Papers by Author: A.M. Botelho do Rego

Abstract: Laser treatment is a promising technique for dental applications such as caries prevention, dental hypersensitivity reduction and improvement of bond strength of restoration materials. In this study the morphological, structural and chemical changes of enamel surface due to treatment with KrF excimer laser radiation were evaluated using scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. For radiation fluences near 1 J/cm², laser processing originates a relatively porous surface due to preferential removal of material in the enamel prism sheaths. Increasing the fluence leads to a relatively flat surface with clear evidence of surface melting. The X-ray diffractograms of both treated and untreated enamel are similar and correspond to hydroxyapatite. The only modification due to the laser treatment is a slight shift of the peaks, probably, due to a loss of the structural water of hydroxyapatite. X-ray photoelectron spectroscopy confirmed that organic matter is removed from the irradiated surface but no significant changes in the mineral phase occur.

Abstract: Gallium arsenide coating by molecular layers is a of increasing interest both for its surface passivation and for its use as a chemical or biochemical sensor. The surface state of GaAs and the nature of the molecular functionality to be bound to the surface are very important to assure good and durable adhesion. This work, using both the vibrational and the electronic energy loss range of high resolution electron energy loss spectra, showed that the water content in the solvent – acetonitrile – has a dramatic effect on the amount of phenylphosphonic acid molecules adsorbed on the GaAs substrate. There is a poor molecular adsorption for water contents ranging from 0 to 4% volume: HREELS spectrum is always a combination of the substrate and the adsorbed molecule spectra. For a water content of 5% there is an abrupt jump in the HREELS spectra shape: they become typical of phenyl groups in the electronic region. In the vibrational region, the typical C-H stretching peaks of aliphatic chains disappear showing that the extreme surface is exclusively covered by phenyl functions. Also for the samples, where a large adsorption occurs, surfaces become negatively charged under electron irradiation showing the existence of a large number of traps for incident electrons. Sonication of such well covered substrates destroys intermolecular bonds but keeps molecules that are chemically bound to the substrate.