F. Lanciotti

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.

Influence of Ca concentration on the electric, morphological, and structural properties of (Pb,Ca)TiO3 thin films

Pb1−xCaxTiO3 (0.10⩽x⩽0.40) thin films on Pt/Ti/SiO2/Si(100) substrates were prepared by the soft solution process and their characteristics were investigated as a function of the calcium content (x). The structural modifications in the films were studied using x-ray diffraction and micro-Raman scattering techniques. Lattice parameters calculated from x-ray data indicate a decrease in lattice tetragonality with the increasing content of calcium in these films. Raman spectra exhibited characteristic features of pure PbTiO3 thin films. Variations in the phonon mode wave numbers, especially those of lower wave numbers, of Pb1−xCaxTiO3 thin films as a function of the composition corroborate the decrease in tetragonality caused by the calcium doping. As the Ca content (x) increases from 0.10 to 0.40, the dielectric constant at room temperature abnormally increased at 1 kHz from 148 to 430. Also calcium substitution decreased the remanent polarization and coercive field from 28.0 to 5.3 μC/cm2 and 124 to 58 kV/c...

Ferroelectric and optical properties of Ba0.8Sr0.2TiO3 thin film

Barium strontium titanate (Ba0.8Sr0.2TiO3) thin films have been prepared on Pt/Ti/SiO2/Si substrates using a soft solution processing. X-ray diffraction and also micro-Raman spectroscopy showed that the Ba0.8Sr0.2TiO3 thin films exhibited a tetragonal structure at room temperature. The presence of Raman active modes was clearly shown at the 299 and 725 cm−1 peaks. The tetragonal-to-cubic phase transition in the Ba0.8Sr0.2TiO3 thin films is broadened, and suppressed at about 35 °C, with a maximum dielectric constant of 948 (100 kHz). Electrical measurements for the prepared Ba0.8Sr0.2TiO3 thin films showed a remnant polarization (Pr) of 6.5 μC/cm2, a coercive field (Ec) of 41 kV/cm, and good insulating properties. The dispersion of the refractive index is interpreted in terms of a single electronic oscillator at 6.97 eV. The direct band gap energy (Eg) and the refractive index (n) are estimated to be 3.3 eV and n = 2.27–2.10, respectively.

Photoluminescence of nanostructured PbTiO3 processed by high-energy mechanical milling

This letter reports on a process to prepare nanostructured PbTiO 3 ~PT! at room temperature with photoluminescence ~PL! emission in the visible range. This process is based on the high-energy mechanical milling of ultrafine PbTiO 3 powder. The results suggest that high-energy mechanical milling modifies the particle’s structure, resulting in localized states in an interfacial region between the crystalline PT and the amorphous PT. These localized states are believed to be responsible for the PL obtained with short milling times. When long milling times are employed, the amorphous phase that is formed causes PL behavior. An alternative method to process nanostructured wide-band-gap semiconductors with active optical properties such as PL is described in this letter.

Visible photoluminescence in amorphous ABO3 perovskites

The photoluminescence observed in ABO3 type perovskite in their highly structural disordered state can be explained by a model in which is assumed a distribution of electronic states localized within the energy band gap coupled to lattice local vibrational states. The model fits very well the experimental results and indicates that photoluminescence in the visible region can be considered as a general behavior of disordered solids.

Correlation between the surface morphology and structure and the photoluminescence of amorphous PbTiO3 thin films obtained by the chemical route

A polymeric precursor method was used to synthesis PbTiO3 amorphous thin film processed at low temperature. The luminescence spectra of PbTiO3 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 on the thermal treatment history. Photoluminescence properties versus different annealing temperatures were investigated. The experimental results (XRD, AFM, PL) indicate that the nature of photoluminescence (PL) must be related to the disordered structure of PbTiO3 amorphous thin films. Copyright

Structural phase evolution of strontium-doped lead titanate thin films prepared by the soft chemical technique

Lab. Interdisc. Eletroquim./Ceram. Ctro. Multidisc. Para D.M.C. Univ. Federal de Sao Carlos (UFSCar), Via Washington, Km 235, CP-676, CEP-13565-905, Sao Carlos, S.P.

Raman characterization of structural disorder and residual strains in micromachined GaAs

Structural disorder and strain effects in ductile-regime single-point-diamond-turned gallium arsenide monocrystalline samples were probed by Raman scattering. The positive frequency shift of the longitudinal and transverse optical phonons observed in the machined samples indicate a residual compressive stress of about 1.5 GPa. This residual strain was attributed to the hysteresis of phase transformation generated by the high pressure imposed by the cutting tool tip during the machining process. The broadening of the Raman peaks indicate a high degree of structural disorder in the GaAs lattice. Moreover, the Raman spectrum of annealed samples, after machining, shows a less disordered but still misoriented matrix. In addition, it was found that crystalline arsenic formed into the surface vicinity.

ALLOYING EFFECTS ON THE CRITICAL LAYER THICKNESS IN INXGA1-XAS/INP HETEROSTRUCTURES ANALYZED BY RAMAN SCATTERING

Raman scattering has been used to estimate the critical layer thickness and to analyze the alloying effect on strain relaxation in InxGa1−xAs layers grown by molecular beam epitaxy on InP [001]-oriented substrate, for x ranging from 0.0 to 1.0. Measurements of longitudinal optical GaAs-like phonon frequency and Raman linewidth showed that the indium/gallium ratio contents greatly influences the strain relaxation. A comparison between Raman and x-ray diffraction measurements of relaxation ratios as a function of layer thickness is presented. The results can be explained in terms of the combined effect of strain and chemical and structural disorder.