Efeitos da adição de átomos de Mn na rede do GaN via métodos de estrutura eletrônica

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

Theoretical and experimental study of the infrared spectrum of isonicotinamide

The IR-spectrum of the isonicotinamide molecule (C(2)H(2)NC(3)H(2)CONH(2)) is studied by means of theoretical and experimental methods. For an appropriate representation of the molecular environment, Gaussian basis sets to the atoms of these molecule are built and then contracted (5s and 6s5p). For evaluation of the quality of contracted basis sets in molecular calculations, we have accomplished calculations of the total and the orbital (HOMO and HOMO-1) energies in the HF-Roothaan method for the molecule studied. The results obtained with the contracted basis sets [5s/6s5p] are compared to values obtained with our (21s/22s14p) basis sets and with those obtained with the D95, 6-31G, and 6-311G basis sets from literature. It was added one d polarization function in the [6s5p] contracted basis set for C ((3)P) atom, which was used in combination with the basis sets for H ((2)S), N ((4)S). and O((3)P) atoms to calculate the infrared spectrum of isonicotinamide. The calculations were performed at B3LYP level and were compared to corresponding experimental values also obtained in our laboratory. The theoretical results in comparison with the corresponding experimental values indicate a very good interpretation of the IR-spectrum and that the strategy of an appropriate representation of the molecular environment through the basis sets is an effective alternative to investigate vibrational theoretical properties of the nicotinamide molecule. (c) 2006 Published by Elsevier B.V.

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.

A theoretical analysis of the TiO2/Sn doped (110) surface properties

We have used the periodic quantum-mechanical method with density functional theory at the B3LYP level in order to study TiO2/Sn doped (1 1 0) surfaces and have investigated the structural, electronic and energy band properties of these oxides. Our calculated relaxation directions for TiO2 is the experimental one and is also in agreement with other theoretical results. We also observe for the doped systems relaxation of lattice positions of the atoms. Modification of Sri, O and Ti charges depend on the planes and positions of the substituted atoms. Doping can modify the Fermi levels, energy gaps as well as the localization and composition of both valence and conduction band main components. Doping can also modify the chemical, electronic and optical properties of these oxides surfaces increasing their suitability for use as gas sensors and optoelectronic devices. (c) 2005 Elsevier B.V. All rights reserved.

Combined experimental and theoretical study to understand the photoluminescence of Sr1-xTiO3-x

A joint experimental and theoretical study has been carried out to rationalize the results of visible photoluminescence measurements at room temperature on Sr1-xTiO3-x (ST) perovskite thin films. From the experimental side, ST thin films, x = 0 to 0.9, have been synthesized following soft chemical processing, and the corresponding photoluminescence properties have been measured. First principles quantum mechanical techniques, based on density functional theory at the B3LYP level, have been employed to study the electronic structure of a crystalline, stoichiometric (x = 0) ST-s model and a nonstoichiometric (SrO-deficient, x not equal 0) and structurally disordered ST-d model. The relevance of the present theoretical and experimental results of the photoluminescence behavior of ST is discussed. The optical spectra and the calculations indicate that the symmetry-breaking process on going from ST-s to ST-d creates electronic levels in the valence band. Moreover, an analysis of the Mulliken charge distribution reveals a charge gradient in the structure. These combined effects seem to be responsible for the photoluminescence behavior of deficient Sr1-xTiO3-x.

Photoluminescence of crystalline and disordered BTO : Mn powder: Experimental and theoretical modeling

Disordered and crystalline Mn-doped BaTiO3 (BTO:Mn) powders were synthesized by the polymeric precursor method. After heat treatment, the nature of visible photoluminescence (PL) at room temperature in amorphous BTO:Mn was discussed, considering results of experimental and theoretical studies. X-ray diffraction (XRD), PL, and UV-vis were used to characterize this material. Rietveld refinement of the BTO:Mn from XRD data was used to built two models, which represent the crystalline BTO:Mn (BTO:Mn,) and disordered BTO:Mn (BTO:Mn-d) structures. Theses models were analyzed by the periodic ab initio quantum mechanical calculations using the CRYSTAL98 package within the framework of density functional theory at the B3LYP level. The experimental and theoretical results indicated that PL is related with the degree of disorder in the BTO:Mn powders and also suggests the presence of localized states in the disordered structure. (c) 2006 Elsevier B.V. All rights reserved.

Room-temperature photoluminescence of BaTiO3: Joint experimental and theoretical study

Strong photoluminescent emission has been measured at room temperature for noncrystalline BaT'O-3 (BT) perovskite powders. A joint experimental and theoretical study has been carried out to rationalize this phenomenon. From the experimental side, BT powder samples have been synthesized following a soft chemical processing, their crystal structure has been confirmed by x-ray data and the corresponding photoluminescence (PL) properties have been measured. Only the structurally disordered samples present PL at room temperature. From the theoretical side, first-principles quantum-mechanical techniques, based on density-functional theory at the B3LYP level, have been employed to study the electronic structure of crystalline (BT-c) and asymmetric (BT-a) models. Theoretical and experimental results are found to be consistent and their confrontation leads to an interpretation of the PL apparition at room temperature in the structurally disordered powders.

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.

Electronic and structural properties of the (001) SrZrO3 surface

The structural and electronic properties of SrZrO3 selected surfaces were investigated by means of density functional theory applied to periodic calculations at B3LYP level. The relaxation effects for two symmetric and asymmetric terminations are analyzed. The electronic and energy band properties are discussed on the basis of band structure as well density of states. There is a more significant rumpling in the SrO as compared to the ZrO2 terminated surfaces. The calculated indirect gap is 4.856, 4.562, 4.637 eV for bulk, ZrO2 and asymmetric terminations, respectively. The gap becomes direct; 4.536 eV; for SrO termination. The contour in the (110) diagonal plane indicates a partial covalent character between Zr and 0 atoms for the SrO terminated surface. (c) 2007 Elsevier B.V. All rights reserved.

Effect of Different Solvent Ratios (Water/Ethylene Glycol) on the Growth Process of CaMoO4 Crystals and Their Optical Properties

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

Gaussian basis sets by generator coordinate Hartree-Fock method to ab initio calculations of electron affinities of enolates

The Generator Coordinate Hartree-Fock (GCHF) method is employed to generate uncontracted 15s and 18s11p gaussian basis sets for the H, C and O atoms, respectively. These basis sets are then contracted to 3s and 4s H atom and 6s5p, for C and O atoms by a standard procedure. For quality evaluation of contracted basis sets in molecular calculations, we have accomplished calculations of total and orbital energies in the Hartree-Fock-Roothaaii (HFR) approach for CH, C(2) and CO molecules. The results obtained with the uncontracted basis sets are compared with values obtained with the standard D95, 6-311G basis sets and with values reported in the literature. The 4s and 6s5p basis sets are enriched with polarization and diffuse functions for atoms of the parent neutral systems and of the enolates anions (cycloheptanone enolate, 2,5-dimethyleyelopentanone enolate, 4-heptanone enolate, and di-isopropyl ketone enolate) from the literature, in order to assess their performance in ab initio molecular calculations, and applied for calculations of electron affinities of the enolates. The calculations were performed at the DFT (BLYP and B3LYP) and HF levels and compared with the corresponding experimental values and with those obtained by using other 6-3 1 + +G((*)) and 6-311 + +G((*)) basis sets from literature. For the enolates studied, the differences between the electron affinities obtained with GCHF basis sets, at the B3LYP level, and the experimental values are -0.001, -0,014, -0.001, and -0.001 eV. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Electronic and structural properties of the (10(1)over-bar0) and (11(2)over-bar0) ZnO surfaces

The structural and electronic properties of ZnO (10 (1) over bar0) and (11 (2) over bar0) surfaces were investigated by means of density functional theory applied to periodic calculations at B3LYP level. The stability and relaxation effects for both surfaces were analyzed. The electronic and energy band properties were discussed on the basis of band structure as well as density of states. There is a significant relaxation in the (10 (1) over bar0) as compared to the (11 (2) over bar0) terminated surfaces. The calculated direct gap is 3.09, 2.85, and 3.09 eV for bulk, (10 (1) over bar0), and (11 (2) over bar0) surfaces, respectively. The band structures for both surfaces are very similar.

Density functional theory study on the structural and electronic properties of low index rutile surfaces for TiO2/SnO2/TiO2 and SnO2/TiO2/SnO2 composite systems

The present study is concerned with the structural and electronic properties of the TiO2/SnO2/TiO2 and SnO2/TiO2/SnO2 composite systems. Periodic quantum mechanical method with density functional theory at the B3LYP level has been carried out. Relaxed surface energies, structural characteristics and electronic properties of the (I 10), (0 10), (10 1) and (00) low-index rutile surfaces for TiO2/SnO2/TiO2 and SnO2/TiO2/SnO2 models are studied. For, comparison purposes, the bare rutile TiO2 and SnO2 structures are also analyzed and compared with previous theoretical and experimental data. The calculated surface energy for both rutile TiO2 and SnO2 surfaces follows the sequence (110) < (010) < (101) < (001) and the energy increases as (010) < (101) < (110) < (001) and (010) approximate to (110) < (101) < (001) for SnO2/TiO2/SnO2 and TiO2/SnO2/TiO2 composite systems, respectively. SnO2/TiO2/SnO2 presents larger values of surface energy than the individual SnO2 and TiO2 metal oxides and the TiO2/SnO2/TiO2 system renders surface energy values of the same order that the TiO2 and lower than the SnO2. An analysis of the electronic structure of the TiO2/SnO2/TiO2 and SnO2/TiO2/SnO2 systems shows that the main characteristics of the upper part of the valence bands for all the studied surfaces are dominated by the external layers, i.e., by the TiO2 and the SnO2, respectively, and the topology of the lower part of the conduction bands looks like the core layers. There is an energy stabilization of both valence band top and conduction band bottom for (110) and (010) surfaces of the SnO2/TiO2/SnO2 composite system in relation to their core TiO2, whereas an opposite trend is found for the same surfaces of the TiO2/SnO2/TiO2 composite system in relation to the bare SnO2. The present theoretical results may explain the growth of TiO2@SnO2 bimorph composite nanotape.

Combined (13)C NMR and DFT/GIAO studies of the polyketides Aurasperone A and Fonsecinone A

Structural characterization by NMR spectroscopy and DFT calculations was performed for two dimeric naptho-gamma-pyrones, the polyketides Aurasperone A and Fonsecinone A. Experimental data ((13)C NMR chemical shifts and interatomic geometries) were found to be in reasonable agreement with theoretical ones, obtained at B3LYP level for three different basis sets (6-31G/6-31G(d)/6-31G(d,p)). Additionally, the dipolar moments calculation allowed explaining the different solubility for these molecules. The (13)C NMR theoretical chemical shifts were calculated with the GIAO method and the solvent effects were taken into account by means of the PCM approximation. In this work, the DFT/GIAO methodology shows to be a reliable tool in the assignment of experimental NMR chemical shifts of similar molecules. (C) 2008 Wiley Periodicals, Inc. Int J Quantum Chem 108: 2408-2416, 2008.