Morphology and Photoluminescence of HfO2Obtained by Microwave-Hydrothermal

In this letter, we report on the obtention of hafnium oxide (HfO2) nanostructures by the microwave-hydrothermal method. These nanostructures were analyzed by X-ray diffraction (XRD), field-emission gum scanning electron microscopy (FEG-SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectrometry (EDXS), ultraviolet–visible (UV–vis) spectroscopy, and photoluminescence (PL) measurements. XRD patterns confirmed that this material crystallizes in a monoclinic structure. FEG-SEM and TEM micrographs indicated that the rice-like morphologies were formed due to an increase in the effective collisions between the nanoparticles during the MH processing. The EDXS spectrum was used to verify the chemical compositional of this oxide. UV–vis spectrum revealed that this material have an indirect optical band gap. When excited with 488 nm wavelength at room temperature, the HfO2nanostructures exhibited only one broad PL band with a maximum at around 548 nm (green emission)

Microstructure, dielectric properties and optical band gap control on the photoluminescence behavior of Ba['Zr IND.0.25''Ti IND. 0.75]'O IND.3' thin films

Ba[Zr0.25Ti0.75]O3 (BZT) thin films were synthesized\ud by the complex polymerization method and heat\ud treated at 400 C for different times and at 700 C for 2 h.\ud These thin films were analyzed by X-ray diffraction\ud (XRD), Fourier-transform infrared (FT-IR) spectroscopy,\ud field emission gun-scanning electron microscopy (FEGSEM)\ud and atomic force microscopy (AFM), Ultraviolet–\ud visible (UV–vis) absorption spectroscopy, electrical and\ud photoluminescence (PL) measurements. FEG-SEM and\ud AFM micrographs showed that the microstructure and\ud thickness of BZT thin films can be influenced by the processing\ud times. Dielectric constant and dielectric loss of\ud BZT thin films heat treated at 700 C were approximately\ud 148 and 0.08 at 1 MHz, respectively. UV–vis absorption\ud spectra suggested the presence of intermediary energy\ud levels (shallow and deep holes) within the band gap of BZT\ud thin films. PL behavior was explained through the optical\ud band gap values associated to the visible light emission\ud components.CAPESCNPqFAPES

Photoluminescence property of Ba('Zr IND.0.25''Ti IND.0.75)'O IND.3' powders prepared by solid state reaction and polymeric precursor method

Ba(Zr0.25Ti0.75)O3 (BZT) powders were synthesized by the polymeric precursor method (PPM) at different temperatures (400, 500 and 700 °C) for 2 h and by the solid state reaction (SSR) at 1350 °C for 4 h. These powders were analyzed by X-ray diffraction (XRD), ultraviolet–visible (UV–vis) absorption spectroscopy and photoluminescence (PL) measurements. XRD patterns indicated that the crystalline BZT powders prepared by both methods present a cubic structure. The different optical band gap values were observed from the UV–vis spectra, suggesting the presence of intermediary energy levels (shallow and deep holes) within the band gap. When excited with 350 nm wavelength at room temperature, the BZT powders obtained by SSR exhibited only one broad PL band with a maximum at around 467 nm (blue emission). On the other hand, it was noted the presence of two broad bands when BZT powders were prepared by the PPM, where the correspondent positions are influenced by the heat treatment temperatures. Finally, a model was proposed in order to explain the origin of the PL property in these powders.CAPESCNPQFAPES

Regulamento da Millenium - Journal of Education, Technologies, and Health

PbMoO(4) micro-octahedrons were prepared by the coprecipitation method at room temperature without the presence of surfactants and processed in a conventional hydrothermal at different temperatures (from 60 to 120 degrees C) for 10 min. These micro-octahedrons were structurally characterized by X-ray diffraction (XRD) and micro-Raman (MR) spectroscopy, and its morphology was investigated by field-emission gun scanning electron microscopy (FEG-SEM). The optical properties were analyzed by ultraviolet-visible (UV-vis) absorption spectroscopy and photoluminescence (PL) measurements. XRD patterns and MR spectra confirmed that the PbMoO(4) micro-octahedrons are characterized by a scheelite-type tetragonal structure. FEG-SEM micrographs points, out that these structures present a polydisperse particle size distribution in consequence of a predominant growth mechanism via aggregation of particles. In addition, it was observed that the hydrothermal conditions favored a spontaneous formation of micro-octahedrons interconnected along a common crystallographic orientation (oriented-attachment), resulting in self-organized structures. An intense blue PL emission at room temperature was observed in these micro-octahedrons when they were excited with a 350 nm wavelength. The origin of the PL emissions as well as its intensity variations are explained by means of a model based on both distorted [MoO(4)] and [PbO(8)] clusters into the lattice.CAPESCoordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)CNPqFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)FAPES

Structure and optical properties of [Ba(1-x)Y(2x/3)](Zr0.25Ti0.75)O3 powders

[Ba1–xY2x/3](Zr0.25Ti0.75)O3 powders with different yttrium concentrations (x = 0, 0.025 and 0.05) were prepared by solid state reaction. These powders were analyzed by X-ray diffraction (XRD), Fourier transform Raman scattering (FT-RS), Fourier transform infrared (FT-IR) and X-ray absorption near-edge (XANES) spectroscopies. The optical properties were investigated by means of ultraviolet-visible (UV-vis) absorption spectroscopy and photoluminescence (PL) measurements. Even with the addition of yttrium, the XRD patterns revealed that all powders crystallize in a perovskite-type cubic structure. FT-RS and FT-IR spectra indicated that the presence of [YO6] clusters is able to change the interaction forces between the O-Ti-O and O-Zr-O bonds. XANES spectra were used to obtain information on the off-center Ti displacements or distortion effects on the [TiO6] clusters. The different optical band gap values estimated from UV-vis spectra suggested the existence of intermediary energy levels (shallow or deep holes) within the band gap. The PL measurements carried out with a 350 nm wavelength at room temperature showed that all powders present typical broad band emissions in the blue region.FAPESP (09/50303-4)CNPqCAPESLNLS (D04B - XAFS1 - 8823

Photoluminescence behavior in MgTi'O IND. 3' powders with vacancy/distorted clusters and octahedral tilting

MgTiO3 powders were prepared by the complex polymerization method and heat treated at different temperatures for 2 h. These powders were analyzed by X-ray diffraction (XRD), Rietveld refinements, micro-Raman (MR) spectroscopy, X-ray absorption near edge spectroscopy (XANES), ultraviolet–visible (UV–vis) absorption spectroscopy and photoluminescence (PL) measurements. XRD patterns and MR spectra showed that the crystalline powders have a rhombohedral structure. XANES spectra indicated that the local structure of crystalline MgTiO3 powders is composed only by [TiO6] clusters, while the disordered exhibit the simultaneous presence of both [TiO5] and [TiO6] complex clusters into the lattice. UV–vis spectra revealed different optical band gap values as a function of heat treatment temperature. This result can be an indicative of intermediary energy levels within the band gap of this material because of structural defects into the perovskite-type structure. PL behavior was attributed to the structural order–disorder and/or distortions on the [TiO6]–[TiO6] complex clustersCAPESCNPQFAPESPLNLS (Projeto No. D04B - XAFS1 - 8050

Effect of different solvent ratios (water/ethylene glycol) on the growth process of CaMo'O IND.4' crystals and their optical properties

ABSTRACT: In this paper, calcium molybdate (CaMoO4) crystals (meso- and nanoscale) were synthesized by the coprecipitation\ud method using different solvent volume ratios (water/ethylene glycol). Subsequently, the obtained suspensions were\ud processed in microwave-assisted hydrothermal/solvothermal systems at 140 C for 1 h. These meso- and nanocrystals processed\ud were characterized by X-ray diffraction (XRD), Fourier transform Raman (FT-Raman), Fourier transform infrared (FT-IR),\ud ultraviolet-visible (UV-vis) absorption spectroscopies, field-emission gun scanning electron microscopy (FEG-SEM),\ud transmission electron microscopy (TEM), and photoluminescence (PL) measurements. XRD patterns and FT-Raman spectra\ud showed that these meso- and nanocrystals have a scheelite-type tetragonal structure without the presence of deleterious phases.\ud FT-IR spectra exhibited a large absorption band situated at around 827 cm-1, which is associated with the Mo-O antisymmetric\ud stretching vibrations into the [MoO4] clusters. FEG-SEM micrographs indicated that the ethylene glycol concentration\ud in the aqueous solution plays an important role in the morphological evolution of CaMoO4 crystals. High-resolution\ud TEM micrographs demonstrated that the mesocrystals consist of several aggregated nanoparticles with electron diffraction\ud patterns of monocrystal. In addition, the differences observed in the selected area electron diffraction patterns of CaMoO4\ud crystals proved the coexistence of both nano- and mesostructures. First-principles quantum mechanical calculations based on\ud the density functional theory at the B3LYP level were employed in order to understand the band structure and density of states\ud for the CaMoO4. UV-vis absorption measurements evidenced a variation in optical band gap values (from 3.42 to 3.72 eV) for\ud the distinct morphologies. The blue and green PL emissions observed in these crystals were ascribed to the intermediary energy\ud levels arising from the distortions on the [MoO4] clusters due to intrinsic defects in the lattice of anisotropic/isotropic crystals.FAPESP (09/50303-4)CNPqCAPE

Intense blue and green photoluminescence emissions at room temperature in barium zirconate powders

Intense blue and green photoluminescence (PL) emissions were observed at room temperature in barium zirconate (BaZrO3) powders prepared by chemical method. The powders were analyzed by X-ray diffraction (XRD), X-ray absorption near edge spectroscopy (XANES), extended X-ray absorption spectroscopy (EXAFS) and ultraviolet–visible (UV–vis) absorption spectroscopy. The XRD patterns indicated the presence of a secondary phase of BaCO3 in BaZrO3 powders heat treated at 773 K. K-edge XANES data revealed that Zr atoms presents in at least two Zr environments. EXAFS investigations showed strong bond length dispersion in the first coordination shell around Zr atoms. EXAFS analysis indicated that Zr atoms are coordinated by 6 oxygen in BaZrO3 powders heat treated at 973 K. UV–vis measurements suggested the presence of intermediary energy levels into the band gap of BaZrO3 powders heat treated at 773 K. The intense PL emission in BaZrO3 powders can be attributed to the two Zr environments or ZrO5–ZrO6 clusters.CAPESFAPESPCNPQLaboratório Nacional de Luz Síncrotron (LNLS

Synthesis of (Ca,Nd)Ti'O IND.3' powders by complex polymerization, Rietveld refinement and optical properties

Neodymium calcium titanate, (Ca0.99Nd0.01)TiO3 powders were synthesized by the complex polymerization method and heat treated at different temperatures for 2 h under air atmosphere. The structural evolution of these powders as a function of heat treatment temperature was analyzed by X-ray diffraction (XRD) and micro-Raman (MR) spectroscopy. The optical properties were investigated by Ultraviolet–visible (UV–vis) absorption spectroscopy and Photoluminescence (PL) measurements. XRD patterns, Rietveld refinement and MR spectra indicated that the powders heated treated at 750 °°C for 2 h present an orthorhombic structure without secondary phases. UV–vis measurements suggested the presence of intermediary energy in disordered (Ca0.99Nd0.01)TiO3 powders. Broad and narrow bands were observed in the PL spectra of these powders when excited with 350 nm wavelength. The broad bands were associated to the structural defects and/or p–d electronic transitions while, the narrow bands were ascribed to f–f transitions arising from Nd3+ ions.CAPESCNPQFAPES

Electronic structure, growth mechanism and photoluminescence of CaW'O IND. 4' crystals

In this paper, aggregated CaWO4 micro- and nanocrystals were synthesized by the co-precipitation method and processed under microwave-assisted hydrothermal/solvothermal conditions (160 °C for 30 min). According to the X-ray patterns, all crystals exhibited only the scheelite-type tetragonal structure. The data obtained by the Rietveld refinements revealed that the oxygen atoms occupy different positions in the [WO4] clusters, suggesting the presence of lattice distortions. The crystal shapes as well as its crystallographic orientations were identified by field-emission scanning electron microscopy and high-resolution transmission electron microcopy. Electronic structures of these crystals were evaluated by the first-principles quantum mechanical calculations based on the density functional theory in the B3LYP level. A good correlation was found between the experimental and theoretical Raman and infrared-active modes. A crystal growth mechanism was proposed to explain the morphological evolution. The ultraviolet-visible absorption spectra indicated the existence of intermediary energy levels within the band gap. The highest blue photoluminescence emission, lifetime and quantum yield were observed for the nanocrystals processed in the microwave-assisted solvothermal method.FAPESP (09/50303-4 09/53189-8/ 08/55585-5)CAPESCNP