Photoluminescence characterization of aged and regenerated mesoporous silica

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Formation of citrazinic acid ions and their contribution to optical and magnetic features of carbon nanodots: A combined experimental and computational approach

The molecular model is one of the most appealing to explain the peculiar optical properties of Carbon nanodots (CNDs) and was proven to be successful for the bottom up synthesis, where a few molecules were recognized. Among the others, citrazinic acid is relevant for the synthesis of citric acid-based CNDs. Here we report a combined experimental and computational approach to discuss the formation of different protonated and deprotonated species of citrazinic acid and their contribution to vibrational and magnetic spectra. By computing the free energy formation in water solution, we selected the most favoured species and we retrieved their presence in the experimental surface enhanced Raman spectra. As well, the chemical shifts are discussed in terms of tautomers and rotamers of most favoured species. The expected formation of protonated and de-protonated citrazinic acid ions under extreme pH conditions was proven by evaluating specific interactions with H2 SO4 and NaOH molecules. The reported results confirm that the presence of citrazinic acid and its ionic forms should be considered in the interpretation of the spectroscopic features of CNDs

Phosphorescence by Trapping Defects in Boric Acid Induced by Thermal Processing

The phosphorescence of boric acid (BA, H3BO3) at room temperature is a puzzling phenomenon subject to controversial interpretations although the role of structural defects has not yet been considered. Heat treatments of boric acid cause its transformation into the metaboric phase and amorphous boron oxide (B2O3). The structural changes after thermal processing can create defects that become centers of luminescence and recombination channels in the visible range. In the present work, commercial boric acid is thermally processed at different temperatures. Samples treated between 200 and 400 degrees C exhibit remarkable phosphorescence in the visible range. At approximate to 480 and 528 nm, two distinct phosphorescent emissions occur, associated with trapped charge carriers recombinations identified by thermoluminescence (TL) and electron paramagnetic resonance spectroscopy (EPR). The structural and optical studies suggest that the activation of boric acid phosphorescence after heat treatment is correlated with the presence of defects. The afterglow results from a trapping and detrapping process, which delays the recombination at the active optical centers. Time-dependent density functional study (DFT) of defective BOH molecules and clusters shows the emergence of near UV and blue optical transitions in absorption. These defects trigger the photoluminescence in thermally processed boric acid samples.Boric acid emits phosphorescence in visible range only after dehydration and melting. Defects activate two distinct phosphorescent emissions, with trapping and detrapping process delaying recombination at active optical centers. Time-dependent density functional study reveals near UV and blue optical transitions in absorption due to oxygen vacancies and non-bridging oxygens, triggering photoluminescence.imag

Post-Optimality Pareto-Robustness Analysis of an Earth-Observation Satellite Mission

In this paper a methodology for post-optimality studies to assess the robustness of the Pareto-optimal solutions, computed with a multi-objective optimization algorithm, is presented. The proposed Pareto-Robust Optimization Approach (PROA) is based on factorial design for sampling the design region in the neighborhood of the Pareto-optimal solutions. It allows for estimating a metric for the Pareto-Robustness and contributes to improving convergence of the known Pareto-front towards the true Pareto-front. Further, the sensitivity of the performance (the objectives of the analysis) to the dispersion of the design variables is analyzed using variance decomposition techniques. The shape of the design space is studied using the response surfaces method without additional computational cost. The proposed approach is applied to three validation test cases and to the design of a satellite Earth-observation mission for disaster monitoring. The results show that PROA can correctly detect Pareto-robust solutions on the Pareto front and that it provides additional Pareto-optimal solutions at the same time, eventually improving the original known Pareto front. Further, PROA shows its potentialities in effectively narrowing the search-space to only one limited portion. In the case of the Earth-observation mission, the study demonstrates the possibility to enable and promote trade-offs amongst the engineering-team members to obtain an effective decision-making process. The solution identified as the most Pareto-robust one can be considered quite uncommon, but still very reasonable due to the assumptions, presenting a satellite in a non sun-synchronous Medium Earth Orbi

Cyclic vomiting syndrome in children: a nationwide survey of current practice on behalf of the Italian Society of Pediatric Gastroenterology, Hepatology and Nutrition (SIGENP) and Italian Society of Pediatric Neurology (SINP)

Background: Cyclic Vomiting Syndrome (CVS) is a rare functional gastrointestinal disorder, which has a considerable burden on quality of life of both children and their family. Aim of the study was to evaluate the diagnostic modalities and therapeutic approach to CVS among Italian tertiary care centers and the differences according to subspecialties, as well as to explore whether potential predictive factors associated with either a poor outcome or a response to a specific treatment. Methods: Cross-sectional multicenter web-based survey involving members of the Italian Society of Pediatric Gastroenterology, Hepatology and Nutrition (SIGENP) and Italian Society of Pediatric Neurology (SINP). Results: A total of 67 responses were received and analyzed. Most of the respondent units cared for less than 20 patients. More than half of the patients were referred after 3 to 5 episodes, and a quarter after 5 attacks. We report different diagnostic approaches among Italian clinicians, which was particularly evident when comparing gastroenterologists and neurologists. Moreover, our survey demonstrated a predilection of certain drugs during emetic phase according to specific clinic, which reflects the cultural background of physicians. Conclusion: In conclusion, our survey highlights poor consensus amongst clinicians in our country in the diagnosis and the management of children with CVS, raising the need for a national consensus guideline in order to standardize the practice

On the mixed nature of the 740 cm(-1) band in wurtzite GaN films: A polarized Raman scattering investigation

A detailed study of the polarized Raman scattering of wurtzite GaN films is presented, focusing on the nature of the band centered at 740 cm(-1) observed in the X(Z, Z)X configuration. The origin of this band is ascribed to the mixed contribution of the A I and E, longitudinal phonon modes coupled with the free carrier excitation. The spectral profile of the 740 cm-1 Raman band has been successfully reconstructed through a linear combination of the A(1)-E-1 longitudinal phonon plasmon-coupled modes, leading to a free carrier concentration in good agreement with Hall effect measurements

PHOTOLUMINESCENT AND PARAMAGNETIC CENTERS IN GAMMA IRRADIATED POROUS SILICA

he photolummescence and electron spin resonance properties of gamma irradiated (up to 500 kGy) porous silica are reported. By exciting at 5.6 eV a photoluminescence contribution can be detected before irradiation, peaked at about 4.1 eV. Gamma irradiation causes the generation of the E' centers (about 1 x 10(14) defects cm(-3)) of paramagnetic hole centers and modifies the photoluminescence properties of the sample: the emission amplitude decreases and three contributions can be singled out at about 3.3, 3.8 and 4.4 eV

Absorption spectrum of Ge-doped silica samples and fiber preforms in the vacuum ultraviolet region

We report absorption measurements in the vacuum ultraviolet (VUV) spectral region on Ge-doped silica samples grown through chemical vapor deposition (CVD) and fiber preforms grown through modified chemical vapor deposition (MCVD) with GeO2 doping concentration from 0.3 to 4.0 mol%. We observed in all spectra an absorption band at 5.15 eV and a structure at about 7.0 eV. We observed differences in the absorption spectrum between Ge-doped silica samples and fiber preforms in the shape and relative amplitude of optical absorption bands. The intensity of defect-related bands depends on the Ge concentration and distribution. Differences in Ge concentration and distribution are due to the preparation methods and growing condition

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 characteristics, with peak position around 2.8 eV, full width at half maximum of about 1 eV and exponential lifetime of 840 ns. (C) 2002 American Institute of Physics