http://repositorio.ufc.br/handle/riufc/169 2026-04-09T08:05:29Z http://repositorio.ufc.br/handle/riufc/85588 Título: Estudo das propriedades vibracionais das perovskitas híbridas de azida Autor(es): Hora, Renata Rodrigues da Abstract: Recently, perovskite-type organic-inorganic hybrid compounds have attracted considerable interest due to their structural versatility, which allows for the manifestation of various interesting physical properties for application in devices. These materials exhibit a wide range of functionalities, including photovoltaic, ferroelectric, and multiferroic properties. In this thesis, we used the slow evaporation method to obtain three perovskite-type hybrid compounds, [(CH3)3N][Mn(N3)3] (TrMAMnN3), [(CH3)2NH][Mn(N3)3] (DMAMnN3), [(CH3)2NH]2[CoNa(N3)6], whose structural, thermal, and vibrational properties were investigated as a function of temperature. In the case of the compound [(CH3)3NH][Mn(N3)3], we analyzed in detail the wavenumber and full-width half-height (FWHM) of lattice modes and internal modes of the NC3 skeleton, N3– and CH3 molecular groups. In general, the modes exhibited unusual behavior during phase transitions, including discontinuity in phonon wavenumber, bandwidth, and unconventional changes due to temperature variation. Furthermore, we used differential scanning calorimetry (DSC) to confirm a subtle monoclinic-to-monoclinic phase transition (P21/c →C2/c) around 330 K; and the phase transition to trigonal structure (R m) above 359 K, whose associated entropy variation becomes |ΔS| ~ 22.3 J·kg⁻¹ K⁻¹ and exhibits a barocaloric coefficient (BC) |δTt/δP| ~ 3.17 K kbar⁻¹. For DMAMnN₃, a first-order structural transition from the orthorhombic phase (Cmca) to the monoclinic phase (P2₁) was identified, with an entropy variation of ~37.2 J·kg⁻¹·K⁻¹ and a barocaloric coefficient of ~2.94 K·kbar⁻¹, in agreement with the Clausius-Clapeyron method. Room-temperature Fourier transform infrared (FTIR) reflectivity spectra identified polar phonons and their damping coefficients. Minimal contributions from the DMA+ mode were observed in the intrinsic dielectric constant (~4.3). An anomaly was observed at ~85 K, indicating a transition from the paramagnetic to the antiferromagnetic state (PM-AFM). Finally, we present a new compound obtained through the slow evaporation synthesis [(CH3)2NH]2[CoNa(N3)6], where single-crystal XRD measurements resolved the material's structure, showing that up to 100 K the structure does not exhibit a structural phase transition. A room-temperature Raman spectrum was also obtained, and mode assignment was performed. Tipo: Tese 2026-01-01T00:00:00Z http://repositorio.ufc.br/handle/riufc/85551 Título: Condições de contorno para nanofitas de fosforeno com bordas enviesadas Autor(es): Costa, Tiago da Silva Abstract: Phosphorene, a monolayer of black phosphorus, exhibits a honeycomb-like structure in top view and a puckered structure in other directions. This is due to sp3 hybridization, in which each phosphorus atom establishes three covalent bonds with neighboring atoms in the monolayer. As a result, phosphorene-based nanostructures can present different edge terminations, called normal and skewed, of the zigzag and armchair type. In the effective theoretical description of the phosphorene monolayer, the literature provides tight-binding models that account for two, five, or ten hopping interactions. Initially, this research employed these models to describe the band structure of the infinite system, exploring the role of each energetic connection between sites in the three theoretical models. It was found that adjustments in the interaction strengths, in-plane and out-of-plane, can drastically change the electronic dispersions of the system’s energy bands. Subsequently, using the three tight-binding models, phosphorene nanoribbons with different widths and distinct edge termination types were studied: normal zigzag, normal armchair, skewed zigzag, and skewed armchair. Results on the dispersion relations and wave functions of edge states were discussed. It was verified, for the case of nanoribbons with normal zigzag and skewed armchair edges, that adjustments in the interaction contributions allow quasi-flat edge states to become perfectly flat. As a perspective, it is intended to employ the continuum approximation, derived from the Taylor series expansion around the Γ point of the tight-binding model, to describe the electronic properties of different phosphorene nanoribbons. To this end, suitable boundary conditions will be proposed to describe the electronic properties of phosphorene nanoribbons with skewed and normal edges. The analytical results can then be compared with those provided by the tight-binding models. Finally, we seek to demonstrate the dependence of the bandgap on the width of the nanoribbons. Tipo: Dissertação 2026-01-01T00:00:00Z http://repositorio.ufc.br/handle/riufc/85550 Título: Escoamento bifásico em meio poroso: análise dos regimes do estado estacionário Autor(es): Magalhães, Alessandro Peixoto Abstract: Immiscible displacement in disordered porous media exhibits a wide range of steady-state interfacial morphologies whose collective organization cannot be inferred from transport laws alone. These patterns arise from the competition between viscous, capillary, and inertial forces; however, descriptions that span different regimes and simultaneously connect dimensionless control parameters, morphology, and collective structure remain limited. In this work, we simulate the Navier-Stokes equations with periodic boundary conditions to resolve statistically steady two-phase flow in a disordered porous matrix consisting of a random array of non-overlapping disks, over wide ranges of pressure gradient and interfacial tension, covering multiple orders of magnitude in the Capillary and Forchheimer numbers. Using unsupervised machine learning, we identify three robust steady-state morphologies — bubbles, stripes, and mixtures — organized in a regime diagram (Fo, Ca) with sharp and consistent transitions governed by the balance between capillary, viscous, and inertial effects. Despite these pronounced morphological reorganizations, macroscopic transport varies smoothly: stripe and mixture states follow a Forchheimer-type V versus |∇p| relation, while bubble states deviate systematically due to the additional energy barrier arising from interface deformations. To characterize the collective organization underlying these morphologies, and following the precedent of pseudo-thermodynamic analogies for two-phase flows, we coarse-grain steady-state configurations into binary fields and infer pairwise maximum-entropy models. The inferred Hamiltonians reproduce the fitted first- and second-order statistics and, near some morphological transitions, accurately predict out-of-sample higher-order correlations. Thermodynamic analysis of the inferred models—in which temperature is a parameter of the statistical model, unrelated to the physical temperature of the fluid—reveals regime-dependent specific heat signatures: a considerable fraction of stripe morphologies operate near an order-disorder transition point, while bubble and mixture states correspond to supercritical organization. Taken together, the hydrodynamic regime map and the maximum-entropy representation establish a unified, dimensionless framework connecting interfacial morphology, macroscopic transport, and collective statistics in two-phase porous flows. Tipo: Dissertação 2026-01-01T00:00:00Z http://repositorio.ufc.br/handle/riufc/85547 Título: Síntese e caracterização de nanocristais coloidais de perovskita CsPbBr₃: efeitos da dopagem com Cu²⁺ nas propriedades ópticas Autor(es): Lopes, Oneida Medeiros Abstract: In recent years, colloidal perovskite nanocrystals (NCs) have attracted considerable attention due to their tunable optical properties and their strong potential for optoelectronic applications. In this context, this work investigates the synthesis of CsPbBr₃ NCs and the modulation of their optical properties through control of the synthetic environment and doping with Cu²⁺ ions. Initially, CsPbBr₃ NCs were synthesized using the ligand-assisted reprecipitation (LARP) method, employing different antisolvents, such as ethyl acetate, chlorobenzene, toluene, and chloroform, to evaluate the influence of the colloidal environment on nucleation, growth, and surface passivation processes. The obtained samples were characterized by optical absorption spectroscopy, steady-state photoluminescence (PL), and time-resolved photoluminescence (TRPL). The results revealed significant differences in the optical properties of the NCs as a function of the antisolvent employed. Less polar antisolvents, particularly toluene, favored the formation of NCs with narrower emission bands and a higher contribution of radiative recombination, indicating improved optical quality and colloidal stability. Based on these results, toluene was selected for the subsequent stage of the study, in which Cu²⁺- doped CsPbBr₃ NCs were synthesized at different nominal concentrations. These samples were characterized by optical absorption spectroscopy, PL, TRPL, X-ray diffraction, and Raman spectroscopy. Copper doping induced changes in the charge carrier recombination dynamics, indicating the influence of the dopant on both radiative and nonradiative channels of the material. Overall, the results demonstrate that control of the antisolvent and metallic doping constitutes an efficient strategy for engineering the optical properties of CsPbBr₃ perovskite nanocrystals, contributing to the understanding of these systems and to their potential use in optoelectronic applications. Tipo: Dissertação 2026-01-01T00:00:00Z