Synthesis and optical properties of quantum dots
Graphene as a constituent of graphite was close to us for almost 500 years. However, it is only in 2005 that A. Geim and K. Novoselov (Nobel Prize in 2010) reported for the first time the obtaining of a nanostructure composed by a single layer of carbon atom. The exceptional electronic properties of graphene make it a very promising material for applications in electronic and renewable energies.
For many applications, one should be able to modify and control precisely the electronic properties of graphene. In this context, we propose to synthesize chemically graphene nanoparticles and study their absorption and photoluminescence properties. We will focus on families of elongated nanoparticles, with the aim of studying how size can enable us to observe and control multiexcitonic processes in these materials. This project will be developed in collaboration with Physicists so the candidate will work in a multidisciplinary environment.
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)
TeraHertz Landau emission in HgTe/CdTe topological quantum wells
Quantum well heterostructures of HgTe/CdTe are known as topological insulators. They inherit very peculiar electronic properties. One of them is the ability of producing TeraHertz emission from inter-Landau energy level optical transitions. These transitions can be envisioned to lead to coherent optical sources in spectral range where they are basically absent. The PhD Thesis consists in elaborating and characterizing HgTe/CdTe multiple quantum well structures by epitaxy, process them in order to add functionality through optical cavities metallic report or deposition and electrical gating, and finally carry out full range optical spectroscopy of Landau emission in magnetic fields. The PhD will be carried out in a collaborative environment between CEA-Leti and Institut Néel (CNRS) in Grenoble, France, two leading laboratories in the expertise in material growth and Physics of HgTe/CdTe topological insulator systems. The results will help to understand the potential of application of this peculiar material system in TeraHertz laser sources and hopefully lead to the first demonstration of spontaneous emission in the TeraHerz range.
Electrocatalyzed Reductive Couplings of Olefins and Carbonyls for the synthesis of sustainable molecules.
The LCMCE aims to develop a sustainable method for the reductive functionalization of carbonyl derivatives with olefins via electrochemistry. Traditional redox processes in organic synthesis often rely on thermochemical methods using stoichiometric oxidants or reductants and produce waste products. The electrification of these processes will improve their atom- and energy economy. The novelty of this project lies in the generation of "metal-hydride" catalytic species by cathodic reduction of organometallic complexes in the presence of protons rather than by adding chemical reductants, as described in the literature. Inserting an alkene function into the metal-hydride bond will lead to the formation of reactive intermediates for coupling with electrophilic carbonyls. The substrates for this project have been selected to provide rapid proof of concept and allow the study of more ambitious reactivities, including carboxylation reactions in which CO2 is the electrophile. Particular attention will be paid to the design of homogeneous catalysts and their synergy with electrochemical conditions to lead to active and selective species. The project will also focus on deciphering the mechanisms involved in these reactions.
Theoretical design of quasi-atomic systems in the band gap of semiconductors/insulators for quantum application
The rise of room-temperature applications like single photon emission of the negatively charged nitrogen-vacancy NV center in diamond has renewed the interest in the search for materials having a quasi-atomic system QAS analogous to that of NV, mainly characterized by the presence of well localized in-gap defect levels generate occupied by electrons and leading to high spin states. In this Ph.D. work, theoretical methods will be used to design new QASs analogous to the NV center as well as, in selected QAS, to predict charge states and explore the effect of the proximity of the surface on the thermodynamic stability and on the spin state structure. The objectives are to design new QASs; To predict charge states of selected QASs in the bulk of the host material; To study changes in the charge state brought by the proximity of the surface; To extend the Hubbard model used to compute the excited states and to account for the electron-lattice interaction in the calculation of the excited states; To study the effect of the presence of deep level states in the band gap on the transport of electrons and phonons. The methodology developed at LSI to design new QASs with high spin states will be exploited and new systems analogous to the NV center will be looked for. Density functional theory (DFT) and a Hubbard model developed at LSI will be the main tools of this PhD.
Porphyrin-based nanostructures
The aim of this project is the synthesis of new molecular structures based on porphyrins for the formation of 0D, 1D and 2D nanostructures. Porphyrins are an important class of molecules that are essential to life through oxygen transport or photosynthesis. Beyond, their importance in Nature, porphyrin derivatives exhibit outstanding optical, electronic, chemical and electrochemical properties that make them promising candidates for applications in catalysis, electrocatalysis, optoelectronics and medicine.
In this project, the porphyrins will be studied in collaboration with several groups of Physicists in order to fabricate 1D or 2D covalent networks on surface via the "bottom-up" approach and to study their electronic and optical properties.
Interfaces in super-concentrated aqueous electrolytes: machine learned simulations at the exascale era
Improving the performance of liquid electrolytes is one of today's major challenges in the field of batteries, with the aim of improving efficiency, safety and economy. Recent advances include superconcentrated media such as WIS (“Water-In-Salts”) solutions. Their properties depend crucially on the chemistry and physics of the interfaces between water and ions (Li+ for lithium-ion batteries, but also Na+, K+, Zn2+), both at a distance and close to the electrodes.
Atomic-scale modeling of these superconcentrated liquid electrolytes requires the study of nanoscopic structures and phenomena taking place over long timescales. One relevant solution is to build potentials by machine learning, based on ab initio molecular dynamics (AIMD) trajectories. This method combines an accurate description of the interactions between ions and water molecules, including the breaking and forming of chemical bonds, with fast calculation speed. In particular, the DeePMD kit has recently been successfully ported to GPU architectures, paving the way for calculations on exascale supercomputers (whose power exceeds 10^18 floating-point operations per second).
This theoretical study will be supported by an experimental counterpart, thanks to direct collaboration with a team in the unit specializing in electrochemistry.
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.