Amostras foram preparadas pelo método de difusão a partir dos reagentes químicos SrCO3, Al2O3 e Cr2O3 em proporções estequiométricas. Medidas por difração de raios X mostraram que as amostras possuem 3 fases: Sr3Al2O6, Sr4Al2O7 e uma residual não identificada. Neste trabalho apresentamos medidas de fotoluminescência, excitação da fotoluminescência e absorção fotoacústica, a temperatura ambiente e a 4 K, da amostra Sr3Al2O6 dopada com 1% de íons de Cr3+ em substituição dos íons de Al3+. Os espectros de emissão apresentam três bandas largas com seus baricentros em torno de 760 nm, 841 nm e 675 nm. Estas três bandas de emissão observadas são oriundas de três centros diferentes de íons trivalentes de cromo, cada um deles associados a uma fase presente na amostra e todas as bandas correspondem a transição eletrônica 4T2(4F) → 4A2(4F). Além disso, observa-se a linha de zero-fônon 2E(2G) → 4A2(4F) situada em 745 nm para o caso de íons de Cr3+ na fase do Sr3Al2O6. A partir dos espectros de excitação e absorção fotoacústica, podemos determinar os parâmetros de campo cristalino (Dq) e Racah (B e C) para as três fases presentes na amostra. A razão Dq/B para os íons de Cr3+ na fase Sr3Al2O6, indica que a ação do campo cristalino em torno dos íons de Cr3+ é relativamente forte, como previsto na literatura para matrizes óxidas. Em todos os casos, os resultados mostram que os íons de Cr3+ ocupam sítios octaédricos nas três fases.

The rapidly thawing permafrost in the Arctic and Subarctic regions poses a complex scientific problem, with its mechanisms defying traditional climate modeling. Unlike gradual thaw, abrupt permafrost thaw processes, such as thermokarst lake and pond formation, depend on intricate local geological and environmental interactions, rendering them extremely challenging to integrate into Earth System Models (ESM). This is due to their abundance, small size (< 10,000 m2 ), and high optical and biogeochemical variability. In this study, spanning the different permafrost zones of Canada, a deep learning Mask-Regional-based Convolutional Neural Network (Mask R-CNN) model was trained over PlanetScope-Dove (PS-D) imagery, producing the High Latitude Water (HLWATER) model. The model was then deployed to automatically delineate 335,281 water bodies, 90% smaller than 10,000 m2 . Unmanned Aerial System (UAS) data were used to validate and assess the limitations of the HLWATER-derived products. The resultant HLWATER database was used as a spatial reference for Sentinel-2 (S2) reflectance retrievals, allowing optical assessments of water bodies (HLWATER-Optical). In the continuous permafrost zone of Paulatuk, the number of ponds increased from 164 to 454 between 1975 and 2020. In the regional sector of western Nunavik, spanning the discontinuous and sporadic permafrost zones, some landscapes, although representing only 2-7% of the total area, corresponded to over one-third of the total number of small water bodies (< 10,000 m2 ). The HLWATER-Optical allowed the identification of different Arctic and Sub-Arctic lake dominated landscapes, including thermokarst areas. Water body optical properties, ranging from oligotrophic black to brown and light-brown colors, showcased complex degradation processes of palsas and lithalsas, likely reflecting varying organic and inorganic (mineral) concentrations. The workflow presented in this thesis provides a scalable framework for mapping and characterizing thermokarst landscapes in high-latitude environments. It underlines the importance of contemplating the role of small water bodies in climate change assessments. Further, it advances the understanding of permafrost dynamics, establishing a framework for similar investigations in other vulnerable ecological systems worldwide.

Localized surface plasmon resonance (LSPR) gas sensors are gaining increasing importance due to their unique tuneable functional properties. Au-WO3-x nanocomposite coatings, in particular, can be outstandingly sensitive to many different gases. However, a proper understanding of their optical properties and the way in which those properties are correlated to their structure/microstructure, is still needed. In this work, Au-WO3 nanocomposite coatings, with Au contents between 0-11 atomic percent, were grown using reactive magnetron co-sputtering technique and were characterized concerning their optical response. The precipitation of Au nanoparticles in the oxide matrix was promoted through thermal annealing treatments until 500 °C. Along with the Au nanoparticles' morphological changes, the annealing treatments stimulated the crystallization of WO3, together with the appearance of oxygen-deficient WO3-x phases. Through theoretical simulations, we have related the LSPR effect with the different structural and morphological variations (namely, size and distribution of the nanoparticles and their local environment), which were a function of the Au content and annealing temperature. Our results suggest that local voids were present in the vicinity of the Au nanoparticles, for all temperature range, and that they should be present in a wide variety of Au-WO3 nanocomposites. A theoretical study concerning the refractive index sensitivity was carried out in order to predict the optimal coating design parameters for gas sensing experiments.

Mn doped ZnS samples with composition formula Zn1-xMn xS where x = 0, 0.02, 0.05 and 0.10 were prepared by chemical method. Samples characterized for its structural, morphological and optical properties by X-ray diffraction (XRD), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FTIR) and UV-vis spectrometry. XRD patterns confirm cubic zinc blend structure with no secondary phases for pure and Mn doped ZnS. Lattice constant value increases slightly with Mn concentration due to the substitution of Mn in ZnO lattice. TEM images show that the particles have spherical in shape with an average particle size 3-4 nm. The chemical species of the grown crystals are identified by FTIR spectra. Optical absorption spectra show decrement in band gap with increasing Mn concentration.

PbCrO4 and Pb2CrO5 particles were synthesized by the polymeric precursor method. Structural and microstructural properties of the particles were characterized by scanning electron microscopy with field emission gun, X-ray diffraction, and Raman spectroscopy techniques. The diffuse reflectance technique was employed to study the optical properties in the 400-700 nm range. The optical bandgap of the samples was obtained indirectly. Colorimetric coordinates L*, a*, b* were calculated for the pigment powders as a function of the heat treatment (400-700 ºC). The powders displayed colors ranging from green to red. X-ray diffraction patterns showed the presence of monoclinic PbCrO4 phase in green samples, while red powders had a monoclinic Pb2CrO5 phase structure. The Raman spectra of the PbCrO4 and Pb2CrO5 powders were in good agreement with those reported in the literature. The synthesized compounds can be used as green and red pigments with high thermal stability.

Mn doped ZnS samples with composition formula Zn1-xMn xS where x = 0.00, 0.02, 0.05 and 0.10 were prepared by chemical method. Samples characterized for its structural, morphological and optical properties by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and UV-vis spectrometer. XRD patterns confirm cubic zinc blend structure with no secondary phases for pure and Mn doped ZnS. Lattice constant value increases slightly with Mn concentration due to the substitution of Mn in ZnS lattice. TEM images show that the particles have spherical in shape with an average particle size between 3-4 nm. The chemical species of the grown crystals are identified by FTIR spectra. Optical absorption spectra show decrement in band gap with increasing Mn concentration.

Abstract The first generation of polycrystalline zirconia-based ceramic used to produce dental prosthesis showed excellent mechanical behavior; however, its low translucency limited the indication for aesthetic monolithic restorations. Thus, various strategies have been tested in order to develop a material that has high strength and good optical properties. The aim of this study was to characterize, through a literature review, the translucency of different types of zirconia-based ceramics indicated for monolithic restorations. The literature search was performed in PubMed/Medline database, from 2010 to 2019, using as keywords: zirconia, translucency, and optical properties. The 3rd (monochromatic) and 4th (multichromatic) generations of zirconia, which have high cubic phase content, showed the highest translucency among the zirconia-based ceramics investigated. Additionally, other strategies, such as the addition of lanthanum oxide and the infiltration of feldspathic glass in the zirconia surface (graded-zirconia), resulted in a good balance of optical and mechanical properties.

Abstract FexZn0.95-xCr0.05O nanoparticles have been obtained for the different concentrations of Fe ions by using the sol-gel method. The hexagonal wurtzite structure of all the samples was confirmed by Rietveld refined X-ray diffraction patterns. The addition of Fe ions decreased lattice constants a and c from 3.2487 to 3.2474 Å and 5.2043 to 5.2029 Å, respectively. Uniform deformation, uniform stress deformation, and uniform deformation energy density models were assumed to obtain the crystallite size and strain values from Williamson-Hall analysis. The average crystallite size obtained from all four methods showed increasing variation with the addition of Fe ions in Zn-Cr-O, which was found in good agreement with the Scherrer equation, scanning electron microscopy, and high-resolution transmission electron microscopy results. Energy-dispersive X-ray spectra of typical samples confirmed the stoichiometric proportion of constituent elements. Room temperature ferromagnetism was observed in all the Fe-free and Fe-doped samples. Optical studies were carried out by UV-visible spectrophotometry. The band gap values lay in the range of 3.275 to 3.215 eV.

In this work, we have synthesized CdTe quantum dots (QDs) dispersed in an aqueous medium at ambient temperature, and investigated their optical properties. Synthesis of CdTe QDs in the presence of simple amines removed the need for an additional energy source and inert atmosphere, in a simple and inexpensive experimental setup. The use of ammonia or hydrazine promoted nanoparticle growth by kinetic nanocrystal agglomeration in the initial growth stage. These weak electrolytes acted in the electrical double layer during the growth of the nanocrystals. A comparative study on the concentration of hydrazine in the reaction medium helped to investigate their role in nanocrystal growth. Substitution of hydrazine for ethylenediamine and other electrolytes like sodium chloride and ammonium chloride contributed to a better understanding of the mechanism that underlies the use of primary amines in the synthesis of CdTe. The synthesis conditions afforded the highest photoluminescence quantum yield for CdTe QDs prepared at room temperature (27.5%).

We herein report the preparation of AgAu nanotubes displaying controlled surface morphologies and optical properties by varying the reaction temperature during the galvanic reaction between Ag nanowires and AuCl4-(aq). As the AgAu nanotubes presented similar sizes and compositions, they enabled us to isolate the influence of surface morphology and optical properties over their catalytic and plasmonic photocatalytic activities towards methylene blue oxidation. At 25 °C, AgAu nanotubes (AgAu 25) presented branched walls and surface plasmon resonance (SPR) band with low intensities in the visible were obtained. However, at 100 °C, the AgAu nanotubes (AgAu 100) presented smooth surfaces and SPR bands that closely matched the emission spectra of a commercial halogen-tungsten lamp. The AgAu 25 nanotubes displayed better catalytic performances in classical heterogeneous catalysis as a result of its branched walls that lead to increased surface areas relative to the smooth nanotubes. Conversely, AgAu 100 nanotubes showed better activities in plasmonic photocatalysis due to its broader and more intense SPR bands. Thus, our results demonstrate the potential of the control over morphological and optical features towards the optimization of distinct catalytic phenomena.