C. D. Pinheiro

The role of network modifiers in the creation of photoluminescence in CaTiO3

We discuss the nature of visible photoluminescence (PL) at room temperature in amorphous calcium titanate in the light of the results of recent experimental and quantum mechanical theoretical studies. Our investigation of the electronic structure involved the use of first-principle molecular calculations to simulate the variation of the electronic structure in the calcium titanate crystalline phase, which is known to have a direct band gap, and we also made an in-depth examination of amorphous calcium titanate. The results of our theoretical calculations of amorphous calcium titanate indicate that the formation of fivefold coordination in the amorphous system may introduce delocalized electronic levels in the highest occupied and the lowest unoccupied molecular orbitals. These delocalized electronic levels are related to the formation of a tail in the absorbance spectrum curve. The results indicate that amorphous calcium titanate has the conduction band near the band gap dominated by Ca states contribution. Experimental optical absorption measurements showed the presence of a tail. These results are interpreted by the nature of these exponential optical edges and tails, associated with defects promoted by the disordered structure of the amorphous material. We associate them with delocalized states in the band gap.

Theoretical and experimental study on the photoluminescence in BaTiO3 amorphous thin films prepared by the chemical route

Abstract A polymeric precursor method was used to synthesize BaTiO 3 amorphous thin film processed at low temperature. The luminescence spectra of BaTiO 3 amorphous thin films at room temperature revealed an intense single-emission band in the visible region. The visible emission band was found to be dependent of the thermal treatment history. Photoluminescence (PL) properties for different annealing temperatures were investigated. It was concluded that the intensity of PL is strongly dependent on both the heat treatment of the films and the presence of an inorganic disordered phase. Experimental optical absorption measurements showed the presence of a tail. These results are interpreted by the nature of these exponential optical edges and tails, associated with defects promoted by the disordered structure of the amorphous material. We discuss the nature of visible PL at room temperature in amorphous barium titanate in the light of the results of recent experimental and quantum mechanical theoretical studies. Our investigation of the electronic structure involved the use of first-principle molecular calculations to simulate the variation of the electronic structure in the barium titanate crystalline phase, which is known to have a direct band gap, and we also made an in-depth examination of amorphous barium titanate.

Standard enthalpy of formation of di-isobutyldithiocarbamate complexes of zinc-group elements and mean M-S bond-dissociation enthalpies

The standard enthalpies of formation of crystalline di-isobutyldithiocarbamate complexes of Zn, Cd and Hg(II) have been derived by solution calorimetry at 298.15 K, giving values −526.06±3.30, −468.07±3.23 and −310.15±3.56 kJ mol−1, respectively. The corresponding molar enthalpies of sublimation, 283±2, 281±2 and 247±1 kJ mol−1, were estimated by means of differential scanning calorimetry. From the standard molar enthalpies of formation of the gaseous chelates, −243±4, −187±4 and −63±4 kJ mol−1 the homolytic 137±4, 118±4 and 75±4 kJ mol−1 and heterolytic 700±4, 646±4 and 682±4 kJ mol−1 mean metal-sulphur bond enthalpies were calculated. The homolytic enthalpies decrease from zinc to mercury and depend on the nature of the radical attached to the nitrogen of the ligand.

Metal—sulphur bond enthalpy determination of diethyldithiocarbamate complexes of cadmium and mercury

Abstract The standard molar enthalpy of formation of crystalline diethyldithiocarbamate complexes of Cd and Hg(II) has been derived by solution calorimetry at 298.15 K, giving −227.5 ± 5.0 and −141.4 ± 5.0 kJ mol −1 , respectively. The corresponding standard molar enthalpies of sublimation, 102.0 ± 3.5 and 129.2 ± 4.2 kJ mol −1 , were estimated by means of differential scanning calorimetry. From the standard molar enthalpies of formation of the gaseous chelates (−125.5 ± 6.1 and −12.2 ± 6.5 kJ mol −1 ), the homolytic (167.3 ± 3.6 and 74.6 ± 3.6 kJ mol −1 ) and heterolytic (695.0 ± 3.6 and 733.4 ± 3.6 kJ mol −1 ) mean metal—sulphur bond-dissociation enthalpies were calculated. The results correlate with the characteristics of the metals and cations in the gas phase.

Kinetic study of the thermal decomposition on bis(dialkyldithiocarbamate) Cd(II) complexes by isothermal and non-isothermal thermogravimetry

Abstract The thermal decomposition kinetics of solid Cd(S 2 CNR 2 ) 2 complexes, where R=C 2 H 5 , n -C 3 H 7 , n -C 4 H 9 and iso-C 4 H 9 , has been studied using isothermal and non-isothermal thermogravimetry. Superimposed TG/DTG/DSC curves show that thermal decomposition reactions occur in the liquid phase. The kinetic model that best adjusted the experimental isothermal TG data was the one-dimensional phase-boundary reaction controlling process R 1 . The thermal analysis data suggest the thermal stability sequence Cd(S 2 CNBu n 2 ) 2 >Cd(S 2 CNPr n 2 ) 2 >Cd(S 2 CNBu i 2 ) 2 >Cd(S 2 CNEt 2 ) 2 , according to the order of stability of apparent activation energy.

Standard enthalpies of formation and lattice energies of di-n-butyl ammonium halides and di-2-metrylpropyl ammonium halides

Abstract The standard molar enthalpies of formation of di- n -butyl ammonium halides and di-2-methylpropyl ammonium halides have been derived from solution-reaction calorimetry. From these values the crystal lattice energies of the dialkyl ammonium salts were calculated and found to decrease from chloride to iodide within both series of salts.

Dipropyl ammonium halides and di-1-methylethyl ammonium halides: standard molar enthalpies of formation and lattice energies

Abstract The standard molar enthalpies of formation of dipropyl ammonium halides and di-1-methylethyl ammonium halides have been derived from calorimetric studies. The results lead to the crystal lattice energies of the salts, which decrease from chloride to iodide within both series of compounds.

O papel dos modificadores de rede na produção da fotoluminescência no CaWO4

We discuss the nature of visible photoluminescence at room temperature in amorphous calcium tungstate taking into account the results of recent experimental and quantum mechanical theoretical studies. Our investigation of the electronic structure involved the use of first-principle molecular calculations to simulate the variation of the electronic structure in the calcium tungstate crystalline phase, which is known to have a direct band gap, and we also made an in-depth examination of amorphous calcium tungstate. The results of our theoretical calculations of amorphous calcium tungstate indicate that the formation of threefold coordination in the amorphous system may introduce delocalized electronic levels in the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital). These delocalized electronics levels are related to the formation of a tail in the absorbency spectrum curve. The results indicate that amorphous calcium tungstate has the conduction band near the band gap dominated by Ca states contribution. Experimental optical absorption measurements showed the presence of a tail. These results are interpreted by the nature of these exponential optical edges and tails, associated with defects promoted by the disordered structure of the amorphous material. We associate them with delocalized states in the band gap.

Estudo do comportamento óptico-estrutural do LiNbO3

A study of the crystalline and amorphous structures of lithium niobate (LN) semiconductor was carried out to verify the influence of defects on the optical properties. Crystalline c-LN and amorphous a-LN thin films were prepared by the polymeric precursor method and characterized by Atomic Force Microscopy. Ultraviolet-Visible (UV-VIS) and Raman Spectroscopy were the studied optical properties. The amorphous film showed luminescence with a peak position dependent on the excitation wavelength. The energy difference between the Highest Occupied Molecular Orbital and the Lowest Unoccupied Molecular Orbital levels reveals that the band gap of the crystalline phase is larger than that exhibited by the amorphous phase, in agreement with the UV-VIS data. New electronic levels were observed in the band gap of the amorphous structure and could explain the optical properties observed in the amorphous film.