Johann Bouclé

Vibrational density of states in silicon carbide nanoparticles: experiments and numerical simulations

The vibrational properties of silicon carbide nanoparticles (np-SiC) were investigated as function of the nanocrystal size (5-25 nm) and the features of their outermost surfaces. Raman experiments and numerical methods were conjugated to characterize the signatures from the active SiC normal modes and the vibrational density of states (VDOS). The Raman spectra of the nanopowders were marked by VDOS signals which correlate with the SiC amorphous fractions favoured by the high specific surfaces of the nanoparticles and their surface reconstruction. Quantitative interpretation of the experimental VDOS features, IR absorption and Raman scattering properties in nanosized SiC were carried out by means of numerical methods developed on SiC clusters with suitable structures and sizes.

Local electrooptic effect of the SiC large-sized nanocrystallites incorporated in polymer matrices

Linear electro-optical effect (LEO) is experimentally demonstrated in large-sized (10 and 30 nm) silicon carbide nanocrystallites (nc-SiC) incorporated in poly(N-vinylcarbazole) (PVK) blended with coumarine (MK). By using the angle-dependent polarimetric method at λprobe=694 nm, in both static and photoinduced regimes, we have shown that embedding the nc-SiC within polymer host matrices gives rise to an enhancement of the LEO effective parameters up to 1.325 pm/V. The absence of these phenomena in the separate components—i.e., pure wurtzite-like SiC crystals as well as the organic matrices—suggests that the observed local electro-optical response is intimately driven by the interfacial hyperpolarizabilities appearing between the nc-SiC and the surrounding organic matrix.

Single-Step Preparation of TiO2/MWCNT Nanohybrid Materials by Laser Pyrolysis and Application to Efficient Photovoltaic Energy Conversion

This paper presents the continuous-flowand single-step synthesis of a TiO2/MWCNT (multiwall carbon nanotubes) nanohybrid material. The synthesis method allows achieving high coverage and intimate interface between the TiO2particles and MWCNTs, together with a highly homogeneous distribution of nanotubes within the oxide. Such materials used as active layer in theporous photoelectrode of solid-state dye-sensitized solar cells leads to a substantial performance improvement (20%) as compared to reference devices.

Stoichiometry and interface effects on the electronic and optical properties of SiC nanoparticles

The physical properties of nanosized SiC particles are investigated as a function of the C/Si ratios. The synthesis by the laser pyrolysis process monitors the particle stoichiometry by the initial fluxes of the reactants (SiH4 and C2H2) and leads to Si- or C-rich batches. Spectroscopic methods such as EPR and Raman are used to probe the paramagnetic active electronic centres as well as the role played by the external surface composition on the effective vibrational and optical properties of the nanoparticles. Numerical methods are undertaken in order to point out the role played by the carbon or silicon excess on the interface properties. In particular, simulations by quantum chemistry codes within the configuration interaction and the parametric method (PM3) evidence the relevant features of the Raman spectra induced by a suitable distribution of C or Si excess within the SiC nanoparticles.

Influence of Nitrogen Doping on Device Operation for TiO2-Based Solid-State Dye-Sensitized Solar Cells: Photo-Physics from Materials to Devices

Solid-state dye-sensitized solar cells (ssDSSC) constitute a major approach to photovoltaic energy conversion with efficiencies over 8% reported thanks to the rational design of efficient porous metal oxide electrodes, organic chromophores, and hole transporters. Among the various strategies used to push the performance ahead, doping of the nanocrystalline titanium dioxide (TiO2) electrode is regularly proposed to extend the photo-activity of the materials into the visible range. However, although various beneficial effects for device performance have been observed in the literature, they remain strongly dependent on the method used for the production of the metal oxide, and the influence of nitrogen atoms on charge kinetics remains unclear. To shed light on this open question, we synthesized a set of N-doped TiO2 nanopowders with various nitrogen contents, and exploited them for the fabrication of ssDSSC. Particularly, we carefully analyzed the localization of the dopants using X-ray photo-electron spectroscopy (XPS) and monitored their influence on the photo-induced charge kinetics probed both at the material and device levels. We demonstrate a strong correlation between the kinetics of photo-induced charge carriers probed both at the level of the nanopowders and at the level of working solar cells, illustrating a direct transposition of the photo-physic properties from materials to devices.

Some fundamental and applicative properties of (polymer/nano-SiC) hybrid nanocomposites

Hybrid nanocomposites which combine polymer as host matrix and nanocrystals as active elements are promising functional materials for electronics, optics or photonics. In these systems, the physical response is governed by the nanocrystal features (size, surface and defect states), the polymer properties and the polymer-nanocrystal interface. This work reviews some selective nanostructured architectures based on active elements such as silicon carbide (SiC) nanocrystals and polymer host matrices. Beyond an overview of some key properties of the nanocrystals, a main part will be devoted to the electro-optical (EO) properties of SiC based hybrid systems where SiC nanocrystals are embedded in polymer matrices of different chemical nature such as poly-(methylmethacrylate) (PMMA), poly-vinylcarbazole (PVK) or polycarbonate. Using this approach, the organic-inorganic interface effects are emphasised with regard to the dielectric or hole transporting behaviour of PMMA and PVK respectively. These effects are illustrated through different EO responses associated with hybrid composites based on PMMA or PVK.