Navegando por Autor "Pizani, Paulo Sérgio"

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An investigation of metal oxides which are photoluminiscent at room temperature
(Elsevier, 2004-01) Leite, Edson Roberto; Pontes, Fenelon M.; Lee, Eduardo J. H.; Aguiar, Rosiana; Silva, Elson Longo da; Pontes, D. S. L.; Nunes, M. S. J.; Pizani, Paulo Sérgio; Lanciotti Júnior, Franchesco; Boschi, Tania M.; Varela, José A.; Paskocimas, Carlos Alberto; Pinheiro, Carlos Davidson; Taft, Carlton Anthony
Amorphous thin films, based on different network formers, were processed by a soft chemical process called the polymeric precursor method. The resultant amorphous metal oxides, displayed intense photoluminescence (PL) at room temperature. Heat treatment increases the PL intensity of these materials. Theoretical ab initio calculations are correlated with the observed experimental trends
Experimental and theoretical correlation of very intense visible green photoluminescence in BaZrO3 powders
(American Institute of Physics, 2008) Cavalcante, L. S.; Zampieri, M.; Espinosa, J. W. M.; Pizani, Paulo Sérgio; Paskocimas, Carlos Alberto; Sambrano, Julio Ricardo; Silva, Elson Longo da; Varela, José A.; Simões, M. L.; Longo, V. M.
Very intense visible green photoluminescence PL was observed at room temperature in structurally ordered-disordered BaZrO3 powders. Ab initio calculations, ultraviolet-visible absorption spectroscopy, electron paramagnetic resonance, and PL were performed. Theoretical and experimental results showed that local defects in the cubic structure caused by ZrO5 ·VO z complex clusters, where VO z =VO x , VO • , and VO ••, play an important role in the formation of hole-electron pairs, giving rise to a charge gradient in the structure which is responsible for PL emission
O papel dos modificadores de rede na produção da fotoluminescência no CaWO4
(ABCERAM, 2004-01) Ciaco, F. R. C.; Pontes, F. M.; Pinheiro, C. D.; Leite, Edson Roberto; Lázaro, S. R. de; Varela, José A.; Pizani, Paulo Sérgio; Paskocimas, Carlos Alberto; Souza, Antônio Gouveia de; Silva, Elson Longo da
Discutiu-se a fotoluminescência para o tungstato de cálcio amorfo, observada à temperatura ambiente. Verificou-se que há concordância entre os resultados experimentais e teóricos. Neste trabalho foram simuladas as estruturas cristalinas e amorfas do tungstato de cálcio (CaWO4), comparando-se as respectivas estruturas eletrônicas. Os resultados dos cálculos teóricos indicam a formação de novos níveis de energia na banda de valência e de condução do amorfo. Estes níveis eletrônicos extras são os responsáveis pela formação da cauda na curva do espectro de absorção. Correspondentemente, medidas experimentais de absorção óptica mostraram a presença da cauda, com relação ao espectro do cristalino. Desta forma, pode-se interpretar a formação da cauda, como sendo associada aos defeitos promovidos na estrutura desordenada do material amorfo
Photoluminescence in the CaxSr1 xWO4 system at room temperature
(Elsevier, 2008-08) Pôrto, S. L.; Silva, Elson Longo da; Pizani, Paulo Sérgio; Boschi, Tania M.; Simões, Luis Gustavo Pagotto; Lima, Severino Jackson Guedes; Ferreira, Jailson M.; Soledade, Luiz Edmundo Bastos; Espinoza, J. W. M.; Santos, Maria Rita de Cássia; Maurera, Maria Aldeiza M. A.; Paskocimas, Carlos Alberto; Santos, Ieda Maria Garcia dos; Souza, Antônio Gouveia de
In this work, a study was undertaken about the structural and photoluminescent properties, at room temperature, of powder samples from the CaxSr1 xWO4 (x ¼ 0–1.0) system, synthesized by a soft chemical method and heat treated between 400 and 700 1C. The material was characterized using Infrared, UV–vis and Raman spectroscopy and XRD. The most intense PL emission was obtained for the sample calcined at 600 1C, which is neither highly disordered (400–500 1C), nor completely ordered (700 1C). Corroborating the role of disorder in the PL phenomenon, the most intense PL response was not observed for pure CaWO4 or SrWO4, but for Ca0.6Sr0.4WO4. The PL emission spectra could be separated into two Gaussian curves. The lower wavelength peak is placed around 530 nm, and the higher wavelength peak at about 690 nm. Similar results were reported in the literature for both CaWO4 and SrWO4
Photoluminescence properties of BaMoO4 amorphous thin films
(Elsevier, 2005-07) Marques, Ana Paula Azevedo; Melo, Dulce Maria de Araújo; Silva, Elson Longo da; Paskocimas, Carlos Alberto; Pizani, Paulo Sérgio; Leite, Edson Roberto
BaMoO4 amorphous and crystalline thin films were prepared from polymeric precursors. The BaMoO4 was deposited onto Si wafers by means of the spinning technique. The structure and optical properties of the resulting films were characterized by FTIR reflectance spectra, X-ray diffraction (XRD), atomic force microscopy (AFM) and optical reflectance. The bond Mo–O present in BaMoO4 was confirmed by FTIR reflectance spectra. XRD characterization showed that thin films heat-treated at 600 and 200 1C presented the scheelite-type crystalline phase and amorphous, respectively. AFM analyses showed a considerable variation in surface morphology by comparing samples heat-treated at 200 and 600 1C. The reflectivity spectra showed two bands, positioned at 3.38 and 4.37 eV that were attributed to the excitonic state of Ba2+ and electronic transitions within MoO24 , respectively. The optical band gaps of BaMoO4 were 3.38 and 2.19 eV, for crystalline (600 1C/2 h) and amorphous (200 1C/8 h) films, respectively. The room-temperature luminescence spectra revealed an intense single-emission band in the visible region. The PL intensity of these materials was increased upon heat-treatment. The excellent optical properties observed for BaMoO4 amorphous thin films suggested that this material is a highly promising candidate for photoluminescent applications
Photoluminescent BaMoO4 nanopowders prepared by complex polymerization method (CPM)
(Elsevier, 2005-12-28) Marques, Ana Paula de Azevedo; Melo, Dulce Maria de Araújo; Paskocimas, Carlos Alberto; Pizani, Paulo Sérgio; Joya, Miryam Rincón; Leite, Edson Roberto; Silva, Elson Longo da
The BaMoO4 nanopowders were prepared by the Complex Polymerization Method (CPM). The structure properties of the BaMoO4 powders were characterized by FTIR transmittance spectra, X-ray diffraction (XRD), Raman spectra, photoluminescence spectra (PL) and high-resolution scanning electron microscopy (HR-SEM). The XRD, FTIR and Raman data showed that BaMoO4 at 300 1C was disordered. At 400 1C and higher temperature, BaMoO4 crystalline scheelite-type phases could be identified, without the presence of additional phases, according to the XRD, FTIR and Raman data. The calculated average crystallite sizes, calculated by XRD, around 40 nm, showed the tendency to increase with the temperature. The crystallite sizes, obtained by HR-SEM, were around of 40–50 nm. The sample that presented the highest intensity of the red emission band was the one heat treated at 400 1C for 2 h, and the sample that displayed the highest intensity of the green emission band was the one heat treated at 700 1C for 2 h. The CPM was shown to be a low cost route for the production of BaMoO4 nanopowders, with the advantages of lower temperature, smaller time and reduced cost. The optical properties observed for BaMoO4 nanopowders suggested that this material is a highly promising candidate for photoluminescent applications
Room temperature photoluminescence of BCT prepared by Complex Polymerization Method
(Elsevier, 2010) Motta, Fabiana Villela da; Marques, Ana Paula de Azevedo; Espinosa, José Waldo Martínez; Pizani, Paulo Sérgio; Silva, Elson Longo da; Varela, José A.
It was used the Complex Polymerization Method to synthesize barium calcium titanate powders (BCT). Crystalline Ba0.8Ca0.2TiO3 perovskite-type phase could be identified by X-ray diffraction and confirmed by Raman spectroscopy in the powders heat treated at 600 °C. Inherent defects, linked to structural disorder, facilitate the photoluminescence emission. The photoluminescent emission peak maximum was around of 533 nm (2.33 eV) for the Ba0.8Ca0.2TiO3. The photoluminescence process and the band emission energy photon showed dependence of both the structural order–disorder and the thermal treatment history. The results revealed that Ba0.8Ca0.2TiO3 (BCT20) is a highly promising candidate material for optical applications.
Structural refinement, growth mechanism, infrared/Raman spectroscopies and photoluminescence properties of PbMoO4 crystals
(Elsevier, 2013-02-13) Delmonte, Maurício Roberto Bomio; Cavalcante, L. S.; Almeida, Márcio Aurélio Pinheiro; Tranquilin, Ricardo Luis; Batista, Nouga Cardoso; Pizani, Paulo Sérgio; Li, Maximo Siu; Andres, Juan; Silva, Elson Longo da
Lead molybdate (PbMoO4) crystals were synthesized by the co-precipitation method at room temperature and then processed in a conventional hydrothermal (CH) system at low temperature (70 C for different times). These crystals were structurally characterized by X-ray diffraction (XRD), Rietveld refinement, micro-Raman (MR) and Fourier transformed infrared (FT-IR) spectroscopies. Field emission scanning electron microscopy images were employed to observe the shape and monitor the crystal growth process. The optical properties were investigated by ultraviolet–visible (UV–Vis) absorption and photoluminescence (PL) measurements. XRD patterns and MR spectra indicate that these crystals have a scheelite-type tetragonal structure. Rietveld refinement data possibilities the evaluation of distortions in the tetrahedral 1⁄2MoO4 clusters. MR and FT-IR spectra exhibited a high mode m1(Ag) ascribed to symmetric stretching vibrations as well as a large absorption band with two modes m3(Eu and Au) related to anti-symmetric stretching vibrations in 1⁄2MoO4 clusters. Growth mechanisms were proposed to explain the stages involved for the formation of octahedron-like PbMoO4 crystals. UV–Vis absorption spectra indicate a reduction in optical band gap with an increase in the CH processing time. PL properties of PbMoO4 crystals have been elucidated using a model based on distortions of tetrahedral 1⁄2MoO4 clusters due to medium-range intrinsic defects and intermediary energy levels (deep and shallow holes) within the band gap
The origin of photoluminescence in amorphous lead titanate
(Springer, 2003-03) Leite, Edson Roberto; Paris, Elaine Cristina; Pontes, F. M.; Paskocimas, Carlos Alberto; Silva, Elson Longo da; Sensato, F.; Pinheiro, Carlos Davidson; Varela, José A.; Pizani, Paulo Sérgio; Campos, C. E. M.; Lanciotti Júnior, Franchesco
We discuss the nature of visible photoluminescence at room temperature in amorphous lead titanate in the light of the results of recent experimental and theoretical calculations. Experimental results obtained by XANES and EXAFS revealed that amorphous lead titanate is composed of a Ti-O network having fivefold Ti coordination and NBO-type (non-bridging oxygen) defects. These defects can modify the electronic structure of amorphous compounds. Our calculation of the electronic structure involved the use of first-principle molecular calculations to simulate the variation of the electronic structure in the lead titanate crystalline phase, which is known to have a direct band gap, and we also made an in-depth examination of amorphous lead titanate. The results of our theoretical calculations of amorphous lead titanate indicate that the formation of fivefold coordination in the amorphous system may introduce delocalized electronic levels in the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital). A comparison of the experimental and theoretical results of amorphous compounds suggests the possibility of a radiative recombination (electron-hole pairs), which may be responsible for the emission of photoluminescence
The role of the Eu3+ ions in structure and photoluminescence properties of SrBi2Nb2O9 powders
(Elsevier, 2009-04) Volanti, Diogo Paschoalini; Rosa, Ieda Lúcia Viana; Paris, Elaine Cristina; Paskocimas, Carlos Alberto; Pizani, Paulo Sérgio; Varela, José A.; Silva, Elson Longo da
This work rationalized for the first time the key role of Eu3+ ion in the doping process of SrBi2Nb2O9 (SBN) compound. This process allows us to understand the role of Eu3+ as lattice modifier as well as to obtain information on the crystalline structure surrounding. Therefore, the SBN and Eu3+ doped SBN (SBN:Eu) were synthesized using the polymeric precursor method (PPM). Scanning electron microscope (SEM) images reveal the strong influence of europium on the SBN microstructure. Thermogravimetry (TG) and differential thermal analysis (DTA) techniques were used to determine the weight loss and changes associated with phase transitions in SBN during thermal evolution. The behavior of the Eu3+ lattice modifier was followed in a long-range order by X-ray diffraction (XRD), while Fourier transform Raman (FTRaman) spectroscopy was used to analyze the short-range order. To this end, the SBN orthorhombic phase was observed for all samples heat treated from 400 to 700 C for 2 h. In addition, photoluminescence measurements were employed to study the structural modifications in SBN lattice. The characteristic red emission of the Eu3+ using the 488 nm exciting wavelength of an argon–ion laser was distinctly observed for SBN:Eu samples heat treated from 550 to 700 C. Europium characteristic emission bands are related to 5D1?7FJ (J = 0–2) transitions at 538 and 555 nm, as well as the 5D0?7FJ (J = 0–4) ones at 580, 592, 615, 653 and 695 nm. By means of the emission spectra analyses it was possible to predict that the Eu3+ ions are located at sites of higher symmetry, since the relative area of the (5D0?7F2)/ (5D0?7F1) transitions for the SBN:Eu samples decrease from 3.82 to 2.60 with increasing temperature from 550 to 700 °C

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Navegando por Autor "Pizani, Paulo Sérgio"