Combined experimental and theoretical investigations of the photoluminescent behavior of Ba(Ti,Zr)O-3 thin films

The correlation between experimental data and theoretical calculations have been investigated to explain the photoluminescence at room temperature of Ba(Ti0.75Zr0.25)O-3 (BTZ) thin films prepared by the polymeric precursor method. The degree of structural order-disorder was investigated by X-ray diffraction, Fourier transform infrared spectroscopy, ultraviolet-visible absorption spectroscopy and photoluminescence (PL) measurements. First-principles quantum mechanical calculations based on density functional theory (B3LYP level) were employed to study the electronic structure of ordered and deformed asymmetric models. The electronic properties are analyzed and the relevance of the present theoretical and experimental results on the PL behavior is discussed. The presence of localized electronic levels and a charge gradient in the band gap due to a break in symmetry, are responsible for the PL in disordered BTZ lattice. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Effects of Alloyed and Oxide Phases on Methanol Oxidation of Pt-Ru/C Nanocatalysts of the Same Particle Size

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

Very Intense Distinct Blue and Red Photoluminescence Emission in MgTiO3 Thin Films Prepared by the Polymeric Precursor Method: An Experimental and Theoretical Approach

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Color tunability of intense upconversion emission from Er3+-Yb3+ co-doped SiO2-Ta2O5 glass ceramic planar waveguides

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

Propriedades eletrônicas e estruturais do PbTiO3: teoria do funcional de densidade aplicada a modelos periódicos

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

Propriedades eletrônicas, estruturais e constantes elásticas do ZnO

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

Determinação de diagramas de bandas de energia e da borda de absorção em SnO2, depositado via sol-gel, sobre quartzo

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

A DFT rationalization of the room temperature photoluminescence of Li2TiSiO5

Li2TiSiO5 powders were synthesized by the polymeric precursor method. The calcination temperatures were progressively increased until the complete crystallization of the phase occurring at 870 degreesC. For the first time, a strong photoluminescence was measured at room temperature with a 488 nm excitation wavelength for the non-crystalline samples. This photoluminescence in disordered phases has been interpreted by means of high-level quantum mechanical calculations based on density functional theory. Two periodic models have been used to represent the crystalline and disordered powders. They allowed to calculate electronic properties consistent with experimental data and to explain the relations between photoluminescence and structural disorder. (C) 2004 Elsevier B.V. All rights reserved.

Ab initio study of high tridymite by the formalism generator coordinate Hartree-Fock

The Generator Coordinate Hartree-Fock (GCHF) Method is applied to generate extended 14s 8p and 17s 11p Gaussian basis sets for the atoms O and Si, respectively. The role of the weight functions in the assessment of the numerical integration range of the GCHF is shown. The Gaussian basis sets are contracted to [6s4p] O atom and [8s5p] Si atom by the Dunning's segmented contraction scheme. To evaluate the quality of our contracted [6s4p] and [8s5p] bases in molecular calculations we accomplish calculations of total and orbital energies in the Hartree-Fock-Roothaan method for O-2 and SiO molecules. We compare the results obtained with the our (14s 8p) and (17s 11p) bases sets with the of 6-311G basis and with values from the literature. The addition of one d polarization function in the silicon basis and its utilization with the basis for oxygen leads to the calculation of electronic properties and IR Spectrum of high tridymite in space group D-3d. (C) 1999 Elsevier B.V. B.V. All rights reserved.

Phase separation and gap bowing in zinc-blende InGaN, InAlN, BGaN, and BAlN alloy layers

We present first-principles calculations of the thermodynamic and electronic properties of the zinc-blende ternary InxGa1-xN. InxAl1-xN, BxGa1-xN, and BxAl1-xN alloys. They are based on a generalized quasi-chemical approximation and a pseudopotential-plane-wave method. T-x phase diagrams for the alloys are obtained, We show that due to the large difference in interatomic distances between the binary compounds a significant phase miscibility gap for the alloys is found. In particular for the InxGa1-xN alloy, we show also experimental results obtained from X-ray and resonant Raman scattering measurements, which indicate the presence of an In-rich phase with x approximate to 0.8. For the boron-containing alloy layers we found a very high value for the critical temperature for miscibility. similar to9000 K. providing an explanation for the difficulties encountered to grow these materials with higher boron content. The influence of a biaxial strain on phase diagrams, energy gaps and gap bowing of these alloys is also discussed. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Photoluminescence in disordered sm-doped PbTiO3: Experimental and theoretical approach

Sm-doped PbTiO3 powder was synthesized by the polymeric precursor method, and was heat treated at different temperatures. The x-ray diffraction, photoluminescence, and UV-visible were used as a probe for the structural order degree short-, intermediate-, and long-range orders. Sm-3+ ions were used as markers of these order-disorder transformations in the PbTiO3 system. From the Rietveld refinement of the Sm-doped PbTiO3 x-ray diffraction data, structural models were obtained and analyzed by periodic ab initio quantum mechanical calculations using the CRYSTAL 98 package within the framework of density functional theory at the B3LYP level. This program can yield important information regarding the structural and electronic properties of crystalline and disordered structures. The experimental and theoretical results indicate the presence of the localized states in the band gap, due to the symmetry break, which is responsible for visible photoluminescence at room temperature in the disordered structure. (c) 2006 American Institute of Physics.

Origin of photo luminescence in SrTiO3: a combined experimental and theoretical study

A joint experimental and theoretical study has been carried out to rationalize the photoluminescence properties of SrTiO3 perovskite thin films synthesized through a soft chemical processing. Only the amorphous samples present photoluminescence at room temperature. From the theoretical side, first principles quantum mechanical techniques, based on density functional theory at B3LYP level, have been employed to study the electronic structure of a crystalline (ST-c) and an asymmetric (ST-a) model. Electronic properties are analyzed in the light of the experimental results and their relevance in relation to the PL behavior of ST is discussed. (C) 2004 Elsevier B.V. All rights reserved.

BEHAVIOR OF THE ANION-RADICALS ELECTROCHEMICALLY GENERATED IN THE REDUCTION OF NOSYL AMIDES

The behaviour of nitrobenzenesulfonamide anion radicals generated from the electrochemical reduction of aliphatic and aromatic amines protected by nitrobenzenesulfonyl (nosyl) groups in N,N-' dimethylformamide has been reported. The species have been characterized by voltammetry and optical and electron spin resonance spectroscopies. The visible spectra of the anion radicals were recorded and the hyperfine splitting constants were assigned to specific proton positions and nitrogen nuclei of the molecule. The stabilities of the anion radicals are affected by electronic properties of the protecting group and specific features of the amines, which show direct influence on the route of cathodic cleavage of the nitrobenzenesulfonamides.

Towards an insight on the photoluminescence of disordered CaWO4 from a joint experimental and theoretical analysis

A joint experimental and theoretical study has been carried out to rationalize for the first time the photoluminescence (PL) properties of disordered CaWO4 (CWO) thin films. From the experimental side, thin films of CWO have been synthesized following a soft chemical processing, their structure has been confirmed by X-ray diffraction data and corresponding PL properties have been measured using the 488 nm line of an argon ion laser. Although we observe PL at room temperature for the crystalline thin films, the structurally disordered samples present much more intense emission. From the theoretical side, first principles quantum mechanical calculations, based on density functional theory at B3LYP level, have been employed to study the electronic structure of a crystalline (CWO-c) and asymmetric (CWO-a) periodic model. Electronic properties are analyzed in the light of the experimental results and their relevance in relation to the PL behavior of CWO is discussed. The symmetry breaking process on going from CWO-c to CWO-a creates localized electronic levels above the valence band and a negative charge transfer process takes place from threefold, WO3, to fourfold, WO4,. tungsten coordinations. The correlation of both effects seems to be responsible for the PL of amorphous CWO. (c) 2005 Elsevier B.V. All rights reserved.

Experimental and theoretical investigation of the room-temperature photoluminescence of amorphized Pb(ZrTi)O-3

Ultrafine PbZr0.20Ti0.80O3 was omorphized through high-energy mechanical milling. The structural evolution through the omorphization process was accompanied by various characterization techniques, such as X-ray diffraction, Fourier-transformed IR spectroscopy (FTIR), high-resolution transmission electron microscopy (HR-TEM), and Raman spectroscopy. A strong photoluminescence was measured at room temperature for amorphized PbZr0.20Ti0.80O3, and interpreted by means of high-level quantum mechanical calculations in the density functional theory frame-work. Three periodic models were used to represent the crystalline and amorphized PbZr0.20Ti0.80O3, and they allowed the calculation of electronic properties that are consistent with the experimental data and that explain the appearance of photoluminescence.