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Themes

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  • III-V semiconductors

    For the last two decades the members of PM2E have participated in the tremendous development of nitride semiconductors materials and devices. The latestt important project carried out has been the European ITN RAINBOW (2008-2012) which we coordinated ; it had been set up to investigate the growth of indium rich alloys for the potential applications in multicolor lighting. During this period we confirmed the low InN band gap by producing highest quality layers of that compound. The collective work included layers and heterostructures growth using the three main techniques of nitride materials which are Hydride vapour phase, molecular beam, as well as metalorganic vapour phase epitaxy techniques. The growth of InN and ternary alloys (InAlN, InGaN) has been investigated throughout multiscale characterization and modelling. In PM2E, we applied from ab initio to empirical potential modelling methods along with transmission electron microscopy, Raman and Rutherford backscattering spectroscopies, and finally electrical and optical characterization of the layers and devices. Two PhD projects were successifully carried out as well as a 12 months post doc research project. At the same time, we also coordinated the ANR project COSNI which investigated the growth of semipolar GaN layer towards a possible fabrication of LEDs within and past te green gap, exploiting the minimization of the polarization effects. In this instance we succeeded in growing 400 µm thick semipolar GaN layers and the project was chosen by the ANR as one of the most successful from 2008 ITC series. For the antimonium based compounds and alloys, an important effort was made through the ANR project MOS35 towards the understanding of the growth of GaSb on GaAs and InP. In this instance, many tools for the analysis of electron microscopy data have been developed and integrated within the conventional tool "Digital Micrograph". This allowed us, for instance, to point out the fundamental role of Sb in the relaxation of the strain at the GaSb/GaAs interface.
    The work in RAINBOW is now being continued within the ANR project LHOM which we coordinate, as well as in the EU H2020 ECSEL project OSIRIS.

  • Vegetal fibers and nanoparticules reinforced polymer composites

    The improvement of the properties of the composites (polymer/reinforcement) critically depend on the nature of reinforcement (chemical composition, shape of the particles, grain size, surface treatments, ...), as well as on the many factors which are related to the process of mixing (process type, tuning of the process (shear rate, temperature, time of mixture, ...). Indeed, the dispersion of the particles in the matrix come out as a key factor. Of course, under this term, there are obviously two distinct notions : the dispercion of the particles inside the matrix, and the size distribution of the particles themselves. The former is in principle homogeneous, whereas the latter can be very sensitive to the above processing parameters, and as such difficult to forecast and control. Although, being at the center of the problem, the measurement of size distribution of the particles if often inaccurate. As a consequence, one of our aims has been to find a method for the measurement, within a polymer matrix, of the size distribution of particles with a strong size non homogeneity. This method relies on the numerical treatments of successive images recorded at different magnifications from the same area of the sample. The experimental size distribution is approximated by a logarythmic normal Law. As associated to the development of numerical estimation tools by stochastic homogeneisation, the measurement of the distribution and effective morphology of the mixtures polymer/nanoparticles (PP-CaCO3, PVA-CNT) or nanostructured polymers (ITO, PVDC, …) allows us now to closely access the macroscopic useful properties (mechanical, rheologic and thermal behaviour). Moreover, throughout this work, it came out that it was difficult to effiently disperse nanoparticles in a polymer matrix, therefore, we developed a new protocole in order to abtain a chemical adhesion of carbon nanotubes (CNT) on micro-objects with mechanical and electric applications (mechanical properties of CNT renforced composites, electrical conductivity of SiO2 particles charged polymers). The impact of the CTN networks within and around the micro-objects is estimated through experimental measurements at the laboratopry level (mechanical strength, electrical percolation). In parallel, through atomistic simulations, we have investigated the nanotube functionalization using chemical radicals. in addition to the initial objective which was to optimized the adhesion of nanotubes by investigating the various bondings (which should allow us to choose the best radicals in the optimization of our systems), this modelling has allowed to show that the presence of an active radical can critically modify the electrical conductivity of the CTN. Historically, France is the first long flax fibre producer in the world and a major hemp fibre producer. Futhermore, the agriculture in France offers exceptional wastes that can be recycled and turned into biomaterials. (hemp and flax shives, sunflower bark and rape straw). The use of these new agroresources as materials could increase the potential supplies for this application and could provide an additional source of income for farmers. Therefore, starting in 2007, we started to invesigate bioinspired composites, of course this work is carried out in close collaboration with the industry. During the last four years, we our research has covered the following aspects : 1) the analysis of the dissipative properties of the plant fibers at the level of individual plant fiber, 2) determination of thermohygrometric properties of biobased composites, 3) phenomenological modelling of their mechanical properties, 4) study of sorption and mechanical properties of panels made of waste agroresources.