Browsing by Author "Paula, Ana Maria de"

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    Angle-tunable intersubband photoabsorption and enhanced photobleaching in twisted bilayer graphene.
    (2021) Pogna, Eva A. A.; Miao, Xianchong; Dreifus, Driele Von; Souza, Thonimar Vieira de Alencar; Moutinho, Marcus Vinicius de Oliveira; Venezuela, Pedro Paulo de Mello; Manzoni, Cristian; Ji, Minbiao; Cerullo, Giulio; Paula, Ana Maria de
    Van der Waals heterostructures obtained by artificially stacking two-dimensional crystals represent the frontier of material engineering, demonstrating properties superior to those of the starting materials. Fine control of the interlayer twist angle has opened new possibilities for tailoring the optoelectronic properties of these heterostructures. Twisted bilayer graphene with a strong interlayer coupling is a prototype of twisted heterostructure inheriting the intriguing electronic properties of graphene. Understanding the effects of the twist angle on its out-of-equilibrium optical properties is crucial for devising optoelectronic applications. With this aim, we here combine excitation-resolved hot photoluminescence with femtosecond transient absorption microscopy. The hot charge carrier distribution induced by photo-excitation results in peaked absorption bleaching and photo-induced absorption bands, both with pronounced twist angle dependence. Theoretical simulations of the electronic band structure and of the joint density of states enable to assign these bands to the blocking of interband transitions at the van Hove singularities and to photo-activated intersubband transitions. The tens of picoseconds relaxation dynamics of the observed bands is attributed to the angle-dependence of electron and phonon heat capacities of twisted bilayer graphene.
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    Crystal structure of fluorite-related Ln3SbO7 (Ln=La–Dy) ceramics studied by synchrotron X-ray diffraction and Raman scattering.
    (2013) Siqueira, Kisla Prislen Félix; Borges, Raquel Moreira; Granado, Eduardo; Moreira, Leandro Malard; Paula, Ana Maria de; Moreira, Roberto Luiz; Bittar, Eduardo Matzenbacher; Dias, Anderson
    Ln3SbO7 (Ln=La, Pr, Nd, Sm, Eu, Gd, Tb and Dy) ceramics were synthesized by solid-state reaction in optimized conditions of temperature and time to yield single-phase ceramics. The crystal structures of the obtained ceramics were investigated by synchrotron X-ray diffraction, second harmonic generation (SHG) and Raman scattering. All samples exhibited fluorite-type orthorhombic structures with different oxygen arrangements as a function of the ionic radius of the lanthanide metal. For ceramics with the largest ionic radii (La–Nd), the ceramics crystallized into the Cmcm space group, while the ceramics with intermediate and smallest ionic radii (Sm–Dy) exhibited a different crystal structure belonging to the same space group, described under the Ccmm setting. The results from SHG and Raman scattering confirmed these settings and ruled out any possibility for the non-centrosymmetric C2221 space group describing the structure of the small ionic radii ceramics, solving a recent controversy in the literature. Besides, the Raman modes for all samples are reported for the first time, showing characteristic features for each group of samples.
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    Crystal-oriented wrinkles with origami-type junctions in few-layer hexagonal boron nitride.
    (2015) Oliveira, Camilla Karla Brites Queiroz Martins de; Gomes, Egleidson Frederik do Amaral; Prado, Mariana C.; Souza, Thonimar Vieira de Alencar; Nascimento, Regiane do; Moreira, Leandro Malard; Batista, Ronaldo Junio Campos; Oliveira, Alan Barros de; Chacham, Helio; Paula, Ana Maria de; Neves, Bernardo Ruegger Almeida
    Understanding layer interplay is the key to utilizing layered heterostructures formed by the stacking of different two-dimensional materials for device applications. Boron nitride has been demonstrated to be an ideal substrate on which to build graphene devices with improved mobilities. Here we present studies on the morphology and optical response of annealed few-layer hexagonal boron nitride flakes deposited on a silicon substrate that reveal the formation of linear wrinkles along well-defined crystallographic directions. The wrinkles formed a network of primarily threefold and occasionally fourfold origami-type junctions throughout the sample, and all threefold junctions and wrinkles formed along the armchair crystallographic direction. Furthermore, molecular dynamics simulations yielded, through spontaneous symmetry breaking, wrinkle junction morphologies that are consistent with both the experimental results and the proposed origami-folding model. Our findings indicate that this morphology may be a general feature of several two-dimensional materials under proper stress-strain conditions, resulting in direct consequences in device strain engineering.
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    Raman spectroscopy with a 1064-nm wavelength laser as a potential molecular tool for prostate cancer diagnosis : a pilot study.
    (2018) Magalhães, Felipe Lima; Machado, Alexei Manso Corrêa; Paulino Junior, Eduardo; Sahoo, Sangram K.; Paula, Ana Maria de; Garcia, Aloísio Miguel; Barman, Ishan; Soares, Jaqueline dos Santos; Lewer, Marcelo Henrique Mamede
    Raman spectroscopy is widely used to investigate the structure and property of the molecules from their vibrational transitions and may allow for the diagnosis of cancer in a fast, objective, and nondestructive manner. This experimental study aims to propose the use of the 1064-nm wavelength laser in a Raman spectroscopy and to evaluate its discrimination capability in prostate cancer diagnosis. Seventy-four spectra from patients who underwent radical prostatectomy were evaluated. The acquired signals were filtered, normalized, and corrected for possible oscillations in the laser intensity and fluorescence effects. Wilcoxon tests revealed significant differences between the benign and malign samples associated with the deformation vibration characteristic of nucleic acids, proteins, and lipids. A classifier based on support vector machines was able to predict the Gleason scores of the samples with 95% of accuracy, opening a perspective for the use of the 1064-nm excitatory wavelength in prostatic cancer diagnosis.
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    Second harmonic generation imaging of the collagen architecture in prostate cancer tissue.
    (2018) Garcia, Aloísio Miguel; Magalhães, Felipe Lima; Soares, Jaqueline dos Santos; Paulino Junior, Eduardo; Lima, Mario Felix Richard de; Lewer, Marcelo Henrique Mamede; Paula, Ana Maria de
    Optical microscopy has been one of the most important tools for visualizing biological samples since the seventeenth century. Recently, with the advances in femtosecond laser technology, all the nonlinear optical processes have now been included as optical microscopy methods, and second harmonic generation (SHG) microscopy has emerged as a powerful new optical imaging tool with applications in medicine and biology. Here we use SHG microscopy to obtain images of 76 prostate biopsies on histological slides. Multiple samples from the excised prostates of patients who underwent a radical prostatectomy were evaluated. The samples were collected from prostate positions as in needle biopsy procedures. The results show the collagen fiber architecture among malignant acini, and analysis of the fiber orientation in the images reveals that the collagen fibers become more aligned at higher malignancy grades. Furthermore, we find that the degree of fiber alignment correlates directly with the Gleason patterns.
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    Supercollision cooling effects on the hot photoluminescence emission of graphene.
    (2016) Souza, Thonimar Vieira de Alencar; Moreira, Leandro Malard; Paula, Ana Maria de
    We report on hot photoluminescence measurements that show the effects of acoustic phonon supercollision processes in the intensity of graphene light emission. We use a simple optical method to induce defects on single layer graphene in a controlled manner to study in detail the light emission dependence on the sample defect density. It is now well accepted that the graphene photoluminescence is due to black-body thermal emission from the quasi-equilibrium electrons at a temperature well above the lattice temperature. Our results show that as the sample defect density is increased the electrons relax energy more efficiently via acoustic phonon supercollision processes leading to lower electron temperatures and thus lower emission intensities. The calculated intensity decrease due to supercollision energy relaxation agrees well with the experimental data.
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    Synchrotron X-ray diffraction and Raman spectroscopy of Ln3NbO7 (Ln=La, Pr, Nd, Sm-Lu) ceramics obtained by molten-salt synthesis.
    (2014) Siqueira, Kisla Prislen Félix; Soares, J. C.; Granado, Eduardo; Bittar, Eduardo Matzenbacher; Paula, Ana Maria de; Moreira, Roberto Luiz; Dias, Anderson
    Ln3NbO7 (Ln=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) ceramics were obtained by molten- salt synthesis and their structures were systematically investigated by synchrotron X-ray diffraction (SXRD), second harmonic generation (SHG) and Raman spectroscopy. It was observed that ceramics with the largest ionic radii (La, Pr, Nd) crystallized in to the Pmcn space group, while the ceramics with intermediate ionic radii (Sm-Gd) exhibited different crystal structure belonging to the Ccmm space group. For this last group of ceramics, this result was corroborated by SHG and Raman scattering and ruled out any possibility for the non-centrosymmetric C 2221 space group, solving a recent controversy in the literature. Finally, according to SXRD, Tb-Lu containing samples exhibited an average defect fluorite structure (Fm3m space group). Nonetheless, broad scatter in gat forbidden Bragg reflections indicates the presence of short-range domains with lower symmetry. Vibrational spectroscopy showed the presence of six Raman-active modes, inconsistent with the average cubic fluorite structure, and in line with the existence of lower-symmetry nano-domains immersed in the average fluorite structure of these ceramics.
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    Twisted bilayer graphene photoluminescence emission peaks at van Hove singularities.
    (2018) Souza, Thonimar Vieira de Alencar; Dreifus, Driele Von; Moreira, Maria Gabriela Cota; Silva Neto, Eliel Gomes da; Yeh, Chao-Hui; Chiu, Po-Wen; Pimenta, Marcos Assunção; Moreira, Leandro Malard; Paula, Ana Maria de
    We report on photoluminescence emission imaging by femtosecond laser excitation on twisted bilayer graphene samples. The emission images are obtained by tuning the excitation laser energies in the near infrared region. We demonstrate an increase of the photoluminescence emission at excitation energies that depends on the bilayer twist angle. The results show a peak for the light emission when the excitation is in resonance with transitions at the van Hove singularities in the electronic density of states. We measured the photoluminescence excitation peak position and width for samples with various twist angles showing resonances in the energy range of 1.2 to 1.7 eV.