Daniele Chiriu

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...

Raman Study on Pompeii Potteries: The Role of Calcium Hydroxide on the Surface Treatment

Pottery samples from the Pompeii archaeological site were investigated by IR Raman spectroscopy and EDAX measurements. The analysis of the Raman spectra of the surfaces reveals the presence calcium hydroxide (peak at about 780 cm−1) while the calcium carbonate is totally absent. The comparative studies on the carbonation effect of the surfaces were performed on laboratory grown samples of calcium hydroxide. The samples were treated at high temperature and exposed to different ambient conditions, and the analysis suggests that the original surfaces of Roman pottery were scattered by calcium hydroxide (limewash) before the cooking process in the furnace. The result of this surface treatment not only permits a vitrification of the surfaces but also seems to reduce the content of CO2 in the furnace atmosphere and then obtain a more oxidant ambient during the cooking of the pottery. These results give new insights on the real degree of knowledge of the Romans about the art of ceramics and more generally about chemistry and technologies.

Raman identification of cuneiform tablet pigments: emphasis and colour technology in ancient Mesopotamian mid-third millennium

Cuneiform tablets tell the life and culture of Sumerian people in a sort of black and white tale because of the binary engraving technique. A leading question arises: did Mesopotamian people apply some kind of colour to decorate their tablets or to put emphasis on selected words? Some administrative and literary Sumerian cuneiform tablets of mid-third Millennium B.C. from the site of Kish (central Mesopotamia, modern Iraq) were dug up in twentieth-century and stored at the Ashmolean Museum of the Oxford University. Non-destructive micro-Raman spectroscopy is a powerful technique to detect the presence of residual pigments eventually applied to the engraving signs. Yellow, orange, red and white pigments have been detected and a possible identification has been proposed in this work. In particular yellow pigments are identified as Crocoite (PbCrO4), Lead stannate (Pb2SnO4); red pigments − hematite (Fe2O3) and cuprite (Cu2O); White pigments − Lead carbonate (PbCO3), calcium phosphate (Ca3(PO4)2), titanium dioxide (TiO2), gypsum (CaSO4.2H2O); orange pigment a composition of red and yellow compounds. These results suggest that Sumerian people invented a new editorial style, to overcome the binary logic of engraving process and catch the reader’s eye by decorating cuneiform tablets. Finally, the coloured rendering of the tablet in their original view is proposed.

Luminescence enhancement by energy transfer in melamine-Y2O3:Tb3+ nanohybrids

The phenomenon of luminescence enhancement was studied in melamine-Y2O3:Tb hybrids. Terbium doped Y2O3 mesoporous nanowires were synthesized by hydrothermal method. X-ray diffraction patterns and Raman scattering spectra testified the realization of a cubic crystal phase. Organic-inorganic melamine-Y2O3:Tb3+ hybrid system was successfully obtained by vapour deposition method. Vibration Raman active modes of the organic counterpart were investigated in order to verify the achievement of hybrid system. Photoluminescence excitation and photoluminescence spectra, preformed in the region between 250 and 350 nm, suggest a strong interaction among melamine and Terbium ions. In particular, a remarkable improvement of 5D4→ FJ Rare Earth emission (at about 542 nm) of about 102 fold was observed and attributed to an efficient organic-Tb energy transfer. The energy transfer mechanism was studied by the use of time resolved photoluminescence measurements. The melamine lifetime undergoes to a significant decrease when ...

Structurally defective cerium doped lutetium–yttrium oxyorthosilicates for optically stimulated luminescence imaging devices

The paper reports experimental data concerning the feasibility of structurally defective cerium doped Lu1.8Y0.2SiO5 oxyorthosilicates (Ce:LYSO) as optical stimulated luminescence (OSL) devices with emission at the Ce3+ recombination sites. The total OSL signal was evaluated as a function of continuous irradiation at 514.5 nm and compared to the curve obtained with a commercial C:Al2O3 sample. By assuming a single type of trap and a single recombination centre model for the OSL mechanism, the photoionization cross-section was estimated to be 1.7 × 10−17 cm2. The stimulation spectrum of the OSL signal evidences a single maximum at about 470 nm and a smooth decrease down to 650 nm. The recombination mechanism as well as the nature of the trapping site was studied by means of thermo-stimulated luminescence (TSL) measurements and analysed according to the generalized order kinetics model. The results confirm a single type of trap and a thermally activated tunnelling mechanism between the electrons trapped in the oxygen vacancies and holes localized at the Ce3+ centres. The analysis is further confirmed by TSL measurements after illumination with monochromatic lights in the range of the OSL stimulation spectrum. Finally, due to the high localization of trapping sites and recombination centres, we propose cerium doped oxyorthosilicates as efficient OSL imaging devices and UV based optical data storage materials: direct applications of defective Ce:LYSO as OSL imaging devices are shown.

Compositional tuning of photoluminescence properties in Nd-doped YAG-YSGG mixed structures

The photoluminescence (PL) properties of neodymium-doped yttrium aluminum garnet and yttrium scandium gallium garnet mixed structures were investigated as a function of the relative concentration of the two garnets. The blue shift of the emission bands in the 930-950-nm range is ascribed to compositional tuning effect. An analytical model to estimate the variation of the PL position as a function of the compositional host structure is proposed.

Doping porous silicon with erbium: pores filling as a method to limit the Er-clustering effects and increasing its light emission

Er clustering plays a major role in hindering sufficient optical gain in Er-doped Si materials. For porous Si, the long-standing failure to govern the clustering has been attributed to insufficient knowledge of the several, concomitant and complex processes occurring during the electrochemical Er-doping. We propose here an alternative road to solve the issue: instead of looking for an equilibrium between Er content and light emission using 1–2% Er, we propose to significantly increase the electrochemical doping level to reach the filling the porous silicon pores with luminescent Er-rich material. To better understand the intricate and superposing phenomena of this process, we exploit an original approach based on needle electron tomography, EXAFS and photoluminescence. Needle electron tomography surprisingly shows a heterogeneous distribution of Er content in the silicon thin pores that until now couldn’t be revealed by the sole use of scanning electron microscopy compositional mapping. Besides, while showing that pore filling leads to enhanced photoluminescence emission, we demonstrate that the latter is originated from both erbium oxide and silicate. These results give a much deeper understanding of the photoluminescence origin down to nanoscale and could lead to novel approaches focused on noteworthy enhancement of Er-related photoluminescence in porous silicon.

A facile strategy for new organic white LED hybrid devices: design, features and engineering

A facile and ecofriendly strategy to design and engineer new organic white LEDs is tested. The hybrid system was implemented by combining a commercial blue LED, with emission at 405 nm, with a hybrid transparent organic film (polycarbonate, PMMA, PVC) containing two selected organic dyes. The emitting molecules, a home-designed push–pull based coumarin and DCM ([2-[2-[4-(dimethylamino)phenyl]ethenyl]-6-methyl-4H-pyran-4-ylidene]-propanedinitrile), were selected for their optical features and appropriately mixed to obtain white light, perceptible by the naked eye, through the metamerism effect. The proposed strategy shows the critical parameters to account for in the selection of organic dyes, bearing in mind the preservation of the dye emission properties from the solution to the solid state. To this purpose, the contributions of dynamic and static quenching effects were analyzed in detail, to single out the optimum concentrations to assure the compliance with the respective sphere of interaction and guarantee high optical performances. Different new hybrid white-emitting LEDs, with tunable color rendering index (from warm to cold CCT), high energy efficiency (quantum yield larger than 90%), and high photostability, were produced to prove the proposed strategy.

Surface effects and phase stability in metal oxides nanoparticles under visible irradiation

The light induced phase transformation between stable phases of metal oxides nanoparticles is analyzed. The surrounding atmosphere as well as the defect density at the surface play a fundamental role. It has been found that in oxygen poor chamber atmosphere the phase transformation is favored, while the phase transition cannot be achieved if the defects at the surface are properly passivated. The phase transition is activated by intragap irradiation, able to activate the F- center at the surface connected to oxygen vacancies, and promoting the activation of the surface and the nucleation of neighboring crystallites. The phase transition was studied in Titanium oxide (TiO2) and in Iron oxide (Fe2O3): Maghemite is subjected to a phase transformation to α−Fe2O3 (hematite), Anatase nanoparticles converts to Rutile. The general mechanism of the phase transition and, more in general, the possibility to optically control the surface activity of metal oxides is discussed.

On the origin of blue and UV emission bands in mesoporous silica

In this paper we present a review of the main results we obtained studying the emission properties of mesoporous silica in the UV-VIS range. Two main emission bands were investigated, the blue one, peaked in the 400–450 nm range, and the UV one, centered at about 300–350 nm, both excited in the UV and vacuum UV range. The origin of the two bands, investigated through optical spectroscopy techniques, is discussed in terms of possible surface emitting centers probing the samples with different chemical and physical treatments or exploiting different morphological and textural properties of the samples obtained by properly engineering the sol-gel synthesis.