Pascal Puech

Improved one-phonon confinement model for an accurate size determination of silicon nanocrystals

In this article, we show how the well-known one-phonon confinement model can be improved to determine the diameter of silicon nanocrystalline spheres from the optical phonon wave-number shift, even using a physical-meaning weighting function. We show that the fundamental parameter is the knowledge of the phonon dispersion. The accuracy of our approach is supported by experimental data obtained by selective UV Raman scattering on nanocrystalline silicon thin films produced by size-selected silicon cluster beam deposition.

Characterisation and in vivo ecotoxicity evaluation of double-wall carbon nanotubes in larvae of the amphibian Xenopus laevis.

Because of their outstanding properties, carbon nanotubes (CNTs) are being assessed for inclusion in many manufactured products. Due to their massive production and growing number of potential applications, the impact of CNTs on the environment must be taken into consideration. The present investigation evaluates the ecotoxicological potential of double-walled carbon nanotubes (DWNTs) in the amphibian larvae Xenopus laevis at a large range of concentrations in water (from 10 to 500 mgL(-1)). Acute toxicity and genotoxicity were analysed after 12 days of static exposure in laboratory conditions. Acute toxicity was evaluated according to the mortality and the growth of larvae. The genotoxic effects were analysed by scoring the micronucleated erythrocytes of the circulating blood of larvae according to the International Standard micronucleus assay. Moreover, histological preparations of larval intestine were prepared after 12 days of exposure for observation using optical and transmission electron microscopy (TEM). Finally, the intestine of an exposed larva was prepared on a slide for analyse by Raman imaging. The results showed no genotoxicity in erythrocytes of larvae exposed to DWNTs in water, but acute toxicity at every concentration of DWNTs studied which was related to physical blockage of the gills and/or digestive tract. Indeed, black masses suggesting the presence of CNTs were observed inside the intestine using optical microscopy and TEM, and confirmed by Raman spectroscopy analysis. Assessing the risks of CNTs requires better understanding, especially including mechanistic and environmental investigations.

Resonant Raman scattering in polycrystalline silicon thin films

In this letter, we report the results obtained on polycrystalline silicon thin films using Raman spectrometry in resonance with the silicon direct band gap. First, we show that accurate information about crystallites can be obtained in these experimental conditions, without any deconvolution of Raman spectra. Second, we apply the technique to estimate the mechanical stress of polycrystalline silicon thin films.

GaN nanoindentation: A micro-Raman spectroscopy study of local strain fields

We have investigated strain fields around GaN nanoindentations. Stress relaxation around the edges of the nanoindentation was evident in atomic force microscopy images. More detailed information on the strain fields was obtained from Raman scattering, which has been used to analyze the shape of the strain field around the indentation. We find that the Berkovich tip giving a triangular imprint on the sample generates a strain field, which represents a hexagonal pattern. Negative values of the strain indicate that the residual stress is compressive. Strain is larger in the center of the indentation than outside. Analysis of the ratio of the frequency shift of the E2 and A1(LO) modes suggests that the residual strains are close to biaxial state outside the indentation contact zone, and mostly hydrostatic within the indentation center.

Measurement of the in-depth stress profile in hydrogenated microcrystalline silicon thin films using Raman spectrometry

Raman spectrometry is used to measure stress in hydrogenated microcrystalline silicon thin films. Moreover, by the use of different excitation wavelengths, from red to near ultraviolet, we can probe different film depths and get information on the stress distribution along the growth direction. For films deposited by standard rf glow discharge at different substrate temperatures, on glass substrates, we found large stress gradients. Indeed, the high compressive stress (up to 1 GPa) in the bulk of the film, close to the glass substrate, reduces and becomes tensile as the film free surface is approached. Moreover, the higher the substrate temperature, the higher the stress gradient.

Carbon nanotube ecotoxicity in amphibians: assessment of multiwalled carbon nanotubes and comparison with double-walled carbon nanotubes

The potential impact of industrial multiwalled carbon nanotubes (MWNTs) was investigated under normalized laboratory conditions according to the International Standard micronucleus assay ISO 21427-1 for 12 days of half-static exposure to 0.1, 1, 10 and 50 mg/l of MWNTs in water. Three different end points were carried out for 12 days of exposure: mortality, growth inhibition and micronuclei induction in erythrocytes of the circulating blood of larvae. Raman spectroscopy analysis was used to study the presence of carbon nanotubes in the biological samples. Considering the high diversity of carbon nanotubes according to their different characteristics, MWNTs were analyzed in Xenopus larvae, comparatively to double-walled carbon nanotubes used in a previous study in similar conditions. Growth inhibition in larvae exposed to 50 mg/l of MWNTs was evidenced; however, no genetoxicity (micronucleus assay) was noticed, at any concentration. Carbon nanotube localization in the larvae leads to different possible hypothesis of mechanisms explaining toxicity in Xenopus.

Surfactant-free CZTS nanoparticles as building blocks for low-cost solar cell absorbers

A process route for the fabrication of solvent-redispersible, surfactant-free Cu₂ZnSnS₄ (CZTS) nanoparticles has been designed with the objective to have the benefit of a simple sulfide source which advantageously acts as (i) a complexing agent inhibiting crystallite growth, (ii) a surface additive providing redispersion in low ionic strength polar solvents and (iii) a transient ligand easily replaced by an carbon-free surface additive. This multifunctional use of the sulfide source has been achieved through a fine tuning of ((Cu²⁺)(a)(Zn²⁺)(b)(Sn⁴⁺)(c)(Tu)(d)(OH⁻)(e))(t⁺), Tu = thiourea) oligomers, leading after temperature polycondensation and S²⁻ exchange to highly concentrated (c > 100 g l⁻¹), stable, ethanolic CZTS dispersions. The good electronic properties and low-defect concentration of the sintered, crack-free CZTSe films resulting from these building blocks was shown by photoluminescence investigation, making these building blocks interesting for low-cost, high-performance CZTSe solar cells.

Controlled laser heating of carbon nanotubes

We investigate laser heating of double wall carbon nanotubes deposited on surfaces and immerged in liquids as a function of laser wavelength. Observing the Raman spectrum we find that laser heating of agglomerated double wall carbon nanotubes is six times larger at 488nm than at 647nm. The wavelength dependence of the Raman G band is linear in the visible spectral range. The frequency shift of the Raman G band obtained in methanol as a function of temperature is close to what is observed for graphite.

International amphibian micronucleus standardized procedure (ISO 21427-1) for in vivo evaluation of double-walled carbon nanotubes toxicity and genotoxicity in water

Considering the important production of carbon nanotubes (CNTs), it is likely that some of them will contaminate the environment during each step of their life cycle. Nevertheless, there is little known about their potential ecotoxicity. Consequently, the impact of CNTs on the environment must be taken into consideration. This work evaluates the potential impact of well characterized double‐walled carbon nanotubes (DWNTs) in the amphibian larvae Xenopus laevis under normalized laboratory conditions according to the International Standard micronucleus assay ISO 21427‐1:2006 for 12 days of half‐static exposure to 0.1–1–10 and 50 mg L−1 of DWNTs in water. Two different endpoints were carried out: (i) toxicity (mortality and growth of larvae) and (ii) genotoxicity (induction of micronucleated erythrocytes). Moreover, intestine of larvae were analyzed using Raman spectroscopy. The DWNTs synthetized by catalytic chemical vapor deposition (CCVD) were used as produce (experiment I) and the addition of Gum Arabic (GA) was investigated to improve the stability of the aqueous suspensions (experiment II). The results show growth inhibition in larvae exposed to 10 and 50 mg L−1 of DWNTs with or without GA. No genotoxicity was evidenced in erythrocytes of larvae exposed to DWNTs, except to 1 mg L−1 of DWNTs with GA suggesting its potential effect in association with DWNTs at the first nonacutely toxic concentration. The Raman analysis confirmed the presence of DWNTs into the lumen of intestine but not in intestinal tissues and cells, nor in the circulating blood of exposed larvae.

Chirality of internal metallic and semiconducting carbon nanotubes

We have assigned the chirality of the internal tubes of double walled carbon nanotubes grown by catalytic chemical vapor deposition using the high sensitivity of the radial breathing ~RB! mode in inelastic lightscattering experiments. The deduced chirality corresponds to several semiconducting and only two metallic internal tubes. The RB modes are systematically shifted to higher energies when compared to theoretical values. The difference between experimental and theoretical energies of the RB modes of metallic tubes and semiconducting tubes are discussed in terms of the reduced interlayer distance between the internal and the external tube and electronic resonance effects. We find several pairs of RB modes corresponding to different diameters of internal and external tubes.