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.
Design of plasmonic nanocomposite membranes for biomolecule detection
Detection of specific small biomolecules amounts is usually challenging. Recently, nanomaterials have provided new materials with interesting optical properties for such an application, especially plasmonic nanomaterials.
In this project, we propose the design of a specific type of nanocomposite made from the incorporation of plasmonic nanoparticles (NPs) within track-etched functionalized polymer membranes. The tuning of the material plasmonic response will be achieved by a controlled in situ NP synthesis directly within the membrane nanopores, through chemical and physico-chemical processes. Especially, the use of radiation (electron beam, ?-rays) to induce the in situ reduction of the metallic precursor will be studied. Ionizing beams (Swift Heavy ions) will also serve to structure the polymer matrix in nanoporous membrane with controlled nanoporosity. The relation between the composite nanostructure and its optical properties will be thoroughly investigated in order to determine the ideal material for biomolecule detection, which will be tested on model molecules such as proteins or virus-like particles (VLPs) in the final part of the project.
Experimental study of boundary layers in turbulent convection by diffusive waves spectroscopy
Turbulent convection is one of the main drivers of geophysical and astrophysical flows, and is therefore a key element in climate modeling. It is also involved in many industrial flows. Transport efficiency is often limited by boundary layers whose nature and transitions as a function of control parameters are poorly understood.
The aim of this thesis will be to set up a convection experiment to probe the dissipation rate in boundary layers in the turbulent regime, using an innovative technique developed in the team: multi-scattered wave spectroscopy.
Topological superconductivity and Fermi surface in spin-triplet superconductors
Topological superconductivity has become a subject of intense research due to its potential for breakthrough in the field of quantum information. Bulk systems are a promising possibility, with candidates found mainly among unconventional superconductors, which are also strongly correlated electron systems. Today, only a few candidate compounds for topological bulk superconductivity exists, and they are mostly uranium-based heavy fermion superconductors. UTe2 is one of the most prominent candidates. The topological properties of the superconductors depends crucially on the topology of the Fermi surface.
In this project we want to set up a novel technique (for our team) relying on a tunnel diode oscillator circuit. This techniques is very sensitive to quantum oscillations, and to be well both to high magnetic fields and to high-pressure studies. First experiments concentrate on the novel superconductor UTe2, where the Fermi surface is only partly known. In further studies the topological properties of the ferromagnetic superconductors UCoGe and URhGe will be revised.
Sub-critical crack growth in oxide glasses
Material failure is a concern for scientists and engineers worldwide. This includes oxide glasses, which are integral parts of building, electronics, satellites due to multiple advantageous features, including optical transparency, elevated mechanical and thermal properties, chemical durability, biocompatibility and bioactivity, etc. Despite this, oxide glasses have a significate drawback: they are inherently brittle. Oxide glasses are well known to undergo dynamic fracture (crack propagation velocity of ~km/s – as in the case of a glass crashing to the floor and shattering); yet, there is another fracture mode less noticeable that will be studied during this thesis, where crack fronts grow sub-critically. The growth of these crack fronts is aided by environmental parameters including atmospheric humidity and temperature, and the crack front velocity depends on the local stress felt by a crack tip, coined the stress intensity factor.
Currently, our experimental setup tracks the crack front position in time via a tubular microscope equipped with a camera. Post-analysis of images provides the crack front velocity and reveals the environmental limit K_e and region I. However, the current experimental setup cannot capture regions II and III. Several factors play into this limitation: elevated crack front velocity (10e-4 to 1500 m/s), sample size (5×5×25 mm^3), camera acquisition rates, etc.
In recent years, our team has used the potential drop technique to track the crack front velocity when v > 10e-4 m/s in PMMA. This technique involves the deposition of conductive strips on the sample surface. Subsequently, these lines are attached to a high frequency oscilloscope. As the crack front propagates through the sample, the lines are severed resulting in an increase in the electrical resistance. We now wish to adapt this technique to DCDC samples on oxide glasses. The thesis goal is the development and application of the potential drop techniques to DCDC samples. The challenge concerns the spatial temporal resolution (50 µm and 1 ns) in comparison to the crack tip velocity and sample size. The thesis student will take part in all the steps to realize the experiments: designing and depositing patterns (series of strips) on the glass surfaces using a cleanroom, running sub-critical cracking experiments in Region II and III, and analyzing data acquired during the experiment.
Development of solid porous siliceous supports for actinide sorption - Behaviour under irradiation
The aim of this research project is to study the densification of a mesoporous structure under the effect of irradiation damage produced by the presence of an actinide (238Pu) in the porous structure. To achieve this, siliceous materials based on mesoporous silicas modified by the addition of additive elements (B, Al, etc.) will be used. The purpose of adding these elements is to weaken the mesoporous structure in order to promote densification. The characteristics of the mesoporous structure (pore diameter, wall size, symmetry of the pore network) will be other parameters of the study. These materials will be functionalised with phosphonate ligands for actinide adsorption: thorium as a simulant in a preliminary stage, followed by plutonium. The final part of this work, which will continue beyond the thesis, will involve using various techniques (SAXS, BET, microscopy, etc.) to study the evolution of the mesoporous structure under the effect of irradiation damage as the material ages. This fundamental research work could have spin-offs in the field of nuclear waste conditioning materials: ageing of gels on the surface of nuclear glass, support material for decontaminating radioactive effluents. Part of the work will be carried out at CEA Marcoule's Atalante facility.
Microemulsion model: Towards the prediction of liquid-liquid extraction processes
This multi-scale modeling thesis aims to develop innovative theoretical approaches and numerical tools to revolutionize strategic metal extraction processes, such as liquid-liquid extraction, whose underlying mechanisms remain poorly understood. To address these challenges, solvent phases will be represented as microemulsions through a synergy of mesoscopic and molecular modeling approaches.
The mesoscopic approach will involve the development of a code based on microemulsion theory using a random wavelet basis. This code will enable the characterization of the structural and thermodynamic properties of the solutions. The molecular approach will rely on classical molecular dynamics simulations to evaluate the curvature properties of the extractants, which are essential for bridging the two scales.
The new high-performance computational code may integrate artificial intelligence techniques to accelerate the minimization of the system’s free energy while accounting for all chemical species present with a minimal number of parameters. This will pave the way for new research directions, such as predicting speciation and calculating thermodynamic instabilities in ternary phase diagrams, thereby identifying unexplored experimental conditions.
This PhD thesis, conducted at the Mesoscopic Modeling and Theoretical Chemistry Laboratory at the Marcoule Institute for Separation Chemistry, will have applications in the recycling domain and extend to the broader field of nanoscience, thereby expanding the impact of this work.
The PhD candidate, with an academic background in physical chemistry, theoretical chemistry, or physics, and a strong interest in programming, will be encouraged to disseminate his/her scientific results through publications and presentations at national and international conferences. Upon completion of the thesis, the candidate will have acquired a wide range of skills in modeling and physical chemistry, opening numerous professional opportunities in both academic research and industrial R&D.
study of lithium isotopes separation by laser
This thesis concerns the study of different ways of separating lithium isotopes by laser. The work will be conducted both theoretically and experimentally. The objective is to determine an optimal way as well as its performances.
Influence of chromium doping of UO2 fuel on fission product speciation under accidental conditions
The development of nuclear reactors is part of a drive to improve safety, with, for example, the deployment of nuclear fuels with improved properties in terms of their behavior under accident conditions, the so-called E-ATF (Enhanced Accident Tolerant Fuel). Industrial operator FRAMATOME is developing the Cr2O3-doped UO2 fuel as E-ATF. However, very little data is available on the behavior of fission products from Cr-doped fuel under accident conditions.
This thesis proposes to develop a synthesis process for Cr-doped UO2 fuel that simulates irradiated fuel, in order to study the behavior of the elements (Cr and fission products) at different temperatures and under different oxygen partial pressures. The methodology is based on an experimental approach combining synthesis of model materials and in-depth chemical characterization, complemented by a theoretical approach (thermodynamic calculations) enabling thermal sequences to be dimensioned and the proposed reaction mechanisms to be confirmed.
The thesis will be carried out at CEA Cadarache (France), within IRESNE (Research Institute on Nuclear Systems for Low-Carbon Energy Production). The PhD student will be hosted in a laboratory dedicated to the study of uranium-based compounds in the Department of Fuel Studies (DEC). Depending on the densification processes chosen, experiments of various duration may be carried out in other laboratories in France or Europe.
The PhD student will have the opportunity to learn advanced characterization techniques for ceramic materials, gain access to experiments on large-scale instruments (synchrotron) and take part in exchanges with the academic community(CNRS, Universities, JRC). He or she will be able to promote his work through publications and participation in conferences.
At the end of this thesis, the PhD student will have acquired skills in materials science and solid state characterization that he/she will be able to use in various materials fields, as well as experience in the nuclear environment of interest to the nuclear industry.
Development of an automated and miniaturised system for the isotopic analysis of nuclear samples
Miniaturisation, which is the process of reducing an object’s, a method’s or a function’s dimensions while preserving or even upgrading its performances as compared to the classical scale, has a particular interest in the field of analytical chemistry for nuclear applications. Indeed, most of the analyses are performed in gloveboxes where miniaturisation and automation are a direct solution to the need for reduced doses and waste volumes. This PhD aims at developing a miniaturised and automated system, in a glove box, for performing high-precision isotopic measurements. This system will use capillary electrophoresis (CE) hyphenated with a nuclearised multicollector ICP-MS (MC-ICP-MS). During this PhD, the student will make use of micro-machining machines and 3D printers to develop an ergonomic system which will then be coupled to last generation MC-ICP-MS instruments available in our laboratory. The project will be focused on the conception of the automated system and its integration in the glove box, and on the further development of the existing CE method in order to perform isotopic and elemental analyses with nuclear samples. This PhD is hosted in a laboratory internationally recognized for its ability to carry out high precision isotopic measurements. An analytical chemistry curriculum is expected and a Master 2 internship is available before this PhD.