Metasurfaces based on plasmonic nanoparticles, which collect and concentrate light into subwavelength volumes, have enabled a whole new family of planar optical devices including light sources, energy harvesting systems, light wavefront engineering components, non-linear systems, biosensors and photocatalysis devices. Indeed, from a technological perspective, they offer several advantages over pure dielectric devices: reduced pixel area, sub-wavelength resolution and locally large field enhancement leading to low power consumption, high amplitude signals and local heating. The first successful plasmonic metasurfaces were made out of noble metals and patterned using lithography techniques. The situation might be about to change, however. First, in order to pattern the structures, it has been recently proposed to use chemically synthesized, randomly deposited metallic nanoparticles instead of the expensive top-down lithography techniques. Second, new plasmonic materials have recently emerged and open radically new possibilities. Heavily-doped semiconductors, in particular, have been put forward for two reasons: their lower carrier density compared to noble metals shifts the plasma frequency towards the infrared, where the applications are numerous but the devices are scarce and expensive; in addition, their optical properties can be tuned over a large range of wavelengths by dynamic free carrier modulation. Combining the advantages of chemically synthesized nanoparticles and new plasmonic materials, colloidal heavily-doped semiconductor nanocrystals may thus revolutionize the design of plasmonic metasurfaces. In order to fully exploit their potential, however, a leap in our understanding of their optical properties is required. A few recent experiments have provided first sets of data, but most of them suffer from a crippling limitation: they were performed on nanocrystal ensembles and could not disentangle the intrinsic properties of individual nanocrystals from statistical effects due to the nanocrystal heterogeneity, or from collective effects due to optical coupling between the particles. The only notable exception is a spectroscopy measurement of the optical response of individual ZnO:Al nanocrystals realized with a synchrotron infrared source coupled to a nanoscale Fourier transform infrared spectroscopy (nano-FTIR), a technique that can obviously not be applied systematically owing to the requirement of accessing a large facility. The objective of the MOSAIC project is to solve these issues by using a unique combination of state-of-the-art experiments and numerical simulations. We will first build a table-top, high-sensitivity, background-free nano-FTIR experiment to directly measure the complex polarizability of individual nanocrystals. By combining these measurements with advanced numerical calculations, we will be able to precisely model the intrinsic dielectric permittivity of several families of heavily-doped semiconductor nanocrystals beyond the classical Drude model. We will then extend our study to ensembles of nanocrystals using classical FTIR spectroscopy for the experiment and a multiple-scattering code that we recently developed for the simulations. This code has the unique capability of describing accurately the collective scattering of complex arrangements of particles, even in the presence of a substrate. Going beyond the local Drude model, it also takes into account the size-dependent effects appearing in nanoparticles when the electron gas is confined to a volume much smaller than the wavelength of light to the cube. Our original approach, breeding theory and experiment, is expected to have a strong impact by providing researchers and industrial actors with the fundamental knowledge required to engineer the optical properties of heavily-doped semiconductor metasurfaces, and open the door to a new generation of dynamical plasmonic devices.

This project is built on the concept of a novel type of quantum dots: crystal-phase quantum dots (CPQDs), which consist of heterostructures made with a single material but having different crystal phases. Compared to conventional quantum dots, formed using different materials, CPQDs have a unique advantage: they can be grown with the ultimate accuracy of a single atomic layer. Even though CPQDs have several superior characteristics with respect to conventional quantum-dots, their technological application has been severely limited by the poor understanding of the phase switching mechanism and the difficulty of controlling their formation. In this project, we will break this limitation by using an electric field to trigger the phase change in GaAs nanowires and in doing so, to create CPQDs with the ultimate monolayer precision. This will represent a breakthrough in the fabrication of CPQDs and will unlock their potential for several applications in quantum optics.

Record efficiency solar cells are made of III-V materials, but their usage is limited to niche applications due to their high cost. More than 80% of this one is made up by costly substrates, so that a method to recycle them for several consecutive growths would constitute a breakthrough for high-efficiency low-cost devices. As an appealing solution to answer this technological problem, this project aims at developing the remote epitaxy. It consists in the epitaxy on a crystalline substrate covered by a monolayer of graphene and was shown to allow the growth of transferable epilayers. While providing convincing results, the method raises fundamental questions regarding the particle interactions during growth. This project provides with a methodology to clarify those phenomena, as well as original developments for robust and controllable fabrication processes and ambitious objectives in terms of device performances. Beyond photovoltaics, this project also opens perspectives in fields such as silicon photonics or flexible opto-electronic devices.

Moving from exfoliation to the reproducible direct growth of transition metal dichalcogenides, MX2 [M=W or Mo, X=S, Se or Te], on conventional or van der Waals substrates is the current challenge in the moving field of two-dimensional (2D) materials research. S tarting from WSe2 and WTe2, we first focus improving the Chemical Vapour Deposition (CVD) of 2D mono-layers, before investigating the growth of WSeTe alloys of controlled composition. Being stuck between WSe2, direct band gap semiconductor with hexagonal crystal structure (1H), and WTe2, semi-metal with a monoclinic lattice (1T'), the 2D WSeTe monolayer presents naturally a crystal phase transitions (1H-1T'). At the critical composition, the phase transition occurs reversibly, at minimal energy cost, leading to potential device applications. In this work, we propose to use an external electrical field to drive and control the phase transition between a conductive (semi-metal) and insulating (semi-conductor) state of a 2D WSeTe monolayer, mimicking the geometry of standard field effect transistor devices. The fabrication of this "Mott transition-like" field effect transistor will permit to investigate the dynamics of the phase transition and test its practical implementation.

Over the past few years, it has become clear that the mandatory reduction of energy consumption, the desire to rely less on fossil fuels and environmental pollution necessitate the development of sustainable and alternative energy sources. Technologies based on nanomaterials have proven to be promising in the field of renewable and sustainable energy in terms of optical and thermal properties, long-term stability and cost. However, it is essential to find suitable materials and then evaluate their performance by simulating them at the device level, offering a fast and inexpensive way to check device designs and processes. By exploiting first-principles simulation techniques from theoretical physics and chemistry, the TyLDE project aims to understand and rationalize the correlation between band topology and quantum confinement on their applications in the fields of photovoltaics (PV, direct conversion of energy between light and electricity) and thermoelectric (TE, direct conversion of energy between heat and electricity) in order to propose new interesting materials which will then be transferred to the level of device simulation. Our goal will be to exploit band topology and system dimensionality in order to: 1) simultaneously optimize electric and thermal conductivities in TE materials (leading to a boost in their performances way higher with respect the ones known at the present day); 2) engineer the size of excitons’ wave functions as well as their dispersions in PV systems, boosting exciton photogeneration of carriers and optimizing their diffusion. The success of this project will be a significant step forward in optimizing material properties for a new generation of devices for low power consumption and energy harvesting.