise of discoveries of new solid properties. Thus, we will explore the thermal response of liquids [2,3], exploit the capacity of conversion of mechanical energy into temperature variations and develop a new generation of micro-hydrodynamic tools.
At the nanoscopic scale, we will study the influence of a solid surface in contact with the liquid. It will be a question of studying by unique methods such as Inelastic Neutron Scattering and Synchrotron radiation, the dynamics of the solid-liquid interface using Very Large Research Facilities such as the ILL or the ESRF, as well as by microscopy (AFM). Finally, we will strengthen our collaborations with theoreticians, in particular with K. Trachenko of the Queen Mary Institute "Top 10 Physics World Breakthrough" and A. Zaccone of the University of Milan.
The PhD topic is related to wetting, macroscopic thermal effects, phonon dynamics and liquid transport.
Analysis of solid oxide cell degradation by transmission electron microscopy and atomic probe tomography
Nowadays, high-temperature electrolysis is considered as one of the most promising technology for producing green hydrogen. The electrolysis reaction takes place in a Solid Oxide Cell (SOC) composed of an oxygen electrode (made of LSCF or PrOx) and a hydrogen electrode (made of Ni-YSZ) separated by an electrolyte (made of YSZ). To accompany industrialization f SOCs, the durability still needs to be improved. The main performance losses are due to the degradation of the two electrodes. In order to propose an improvement, it is essential to gain a precise understanding of electrode degradation mechanisms. In this thesis, we thus propose to apply high-resolution transmission electron microscopy and atom probe tomography (SAT) to study electrode degradation after aging under current. On the one hand, advanced electron microscopy techniques coupled with energy dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS) will be applied. In addition, analyses carried out on a SAT will provide three-dimensional information particularly suited to the complex structure of the electrodes.
This work should provide a better understanding of the degradation mechanisms of high-temperature electrolysis cells. Recommendations for their manufacture can then be made to improve their lifespan.
Advanced characterization of ferroelectric domains in hafnia-based thin films
Les mémoires ferroélectriques à accès aléatoire (FeRAM en anglais) à base d'oxyde d’hafnium et de zirconium (HZO) sont intrinsèquement ultra-faibles en consommation grâce au mécanisme de changement de tension, au potentiel de mise à l'échelle du HZO en dessous de 10 nm et à la compatibilité CMOS complète. De plus, elles présentent une faible latence nécessaire à une grande variété d'applications de logique et de mémoire. La compréhension des mécanismes sous-jacents et de la cinétique du ‘switching’ des domaines ferroélectriques est essentielle pour une conception intelligente des FeRAMs avec des performances optimales.
Cette thèse porte sur la caractérisation complète des domaines ferroélectriques (FE) dans des films HZO ultra-minces. L'étudiant utilisera plusieurs techniques d'imagerie de surface (microscopie à force piézoélectrique, PFM, microscopie électronique à basse énergie, LEEM, et microscopie électronique à photoémission de rayons X, PEEM) combinées à des méthodes avancées de caractérisation operando (détection résolue dans le temps couplée au rayonnement synchrotron). Ce projet marquera une avancée importante dans la recherche fondamentale des mécanismes de basculement de polarisation des couches FE ultra-minces à base d'hafnium, en élucidant les effets spécifiques de l'interface électrode métallique/couche FE dans le comportement électrostatique des condensateurs étudiés. Il permettra à terme une avancée significative dans le développement industriel des mémoires émergentes ferroélectriques, essentielles pour les applications d'intelligence artificielle (IA) à grande échelle.
Thermally activated glide of screw dislocations in bcc metals
Thermally activated glide of dislocation is a key point for understanding the plastic deformation of metals. The screw dislocation in bcc metals is an archetypical case for which a large quantity of experimental data has been published in the scientific literature. It is then possible to compare these data to the theoretical predictions realized from the Vineyard statistical theory [1,2]. Such a theory is an essential tool allowing to perform a scale transition from atomistic computations toward macroscopic scale at which are realized the deformation tests.
The aim of our research will be to test Vineyard theory in comparison with molecular dynamics simulations [3]. Some preliminary computations have shown a significant discrepancy that is not present when we repeat the comparison for point-like defect as vacancies or self-interstitial atoms.
[1] Vineyard G.H., J. Phys. Chem. Solids 3, 121 (1957).
[2] Proville L., Rodney D., Marinica M-C., Nature Mater. 11, 845 (2012).
[3] Proville L., Choudhury A., Nature Mater. 23, 47 (2024).
Reactive metals corrosion in innovative binders – Experimental study and hydro-chemo-mechanical modelling
Nuclear waste management requires the packaging of several kinds of metal wastes for long-term storage. These wastes, which can be very reactive metals, are prone to corrosion and commonly immobilised into containers with hydraulic binders as embedding matrices. Innovative binders (low carbon cements, alkali activated materials) are thus developed to increase the packaging performances. The main objective of the European project STREAM (in the frame of the Eurad-2 program) is to evaluate the interactions between these metal wastes and the selected cement matrices. The PhD thesis purpose is to investigate the reactive metal corrosion in the selected binder with electrochemical techniques. A generic experimental protocol will be developed in order to determine the impact of the corrosion products growth at the metal/binder interface on the global mechanical behaviour of the binder-waste composite and possible micro-cracks occurrence. A post-mortem characterisation will be performed on the metal/binder microstructure with mechanical properties measurements of the materials at the interface, especially the corrosion products. Afterwards, these results will feed a simplified Hydro-Chemo-Mechanical (HCM) model aiming the simulation of corrosion consequences on the composite material behaviour. Subsequently, this model will be used for long-term simulation at the waste package scale.
This research project is aimed at a PhD student wishing to improve his/her skills in materials science both in the experimental field and in the modelling/simulation of coupled physicochemical phenomena.
Covalent 2D organic nanostructures by optically controlled cross-linking of molecular self-assemblies
The self-assembly of molecules on crystalline substrates leads to non-covalent 2D structures with interesting properties for various fields such as optoelectronics and sensors. The stabilization of these 2D networks into covalent networks, while preserving these properties, is a major challenge and a topical issue. Various demonstrations show that crosslinking can be triggered by thermal processes. Photocrosslinking, on the other hand, is poorly described and the few examples that have been found involve ultra-high vacuum conditions.
Building on our previously developed know-how and the additional expertise of chemist collaborators, we therefore propose to carry out photocrosslinking of 2D networks at atmospheric pressure. We will use a model oligophenyl system that will be functionalized to allow photocrosslinking towards the production of a covalent 2D network. The resulting networks will be characterized through the correlation of optical spectroscopy and local probe microscopy to monitor and highlight photo-induced cross-linking processes at wavelength scale.
Control of two-dimensional magnetism by structural and chemical engineering of van der Waals interfaces
2D materials exhibit tunable interlayer interactions due to weak van der Waals bonding, which influences magnetic ordering in 2D magnets. The stacking sequence and internal chemistry impact ferromagnetic (FM) or antiferromagnetic (AFM) ordering, as seen in materials like CrBr3, CrI3, and Fe5GeTe2, where doping with Co raises the Curie temperature and alters magnetic phases. Chemical disorder also affects magnetic properties, with Mn/Sb substitution promoting FM ordering in Mn(Bi,Sb)2Te4. However, understanding how the atomic structure affects macroscopic magnetic properties remains limited due to the coexistence of metastable configurations. Precise control over stacking and chemical order is needed to harness 2D materials' magnetic and quantum properties. Transmission electron microscopy (TEM), especially aberration-corrected STEM, is today one of the most powerful techniques, enabling atomic-scale imaging and spectroscopy, for studying structural and chemical properties of 2D materials. This PhD project aims to study the relationship between atomic structure, chemistry, and magnetic properties in epitaxial 2D layers like (Fe,Co)5GeTe2, combining growth via molecular beam epitaxy (MBE) with STEM-based structural and chemical analysis.
Magneto-ionic gating of magnetic tunnel junctions for neuromorphic applications
Magneto-ionics is an emerging field that offers great potential for reducing power consumption in spintronics memory applications through non-volatile control of magnetic properties through gating. By combining the concept of voltage-controlled ionic motion from memristor technologies, typically used in neuromorphic applications, with spintronics, this field also provides a unique opportunity to create a new generation of neuromorphic functionalities based on spintronics devices.
The PhD will be an experimental research project focused on the implementation of magneto-ionic gating schemes in magnetic tunnel junction’s spintronics devices. The ultimate goal of the project is to obtain reliable and non-volatile gate-control over magnetisation switching in three-terminal magnetic tunnel junctions.
One major challenge remains ahead for the use of magneto-ionics in practical applications, its integration into magnetic tunnel junctions (MTJ), the building blocks of magnetic memory architectures. This will not only unlock the dynamic control of switching fields/currents in magnetic tunnel junctions to reduce power consumption, but also allow for the control of stochasticity, which has important implications in probabilistic computing.
Study of electronic processes in nitride LEDs by electro-emission microscopy
Nitride LEDs are universally used for energy-efficient lighting. They are extremely efficient at low indium content and low current density, allowing to produce commercial white LEDs from a blue LED and a phosphor that absorbs blue and re-emits a broad spectrum in the visible range. However, nitride LEDs suffer from a drastic drop in efficiency at higher current densities and higher indium concentrations, for emission in the green or red. This is an obstacle to extending their use, in order to obtain higher efficiencies with less material, as well as better color rendering. These efficiency drops are partly due to an increase in three-particle Auger-Meitner processes, which are strongly impacted by local device heterogeneities, and can be reduced by specific engineering of structural defects in nitride materials. This thesis proposes to study the electronic processes in nitride LEDs in operando, using electro-emission microscopy. In particular, charge injection mechanisms in the active part of the LEDs and Auger-Meitner processes will be investigated and quantified. The spatial resolution of the technique will allow to characterize the role of heterogeneities (defects or alloy disorder) in the loss processes.