P. Roura

Blackbody emission under laser excitation of silicon nanopowder produced by plasma-enhanced chemical-vapor deposition

Previous results concerning radiative emission under laser irradiation of silicon nanopowder are reinterpreted in terms of thermal emission. A model is developed that considers the particles in the powder as independent, so under vacuum the only dissipation mechanism is thermal radiation. The supralinear dependence observed between the intensity of the emitted radiation and laser power is predicted by the model, as is the exponential quenching when the gas pressure around the sample increases. The analysis allows us to determine the sample temperature. The local heating of the sample has been assessed independently by the position of the transverse optical Raman mode. Finally, it is suggested that the photoluminescence observed in porous silicon and similar materials could, in some cases, be blackbody radiation

Si3N4 single-crystal nanowires grown from silicon micro- and nanoparticles near the threshold of passive oxidation

A simple and most promising oxide-assisted catalyst-free method is used to prepare silicon nitride nanowires that give rise to high yield in a short time. After a brief analysis of the state of the art, we reveal the crucial role played by the oxygen partial pressure: when oxygen partial pressure is slightly below the threshold of passive oxidation, a high yield inhibiting the formation of any silica layer covering the nanowires occurs and thanks to the synthesis temperature one can control nanowire dimensions.

Numerical model of solid phase transformations governed by nucleation and growth: Microstructure development during isothermal crystallization

A simple numerical model which calculates the kinetics of crystallization involving randomly distributed nucleation and isotropic growth is presented. The model can be applied to different thermal histories and no restrictions are imposed on the time and the temperature dependencies of the nucleation and growth rates. We also develop an algorithm which evaluates the corresponding emerging grain size distribution. The algorithm is easy to implement and particularly flexible making it possible to simulate several experimental conditions. Its simplicity and minimal computer requirements allow high accuracy for two- and three-dimensional growth simulations. The algorithm is applied to explore the grain morphology development during isothermal treatments for several nucleation regimes. In particular, thermal nucleation, pre-existing nuclei and the combination of both nucleation mechanisms are analyzed. For the first two cases, the universal grain size distribution is obtained. The high accuracy of the model is stated from its comparison to analytical predictions. Finally, the validity of the Kolmogorov-Johnson-Mehl-Avrami model is verified for all the cases studied.

Unusual photoluminescence properties in amorphous silicon nanopowder produced by plasma enhanced chemical vapor deposition

The supralinear dependence of visible photoluminescence intensity from amorphous silicon nanopowder produced in plasma enhanced chemical vapor deposition on excitation power and its exponential dependence on pressure is reported. It is shown that this new material emits two different kinds of photoluminescence that dominate at different pressures.

Mechanical properties of nanometric structures of Si/SiC, C/SiC and C/SiN produced by PECVD

Abstract Nanometric multilayered structures were deposited by PECVD. Deposition on the cathode resulted in amorphous multilayers (a-C/a-SiC/…/a-SiC and a-C/a-SiN/…/a-SiN), whereas nanostructured multilayers (ns-Si/ns-SiC/…/ns-Si) were obtained at the anode. Previous studies on multilayer structures of ns-Si/ns-SiC deposited at room temperature on c-Si wafers by modulated PECVD revealed the mechanical and wear characteristics of these structures, which showed improved adherence to the substrate and blocking of the cracks induced by nanoindentation. Another characteristic of these structures was the absence of oxygen in the SiC layers after exposure to the atmosphere. In the present study, the mechanical characterization of the nanometric multilayer ns-Si/ns-SiC structures, after annealing under an inert atmosphere, has shown an increase in hardness due to: (a) material densification, with an increase in density after dehydrogenation; and (b) crystallization of the layers. Although the films were deposited at low temperature, the need to anneal them to improve their mechanical properties requires the use of temperature-resistant substrates. To avoid the need for post-thermal treatments, we have chosen the deposition of nanometric multilayer structures of a-C/a-SiC and a-C/a-SiN (5–10 nm/layer) using PECVD at room temperature and depositing them on the cathode (higher ion bombardment). The microstructure and morphology of the hybrid amorphous layers were examined by TEM. Hardness and Youngs modulus were measured by the nanoindentation technique. Wear properties were evaluated using a pin-on-disc system. The structures containing approximately 20 layers had better mechanical properties than the corresponding thick monolayers of their components. Their mechanical characteristics, along with their ability to block crack propagation and wear resistance, are useful for applications such as protective coatings for optical (fibers and lenses), electronic and magnetic devices.

Black-body emission from nanostructured materials

Photoluminescence (PL) experiments on materials of low thermal conductance can cause black-body emission from the sample even at low intensities of laser excitation. This thermal emission may be misinterpreted in terms of quantum emission. Although the quantum origin of most radiative emissions in nanostructured materials such as porous silicon is well established, we show in this paper that SiC nanoparticles and mechanically milled Si do exhibit thermal emission at typical excitation intensities for PL measurements provided the samples are under vacuum. An Si membrane was also investigated and the fact that it did not emit black-body radiation is explained with a simple analysis of the heating in materials of reduced dimensionality.

Quantification of the bond-angle dispersion by Raman spectroscopy and the strain energy of amorphous silicon

A thorough critical analysis of the theoretical relationships between the bond-angle dispersion in a-Si,Δθ, and the width of the transverse optical Raman peak, Γ, is presented. It is shown that the discrepancies between them are drastically reduced when unified definitions for Δθ and Γ are used. This reduced dispersion in the predicted values of Δθ together with the broad agreement with the scarce direct determinations of Δθ is then used to analyze the strain energy in partially relaxed pure a-Si. It is concluded that defect annihilation does not contribute appreciably to the reduction of the a-Si energy during structural relaxation. In contrast, it can account for half of the crystallization energy, which can be as low as 7 kJ/mol in defect-free a-Si.

Non-isothermal model-free predictions

Suitable thermal treatment of metal organic precursors is a key process to obtain oxide films. To this purpose, non-isothermal model-free predictions are specially suited. In this article we will explore the ability of these methods to provide an accurate prediction of the evolution of the decomposition of yttrium trifluoroacetate, a precursor used in the synthesis of YBaCuO superconducting thin-films. A good agreement has been obtained between the predicted and the measured reaction courses.

Ene reactions between two alkynes? Doors open to thermally induced cycloisomerization of macrocyclic triynes and enediynes

A domino process is described combining an ene reaction between two alkynes and a Diels-Alder cycloaddition of the vinylallene formed. The process accounts for the thermally induced cycloisomerization of macrocyclic triynes and enediynes to give fused tetracycles in a stereoselective manner.

Cell size distribution in a random tessellation of space governed by the Kolmogorov-Johnson-Mehl-Avrami model: Grain size distribution in crystallization

The space subdivision in cells resulting from a process of random nucleation and growth is a subject of interest in many scientific fields. In this paper, we deduce the expected value and variance of these distributions while assuming that the space subdivision process is in accordance with the premises of the Kolmogorov-Johnson-Mehl-Avrami model. We have not imposed restrictions on the time dependency of nucleation and growth rates. We have also developed an approximate analytical cell size probability density function. Finally, we have applied our approach to the distributions resulting from solid phase crystallization under isochronal heating conditions.