Estudo da evolução microestrutural de sistemas nanoestruturados em função da metodologia de preparo

Abstract

Nanostructured materials receive great attention because they often have physical properties different from those observed when analyzed in a bulk configuration. The search for systems with reduced dimensionality and well-defined physical properties, which can be controlled, stands out as an area of great interest, promoting the research of new two-dimensional materials and the reduction of already known bulk systems to the micro or nanometric scale. In this scenario, the need to develop new synthesis techniques, with a focus on morphological and nanostructural control is growing, driving the improvement of characterization techniques, physical, mathematical and computational models capable of robustly describing the evolution of the properties of interest. This dissertation is centered on the application of a methodology for the characterization of nanostructured systems, aiming to study the effects of particle size, morphology and crystalline defects on the physical properties of interest. The systems of metallic gold nanoparticles (Au), magnetic nanoparticles of nickel oxide (NiO), rare-earth doped systems (BaLa2-x1-x2Erx1Ybx2ZnO5) and metal oxides Ba1-xLaxTi0.5Mn0.5O3 (x = 0;0.5), were chosen for the study due to their potential technological applications as magnetic nanodevices, optoelectronics and biological markers. The samples were characterized by applying a set of procedures ranging from the control of the synthesis parameters to the implementation of a microstructural analysis routine, through the use of direct characterization techniques (Transmission Electron Microscopy - TEM), indirect techniques of X-ray diffraction (XRD), photoluminescence spectroscopy, Raman scattering and magnetization measurements, together with computational tools based on the X-Ray Profile Analysis (XPA) technique. Gold nanoparticles with sizes ranging from ~ 16 nm to ~ 72 nm were obtained, whose analysis by XRD and TEM showed an increase in \textit{twinning} density with the reduction in particle size. For the NiO samples, nanoparticles with sizes ranging from ~ 5.6 nm to ~ 57.7 nm were obtained, with an increase in stacking faults density for smaller particles and an increase in the ferromagnetic characteristic, different from the antiferromagnetic behavior presented by the system in its bulk configuration. In the BaLa2-x1x2Erx1Ybx2ZnO5 case system, samples with approximate sizes of 140 nm were obtained, showing variations in the photoluminescence spectra as a function of the different co-doping with Er+3 and Yb+3 ions in the replacement of La+3 ions in the crystal lattice. The evolution of the photoluminescence data suggests an increase in the excitation energy migration processes in the crystalline lattice, promoted by the increase in concentration of Er and reinforced by the reduction in the unit cell volume, allowing resonant energy transfer processes by cross relaxation between Er+3 ions. Finally, qualitative analyzes were presented for the metal oxide systems Ba1-xLaxTi0.5Mn0.5O3 (x = 0;0.5).