Papers by Author: Sonia S. Nobre

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: 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.