A. Baraldi

High-efficiency SiO2:Ce3+ glass scintillators

We present the effect of a rapid thermal treatment (RTT) at high temperature (1800 °C) on the radio-luminescence properties of Ce-doped SiO2 glasses prepared by the sol–gel method and previously densified at 1050 °C. Cerium concentrations ranging from 0.05 up to 1 mol % were considered. We found that, for all concentrations, the RTT induces a strong increase of the Ce3+ radio-luminescence efficiency; the x-ray-induced luminescence intensity of the SiO2:0.1% Ce is about twice that of Bi3Ge4O12. The decay time of the scintillation response, evaluated as ≈50 ns, is not affected by RTT. Infrared absorption measurements indicate that the luminescence increase cannot be related to significant release of OH groups during RTT. The conversion of Ce4+ ions into Ce3+ ions can also be ruled out since an increase of about 20% of the intensity of the 4.8 eV optical absorption band related to Ce4+ was observed after RTT. The occurrence of dissolution of rare-earth aggregates is suggested.

Superposition-model analysis of rare-earth doped BaY 2 F 8

The energy level schemes of four rare-earth dopants (Ce 3+ , Nd 3+ , Dy 3+ , and Er 3+ ) in BaY 2 F 8 , as determined by optical absorption spectra, were fitted with a single-ion Hamiltonian and analysed within Newmans Superposition Model for the crystal field. A unified picture for the four dopants was obtained, by assuming a distortion of the F m ligand cage around the RE site; within the framework of the Superposition Model, this distortion is found to have a marked anisotropic behaviour for heavy rare earths, while it turns into an isotropic expansion of the nearest-neighbours polyhedron for light rare earths. It is also inferred that the substituting ion may occupy an off-center position with respect to the original Y 3+ site in the crystal.

Growth of SnO2 nanocrystals controlled by erbium doping in silica

We investigate the effects of erbium doping on SnO(2) nanoclustering in Sn-doped silica. Vibrational spectroscopy data from Raman and infrared absorption measurements show nanostructuring effects on the SnO(2) nanophase. Ultraviolet absorption spectra evidence a gap shift ascribable to size-dependent quantum confinement, also suggesting a role of erbium doping in determining cluster sizes and the amount of localized states on the nanophase boundary. Transmission electron microscopy confirms and details the spectroscopic data. As a result of these measurements, we find that the nanocrystal size distribution becomes narrower, increasing the erbium concentration, while the density of localized states at the nanocrystal surface decreases. The distribution of erbium ions among the possible environments is then examined through simultaneous spectroscopy of luminescence excited by nanocrystal-to-erbium energy transfer and the absorption of nanocrystal luminescence by erbium ions. This analysis shows that erbium behaves as an extrinsic nucleation centre of the SnO(2) nanophase at low doping levels, whereas at high concentrations it modifies the matrix, hindering the growth of SnO(2) crystals and passivating the interface.

Spectroscopic investigation and crystal field modelling of Dy3+ and Er3+ energy levels in yttrium aluminium borate (YAB) single crystals

High resolution (0.04 cm−1) absorption spectra of Dy3+-doped YAl3(BO3)4 (YAB) single crystals were measured by Fourier transform spectroscopy in the spectral (2000–23 000 cm−1) and temperature (9–300 K) ranges. Samples with nominal 1 and 4% Dy/Y molar ratios were studied. Dy3+ transitions from the 6H15/2 ground state to the 6H13/2, 6H11/2, 6H9/2+ 6F11/2, 6H7/2+ 6F9/2, 6H5/2, 6F7/2, 6F5/2, 6F3/2, and 4F9/2 excited states were analysed. A small (~3.3 cm−1) splitting between the first two sublevels of the 6H15/2 ground manifold was detected. The experimentally determined energy levels were fitted with a single-ion Hamiltonian and the crystal-field parameters were obtained. The same procedure was applied to analyse the previously published high resolution spectra of YAB:Er3+, leading to a reliable unified picture for the two dopants.

Vibrational spectroscopy of OH-related groups in Ce3+- and Gd3+-doped silicate glasses

FTIR spectroscopy was applied in the 500–14,000 cm � 1 range to detect OH vibrational modes (stretching, bending, combination, first and second overtone) in Ce 3+ - and Gd 3+ -doped silicate glasses, prepared by means of the sol–gel technique and aimed to applications as scintillators. The presence of OH groups, belonging either to silanol (Si-OH) or to H2O should be limited, since they could cause radiative emission quenching. The samples, doped with Ce 3+ or Gd 3+ (0.05–3% m.f.), were submitted to thermal treatments at 4501C, 7501C, and 10501C, to remove water and organic groups for producing a dense glass. A large amount of H2O and Si-OH is still present in samples annealed at 4501C and 7501C, while only a reduced quantity of Si-OH (evaluated as o1% m.f.) is detected after annealing at 10501C, i.e. in the final dense glasses (for all concentrations of both dopants). r 2002 Elsevier Science B.V. All rights reserved.

Rare-Earth Doped Sol-Gel Silicate Glasses for Scintillator Applications

Radio-, photo- and thermally stimulated—luminescence (RL, PL, TSL) measurements have been performed on SiO2 sol-gel glasses doped by 0.1 mol% Ce and 3 mol% Gd, and on (0.1 mol% Ce, 3 mol% Gd) co-doped samples. Ce3☎ 5d-4f emission peaking at about 2.7 eV has been observed in the RL of SiO2: 0.1 mol% Ce, while the typical 6P-8S emission of Gd3☎ centred at 3.97 eV has been detected in SiO2: 3 mol% Gd. The co-doped sample displays both 5d-4f Ce3☎ and 6P-8S Gd3☎ emissions with reduced intensities with respect to those observed in the singly doped glasses. Moreover, in co-doped glasses the PL time decay patterns of both rare earth ions show a non exponential dependence and are significantly shortened. To explain such an effect non radiative de-excitation of both RE ions excited states involving energy transfers to defect levels is suggested. Bidirectional Gd3☎ ↔ Ce3☎ energy transfers could also occur. Complementary TSL measurements put in evidence the existence of broad glow peaks at about 100 K and 220 K. The TSL spectra feature the RE ions emissions.

Effects of composition and catalyst on the optical properties of ZrO2-based Ormosil films

Abstract FTIR spectroscopy measurements were performed to investigate zirconia-based Ormosil thin films. Samples synthesized with a sol–gel technique were prepared starting from 3-glycidoxypropil-trimethoxysilane (GLYMO) and zirconium(IV)-propoxide (Zr–OPr). Acetic or nitric acid were used as a catalyst. The effects induced by the Zr content and the catalyst type on the mid-infrared spectra of the sol–gel hybrid films were studied and a detailed assignment of the vibrational absorption bands is presented. The thermal evolution of the hybrid systems was also followed by monitoring the release of the organic groups, the thickness, and refractive index changes by means of FTIR, diffuse reflectance spectroscopy, and ellipsometry.

Microstructure and silver nanoparticles in ion-exchanged and deformed soda-lime silicate glasses

Abstract Optical absorption, microspectrophotometry, thermally stimulated depolarization current, and transmission electron microscopy were applied to a commercial multicomponent soda-lime silicate glass subjected to the Na + ⇔Ag + ion-exchange, annealing in air or hydrogen, and to mechanical stretching. The treatments affect both the matrix microstructure and the morphology of the silver particles. Mechanical stretching of the exchanged specimens annealed in hydrogen induces the most interesting effects. Secondary phase separation occurs within the glassy matrix, and causes drastic changes of the thermocurrent data, which are interpreted in terms of the Maxwell–Wagner interfacial polarization. Elongation of the spherical silver nanoparticles produces a remarkable dichroism of the optical absorption, which could be exploited for new applications. The sample stretching affects only particles with a diameter larger than 5 nm interpreted as a threshold size, at least at the stresses used in the present work.

Evidences of Rare-Earth Nanophases Embedded in Silica Using Vibrational Spectroscopy

Transmission electron microscopy, Raman scattering, Fourier transform infrared spectroscopy, and radio-luminescence are employed to investigate rare-earth (RE) incorporation and aggregate formation in silica glasses prepared by the Sol-Gel method and doped with Ce3+, or Tb3+, Gd3+, Yb3+ with concentrations up to several mol%. The results demonstrate that rare-earth aggregates with a mean diameter extending up to several tens of nanometers occur, further increasing their size after post-densification high temperature treatments. Rare-earth segregation causes a reduction of the OH content of glasses. Nanoclusters are amorphous, possibly close to a (RE)2SiO5 stoichiometry. Room temperature radio-luminescence measurements reveal that the emission spectra are dominated by RE3+ emissions and no bands due to silica matrix defects are detected.

Theoretical and experimental investigation of optical absorption anisotropy in β-Ga2O3

The question of optical bandgap anisotropy in the monoclinic semiconductor β-Ga2O3 was revisited by combining accurate optical absorption measurements with theoretical analysis, performed using different advanced computation methods. As expected, the bandgap edge of bulk β-Ga2O3 was found to be a function of light polarization and crystal orientation, with the lowest onset occurring at polarization in the ac crystal plane around 4.5-4.6 eV; polarization along b unambiguously shifts the onset up by 0.2 eV. The theoretical analysis clearly indicates that the shift in the b onset is due to a suppression of the transition matrix elements of the three top valence bands at Γ point.