Abstract

Luminescent nanostructured oxides doped with lanthanide ions have a great potential in a wide range of conventional applications, such as lighting, displays and laser devices. Materials doped with lanthanide (Ln3+) ions are interesting due to their unique optical properties, since they undergo a process called upconversion, which is the conversion of a low energy source (usually near-infrared) to a higher energy emission (i.e. visible or UV) through a multiphoton process. In addition to this, with the continuous increase of demand for technology miniaturization, the use of nanosized phosphors with high emission efficiencies becomes essential to replace commercially available micron-sized counterparts. In order to achieve this, it is very important to engineer phosphors for their specific application and controlling the physical and optical properties of the materials directly during their synthesis.
In this thesis, a flame spray pyrolysis (FSP) synthesis has been developed for the synthesis of luminescent nanostructures oxides. This synthesis is a one step process that allows the preparation of nanoparticles with homogeneous size distribution and the direct control of their physical properties during the synthesis. Firstly, Gd2O3: Er3+, a system whose optical behaviour is well known, was chosen as a model to compare the properties of materials prepared by combustion synthesis with the ones prepared by FSP. The materials prepared by the latter showed higher luminescence intensity and better morphological homogeneity, due to the control achieved during the preparation. Secondly, a systematic study of the FSP method showed how the morphological and optical properties change with the variation of the synthetic parameters. In particular, it has been observed that as the temperature of the flame increases, the upconversion emission increases, along with the particle size and thermal stability. Thirdly, the luminescence intensity of the nanoparticles has been enhanced, choosing different doping combinations (Yb3+, Er3+ and alkali metal ions) and hosts materials (GdVO4). Finally, the synthesis of complex systems has been explored: core-shell (Gd2O3@SiO2) nanoparticles have been prepared more rapidly by FSP than with conventional wet synthesis methods. The versatility of FSP makes it a very suitable technique for the preparation of various nanomaterials tailored directly during the synthesis.