https://ri.ufs.br/jspui/handle/riufs/2466 2026-04-05T10:42:26Z 2026-04-05T10:42:26Z Maia, Luciano Paulo de Araújo https://ri.ufs.br/jspui/handle/riufs/24449 2026-01-29T18:51:41Z 2025-10-23T00:00:00Z

Título: Estudo Ab-Initio das propriedades estruturais, eletrônicas, ópticas e fotovoltaicas dos compostos NaBiS2 e NaBiSe2 Autor(es): Maia, Luciano Paulo de Araújo Abstract: Ferroelectric semiconductor compounds with photovoltaic properties (photoferroics) represent promising alternatives to conventional semiconductors used in solar cells, as they are not subject to the energy conversion efficiency limitation imposed by the ShockleyQueisser limit. In this context, this thesis investigates the potential of the compounds NaBiS2 and NaBiSe2, both with orthorhombic and ferroelectric structures, for applications in photoferroic devices. Density Functional Theory (DFT)-based calculations were conducted to explore their structural, electronic, and optical properties, employing the modified Becke-Johnson exchange potential for a more accurate description of the electronic structure and optical properties. Additionally, energy conversion efficiencies were estimated using the spectroscopic limited maximum efficiency method to assess the potential performance of these materials in photovoltaic cells. The results indicate that the direct band gap energies of NaBiS2 and NaBiSe2 are approximately 1.2 eV and 0.75 eV, respectively. NaBiS2 exhibits a band gap value considered ideal for photovoltaic applications under solar illumination, while NaBiSe2 shows a lower-than-expected value from a theoretical standpoint, yet still relevant for absorption in the near-infrared region. Both compounds display absorption coefficients in the visible spectrum range with intensities on the order of 105 cm−1, a favorable optical characteristic that allows the use of relatively thin lms to efficiently absorb sunlightan essential factor for high-performance devices. A pronounced optical anisotropy was also observed, associated with the electronic structure of both compounds. Visible light absorption mainly arises from transitions between the 3px,y states of S to the 6x,y states of Bi, and from the 4x,y states of Se to the 6x,y states of Bi. Thus, the local BiS(Se)6 structure of the studied compounds is crucial for their optical properties. The spectroscopic limited maximum efficiencies obtained were 32.2% for NaBiS2 and 23.3% for NaBiSe2. The value for NaBiS2 surpasses that of the perovskite MAPbI2 (29.5%), which is currently one of the most efficient photoferroics, with an experimental efficiency around 20.3%. Combining these results with previously published estimates for ferroelectric polarization and carrier mobility, this thesis reveals the unexplored potential of the orthorhombic compounds NaBiS2 and NaBiSe2 for application in photoferroic devices and next-generation solar cells.

2025-10-23T00:00:00Z Lima, Jaelsson Silva https://ri.ufs.br/jspui/handle/riufs/23626 2025-10-24T10:26:56Z 2025-07-23T00:00:00Z

Título: Modelo com interação do sector escuro do Universo com acoplamento quadrático: viabilidade teórica e observacional Autor(es): Lima, Jaelsson Silva Abstract: Models proposing a non-gravitational interaction between dark energy (DE) and cold dark matter (CDM) have been extensively studied as alternatives to the standard cosmological model. A common approach to describing the DE-CDM coupling assumes it to be linearly proportional to the dark energy density. In this thesis, we consider the model with the interaction term Q = 3Hγρ2 x /(ρc +ρx). We show that for positive values of γ, this model predicts a future violation of the Weak Energy Condition (WEC) for the dark matter component, and for a specific range of negative values of γ, the CDM energy density can be negative in the past. In this work, we conducted two different analyses. In the first, we performed a parameter selection analysis for this model using data from Type Ia supernovae (SNe Ia) from the Pantheon sample, 30 measurements of H(z) from the Cosmic Chronometers (CC) sample, Baryon Acoustic Oscillations (BAO) from the DESI Data Release 1 (DR1), and Cosmic Microwave Background (CMB) data from the Planck collaboration, combined with a prior on the Hubble constant H0. Imposing a prior to ensure that the WEC is not violated, our model is consistent with ΛCDM within 2σ confidence levels (C.L.), but it exhibits a preference for smaller values of σ8, alleviating the σ8 tension between the CMB results from Planck 2018 and the cosmic shear observations from KiDS-1000. In the second analysis, we used data from SNe Ia from the Pantheon+ sample, 32 measurements of H(z) from the CC sample, BAO from DESI Data Release 2 (DR2), and CMB data from Planck. In this analysis, we considered the Equation of State (EoS) parameter ωx as a free parameter and included the WEC prior in the analyses as well. Our results are consistent within 2.5σ C.L. with the results from the DESI DR2 analysis for the ωCDM model.

2025-07-23T00:00:00Z Silveira, Wellisson Santos https://ri.ufs.br/jspui/handle/riufs/23581 2025-10-20T13:44:25Z 2022-08-26T00:00:00Z

Título: Efeito dos íons codopantes (Tb3+, Ca2+) nas propriedades luminescentes do Y3Al5O12: Eu3+ Autor(es): Silveira, Wellisson Santos Abstract: The structural and luminescent properties of YAG compounds (Y3Al5O12) – pure, Eu3+ doped and Tb3+ or Ca2+ codoped were investigated, for applications as a light source through WLEDs. To study such materials, two sets of samples (YAG:Eu,Tb and YAG:Eu,Ca) were produced using the modified sol-gel method, which uses glucose as a polymerizing agent. For the characterization of these samples, different techniques were used, such as X-ray diffractometry (XRD), X-ray absorption spectroscopy (XAS), X-ray excited optical luminescence (XEOL), photoluminescence spectroscopy (PL), radioluminescence (RL) and luminescent decay time. The XRD results revealed the formation of the desired phase in all samples, without the formation of secondary phases, proving that the production method was quite efficient to obtain the desired compound. Through of the Rietveld refinement and the Scherrer equation, the lattice parameters and the medium size of the crystallite were determined, showing that the values changed with the variations of the concentrations of dopants/codopants. XAS measurements confirmed the predominantly trivalent oxidation state of europium ions, not having evidence of a possible reduction of this valence due to ionizing radiation or incorporation of these ions in the Y3+ site. The results of the PL emission measurements, obtained by exciting the samples YAG:Eu,Tb and YAG:Eu,Ca with wavelengths of 230 nm and 225 nm, respectively, showed the typical transitions of the ions involved, except for the ions of Ca2+, which transitions were not evidenced. Furthermore, through these spectra, the chromaticity diagrams were obtained, which made it possible to determine how the molar proportions of the dopants can influence the color coordinates emitted by the samples. Most of these transitions, observed in the PL spectra, were also observed in the XEOL and RL spectra. These corroborated the high stability of the compounds when subjected to high energy radiation, not showing transitions related to the Eu2+ ion. All emission spectra confirmed that their intensities were influenced by the concentrations of Eu, Tb and Ca ions, in addition, they allowed an investigation into the local symmetry around Eu3+ ions. Finally, the luminescent decay time was strongly influenced by the variation of dopants and codopants.

2022-08-26T00:00:00Z Santos, Willian Oliveira https://ri.ufs.br/jspui/handle/riufs/23348 2025-10-06T11:49:39Z 2025-05-30T00:00:00Z

Título: Estudo das propriedades estruturais, optoeletrônicas, térmicas e vibracionais de MB4O7 (M= Si, Ge, Yb e Ce) via DFT e métodos experimentais Autor(es): Santos, Willian Oliveira Abstract: Technological evolution in recent decades has brought notable advances in electronic devices, optoelectronics and renewable energy systems. Tetraborates, for instance, have attracted great interest within the scientific community due to their unique optoelectronic properties, standing out for their high transparency, wide band gap, intense luminescence, and remarkable stability, which make them promising candidates for applications in photonics, dosimetry, and light- emitting devices. In this work, we carried out a theoretical and experimental study to understand the properties of new, unprecedented tetraborates. The first part of this work was to carry out calculationsab initio, based on the density functional theory implemented CASTEP code, for theoretical understanding of crystalline systems of the MB4O7 type with (M= Si, Ge, Yb and Ce). We investigate structural, electronic, vibrational, thermodynamic and optical properties. As a result, it was seen that systems of the MB4O7 type with (M= Si, Ge, Yb and Ce) present insulating and direct and indirect gap semiconductor characteristics. Moreover, the mechanical stability, a crucial aspect assessed theoretically through the phonon spectrum, was confirmed by the absence of negative frequencies in the investigated structures. Through thermodynamic functions (enthalpy, entropy, free energy and thermal capacity) we have evidence of how the structure formation process occurs (free from impurity and with impurity).In the second part of this study, the experimental characterization of the GeB4O7 system was carried out. Thermal analysis (DTA/TG) revealed that the material has good thermal stability, while X-ray diffraction confirmed its well-defined crystalline structure, compatible with standards reported in the literature.

2025-05-30T00:00:00Z