Size and optical versatility in rare earth oxysulfide photonic materials

• Autor(a) principal: Machado, Ian Pompermayer
• Data de Publicação: 2021
• Tipo de documento: Tese
• Idioma: eng
• Título da fonte: Biblioteca Digital de Teses e Dissertações da USP
• Texto Completo: https://www.teses.usp.br/teses/disponiveis/46/46136/tde-11112021-103329/
• topic: Cintilação; Colloidal synthesis; Luminescência persistente; Luminescent materials; Materiais luminescentes; Microwave synthesis; Oxissulfetos de terras raras; Persistent luminescence; Rare earth oxysulfides; Scintillation; Síntese coloidal; Síntese em micro-ondas; Upconversion; Upconversion

Luminescent materials based on rare earth oxysulfides RE2O2S (RE: Sc, Y, La Lu) have been extensively researched due to their high chemical/thermal stability, additionally to their unique crystalline and electronic structures, which allow easy incorporation of lanthanide ions to generate highly luminescent materials. However, synthesizing these materials is a task far from trivial. The conventional solid state synthesis method, widely used by industry, employs high temperatures (>1000 °C) for long periods (>6 h), resulting in a high energy consumption, therefore increasing the costs of production. In addition, RE2O2S-based luminescent nanomaterials are of great interest for applications in the medical field, such as bioimaging, but they cannot be obtained by conventional methods. In this context, the objective of this doctorate was to synthesize photonic materials based on rare earth oxysulfides by exploring two distinct methodologies: i) the microwave-assisted solidstate (MASS) synthesis, aiming at optically versatile bulk materials, and ii) the colloidal synthesis in organic solvents, aiming at nanocrystals with high colloidal stability and high quantum efficiency. Initially, the MASS synthesis parameters were investigated and optimized for from an extensive ex-situ characterization of RE2O2S matrices, using techniques such as X-ray diffraction and synchrotron radiation X-ray absorption spectroscopy. The optimal synthesis condition was shown to be two heat treatments of 25 minutes each, using activated carbon as a microwave susceptor. Thus, several bulk (~1 µm) materials were prepared by this methodology, designed to exhibit versatile photonic properties: scintillation Gd2O2S:Tb, upconversion (UC) Gd2O2S:Er(,Yb), and persistent luminescence (PersL) Y2O2S:(Eu,Yb),Ti,Mg. Scintillating Gd2O2S:Tb3+ materials exhibited high emission efficiency (546 nm, 5D4→7F5) over a wide range of excitation energies, from UV (4 eV) to X-rays (8000 eV). For UC phenomenon, it was demonstrated that substituting the oxide Ln2O3 precursors by hydroxycarbonates Ln(OH)CO3 (Ln: Gd, Er, and Yb) led to an increase of the UC emission intensity in almost one order of magnitude, making MASS-synthesized materials comparable in efficiency to commercially available products. Furthermore, several PersL materials were prepared aiming at their potential for different applications. For instance, a new LED device was fabricated covering an UV LED chip with RE2O2S:Ti,Mg materials; this device yields warm-white light when turned on and a self-sustaining orange emission when turned off, being useful for safety lighting in cases of power outage. The RE2O2S:Yb,Ti,Mg materials were also synthesized, which display near-infrared (NIR - 983 nm) PersL, which are important for bioimaging applications. The PersL mechanism of such systems was investigated through a series thermoluminescence experiments on Y2O2S:Eu and Y2O2S:Eu,Ti,Mg materials, demonstrating that a synergetic effect among the three doping ions and the matrix is responsible for the supremacy of this red-emitting PersL material. The preparation of RE2O2S photonic materials by the MASS method consisted in a reduction of ≥ 85% in processing time and ≥ 95% in energy consumption compared to conventional solid-state synthesis methods. Finally, the colloidal synthesis method in organic solvents was developed and proved to be reproducible for the preparation of monodisperse Gd2O2S:Eu3+ and Y2O2S:Eu3+ nanomaterials. These nanocrystals were synthesized in the 20-30 nm size range, both exhibiting high luminescence efficiency in the red spectral region (626 nm, 5D0→7F2) in colloidal form. In addition, it has been shown that both oleylamine and oleic acid act as nucleation and crystal growth agents. Perspectives include the development of core-shell nanomaterials showing NIR absorption/emission, which are promising for monitoring/imaging biological processes.