Nanostructured materials based on SiO2, GeO2, Nb2O5 and Ta2O5 doped with rare earths for applications in photonics as optical amplifiers, energy converters, and nanothermometry

Author(s)

Fábio José Caixeta

• Document type: Doctoral Thesis
• Press: Ribeirão Preto.
• Institution: Universidade de São Paulo (USP). Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto (PCARP/BC)
• Defense date: 2022-03-18

Examining board members

Rogéria Rocha Gonçalves; Maria Rute de Amorim e Sá Ferreira André; Hermi Felinto de Brito; Danilo Manzani; Sidney José Lima Ribeiro; Lucas Carvalho Veloso Rodrigues

• Advisor: Rogéria Rocha Gonçalves

Abstract

This work describes and discusses the preparation and structural, morphological, spectroscopic, and optical characterization of nanostructured materials based on silicates and germanates of Nb2O5 and Ta2O5 doped with rare earths (RE) ions and prepared by the sol-gel methodology as powder, film, or planar waveguide. We detail how the structure of the material is related to its thermal, photoluminescent, and optical properties by conducting X-ray diffraction, Raman spectroscopy, transmission electron microscopy, optical and hyperspectral microscopy, specular and diffuse reflectance, FTIR, M-line spectroscopy, UV-Vis-NIR electronic absorption spectroscopy, and photoluminescence spectroscopy. Initially, we prepared RE3+ ion-doped nanocomposites as powder, film, or active planar waveguide based on SiO2-M2O5 (M = Nb, Ta) and characterized them. Annealing the materials at 900 ºC efficiently eliminated photoluminescent quenchers and allowed crystallization to be controlled. The materials presented wide transparency window and intense and broad emission in the 1.5-µm region with full width at half maximum (FWHM) up to 173 nm. Moreover, we obtained active planar waveguides based on SiO2-Nb2O5:Nd3+/Er3+/Tm3+, which displayed excellent optical properties with potential application as broadband optical amplifiers working in the third telecommunication window in the region of the O, E, S, C, L, and U bands. We prepared Eu3+-doped GeO2-M2O5 (M = Nb, Ta) nanocomposites as powder and characterized them. The Eu3+ ion served as a structural probe and helped to assess the chemical environment of the RE3+ ion. A complex crystalline structure containing mainly trigonal GeO2, orthorhombic Ta2O5, and tetragonal GeO2.9Nb2O5 emerged. The structure of these materials influenced their photoluminescent properties. We observed emission coming from the Eu3+:5D1 excited state, which showed that a material with negligible deactivation of the excited states due to luminescence quenchers and phonon energy was formed. In this sense, the results revealed that the structure of the GeO2.9Nb2O5 mixed oxide affected the photoluminescent properties of the resulting material. Based on the results obtained with the Eu3+-doped materials, we also prepared Er3+/Yb3+-co-doped GeO2-M2O5 (M = Nb, Ta) materials. These co-doped nanocomposites crystallized in the same way as the Eu3+-doped nanocomposites; i.e., with trigonal GeO2, orthorhombic Ta2O5, and tetragonal GeO2.9Nb2O5 being formed. We also observed that a considerable amount of monoclinic YbTaO4 was formed in the GeO2-Ta2O5:Er3+/Yb3+-based material. As in the case of Eu3+-doped GeO2-Nb2O5 nanocomposites, formation of tetragonal GeO2.9Nb2O5 in the Er3+/Yb3+-co-doped GeO2-Nb2O5 materials affected the photoluminescent properties. More specifically, when such mixed oxide was present in an appreciable amount, at 1100 ºC, there was intense upconversion (UPC) emission, mainly in the green region, and the emission quantum yield (EQY) was high. Similarly, formation of monoclinic YbTaO4 in the GeO2-Ta2O5:Er3+/Yb3+ strongly affected the photoluminescent properties. Intense green emission arose in the UPC spectra. In addition, we detected blue emission as well as EQY in the same order of magnitude as fluorides. Given the UPC results obtained with the GeO2-Ta2O5:Er3+/Yb3+-based material, we evaluated its use as a luminescent thermometer. The results showed that such material acted as an efficient primary thermometer not only in the powder form, but also when dispersed in polymethylmethacrylate (PMMA) films. Moreover, these GeO2-Ta2O5:Er3+/Yb3+@PMMA hybrid materials proved to be efficient phothermal converters. Finally, through hyperspectral microscopy, we studied the heat flux at the sub-micrometer level in the hybrid films. (AU)

• FAPESP's process: 17/10423-7 - Broad band NIR emission from nanostructured Rare earth doped Nb2O5 and Ta2O5 derived materials for photonic applications as optical amplifiers and energy converters.
• Grantee: Fábio José Caixeta
• Support Opportunities: Scholarships in Brazil - Doctorate