Riccardo Corpino

Optical properties of α-ZnAl2S4:Cr single crystals

The optical properties of the chromium doped spinel type semiconductor α-ZnAl2S4 have been examined over the temperature range 2–540 K. The intrinsic photoluminescence (PL) showed an intense ultraviolet peak at 381 nm at 296 K. From extrinsic absorption and emission spectra the transitions between the ground state 4A2g and the excited levels 2Eg, 2T1g, 2T2g, 4T2g, and 4T1g of Cr3+ ions were observed. A fit of the measured temperature dependence of the PL decay, using a four level model, yielded the lifetimes of the 2Eg, 2T1g, and 4T2g levels and the energy gaps between the 2Eg–2T1g and 2Eg–4T2g states which were found to be very close to the values obtained from steady-state measurements. A configurational coordinate diagram for Cr3+ ions in a α-ZnAl2S4:Cr spinel host has been constructed. Optical gain measurements at 198, 298, and 380 K displayed two separate spectral regions associated to the Cr3+ ions and possibly Cr4+ impurity centers. Gain values of up to 25 cm−1 at 198 K were obtained.

Photoconductive properties of HgGa2S4

Abstract Mercury thiogallate, HgGa 2 S 4 is a defect chalcopyrite semiconductor with the space group S 4 2 which offers a combination of attractive properties for applications. In order to obtain information about the electron states in the energy gap, photoconductivity measurements are performed in the 80–300 K range. Photoconductivity spectra show two peaks related to intrinsic and extrinsic excitation at about 410 and 500 nm, respectively; these maxima show a temperature dependence similar to the linear coefficient of the energy gap. Thermally stimulated currents have been studied by exciting the samples with intrinsic light at different temperatures. For all excitation temperatures a single TSC peaks were obtained. The analysis of TSC curves allowed one to estimate the kinetics of the trap emptying, trap energy distribution and thermal activation energy. A model for the level distribution in the semiconductor energy gap is suggested which in good agreement with the results of a previous photoluminescence study.

A kinetics model for Tb3+ recombinations in low doped Tb:Lu1.8Y0.2SiO5 crystals

The detailed characterization of the terbium related optical properties of low doped oxyorthosilicates of lutetium and yttrium is reported. The introduction of terbium ions generates an ultraviolet absorption band peaked at about 242 nm and line shaped emissions in the 350–600 nm range. The transitions are related to the D53 and D54 levels and the analysis of the decay time measurements allows to individuate a cross relaxation mechanism among Terbium ions. We propose a three level kinetic model which is able to reproduce the experimental data allowing to discriminate among the radiative and nonradiative contributions to the transitions in the case of low content of Tb ions (nominal content 10 ppm). The reported study addresses two important goals, providing, from one side, a detailed characterization of possible inorganic phosphors, and from the other side, since traces of unwanted elements were identified in numerous commercial samples of cerium doped oxyorthosilicates, it can contribute to increase the ...

OH-dependence of ultraviolet emission in porous silica

Abstract Photoluminescence and Raman measurements have been performed on sol–gel synthesized porous silica monoliths. Porous silica and polyethilenglycole/silica hybrids thermally treated at different temperatures have been analyzed. By exciting at 5.6 eV a structured photoluminescence emission centered at 3.7 eV is found in the whole set of samples. Hydration treatment of the pore surfaces increases the H-bonding interaction between silanols, inhibits the isolated silanols vibrations and changes the photoluminescence spectrum increasing the emission peaked at 3.7 eV.

Ultraviolet excitation fine tuning of luminescence bands of oxygen-deficient centers in silica

The photoluminescence (PL) activity of oxygen deficient centers in commercial silica samples has been reconstructed by exciting in the spectral range of their optical absorption band around 5 eV. The fine tuning of the excitation was provided by a tunable pulsed optical parametric oscillator seeded by a Nd:yttrium–aluminum–garnet laser. The experimental results, analyzed in the spectral moment framework, indicate the presence of different centers whose spectral features are modulated by structural disorder typical of the glassy matrix. The photoluminescence excitation spectra at different emission energies evidence the composite structure of the luminescence bands around 3.1 and 4.2 eV, usually ascribed to triplet and singlet transitions of the same oxygen deficient center. Besides the well-known emission bands centered at 3.15 and 4.20 eV, the contribution of a new PL band below 3 eV is reported. Time resolved photoluminescence measurements of the low energy band give an estimate of its spectral characte...

Optical and structural characterization of cerium doped LYSO sol-gel polycrystal films: potential application as scintillator panel for X-ray imaging†

The optical and structural properties of sol–gel prepared polycrystalline films of Cerium doped Lutetium and Yttrium oxyorthosilicates (Ce:LYSO) are investigated by means of optical spectroscopy, XRD diffraction and Raman spectroscopy. The sol–gel samples are compared to commercially available Ce-doped LYSO single crystals, and a detailed study of the emission properties from the different cerium sites is performed. The main result is that the polycrystalline films do show very good radioluminescence properties for possible applications as X-ray scintillating panels. The structural characterization indicates that the sol–gel polycrystals have similar phase composition to commercial monocrystals; concerning the excitation and emission properties, the same radiative recombination channels are observed in the polycrystals and commercial monocrystals with a slightly faster decay time in the first case. In addition, low temperature measurements (8 K) indicate the presence of a non-radiative cross-relaxation mechanism among cerium ions in the sol–gel grown samples in the two possible crystallographic sites. The results indicate the possibility to develop sol–gel synthesized polycrystalline thin films as scintillating materials in the X-ray energy range.

Ultraviolet excitation of photoluminescence of porous silica under vacuum conditions

Abstract The optical properties of porous sol–gel silica are investigated by measuring the absorbance spectrum in the vacuum ultraviolet energy range and the excited photoluminescence. The emission spectrum shows a luminescence with two maxima at about 3.5 eV and

On the formation of aggregates in silica – rhodamine 6G type II hybrids

The spectroscopic features of dye aggregates in organic–inorganic Rhodamine 6G–silica hybrids are investigated in sol–gel prepared type II bulk and thick film samples in the 10−4–10−3 mol l−1 concentration range. No aggregates are observed in film samples irrespective of the dye concentration and the spectroscopic features are ascribed to fluorescent monomers. On the contrary, by means of excitation and emission fluorescence measurements and the analysis of fluorescence decay kinetics the formation of fluorescent aggregates is reported in bulk samples and the geometry of the formed aggregates at the silica surface is discussed on the single exciton theory framework. Beside the distribution of a single emitting specie observed in bulk samples and individuated as oblique J dimers, the analysis of the excitation spectra indicate also the presence of “dark” monomer units. An energy transfer mechanism is hypothesized to explain the interaction of the monomer–dimer pair.

Vacuum ultraviolet absorption of silica samples

Abstract Optical absorption analysis in the ultraviolet and vacuum ultraviolet region from 4.0 to 9.0 eV has been performed on different silica materials. We analysed natural and synthetic silica samples, both as grown and irradiated with different irradiations, neutrons, γ, and ultraviolet photons. Regardless of irradiation we always observed in the spectrum of irradiated samples the growth of the band at 5.8 eV, a band attributed to the E′ paramagnetic center. Comparing this absorption under different irradiation, we suggest the B2 and E absorption bands are not correlated and therefore cannot be assigned to the same defect.

Low temperature time resolved photoluminescence of the 3.1 and 4.2 eV emission bands in Ge-doped silica

Abstract Time resolved photoluminescence (PL) has been performed on Ge-doped silica preform in the temperature range 10 to 295 K. Under pulsed KrF laser (5 eV) the well known α and β emissions have been recorded at different delays from excitation. An accurate analysis of the time resolved spectra taken at different temperatures has shown the composite property of the two PL structures. At room temperature α components (α1, α2) are peaked at 4.09 and 4.26 eV with a decay time of about 10 ns. The peak energies of β1 and β2 components are calculated at 3.03 and 3.21 eV with lifetimes of 111 and 94 μs, respectively. As temperature is decreased, α1 and α2 display the normal behaviour increasing in intensity down to 125 K; on the contrary, in the same temperature range, β1 and β2 undergo a quenching of their intensities. Taking into account their mutual spectral characteristics, α1 has been correlated to β1 and α2 to β2. The two sets of emission bands are tentatively attributed to a single center stabilized in different environments of the glassy matrix.