S. Chiussi

Influence of the non-bridging oxygen groups on the bioactivity of silicate glasses

The effect of the composition and bonding configuration of the bioactive silica-based glasses on the initial stage in vitro bioactivity is presented. Information of the IR active Si–O groups of glass in the system SiO2–P2O5–CaO–Na2O–K2O–MgO–B2O3 was obtained by fourier transform Infrared (FTIR) spectroscopy. Two different bands associated to non-bridging oxygen stretching vibrations (Si–O–1NBO and Si–O–2NBO) and a gradual shifting of the bridging oxygen stretching vibration (Si–O) have been observed and evaluated. Both effects are attributed to a decrease of the local symmetry originating from the incorporation of alkali ions into the vitreous silica network. The Si–O–NBO(s)/Si–O(s) absorbance intensity ratio increases with a gradual incorporation of the alkali ions (diminution of SiO2 content) following a linear dependence up to values close to 50 wt % of SiO2. In vitro test analysis by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXA) showed a correlation between the amount and type of the non-bridging oxygen functional groups and the growth of the silica-rich and CaP layers. It was found that a minimum concentration of Si–O–NBO bonds in the glass network is required in order to have an efficient ion exchange and dissolution of the silica network. Finally, the bioactivity of the glass is favored by the presence of the Si–O–2NBO groups in the glassy network. The role of these functional groups in the dissolution of the silica network through the formation of silanol groups and the adsorption of water is discussed.

Raman spectroscopic study of bioactive silica based glasses

Abstract A study of the structure and bonding configuration of the bioactive glasses in the system Na2O–CaO–P2O5–SiO2 by Fourier transform Raman spectroscopy is presented. The assignment of the Raman lines, the changes in the Si–O–Si bond environment and the identification of the non-bridging silicon–oxygen groups (Si–O–NBO) for a wide range of silicate glasses are discussed. The frequency shifting and intensity variations of the Raman lines as a function of the bioactive glass composition are attributed to a decrease of the local symmetry originated by the addition of alkali and alkali earth oxides to the vitreous silica network. Correlation plots for the quantification of the Si–O–NBO groups as a function of the glass composition are also presented. These Raman analyses contribute to a better knowledge of the structural role of the network modifiers in the bioactive glasses and, as a consequence, improve the understanding of the bioactive process and the chemical routes of the CaP layer formation when exposed body fluids.

Physicochemical properties of calcium phosphate coatings produced by pulsed laser deposition at different water vapour pressures.

Calcium phosphate coatings were produced by pulsed laser deposition from targets of non-stoichiometric hydroxyapatite (Ca/P = 1.70) at a substrate temperature of 485 degrees C and different processing pressures of water vapour: 0.15, 0.30, 0.45, 0.60 and 0.80 mbar. The physicochemical properties of these coatings were studied using Fourier-transform IR spectroscopy (FT-IR) and energy dispersive X-ray analysis (EDX). A minimum pressure of water vapour was necessary in order to obtain a crystalline coating, as deduced from the FT-IR spectroscopy of these coatings. This analysis also revealed that when the deposition pressure of water vapour was further increased, the coatings were less crystalline and the content of hydroxyl groups, the carbonate substitution for phosphate, and the Ca/P ratio, as measured by EDX, were lower. These effects can be explained by a combined substitution of carbonate and HPO4(2-) for phosphate, being predominant the carbonate substitution at low pressures and the HPO4(2-) substitution at high pressures.

Tensely strained GeSn alloys as optical gain media

This letter presents the epitaxial growth and characterization of a heterostructure for an electrically injected laser, based on a strained GeSn active well. The elastic strain within the GeSn well can be tuned from compressive to tensile by high quality large Sn content (Si)GeSn buffers. The optimum combination of tensile strain and Sn alloying softens the requirements upon indirect to direct bandgap transition. We theoretically discuss the strain-doping relation for maximum net gain in the GeSn active layer. Employing tensile strain of 0.5% enables reasonable high optical gain values for Ge0.94Sn0.06 and even without any n-type doping for Ge0.92Sn0.08.

Calcium phosphate coatings grown at different substrate temperatures by pulsed ArF-laser deposition

Due to their similarity with the mineral part of bone, calcium phosphate coatings, specially hydroxylapatite, are commonly used to enhance the osteointegration of orthopaedical and dental titanium implants. To obtain more homogeneous and adherent coatings than the commercial ones produced by plasma spraying, pulsed ArF-laser deposition has been investigated. With this technique, hydroxylapatite sintered pellets have been ablated in water atmosphere at different substrate temperatures to produce calcium phosphate coatings. The characterization of the obtained films by profilometry, ellipsometry, Fourier Transformed Infrared Spectroscopy, XRD and SEM with EDX shows a strong dependence in the amount of apatite structure on the substrate temperature used during the deposition process.

A new generation of bio-derived ceramic materials for medical applications

A new generation of bio-derived ceramics can be developed as a base material for medical implants. Specific plant species are used as templates on which innovative transformation processes can modify the chemical composition maintaining the original biostructure. Building on the outstanding mechanical properties of the starting lignocellulosic templates, it is possible to develop lightweight and high-strength scaffolds for bone substitution. In vitro and in vivo experiments demonstrate the excellent biocompatibility of this new silicon carbide material (bioSiC) and how it gets colonized by the hosting bone tissue because of its unique interconnected hierarchic porosity, which opens the door to new biomedical applications.

Carbon nitride films prepared by excimer laser ablation

The preparation of carbon nitride films by laser ablation of graphite in ammonia atmosphere is reported. Experiments were performed at room temperature under different NH3 total pressures using an ArF excimer laser (193 nm). The films were deposited on silicon and metal substrates from the ablated carbon radicals and the species generated by the ammonia photodissociation under the laser VUV photons. Profilometry and ellipsometry show an evolution of the growth rate and refractive index. Energy dispersive X-ray and Fourier transform infrared spectroscopies reveal a gradual incorporation of nitrogen in the films for increasing ammonia concentration. Furthermore, infrared spectra show the presence of CN and C–N groups and the incorporation of hydrogen bonded to carbon and nitrogen. These observations were corroborated by Raman spectroscopy and hydrogen effusion.

Extensive studies on biomorphic SiC ceramics properties for medical applications

Biomorphic silicon carbide ceramics are light, tough and high-strengt h materials with interesting biomedical applications. The fabrication method of the biomor phic SiC is based in the infiltration of molten-Si in carbon preforms with open porosity. The fina l product is a biostructure formed by a tangle of SiC fibers. This innovative process allows the fabrication of complex shapes and the tailoring of SiC ceramics with optimised properties and cont rollable microstructures that will match the biomechanical requirements of the natural host tiss ue. An interdisciplinary approach of the biomorphic SiC fabricated from beech, sapelly and eucalyptus is presented. Their mechanical properties, microstructure and chemical composition were evaluated. The biocompatible behaviour of these materials has been tested in vitro .

Laser synthesis of germanium tin alloys on virtual germanium

Synthesis of heteroepitaxial germanium tin (GeSn) alloys using excimer laser processing of a thin 4 nm Sn layer on Ge has been demonstrated and studied. Laser induced rapid heating, subsequent melting, and re-solidification processes at extremely high cooling rates have been experimentally achieved and also simulated numerically to optimize the processing parameters. “In situ” measured sample reflectivity with nanosecond time resolution was used as feedback for the simulations and directly correlated to alloy composition. Detailed characterization of the GeSn alloys after the optimization of the processing conditions indicated substitutional Sn concentration of up to 1% in the Ge matrix.

Amorphous germanium layers prepared by UV-photo-induced chemical vapour deposition

Abstract Hydrogenated amorphous germanium (a-Ge:H) films have been deposited via laser induced chemical vapour deposition (LCVD) by irradiating germane/helium mixtures with an ArF excimer laser beam passing parallel above Si(100) wafers, metal plates and glass substrates. The analysis of the film thicknesses by profilometry and the characterisation of the material properties by scratch testing, FTIR spectroscopy, Raman spectroscopy, and hydrogen effusion measurements, showed significant dependencies of the growth rate and of the material properties on various processing parameters such as laser power, total pressure, and substrate temperature. Very homogeneous, adherent, and low impurity a-Ge:H films with thicknesses from 10 to 100 nm have been obtained at laser power around 1 W, total pressure of 40 Torr and substrate temperature of 250°C. Higher laser power, total pressure and especially the enhancement of the substrate temperature above the threshold for the thermally induced deposition process lead to higher growth rates but also to the formation of less adherent films and powdery deposits with microcrystalline components.