Spectrometry and Artificial Intelligence: development of explainable, sober and reliable AI models for materials analysis
The discovery of new materials is crucial to meeting many current societal challenges. One of the pillars of this discovery capacity is to have means of characterizing these materials which are rapid, reliable and whose measurement uncertainties are qualified, even quantified.
This PhD project is part of this approach and aims to significantly improve the different ion beam induced spectrometry (IBA) techniques using advanced artificial intelligence (AI) methods. This project aims to develop explainable, sober and reliable AI models for materials analysis.
The PhD project proposed here has three main objectives:
- Develop an uncertainty model using probabilistic machine learning techniques in order to quantify the uncertainties associated with a prediction.
- Due to the very large number of possible combinatory-generated configurations, it is important to understand the intrinsic dimensionality of the problem. We wish to implement means of massive dimensionality reduction, in particular non-linear methods such as autoencoders, as well as PIML (Physics Informed Machine Learning) concepts.
- Evaluate the possibility of generalization of this methodology to other spectroscopic techniques.
Theoretical studies of orbitronic and spin-orbit phenomena in heterostructures comprising van der Waals materials, metals and oxides
The proposed PhD thesis aims at finding the best-unexplored combinations of transition metals, oxides and 2D materials (transition metal dichalcogenides, 2D magnets, graphene…) to help optimizing and providing scientific underpinnings of next generation energy efficient spintronic storage and memory devices based on emerging fields of spin-orbitronics and orbitronics. The latter is a fascinating new field of research that exploits orbital currents and their interaction with spin currents mediated by spin-orbit coupling.
Namely, using first principles calculations combined with tight-binding approach and linear response theory, we will screen the potential of aforementioned heterostructures not only for spin-orbit phenomena such as Dzyaloshinskii-Moriya interaction (DMI), perpendicular magnetic anisotropy (PMA) and spin-charge interconversion based on Rashba and Rashba-Edelstein effects (REE), but also focus on Orbital Rashba Edelstein Effect (OREE). Furthermore, the mechanisms of control of these phenomena via external stimuli (strain, external electric and magnetic fields) will be investigated as well. These studies will help finding optimal material combination to tune DMI, PMA and spin-charge interconversion efficiency to help optimizing spintronic devices making thereby a significant contribution to the development of sustainable microelectronics.
The PhD will be based on a multiscale approach including ab initio, tight-binding and atomistic approaches thus highly motivated candidate with a good background in solid state physics, condensed matter theory and numerical simulations is required. He/she will perform his/her calculations on Spintec computational cluster nodes using first-principles packages based on density functional theory (DFT) combined with other simulation codes/tools. Results obtained will be carefully analyzed with the possibility of publication in international scientific journals. Strong collaboration with labs in France (CEA/LETI, Laboratoire Albert Fert (CNRS,Thales), Aix-Marseille Univ…) and abroad (ICN2-Barcelona, PGI Forschungszentrum Jülich, Osaka University) are previewed.
Deciphering the roles of surface chemistry and multi-scale structuration in controlling the storage performances of graphene-based supercapacitors
Summary of the project: The project’s objective is to advance fundamental research by elucidating the intrinsic relationship between the properties of graphene-based material and their electrochemical storage performances in supercapacitor cells, thanks to the combination of basic and advanced characterization techniques, particularly adapted to the investigation of the evolutions of the surface chemistry and multi-scale structure upon cycling. These findings will enable to provide a multi-scale understanding of storage mechanism and will help to further design materials with enhanced storage properties.
Fluorescence photoswitching for excitonic gate
Förster Resonance Energy Transfer (FRET) enables the exciton diffusion between molecules through a characteristic distance of 1 to 10 nm. The association of multiple fluorophores represents a solution to facilitate exciton diffusion over a longer range, taking profit of homo-FRET and hetero-FRET phenomena. FRET is a fundamental aspect in the development of photo-switchable luminescent devices. At the molecular level, the design of photo-switchable systems relies on the association of two components: a luminescent material and a photochromic compound. The formation of nano-objects with similar molecules leads to intriguing responses in fluorescence and photochromic behavior due to multiple energy transfers. However, these systems are poorly used in molecular logic, and they switch between bright and dark states. Considering an emissive acceptor (a second bright state) would allow exciton diffusion over longer distances and enable its detection.
The FLUOGATE project objective is the preparation and characterization of photoswitchable luminescent molecular nanostructures that behaves as an excitonic gate. The initial step is the preparation and study of 2D photoswitchable monolayers with controlled organization. The combination of optical and local probe measurements will permit the characterization of fluorescence photoswitching following the structural change at the single molecule scale and determination of the quenching radius. Then, the preparation and study of 3D architectures will be undertaken. The strategy entails the successive deposition of various dyes. Layers of the donor fluorophore will be deposited just above the substrate, followed by layers of the photochromic compound and finally layers of the acceptor fluorophore. The ultimate goal will consist in exploring the replacement of the photochromic layer by a photochromic nanoparticle in a polymer matrix.
Understanding the fundamental properties of PrOx based oxygen electrodes through ab-initio and electrochemical modelling for solid oxide cells application
Solid Oxide Cells (SOCs) are reversible and efficient energy-conversion systems for the production of electricity and green hydrogen. Nowadays, they are considered as one of the key technological solutions for the transition to a renewable energy market. A SOC consists of a dense electrolyte sandwiched between two porous electrodes. To date, the large-scale commercialization of SOCs still requires the improvement of both their performances and lifetime. In this context, the main limitations in terms of efficiency and degradation of SOCs have been attributed to the conventional oxygen electrode in La0.6Sr0.4Co0.2Fe0.8O3. To overcome this issue, it has recently been proposed to replace this material with an alternative electrode based on PrOx. Indeed, this material has a high electro-catalytic activity for the oxygen reduction and good transport properties. The performance of cells incorporating this new electrode is promising and might enable to reach the targets required for large-scale industrialization (i.e. -1.5A/cm2 at 1.3V at 750°C and a degradation rate of 0.5%/kh). However, it has been shown that PrOx undergoes phase transitions depending on the cell operating conditions. The impact of these phase transitions on the electrode properties and on its performance and durability are still unknown. Thus, the purpose of the PhD is to gain an in-depth understanding of the physical properties for the different PrOx phases in order to investigate their role in the electrode reaction mechanisms. The study will contribute to validate whether PrOx based electrodes are good candidates for a new generation of SOCs and help to identify an optimized electrode using a methodology combining ab-initio calculation with electrochemical modelling.
Fracture dynamics in crystalline layer transfer technology
Smart Cut™ is a technology discovered at CEA and now industrially used for the manufacture of advanced substrates for electronics. However, the physical phenomena involved are still the focus of numerous studies at CEA. In Smart Cut™, a thin material layer is transferred from one wafer to another using a key fracture annealing step upon which a macroscopic fracture initiate & propagates at several km/s [i].
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Improving technology requires a solid understanding of the physical phenomena involved in the fracture step. The aim of this PhD project is thus to address the mechanisms involved in fracture initiation, propagation and post-fracture vibrations
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On the CEA-Grenoble site, with industrial interest, the student will use and further develop existing experimental setups to investigate the fracture behavior in brittle materials, including optical laser reflections [iv], time-resolved synchrotron diffracting imaging [iii], and ultra-fast direct imaging [ii].
In addition, python-based data analysis algorithms will be developed to extract quantitative information from the different datasets. This will enable the student to determine involved mechanisms and evaluate the influence of the wafer processing parameters on the fracture behavior, and thus propose improvement methods.
References :
[i] https://pubs.aip.org/aip/apl/article/107/9/092102/594044
[ii] https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.15.024068
[ii] https://journals.iucr.org/j/issues/2022/04/00/vb5040/index.html
[iv] https://pubs.aip.org/aip/jap/article/129/18/185103/158396
Chemical recycling of oxgenated and nitrogenated plastic waste by reductive catalytic routes
Since the 1950s, the use of petroleum-based plastics has created a modern consumerist world based on the use of disposable products. Global production of plastic waste is therefore considerable, and has almost doubled 20 years, now reaching 468 million tons/year. This non-biodegradable plastic waste causes a great deal of environmental pollution (disturbance of flora and fauna, water and soil pollution, etc.). Barely 9% of this waste is recycled, the rest being burnt or landfilled. The health, climate and social problems inherent in this linear economy mean that we need to create a circularity for these materials by developing effective and robust recycling routes. While current recycling methods rely mainly on mechanical processes and are limited to specific types of waste (e.g. plastic water bottles), the development of chemical recycling methods seems promising for treating waste for which there are no recycling channels. Such chemical processes make it possible to recover the carbonaceous matter in plastics in order to regenerate new plastics.
Within this objective of material circularity, this doctoral project aims to develop new chemical recycling routes for mixed oxygen/nitrogen plastic waste such as polyurethanes (insulation foam, mattresses, etc.) and polyamides (textile fibres, circuit breaker boxes, etc.), for which recycling routes are virtually non-existent. This project is based on a strategy of depolymerizing these plastics by the selective cleavage of the carbon-oxygen and/or carbon-nitrogen bonds to form the corresponding monomers or their derivatives. To do that, catalytic systems involving metal catalysts coupled with abundant and inexpensive reducing agents, such as alcohols and formic acid, will be developed. The use of dihydrogen, an industrial reducing agent, will also be considered. In order to optimize these catalytic systems, we will seek to understand how they proceed and the mechanisms involved.
Two-qubit gate made with Germanium heterostructures
We are working on germanium spin qubits, a promising and versatile base material to engineer spin quantum bits. In these "heterostructures", holes are hosted in a germanium layer sandwiched between two layers of silicon/germanium. These holes exhibit a very high mobility and unlike electron spins which are only sensitive to magnetic fields, hole spins can be manipulated by an electric field, ie by voltages on a gate. The all-electrical control comes with its own drawback: spins become sensitive to electrical, and therefore charge noise in the devices. The germanium heterostructures feature metallic top gates that mostly screen the charge noise from defects they covered; however, in regions not covered by top gates, unscreened charges are responsible for charge noise limiting the coherence time.
We are acquiring a world unique cleanroom equipment combining atomic layer deposition and atomic layer etching, which will allow for the development of original structures where the gates are penetrating deep within the heterostructure, in order to circumvent the effect of these lone charges on the surface in the case of top gates. With this novel scheme, the definition and manipulation of quantum dots will be extremely simplified, and we plan to obtain two-qubit gate devices well within the scope of this PhD.
Study of catalysis on stainless steels
The materials (mainly stainless steels) aging of the spent nuclear fuel reprocessing plant is the focus of an important R&D activity at CEA. The control of this aging will be achieved by a better understanding the corrosion mechanisms the stainless steels in nitric acid (the oxidizing agent used in the reprocessing steps).
The aim of the PhD is to develop a model of corrosion on a stainless steel in nitric acid as a function of temperature and the acid nitric concentration. This PhD represents a technological challenge because currently few studies exist on in situ electrochemical measurements in hot and concentrated nitric acid. The PhD student will carry out by coupling electrochemical measurements, chemical analyses (UV-visible-IR spectrometry...) and surfaces analyses (SEM, XPS,…). Based on these experimental results, a model will be developed, which will be incorporated in the future in a more global model of the industrial equipments aging of the plant.
The laboratory is specialized in the corrosion study in extreme conditions. It is composed of a very dynamic and motivated scientific team which has the habit to receive students.
Topological magnons in quantum materials
Topology has become an essential paradigm in condensed matter, making it possible to classify phases of matter according to properties that are invariant under continuous deformations. Early research has mainly focused on electronic band structures, leading to the discovery of “topological insulators” for example. However, there is growing interest in applying topological concepts to bosons, in particular magnons. Magnons, which are collective excitations in magnetic materials, illustrate how topology influences magnetic dynamics and affects heat and spin transport. Analogues of topological insulators and semi-metals appear in their band structures. Magnons thus offer a platform for studying the interplay between magnetic symmetries and topology, examining the effect of interactions on topological bands, and generating protected spin currents at interfaces. The search for materials containing topological magnons is therefore crucial, especially for applications in magnonics, which exploit spin waves for fast data storage and processing.
This thesis project is dedicated to exploring these topological aspects in candidate quantum materials using neutron and X-ray scattering techniques in large scale facilities (ILL, ESRF, SOLEIL) to probe the magnon band structure in search of topological features such as Dirac or Weyl points. Experimental results will be supported by numerical and theoretical calculations of magnonic bands incorporating topological concepts.