Papers by Author: Luís D. Carlos

Abstract: Novel polynuclear tungsten and molybdenum (VI) complexes with 3-hydroxypicolinic acid (HpicOH) and europium(III) [M4O12Eu(picOH)3] (M(VI) = W, Mo) were prepared by hydrothermal methods. The solid compounds were characterized by FT-IR and FT-Raman spectroscopy and elemental analysis. The photoluminescent properties of the materials were investigated, showing sensitization of the europium(III) luminescence by both the 3- hydroxypicolinate ligand and the polyoxometalate moiety

Abstract: A layer-by-layer assembly method was employed to fabricate multilayered films containing the terbium heteropolyanions K6[Tb(BW11O39)(H2O)3]·32H2O and Ky[Tb(XW11O39)2]·nH2O (X = Si and P) and the polyelectrolytes poly(4-styrenesulfonate) and poly(allylamine hydrochloride). A comparative study on the optical properties of the films containing distinct polyoxometalates and different types of multilayers was carried out. The photoluminescence of the solid polyoxometalates and of the resulting films was investigated.

Abstract: The main information transmission limitation in optical communications employing SiO2 optical fibres is imposed by the optical pulses temporal broadening, resulting from the dispersive properties of the transmission medium (chromatic dispersion). Usually, this dispersion is modelled assuming a -0.4 meV/°C dependence of the 11 eV gap energy. We have monitored the 4.3 eV absorption band, with temperature and obtained a thermal dependence of -0.026 meV/°C. With this value we can predict the first and second order chromatic dispersion for a wavelength of 1550 nm.

Abstract: Nanometric ferrihydrite, maghemite and magnetite particles formed within an organicinorganic hybrid matrix were obtained by the sol-gel process. In contrast to precipitation techniques, sol-gel process appears as suitable way to achieve size-controlled nanoscopic magnetic particles anchored in a hybrid structure. The hybrid matrix here reported, named di-ureasil, is composed of poly(oxyethylene) chains grafted to siloxane groups by means of urea cross-linkages. The formation of ferrihydrite particles was achieved incorporating iron nitrate during the sol-gel process, at low pH. The formation of maghemite takes place after the incorporation of a mixture of Fe3+ and Fe2+ ions and treatment with an ammonia solution, after the sol-gel process. Magnetite nanoparticles are formed after the incorporation of Fe2+ ions and treatment with ammonia at 80°C. The AC magnetic susceptibility shows thermal irreversibility with a blocking temperature TB≈13K and ≈25K depending on frequency for the ferrihydrite and maghemite particles, respectively. The magnetite nanoparticles are blocked at room temperature. Above the irreversibility the magnetization of ferrihydrite and maghemite follows a Langevin function modified with a linear term, as found in antiferromagnetic and ferrimagnetic particles.

Abstract: The thermal transformation of Eu3+-doped and undoped microporous titanosilicate AM-3 has been reported. AM-3 is stable up to 600 °C and transforms into the analogue of the mineral narsarsukite at 800 °C. The narsarsukite obtained from AM-3 is purer than that prepared from the titanosilicates ETS-10, ETS-4 and AM-1, and is suitable for hosting optically-active Ln3+ ions. The materials have been studied by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR), Raman and photoluminescence pectroscopies.

Abstract: Two organo-bridged silsequioxanes derived from the same molecular precursor were synthesized with completely different local structure, namely long-range ordering. The photoluminescence features (emission, excitation, and time-resolved modes) were studied in the temperature range 13-300 K and compared with those of the molecular precursor. The effects of the self-assembling of the nanobuild blocks on these emission properties were discussed in terms of the magnitude of the hydrogen bonds between adjacent organic groups.

Abstract: Photoluminescence of di-ureasil hybrids containing homogeneously dispersed Ag nanoparticles, covered with silica shells was investigated at room temperature and compared with emission of undoped hybrids. Metal-doped di-ureasils show a broad emission band in the blue spectral region similar to what has been observed in pure di-ureasils but with lower relative intensity. A broad minimum arises around 438 nm for Ag@SiO2 - containing di-ureasils which coincides with the peak of plasmon resonance band in the absorption spectrum. This behaviour can be attributed to inner filtering effect, which manifests itself as absorption by metal nanoparticles of light emitted from the matrix. Low-temperature time-resolved spectroscopy and lifetime measurements demonstrate that changes are induced in the time-scale behind the di-ureasil host emission when the nanoparticles are incorporated. In particular, the lifetime values for Ag@SiO2- doped di-ureasils are smaller than those measured for the undoped host, suggesting that energy transfer might exist between the hybrid's emitting centres (NH groups and siliceous nanodomains) and the nanoparticles.

Abstract: Organically-modified silica xerogels from 3-aminopropyltrimethoxysilane (APTES) and 3-isocyanatepropyltriethoxysilane (ICPTES) have been synthesized through carboxylic acid (formic acid, acetic acid and valeric acid) solvolysis. The resulting hybrid materials have been characterized by powder X-ray diffraction, mid-infrared spectroscopy, 29Si and 13C nuclear magnetic resonance, and photoluminescence spectroscopy. The results show that urea cross-links have been formed in these hybrids. The luminescence features depend on the selected carboxylic acids. For example, comparatively to the hybrids derived from formic and acetic acid solvolysis, valeric acid shows a red-shift of the emission features.