Brigida Allieri

Synthesis and optical properties of nanosized powders: lanthanide-doped Y2O3

Abstract Y1.8Nd0.2O3 and Y1.8Er0.2O3 powder samples were prepared by combustion synthesis. X-ray diffraction showed that the materials are nanostructured, with average coherence length 5–10 nm. The visible and infrared luminescence spectra of nanometric cubic Y1.8Nd0.2O3 and Y1.8Er0.2O3 show some important differences from those of the bulk materials. In the case of Y1.8Er0.2O3, faster relaxation of some excited states occurs, leading to changes in the relative intensities of several optical transitions. As far as nanostructured Y1.8Nd0.2O3 is concerned, the 4 F 3/2 → 4 I 9/2 transition is characterized by a perturbed intensity pattern of the Stark components and by an anomalous integrated intensity. Possible explanations for this behaviour are proposed.

Oxidation of Sn Thin Films to SnO 2 . Micro-Raman Mapping and X-ray Diffraction Studies

The oxidation of tin layers deposited onto alumina substrates is investigated with the aim to identify the different steps of the process and obtain information on the sample homogeneity, phase segregation, and degree of oxidation. It is shown that at least three phases coexist at 450 °C, Sn, SnO, and SnO2, and remarkable inhomogeneities, already visible at an optical inspection, are found in the thin film. A micro-Raman mapping of the layer shows that these inhomogeneities are related to the presence of different Sn oxidation states, as evidenced by the inhomogeneous distribution of SnO and SnOx Raman bands. The thin film becomes homogeneous after annealing treatments above 550 °C, where only the SnO2 cassiterite phase is detected.

Morphology and microstructural properties of TiO2 nanopowders doped with trivalent Al and Ga cations

The effects of doping on the morphological and microstructural properties of TiO2 nanopowders produced by laser pyrolysis were investigated mainly by x-ray diffraction (XRD) and electron microscopy. Samples of TiO2 powders were prepared by doping with different trivalent cations (Al and Ga). The powders were calcined at different temperatures in the range 400–1000 °C for 18 h, as well as at constant T = 700 °C up to 160 h. After each thermal treatment, XRD patterns were collected. The analysis of XRD patterns allowed us to estimate the microstrains and average crystallite size and to observe the evolution of the microstructural parameters with temperature. Both Al and Ga inhibited the crystallite growth of TiO2 anatase and the rutile phases, this effect being larger in the Al-doped powders.

Niobium-titanium oxide powders obtained by laser-induced synthesis: Microstructure and structure evolution from diffraction data

The influence of the niobium content on the anatase-to-rutile phase transition in nanopowders of Nb–Ti oxides was studied and the changes in the particle size and microstrain distribution obtained at different temperatures were analyzed. A correlation is found between the initial microstructure in the Ti 1 – x Nb x O 2 ( x = 0.03, 0.2) powder and the niobium content. The presence of Nb was found to inhibit the growth of both the anatase and the rutile phases.

ELECTRICAL AND STRUCTURAL PROPERTIES OF RGTO-IN2O3 SENSORS FOR OZONE DETECTION

Abstract Bare and Au-doped indium oxide thin films have been characterized as ozone sensors. Active layers have been obtained by oxidation of a metallic In thin film through the RGTO technique. Structural characterization showed that only the In2O3 phase is present for oxidation temperatures greater than 250°C. At the operating temperature of 400°C, response to ozone was greater, stable and reproducible. Au introduction as a catalyst increased the overall response and reduced to about 300°C the temperature where ozone/surface reactions are enhanced.

Optical and morphological characterization of Si nanocrystals/silica composites prepared by sol-gel processing

Abstract Linear (visible photoluminescence) and non-linear (third-order susceptibility) optical properties have been measured for laser-synthesized crystalline Si nanoparticles. The observed luminescence is compared to similar measurements on nanocomposite aerogels prepared integrating the Si-nanoparticles into a continuous silica phase by sol–gel processing. The results show that incorporation of the Si-nanoparticles into a silica matrix by sol–gel technology is not detrimental to the PL (photoluminescence) emission intensity. The effect on the nanocomposites of annealing in air (at temperatures in the range of 300–500°C) has been investigated by means of PL and Raman spectroscopy. The obtained results indicate that the oxidation treatment induces the bleaching of the smaller luminescent particles and is not effective in reducing the average size. Micro-Raman mapping and TEM analysis of the nanocomposites have shown the presence of agglomerated nanoparticles (in the sub-micrometer range), as confirmed by dynamic light scattering (DLS) measurements on colloidal suspensions of the as-synthesized powders.

Correlation between crystallite sizes and microstrains in TiO2 nanopowders

Abstract Nanopowders of TiO 2 doped with by V, Nb, Ta were obtained by laser-induced pyrolysis. The profiles of the diffraction peaks were Fourier analysed with a single-peak method and the microstructural parameters 〈 M 〉 (average crystallite size) and 〈 e 2 〉 1/2 (mean-square root microstrain) were estimated. A linear correlation between 〈 e 2 〉 1/2 〈 M 〉 and 1/〈 M 〉 was observed in both anatase and rutile phases. For a given crystallite size, the microstrains as well as the dispersion of data were larger in anatase then in rutile. Indeed, one of the factors inducing the anatase-to-rutile transformation may be the presence of larger microstrains in anatase. In rutile, the high correlation found between 〈 e 2 〉 1/2 〈 M 〉 and 1/〈 M 〉 suggests that in this phase the microstrains present in the structure are due only to the geometrical constraints of the surface. Indeed, for infinite crystallite size no microstrains are expected. In anatase, microstrains depend also on the specific cation present in the crystallite.

Growth and microstructural analysis of nanosized Y2O3 doped with rare-earths

Nanosized cubic Y2O3 samples, undoped and doped with 10 mol% Nd2O3 ,E u 2O3 ,G d 2O3 ,T b 2O3 ,H o 2O3 and Er2O3 (Y1:8Ln0:2O3, where LnDNd, Eu, Gd, Tb, Ho or Er), were prepared by means of a controlled hydrolysis method in an aqueous solution containing ammonia, Y(NO3)3 and Ln(NO3)3 as precursors, and a surface modifier. The microstrain and the average size of the diffraction domains have been calculated from the XRD patterns and the results have been compared with those obtained by a combustion synthesis. It is shown that the cell parameter of the C-M2O3 (bcc structure related to the CaF2 structure; the M atom is 6-coordinated) structure of doped Y2O3 is correlated to the ion size of the dopant. The shape of the crystallites appears to be needle-like in all cases, while the microstrains depend on the dopant and are probably due to surface effect. XRD and Raman analysis show that, despite the heavy doping, only one phase in the Y2O3 powders is present.

Spectroscopic characterisation of alternate current electroluminescent devices based on ZnS–Cu

Abstract The present study is focused on the spectroscopic properties of ac powder electroluminescent lamps, in which the electroluminescent materials are based on ZnS–Cu, in order to understand the mechanisms of the electroluminescence and improve their performances. To this purpose ad hoc sets of devices have been built and their performances have been evaluated by comparing spectroscopic measurements performed by changing alternatively the applied frequency and the voltage. The emission wavelength of these devices (i.e. the color of the lamps) is found to be strictly related to the applied frequency, while the voltage mainly determines the intensity of the emission.

An X-ray study of the trimetallic LaxSm1−xFeO3 orthoferrites

Abstract The structural properties of the LaxSm1−xFeO3 trimetallic orthoferrites are described on the basis of a Rietveld analysis of X-ray diffraction pattern. Ultrafine powders of these oxides are prepared by the thermal decomposition at low temperatures of the corresponding hexacyanocomplexes. Single-phase compounds are obtained. Their structural parameters can be tuned by properly changing the Sm/La stoichiometric ratio.