Dans ce travail, nous avons étudié la relation entre la viscosité et la dynamique microscopique (caractérisée par le temps de relaxation structurale) d'une suspension colloïdale en fonction de la fraction volumique. Nous avons mis au point une expérience originale nous permettant de mesurer la viscosité et le temps de relaxation simultanément sur le même échantillon. La dynamique microscopique est mesurée à l'aide de techniques conventionnelles de diffusion dynamique de la lumière "multi-speckles" ; la viscosité est quant à elle obtenue en mesurant la vitesse de sédimentation de particules-sondes micrométriques. Ces mesures nous ont permis d'étendre d'au moins deux décades la gamme de viscosités à taux de cisaillement nul préalablement explorée. Nous montrons que la viscosité et le temps de relaxation structurale mesuré au pic du facteur de structure statique, sont couplés jusqu'à des fractions volumiques situées bien au-delà de la transition vers un régime surfondu. Par ailleurs, nous avons constaté que le fort accroissement du temps de relaxation à l'approche de la transition vitreuse était bien décrit par une divergence exponentielle, plutôt que par une loi de puissance critique prévue par la Théorie de Couplage des Modes (MCT).

A biosensor taking advantage of the optical properties of sorted carbon nanotubes has been developed. A polyfluorene polymer bearing azido groups was synthesized and used for the selective extraction of semi-conducting nanotubes from the bulk population. The resulting polymer-decorated nanotubes were then conjugated by click-chemistry to a ligand unit (biotin), and the sensing properties of the biotinylated nanotubes were investigated by photoluminescence measurements, upon interaction with the streptavidin target.

A deep-subwavelength imaging of the optical-guided modes localized in silicon microring resonators, obtained with a polarization-sensitive Fano-imaging technique, is demonstrated. We merge together near-field scanning optical microscopy and resonant forward scattering spectroscopy, leading to near-field hyperspectral imaging without the need of embedded light emitters or evanescent light coupling into the microring. The combined analysis of the observed Fano-like spectral line shapes and of the near-field intensity spatial distributions, supported by accurate numerical calculations, gives a clear discrimination between the TE and the TM modes.

Strategic approaches for dispersing and/or orienting SWNT in films and composites are essential for controling and optimizing their properties [1,2]. Dispersion in a liquid phase is the most efficient way to exfoliate the bundles into individual SWNT. Additives, most often surfactants, have to be used because of the poor solubility of SWNT in all solvents. However, the presence of surfactants in the final films/composites can drastically change their properties. In this talk, we show that a simple dialysis process opens a new route for stabilizing aqueous inks with high concentrations of individual SWNT without surfactant, and for preparing surfactant-free polymer-based composites [3]. Coupled Raman/ photoluminescence spectroscopy studies allow to probe in situ both the dispersion of the nanotubes and the nature of the molecules adsorbed on their surface. We demonstrate a spontaneous exchange between surfactant molecules (bile salts) and polyvinyl alcohol (PVA) chains at the surface of the nanotubes, and a forced exchange by dialysis for both PVA and polyvinyl pyrrolidone (PVP) [3].
On the other hand, we study the processing of SWNT aqueous inks into patterned deposits by inkjet printing. We show that a good control of both the morphology of inkjet printing lines and of the alignment of SWNT in these lines can be achieved by tuning the SWNT concentration, inter-drop distance and substrate temperature. Using polarized micro-Raman spectroscopy, we present an original method to determine the spatial distribution of nanotubes in the deposits as well as their orientation. We show that two types of line morphologies can be obtained: homogeneous deposits or pairs of ‘‘railway tracks’’. Highly oriented SWNT patterns can be observed in the “railways tracks” and at the edge of the homogeneous lines.
References:
[1] Ph. Avouris. Acc. Chem. Res. (2002) 35, 1026
[2] P. M Ajayan et al, Nature. (2007) 447, 1066
[3] F. Torres-Canas et al, J. Phys. Chem. C 119, 703 (2015)
[4] F. Torres-Canas et al, in preparation

In this paper, I will review our recent results demonstrating that hBN has an indirect bandgap at 5.9 eV. I will show that the optical properties of hBN are profoundly determined by phonon-assisted transitions with a mirror symmetry between emission and absorption around the indirect exciton at 5.9 eV (Figure 1). I will provide a comprehensive analysis of the emission spectrum in the deep ultraviolet in terms of phonon-assisted transitions involving either virtual or real excitonic states, the latter being provided by structural defects. I will finally point out the complex relaxation dynamics of the quantum gas formed by the reservoir of indirect excitons.

In this paper, I will review our recent results demonstrating that hBN has an indirect bandgap at 5.9 eV. I will show that the optical properties of hBN are profoundly determined by phonon-assisted transitions with a mirror symmetry between emission and absorption around the indirect exciton at 5.9 eV (Figure 1). I will provide a comprehensive analysis of the emission spectrum in the deep ultraviolet in terms of phonon-assisted transitions involving either virtual or real excitonic states, the latter being provided by structural defects. I will finally point out the complex relaxation dynamics of the quantum gas formed by the reservoir of indirect excitons.

Bulk films and heterostructures based on HgCdTe compounds can be engineered to fabricate “gapped-at-will” structures. Therefore, 1D [1], 2D [2] and even 3D [3] massless particles can be observed in topological phase transitions driven by intrinsic (quantum well thickness, Cd content) and external (magnetic field, temperature or pressure) physical parameters. So far, the phases of 2D [1] and 3D [4] topological insulator have already been experimentally demonstrated in HgCdTe-based heterostructures. More recently, clear experimental evidence of the existence of 3D electronic states with conical-like spectrum was obtained in HgCdTe bulk films at specific Cd content [3]. These 3D massless particles, called Kane fermions, have unique symmetry properties, which are not equivalent to any well-known case of massless particles in the ultrarelativistic limit of the quantum electrodynamics. In this talk, I will report our recent results on topological phase transitions driven by external physical parameters in HgCdTe-based QWs, as well as the first experimental demonstration of the rest-mass collapse of Kane fermions induced by temperature in HgCdTe bulk films.