Structural refinement, optical and microwave dielectric properties of BaZrO3

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Photoluminescence in quasi-amorphous Pb0.8X0.2Zr0.53Ti0.47O3 (X = Ca, Sr and Ba) powders: An optical and structural study

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Electrical and optical characteristics of SnO2 thin films prepared by dip coating from aqueous colloidal suspensions

Starting from aqueous colloidal suspensions, undoped and Nb5+ doped SnO2 thin films have been prepared by using the dip-coating sol gel process. X-ray diffraction results show that films are polycrystalline with crystallites of average size1-4nm. Decreasing the thickness of the films and increasing the Nb5+ concentration limits the crystallite size growth during firing. Complex impedance measurements reveal capacitive and resistive effects between adjacent crystallites or grains, characteristic of electrical potential barriers. The transfer of charge throughout these barriers determines the macroscopic electrical resistance of the layer. The analysis of the optical absorption spectra shows that the samples present more than 80% of their transmittance in the visible region and the value of the band gap energy increases with decreasing crystallite size. © 1997 Chapman & Hall.

Effect of pressure-assisted thermal annealing on the optical properties of ZnO thin films

ZnO thin films were prepared by the polymeric precursor method. The films were deposited on silicon substrates using the spin-coating technique, and were annealed at 330°C for 32h under pressure-assisted thermal annealing and under ambient pressure. Their structural and optical properties were characterized, and the phases formed were identified by X-ray diffraction. No secondary phase was detected. The ZnO thin films were also characterized by field-emission scanning electron microscopy, Fourier transform infrared spectroscopy, photoluminescence and ultraviolet emission intensity measurements. The effect of pressure on these thin films modifies the active defects that cause the recombination of deep level states located inside the band gap that emit yellow-green (575nm) and orange (645nm) photoluminescence. © 2012 John Wiley & Sons, Ltd.

Optical properties of the MoO3-TiO2 particulate system and its use as a ceramic pigment

Mo-doped TiO2 powders were prepared using a dry mixture of TiO2 and MoO3 oxides with several compositions, followed by a calcination step at several temperatures. The resulting oxide system develops yellow and green tones. The XRD patterns showed only traces of MoO 3; however, EDS results, combined with TG/DTA data, confirmed the presence of molybdenum ions, suggesting that the changes in optical properties of the oxide system is due to the incorporation of Mo ions into the TiO 2 matrix, substituting Ti+4 with Mo+6 ions. The band gap decreased with increasing of MoO3 content; on the other hand, the band gap reached a maximum value at about 850°C to 910°C when plotted as a function of the calcination temperature. The glazes produced showed that the oxide system under study is a potential material for use as abinary ceramic pigment. Copyright © 2013 Taylor & Francis Group, LLC.

Effect of Fe-doping on the structural, microstructural, optical, and ferroeletric properties of Pb1/2Sr1/2Ti1-xFexO3 oxide prepared by spin coating technique

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Structural and optical properties of Cu2O crystalline electrodeposited films

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Correlation between structural and electronic order-disorder effects and optical properties in ZnO nanocrystals

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

A relationship between structural and electronic order-disorder effects and optical properties in crystalline TiO2 nanomaterials

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Structural refinement, optical and microwave dielectric properties of BaZrO3

In this work, barium zirconate (BaZrO3) ceramics synthesized by solid state reaction method and sintered at 1670 degrees C for 4 h were characterized by X-ray diffraction (XRD), Rietveld refinement, and Fourier transform infrared (FT-IR) spectroscopy. XRD patterns, Rietveld refinement data and FT-IR spectra which confirmed that BaZrO3 ceramics have a perovskite-type cubic structure. Optical properties were investigated by ultraviolet-visible (UV-vis) absorption and photoluminescence (PL) measurements. UV-vis absorption spectra suggested an indirect allowed transition with the existence of intermediary energy levels within the band gap. Intense visible green PL emission was observed in BaZrO3 ceramics upon excitation with a 350 nm wavelength. This behavior is due to a majority of deep defects within the band gap caused by symmetry breaking in octahedral [ZrO6] clusters in the lattice. The microwave dielectric constant and quality factor were measured using the method proposed by Hakki-Coleman. The dielectric resonator antenna (DRA) was investigated experimentally and numerically using a monopole antenna through an infinite ground plane and Ansoft's high frequency structure simulator software, respectively. The required resonance frequency and bandwidth of DRA were investigated by adjusting the dimension of the same material. (C) 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Optical third harmonic generation in the magnetic semiconductor EuSe

Third harmonic generation (THG) has been studied in europium selenide EuSe in the vicinity of the band gap at 2.1-2.6 eV and at higher energies up to 3.7 eV. EuSe is amagnetic semiconductor crystalizing in centrosymmetric structure of rock-salt type with the point group m3m. For this symmetry the crystallographic and magnetic-field-induced THG nonlinearities are allowed in the electric-dipole approximation. Using temperature, magnetic field, and rotational anisotropy measurements, the crystallographic and magnetic-field-induced contributions to THG were unambiguously separated. Strong resonant magnetic-field-induced THG signals were measured at energies in the range of 2.1-2.6 eV and 3.1-3.6 eV for which we assign to transitions from 4 f(7) to 4 f(6)5d(1) bands, namely involving 5d(t(2g)) and 5d(e(g)) states.

CdSe/CdTe interface band gaps and band offsets calculated using spin-orbit and self-energy corrections

We performed ab initio calculations of the electronic structures of bulk CdSe and CdTe, and their interface band alignments on the CdSe in-plane lattice parameters. For this, we employed the LDA-1/2 self-energy correction scheme [L.G. Ferreira, M. Marques, L.K. Teles, Phys. Rev. B 78 (2008) 125116] to obtain corrected band gaps and band offsets. Our calculations include the spin-orbit effects for the bulk cases, which have shown to be of importance for the equilibrium systems and are possibly degraded in these strained semiconductors. Therefore, the SO showed reduced importance for the band alignment of this particular system. Moreover, the electronic structure calculated along the transition region across the CdSe/CdTe interface shows an interesting non-monotonic variation of the band gap in the range 0.8-1.8 eV, which may enhance the absorption of light for corresponding frequencies at the interface between these two materials in photovoltaic applications. (C) 2012 Elsevier B.V. All rights reserved.

SnO2 nanocrystals synthesized by microwave-assisted hydrothermal method: towards a relationship between structural and optical properties

The exploration of novel synthetic methodologies that control both size and shape of functional nanostructure opens new avenues for the functional application of nanomaterials. Here, we report a new and versatile approach to synthesize SnO2 nanocrystals (rutile-type structure) using microwave-assisted hydrothermal method. Broad peaks in the X-ray diffraction spectra indicate the nanosized nature of the samples which were indexed as a pure cassiterite tetragonal phase. Chemically and physically adsorbed water was estimated by TGA data and FT-Raman spectra to account for a new broad peak around 560 cm(-1) which is related to defective surface modes. In addition, the spherical-like morphology and low dispersed distribution size around 3-5 nm were investigated by HR-TEM and FE-SEM microscopies. Room temperature PL emission presents two broad bands at 438 and 764 nm, indicating the existence of different recombination centers. When the size of the nanospheres decreases, the relative intensity of 513 nm emission increases and the 393 nm one decreases. UV-Visible spectra show substantial changes in the optical absorbance of crystalline SnO2 nanoparticles while the existence of a small tail points out the presence of localized levels inside the forbidden band gap and supplies the necessary condition for the PL emission.

Nanoscale-electrical and optical properties of iii-nitrides

III-nitrides are wide-band gap materials that have applications in both electronics and optoelectronic devices. Because to their inherent strong polarization properties, thermal stability and higher breakdown voltage in Al(Ga,In)N/GaN heterostructures, they have emerged as strong candidates for high power high frequency transistors. Nonetheless, the use of (Al,In)GaN/GaN in solid state lighting has already proved its success by the commercialization of light-emitting diodes and lasers in blue to UV-range. However, devices based on these heterostructures suffer problems associated to structural defects. This thesis primarily focuses on the nanoscale electrical characterization and the identification of these defects, their physical origin and their effect on the electrical and optical properties of the material. Since, these defects are nano-sized, the thesis deals with the understanding of the results obtained by nano and micro-characterization techniques such as atomic force microscopy(AFM), current-AFM, scanning kelvin probe microscopy (SKPM), electron beam induced current (EBIC) and scanning tunneling microscopy (STM). This allowed us to probe individual defects (dislocations and cracks) and unveil their electrical properties. Taking further advantage of these techniques,conduction mechanism in two-dimensional electron gas heterostructures was well understood and modeled. Secondarily, origin of photoluminescence was deeply investigated. Radiative transition related to confined electrons and photoexcited holes in 2DEG heterostructures was identified and many body effects in nitrides under strong optical excitations were comprehended.

Optical hysteresis of ridge waveguide magnetic garnet films and cavities in two-dimensional magneto-photonic crystal slabs

Magnetic iron garnets as well as magnetic photonic crystals are of great interests in magneto-optic applications such as isolators, current captors, circulators, TE-TM mode conversion, wavelength accordable filters, optical sensors and switches, all of which provide a promising platform for future integrated optical circuits. In the present work, two topics are studied based on magnetic iron garnet films. In the first part, the characteristics of the magnetization are investigated for ridge waveguides fabricated on (100) oriented iron garnet thin films. The magnetic response in magneto-optic waveguides patterned on epitaxial magnetic garnet films depends on the crystallographic orientation of the waveguides and the magnetic anisotropy of the material. These can be studied by polarization rotation hysteresis loops, which are related to the component of magnetization parallel to the light propagation direction and the linear birefringence. Polarization rotation hysteresis loops for low birefringence waveguides with different orientations are experimentally investigated. Asymmetric stepped curves are obtained from waveguides along, due to the large magnetocrystalline anisotropy in the plane. A model based on the free energy density is developed to demonstrate the motion of the magnetization and can be used in the design of magneto-optic devices. The second part of this thesis focuses on the design and fabrication of high-Q cavities in two-dimensional magneto-photonic crystal slabs. The device consists of a layer of silicon and a layer of iron garnet thin film. Triangular lattice elliptical air holes are patterned in the slab. The fundamental TM band gap overlaps with the first-order TE band gap from 0374~0.431(a/λ) showing that both TE and TM polarization light can be confined in the photonic crystals. A nanocavity is designed to obtain both TE and TM defect modes in the band gaps. Additional work is needed to overlap the TE and TM defect modes and obtain a high-Q cavity so as to develop miniaturized Faraday rotators.