Influência do pH na formação dos defeitos pontuais responsáveis pela luminescência intrínseca do CdSiO3

Abstract

Cadmium Metasilicate (CdSiO3) is a material with luminescent properties that has great potential for applications in several areas, such as emergency lights, decoration, luminescent pigments, ceramic coatings for use in radiodiagnosis rooms, scintillators, optoelectronics devices, thermometers for cryogenic temperatures, among others. The main aim of this work was to investigate the best synthesis condition using the sol-gel synthesis with pH variation of the precursor solution, as well as to study the resulting optical properties in order to understand the influence of the hydrolysis and polycondensation mechanisms in the active luminescent channels present in this material, responsible for its intrinsic luminescence. The single crystalline phase of CdSiO3 was obtained for samples calcined at 1000 °C for 3 h in an open atmosphere, when produced at pH 1 and 3, and mostly, for cases of pH 5, 7, and 9, with about 3% of SiO2 as a secondary phase. These data were obtained through XRD measurements and Rietveld Refinement analysis. The morphology was investigated by means of SEM and TEM being observed depending on the pH. The optical properties were investigated through measurements of optical absorption (AO), in order to understand how the samples behaved when they were irradiated with different doses. Photoluminescence (PL) in the temperature range between 62 K and 300 K, was performed at LNLS. PL measurements were performed on excitation ranges from 4.5 to 7.0 eV (275 to 165 nm) and emission energies from 1.7 to 4.1 eV (730 to 302 nm). The behavior of PL emission as a function of temperature and excitation energies clearly indicated that the intrinsic luminescence of CdSiO3 is composed of four emission channels with maxima centered around 2.10 eV (peak 1), 2.66 eV (peak 2), 3.07 eV (peak 3), and 3.48 eV (peak 4). Peaks 1 and 2 are due to charges trapped in the Cd and O vacancies, and peaks 3 and 4 are due to self-trapped excitons (STE) generated after excitation and trapped in the Cd and O vacancies. The PL results as a function of temperature indicated that CdSiO3 can be explored as a contactless thermometer for cryogenic applications. Radioluminescence (RL) was used in order to understand the behavior of the luminescent channels when they are being excited by X-rays, and the emission, right after the excitation ceased.