Abstracts

This presentation shall describe the preparation and characterization of new composite materials for continuous bright white light emission by excitation at near-infrared (NIR) via up-conversion guided by a theoretical model. This model was developed by the researchers at Recife, Brazil and Aveiro, Portugal is based on the power-balance equation, and it is capable of several predictions and quantitative relationships. It explains the brightness of the continuous white light emission as well as its dependence on the absorption cross-section and the conductivity of sample and of its environment. Based on these predictions, we developed a composite material of silica xerogel and lanthanide oxide nanoparticles. The silica xerogel was prepared by ageing and drying of a gel formed by mixing TEOS, resorcinol, and formaldehyde at pH 6, followed by 2h calcination at 1000 °C. The nanocomposite was prepared by embedding the silica xerogel into an aqueous solution of the lanthanide nitrate salt, followed by drying at 50 °C, thermal treatment at 300 °C for 30 min and at 800 °C for 4h. The lanthanide oxides used were PrO₂ and Tb₂O₃. Because the xerogel has a very low thermal conductivity, the brightness of the continuous white light emissions was higher than those of the pure nanopowder oxides, after considering the relative concentrations. These results were expected by the model, because in the case of the xerogel nanocomposite, the particles are heated radially and the emission occurs in 3-dimensions, whereas powdered samples emit mostly through the interface with air. This is corroborated by observing the formation of spherical particles (0.08-0.10 cm in diameter) at the spot of the laser. The relationship between the brightness of the emissions and the absorption cross-sections was also established by the integrated emission intensity and the absorbance of the sample at the laser wavelength excitation (980 nm). For PrO₂, this absorption was ascribed to the ligand-to-metal charge transfer (LMCT) states. Quantum chemical calculations were performed on a model of a [Pr-O]²⁺ moiety embedded into point charges mimicking the nanoparticle (10 nm diameter). It was shown that as the [Pr-O]²⁺ pair moves from the center to the surface of the nanoparticle, the wavelength associated with the LMCT state changes from visible to NIR, which explains the intense absorption at 980 nm. So, sunlight might be a possible excitation source for generating continuous bright white light emission, which may improve solar cells.