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.
Innovative syntheses of perovzalates and rationalization of the formation mechanism by synchrotron methods
“Perovzalates” are a new family of hybrid perovskites based on oxalate, with around ten examples listed since 2019 (AILi3MII(C2O4)3, with A = K+, Rb+, Cs+, NH4+; M = Fe2+, Co2+, Ni2+). Just like conventional perovskites, they are potentially interesting for countless applications (catalysis, optics, solar etc.), presenting additional advantages linked to the oxalate anion, which allows the incorporation of larger cations than in other hybrid pervovskites, while preserving a crystal structure similar to oxide perovskites.
However, this class of new materials is still barely explored, and the syntheses far from being mastered: the few reports to date systematically produce mixtures of phases, and relate to single crystals taken from heterogeneous solutions. In this context, the major problem is to synthesize an extended class of pure perovzalates.
This thesis addresses this challenge by exploiting a property discovered in the laboratory: the crystallization of metal oxalates by co-precipitation in water passes through transient “mineral emulsions”, that is to say nano-droplets rich in reagents which separate from water. The originality of this thesis is to exploit the nanostructuring provided by these mineral emulsions, and to test in particular using nanotomographic techniques accessible in synchrotron if they make it possible to confine the cations until crystallization.
Functionalized aluminosilicate nanotubes for photocatalysis
Rising energy demand and the need to reduce the use of fossil fuels to limit global warming have created an urgent need for clean energy collection technologies. One interesting solution is to use solar energy to produce fuels. Low-cost materials such as semiconductors have been the focus of numerous studies for photocatalytic reactions. Among them, 1D nanostructures are promising because of their interesting properties (high and accessible specific surface areas, confined environments, long-distance electron transport and facilitated charge separation). Imogolite, a natural hollow nanotubes clay, belongs to this category. Its particularity does not lies in its chemical composition (Al, O and Si) but in its intrinsic curvature, which induces a permanent polarization of the wall, effectively separating photo-induced charges. Several modifications of these materials are possible (coupling with metal nanoparticles, functionalization of the internal cavity), enabling their properties to be modulated.We have demonstrated that this clay is a nanoreactor for photocatalytic reactions (H2 production and CO2 reduction) under UV illumination. In order to obtain a useful photocatalyst, it is necessary to extend photon collection into the visible range. One strategy considered is to encapsulate and covalently graft dyes acting as antennae in the cavity. The aim of this thesis is to synthesize imogolites with different internal functionalizations, to study the encapsulation and grafting of dyes into the cavity of these functionalized imogolites, and finally to study the photocatalytic properties.
Exploration of Diamond-Based Nanomaterials for (Sono)photocatalysis: Applications in Hydrogen Production and CO2 Reduction
Nanodiamonds (NDs) are increasingly being studied as semiconductors for photocatalysis, thanks in particular to the very specific positions of their valence and conduction bands, which can be modulated. For example, it has recently been shown that NDs can produce hydrogen under sunlight with an efficiency similar to that of TiO2 nanoparticles. Other studies also show the possibility of photogenerating solvated electrons from certain NDs for CO2 reduction or the degradation of stubborn pollutants.
With a view to accelerating the development of nanodiamond-based ‘solar-to-X’ technologies, we propose in this thesis to integrate nanodiamonds as photocatalysts in a sonophotocatalytic approach. Acoustic cavitation, generated by ultrasound, can improve mass transfer by dispersing catalytic particles and can produce additional reactive species (hydroxyl radicals, superoxides). It also emits sonoluminescence, which can promote the photogeneration of charges and should limit the recombination of charge carriers.
The first phase of the work will focus on the synthesis of nanodiamond-based photocatalysts, exploring various surface chemistries and their association with co-catalysts. Both classical and sonochemical synthesis methods will be used, as preliminary studies have shown that sonochemistry can effectively modify the surface chemistry of NDs. The photocatalytic properties of these materials will first be evaluated, leading to the design of a sonophotocatalytic cell. Further studies will explore the synergies between sonochemistry and photocatalysis for hydrogen production or CO2 reduction. This thesis will be carried out as part of a collaboration between the NIMBE at the Saclay CEA centre and the ICSM at the Marcoule CEA centre.
Very high energy electrons radiotherapy with beams from a wakefield accelerator
Research objectives:
Use numerical modelling to optimize the properties of laser-plasma accelerators in the 50 MeV-200 MeV range for VHEE radiotherapy:
(i) optimize the properties of a laser-plasma accelerator (energy spread, divergence) with electron beams injected from a plasma-mirror injector using the WarpX and HiPACE++ codes.
(ii) Study the impact of such electron beams on DNA using Geant4DNA.
This numerical modelling will then be used to guide/design/interpret experiments of radiobiology on in-vitro biological samples that are planned at our in-house 100 TW laser facility at CEA during the project. This will be carried out in the context of research project FemtoDose funded by the French National Research Agency.
The researcher will benefit from a large variety of training available at CEA on HPC and computer programming as well as training at our industrial partners (ARM, Eviden) and Université Paris Saclay, which has MSc courses in radiobiology and also hosts a research centre (INanoTherad) dedicated to novel radiotherapy treatments, gathering physicists, radiobiologists and medical doctors. The activities will be carried out in the framework of the Marie Sklodowska Curie Action Doctoral Network EPACE (European compact accelerators, their applications, and entrepreneurship)
Investigation and use of uranium glasses for optical neutron detection
The Dosimetry, Sensors and Instrumentation Laboratory of the CEA/IRESNE Cadarache develops, manufactures and operates neutron flux detectors used in the vicinity of and inside nuclear reactor cores. In addition to conventional detectors (fission chambers, collectrons, etc.), the laboratory is working on innovative measurement methods such as optical detectors, semiconductors, fiber scintillators, etc. As part of this PhD thesis, the laboratory wants to explore the potential of Uranium-doped glasses. These glasses are known to show bright fluorescence under various types of radiations. The main idea of this thesis is to try to exploit this fluorescence to detect the fission reactions induced when the glass is exposed to a neutron flux. This could enable the development of a new generation of optical neutron detectors halfway between a fission chamber and a scintillator.
The thesis will focus on two main topics:
- firstly, a detailed understanding of fluorescence mechanisms, and the synthesis of uranium glass with properties optimized for our needs (sensitivity, emission spectrum, isotopic vector, etc.). Synthesis will be carried out in partner laboratories;
- secondly, the development of a dedicated instrumentation, probably in the form of optical fibers, to test these prototypes in a reactor.
Exploration of the energy deposition dynamic on short time scale with laser-driven electron accelerator in the context of the Flash effect in radiotherapy
The objective of the thesis project is to analyze the physicochemical processes resulting from the extreme dose rates that can now be obtained in water with the ultra-short (fs) pulses of relativistic electrons produced by laser-plasma acceleration. Indeed, first measurements show that these processes are probably not equivalent to those obtained with longer pulses (µs) in the FLASH effect used in radiotherapy. To achieve this, we propose to analyze the dynamics of formation/recombination of the hydrated electron, an emblematic species of water radiolysis, to qualify and quantify the dose rate effect over increasingly shorter times. This will be done in three stages in support of the necessary and now accessible technological progress, to have a dose per pulse sufficient to directly detect the hydrated electron. First, with the existing UHI100 facility, using the scavenging of the hydrated electron by producing a stable species; then producing a less stable but detectable species in real time and increasing the repetition rate of the electron source. Finally, by using an innovative concept named a “hybrid target”, based on a plasma mirror as an electron injector coupled to a laser-plasma accelerator, delivering larger doses with a narrower energy spectrum, we will be able to develop pump-probe detection allowing access to the shortest times, and to the formation in clusters of ionization, of the hydrated electron and measuring its initial yield.
Plasma Mirrors: towards extreme intensity light sources and high-quality compact electron
Research objectives:
expand the capabilities of the WarpX Partice-In-Cell code for lower cost-to-convergence using mesh refinement.
Devise a high-charge high quality injector for laser-plasma accelerators.
Determine feasibility of the proposed scheme on a 100-TW-class laser system.
The researcher will benefit from a large variety of training available at CEA on HPC and computer programming as well as training at our industrial partners (ARM, Eviden) and Université Paris Saclay. The activities will be carried out in the framework of the Marie Sklodowska Curie Action Doctoral Network EPACE (European compact accelerators, their applications, and entrepreneurship)
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.
Nucleation, Growth, and Multi-Scale Structural Properties of Colloidal Nanoparticles of Actinide Oxides (Pu, U, Th)
Nanocrystalline oxides possess unique physicochemical properties, modulated by their size and local structure, making them promising for various technological applications. However, actinide oxide nanoparticles remain underexplored due to their radioactivity and toxicity. Nonetheless, studies dedicated to these species are growing, driven by environmental and industrial considerations, particularly for their involvement in current and future nuclear fuel cycles. This thesis focuses on plutonium, a key element in nuclear reactors. Its behavior in solution is complex, particularly due to hydrolysis reactions that lead to the formation of highly stable colloidal PuO2 nanoparticles. Although these species are now better described, the mechanisms leading to their formation remain largely unexplored.
The ambitious goal of this thesis is to uncover the fundamental mechanisms involved in the formation of these nanoparticles by adopting a systematic approach that combines a wide range of experimental parameters. These include the synthesis medium, temperature, reactant concentration, reaction time, and the contribution of sonochemistry. The focus will be on studying the nucleation and growth stages of these nanoparticles, as well as their structural properties in relation to the physicochemical conditions that influence their formation. Studies will be conducted at ICSM with Th, U, and Zr as analogs, and at the Atalante facility for Pu. In addition to standard laboratory techniques necessary for characterizing these systems, complementary experiments will be carried out on synchrotron lines (SOLEIL and ESRF) to thoroughly investigate the structural and reactive properties of these species and their precursors.