Pier Carlo Ricci

Persistent photoconductivity and optical quenching of photocurrent in GaN layers under dual excitation

Persistent photoconductivity (PPC) and optical quenching (OQ) of photoconductivity (PC) were investigated in a variety of n-GaN layers characterized by different carrier concentrations, luminescence characteristics, and strains. The relation between PPC and OQ of PC was studied by exciting the samples with two beams of monochromatic radiation of various wavelengths and intensities. The PPC was found to be excited by the first beam with a threshold at 2.0 eV, while the second beam induces OQ of PC in a wide range of photon energies with a threshold at 1.0 eV. The obtained results are explained on the basis of a model combining two previously put forward schemes with electron traps playing the main role in PPC and hole traps inducing OQ of PC. The possible nature of the defects responsible for optical metastability of GaN is discussed.

Light assisted dimer to monomer transformation in heavily doped rhodamine 6G-Porous silica hybrids.

The spectral properties of postdoping prepared type I Rhodamine 6G-silica hybrids were investigated in the case of intentionally large doping (5 x 10(-2) mol/L). Beside expected concentration quenching effects, steady state and time resolved optical spectroscopy measurements displayed the presence of different kind of aggregates, both fluorescent and nonfluorescent. As the irradiation dose on the sample increases, the emission features change: the overall emission increases, the peak of the emission is blue-shifted, and the emission decay time also increases. The modifications of the spectral properties under light excitation are interpreted in terms of dimer to monomer light assisted transformation, and a thermodynamic model based on a monomolecular kinetics is presented.

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 ...

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.

Preparation and characterisation of transparent and flexible PEDOT:PSS/PANI electrodes by ink-jet printing and electropolymerisation

Novel flexible PEDOT:PSS/PANI bilayered thin-film electrodes were successfully prepared by ink-jet printing of PEDOT:PSS on a PEN substrate and subsequent electrochemical polymerisation of PANI. Scanning electron microscopy (SEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to systematically investigate the structure, morphology and electrochemical behaviour of the electrodes. The PEDOT:PSS/PANI bilayer nanofilms have been tested as pH sensors. As a comparison, either PEDOT:PSS or PANI electrodes have been tested for pH measurements. The results show that the sensitivity of the PEDOT:PSS/PANI bilayer film is higher than that of PEDOT:PSS and PANI electrodes, indicating a synergistic effect of the two polymers. A reversible near-Nerstian behaviour of the open circuit voltage with pH was found, with a slope of 58.9 mV pH−1 unit in a wide range of pH values from 2 to 14.

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.

Investigation of energy transfer in terbium doped Y 2SiO5 phosphor particles

The kinetics of luminescence of sol-gel synthesized terbium doped Y (2)SiO(5) (YSO) phosphor particles is investigated in detail with reference to Tb concentration in the 0.001%-10% range. By increasing the dopant concentration, the luminescence profile changes from a blue to a green peaked emission spectrum because of the energy transfer among centers. The inter-center energy transfer mechanism is well accounted for by the Inokuti-Hirayama (IH) kinetic model which is based on a statistical average of inter-center distance dependent decay modes of the donor luminescence. The distribution of the decay modes is implemented from the Förster-Dexter resonance theory of energy transfer by assuming a rate constant for the energy transfer by multipolar interactions between donors and acceptors. However, the experimental results recorded in the low concentration limit show the presence of green emission contributions in the luminescence spectrum which cannot be related to the Tb concentration; for this reason an additional internal energy transfer mechanism, occurring among levels of the same center, is proposed to account for the recorded emission properties. Thus, a new and more exhaustive model which includes both the internal and external energy transfer processes is considered; the proposed model allows a better explanation of the spectroscopic features of Tb related centers in YSO crystals and discloses the critical concentration and the quantum yields of the different energy transfer mechanisms.

Temperature dependence of Raman scattering in porous gallium phosphide

Porous layers fabricated by electrochemical anodization of (111)A-oriented n-GaP:Te substrates were studied by Raman scattering spectroscopy in the temperature interval from 10 to 300 K. Along with the transverse-optical (TO) and longitudinal-optical (LO) modes, the RS spectra of porous layers show Frohlich-type vibrations located in the frequency gap between the bulk optical phonons. A longitudinal-transverse splitting of these surface-related vibrations was evidenced at low temperatures. Apart from that, the porous layers prepared on highly doped substrates were found to show LO-phonon-plasmon coupled (LOPC) modes in the whole temperature interval studied. Observation of LOPC modes at low temperatures is explained taking into account that the GaP skeleton consists of both depleted surface layers surrounding the pores and conductive regions. The free electrons in these regions, originating from the impurities actually located in the depletion layers, are shown to be subject to spatial confinement increasing with decreasing temperature.

Vibrational properties of mixed (Y3Al5O12)x–(Y3Sc2Ga3O12)1−x crystals

The vibrational properties of yttrium aluminum garnet (YAG), yttrium scandium gallium garnet (YSGG), and mixed (YAG)x–(YSGG)1−x structures (0<x<1) were investigated by means of Raman spectroscopy. By comparison with the vibrational spectrum of the YAG structure, the attribution of the Raman modes of the YSGG is proposed. The spectra were analyzed with a model of isolated polyhedra and the formation of mixed structures was verified. The Raman modes in the mixed structures lie between those of the pure garnets indicating the presence of truly mixed and not separated phases. An analytical model based on the free tetrahedral approximation to study the position of the vibration frequencies as a function of the relative concentration of the garnets is proposed and applied to the study of the A1g breathing mode at 785cm−1 in the pure YAG structure and downshifted at 752cm−1 in the pure YSGG structure. The proposed analysis allows to verify both the quality of the grown crystal and to assess the relative concentr...