Stimuli-responsive photoluminescent and structural properties of MIL-53(Al) MOF for sensing applications.

Metal-organic frameworks (MOFs) are an intriguing group of porous materials due to their potential influence on the development of indispensable technologies like luminescent sensors and solid-state light devices, luminescent multifunctional nanomaterials. In this research work we explored MIL-53(Al), an exceptional class of MOF that, along with guest adsorption, undergoes structural transitions exhibiting breathing behavior between narrow pore and large pore under temperature and mechanical stress. Therefore, we opted for the time resolved luminescence and FT-Raman spectroscopy to investigate the mechanochromic and thermochromic response of this material under external stimuli. Intriguingly, when subjected to temperature changes, MIL-53(Al) exhibited a ratiometric fluorescence behavior related to the reversible relationship of photoluminescence emission intensity with respect to temperature. Moreover, under higher mechanical stress MIL-53(Al) displayed turn-on behavior in emission intensity, hence offering a thrilling avenue for the application in mechanically deformed-based luminescent sensors and ratiometric fluorescence temperature sensors.

A way forward for fundamental physics in space.

Space-based research can provide a major leap forward in the study of key open questions in the fundamental physics domain. They include the validity of Einstein's Equivalence principle, the origin and the nature of dark matter and dark energy, decoherence and collapse models in quantum mechanics, and the physics of quantum many-body systems. Cold-atom sensors and quantum technologies have drastically changed the approach to precision measurements. Atomic clocks and atom interferometers as well as classical and quantum links can be used to measure tiny variations of the space-time metric, elusive accelerations, and faint forces to test our knowledge of the physical laws ruling the Universe. In space, such instruments can benefit from unique conditions that allow improving both their precision and the signal to be measured. In this paper, we discuss the scientific priorities of a space-based research program in fundamental physics.

Photocycle of point defects in highly- and weakly-germanium doped silica revealed by transient absorption measurements with femtosecond tunable pump.

We report pump-probe transient absorption measurements addressing the photocycle of the Germanium lone pair center (GLPC) point defect with an unprecedented time resolution. The GLPC is a model point defect with a simple and well-understood electronic structure, highly relevant for several applications. Therefore, a full explanation of its photocycle is fundamental to understand the relaxation mechanisms of such molecular-like systems in solid state. The experiment, carried out exciting the sample resonantly with the ultraviolet (UV) GLPC absorption band peaked at 5.1 eV, gave us the possibility to follow the defect excitation-relaxation dynamics from the femto-picosecond to the nanosecond timescale in the UV-visible range. Moreover, the transient absorption signal was studied as a function of the excitation photon energy and comparative experiments were conducted on highly- and weakly-germanium doped silica glasses. The results offer a comprehensive picture of the relaxation dynamics of GLPC and allow observing the interplay between electronic transitions localized on the defect and those related to bandgap transitions, providing a clear evidence that the role of dopant high concentration is not negligible in the earliest dynamics.

Nanoscale structural and electrical properties of graphene grown on AlGaN by catalyst-free chemical vapor deposition.

The integration of graphene (Gr) with nitride semiconductors is highly interesting for applications in high-power/high-frequency electronics and optoelectronics. In this work, we demonstrated the direct growth of Gr on Al0.5Ga0.5N/sapphire templates by propane (C3H8) chemical vapor deposition at a temperature of 1350 °C. After optimization of the C3H8 flow rate, a uniform and conformal Gr coverage was achieved, which proved beneficial to prevent degradation of AlGaN morphology. X-ray photoemission spectroscopy revealed Ga loss and partial oxidation of Al in the near-surface AlGaN region. Such chemical modification of a ∼2 nm thick AlGaN surface region was confirmed by cross-sectional scanning transmission electron microscopy combined with electron energy loss spectroscopy, which also showed the presence of a bilayer of Gr with partial sp2/sp3 hybridization. Raman spectra indicated that the deposited Gr is nanocrystalline (with domain size ∼7 nm) and compressively strained. A Gr sheet resistance of ∼15.8 kΩ sq-1 was evaluated by four-point-probe measurements, consistently with the nanocrystalline nature of these films. Furthermore, nanoscale resolution current mapping by conductive atomic force microscopy indicated local variations of the Gr carrier density at a mesoscopic scale, which can be ascribed to changes in the charge transfer from the substrate due to local oxidation of AlGaN or to the presence of Gr wrinkles.

Evidence for Tunisian-Like Pestiviruses Presence in Small Ruminants in Italy Since 2007.

The genus Pestivirus, which belongs to the Flaviviridae family, includes ssRNA+ viruses responsible for infectious diseases in pigs, cattle, sheep, goats and other domestic and wild ruminants. Like most of the RNA viruses, pestivirus has high genome variability with practical consequences on disease epidemiology, diagnosis and control. In addition to the officially recognized species in the genus Pestivirus, such as BVDV-1, BVDV-2, BDV and CSFV, other pestiviruses have been detected. Furthermore, most of the ruminant pestiviruses show low or absent species specificity observed in serological tests and are able to infect multiple species. Particularly, small ruminants are receptive hosts of the most heterogeneous group of pestiviruses. The aim of this study was to carry out the molecular characterization of pestiviruses isolated from sheep and goats in Sicily, Italy. Phylogenetic analysis of two viral genomic regions (a fragment of 5'-UTR and the whole Npro regions) revealed the presence of different pestivirus genotypes in the analysed goat and sheep herds. Two of five viral isolates were clustered with BVDV-1d viruses, a strain widespread in Italy, but never reported in Sicily. The other three isolates formed a distinct cluster with high similarity to Tunisian isolates, recently proposed as a new pestivirus species. This represents the first evidence for Tunisian-like pestivirus presence in small ruminants in Italy. Furthermore, one of the isolates was collected from a goat, representing the first isolation of Tunisian-like pestivirus from this species.

Isolation of the CH3˙ rotor in a thermally stable inert matrix: first characterization of the gradual transition from classical to quantum behaviour at low temperatures.

Matrix isolation is a method which plays a key role in isolating and characterizing highly reactive molecular radicals. However, the isolation matrices, usually composed of noble gases or small diamagnetic molecules, are stable only at very low temperatures, as they begin to desegregate even above a few tens of Kelvin. Here we report on the successful isolation of CH3˙ radicals in the cages of a nearly inert clathrate-SiO2 matrix. This host is found to exhibit a comparable inertness with respect to that of most conventional noble gas matrices but it is characterized by a peculiar thermal stability. The latter property is related to the covalent nature of the host material and gives the opportunity to study the confined radicals from a few degrees of Kelvin up to at least room temperature. Thanks to this advantage we were able to explore with continuity for the first time the CH3˙ rotor properties by electron paramagnetic resonance spectroscopy, starting from the quantum rotations which are observable only at the lowest temperatures (T ≈ 4 K), going through the gradual transition to the classical motion (4 K < T < 30 K), and ending with the properties of the fully classical rotor (T > 30 K). The method of isolation presented here is found to be very effective and promising, as it is expected to be applicable to a large variety of different molecular radicals.

Evolution of photo-induced defects in Ge-doped fiber/preform: influence of the drawing.

We have studied the generation mechanisms of two different radiation-induced point defects, the Ge(1) and Ge(2) centers, in a germanosilicate fiber and in its original preform. The samples have been investigated before and after X-ray irradiation using the confocal microscopy luminescence and the electron paramagnetic resonance techniques. Our experimental results show the higher radiation sensitivity of the fiber as compared to the perform and suggest a relation between Ge(1) and Ge(2) generation. To explain our data we have used different models, finding that the destruction probability of the Ge(1) and Ge(2) defects is larger in fiber than in preform, whereas the generation one is similar. Finally we found that the higher radiation sensitivity of the fiber at low doses is essentially related to the presence of germanium lone pair center generated by the drawing.

Dependence of the emission properties of the germanium lone pair center on Ge doping of silica.

We present an experimental investigation regarding the changes induced by the Ge doping level on the emission profile of the germanium lone pair center (GLPC) in Ge doped silica. The investigated samples have been produced by the sol-gel method and by plasma-activated chemical vapor deposition and have doping levels up to 20% by weight. The recorded photoluminescence spectra show that the GLPC emission profile is the same when the Ge content is lower than ∼ 1% by weight, whereas it changes for higher doping levels. We have also performed Raman scattering measurements that show the decrease of the D1 Raman band at 490 cm( - 1) when the Ge content is higher than 1% by weight. The data suggest that both changes can be related to matrix modifications. These findings improve our knowledge of the matrix effects on the physical properties of the point defects and, in particular, for the GLPC they show that variations in their emission properties are induced by the presence of a second Ge atom close to the defect within a sphere with a radius of about 2 nm.

The role of impurities in the irradiation induced densification of amorphous SiO(2).

In a recent work (Buscarino et al 2009 Phys. Rev. B 80 094202), by studying the properties of the (29)Si hyperfine structure of the E'(γ) point defect, we have proposed a model able to describe quantitatively the densification process taking place upon electron irradiation in amorphous SiO(2) (a-SiO(2)). In particular, we have shown that it proceeds heterogeneously, through the nucleation of confined densified regions statistically dispersed into the whole volume of the material. In the present experimental investigation, by using a similar approach on a wider set of materials, we explore how this process is influenced by impurities, such as OH and Cl, typically involved in relevant concentrations in commercial materials. Our results indicate that the degree of local densification within the structurally modified regions is influenced by the OH content of the material: the higher the OH content, the lower the local degree of densification of the irradiated materials. In contrast, no relevant contribution to the densification process induced by electron irradiation in a-SiO(2) can be ascribed to Cl impurities up to [Formula: see text] ppm by weight.

Room temperature instability of E'gamma centers induced by gamma irradiation in amorphous SiO2.

We study by optical absorption measurements the stability of E'(gamma) centers induced in amorphous silica at room temperature by gamma irradiation up to 79 kGy. A significant portion of the defects spontaneously decay after the end of irradiation, thus allowing the partial recovery of the transparency loss initially induced by irradiation. The decay kinetics observed after gamma irradiation with a 0.6 kGy dose closely resembles that measured after exposure to 2000 pulses of pulsed ultraviolet (4.7 eV) laser light of 40 mJ/cm(2) energy density per pulse. In this regime, annealing is ascribed to the reaction of the induced E'(gamma) centers with diffusing H(2) of radiolytic origin. At higher gamma doses, the decay kinetics becomes unexpectedly slower notwithstanding the progressive growth of the concentration of induced defects. In particular, the annealing kinetics of E'(gamma) centers after 79 kGy irradiation is inconsistent with the reaction parameters between the defect and H(2). To explain this result, on the basis of the quantitative analysis of the kinetics, we propose water-related species to be responsible for the slow room temperature annealing of E'(gamma) after irradiation with such a dose. This model is qualitatively supported by results obtained by IR absorption measurements, which show an increase of the absorption in the spectral region of Si-OH groups.

Twofold co-ordinated Ge defects induced by gamma-ray irradiation in Ge-doped SiO2.

We report an experimental study by photoluminescence, optical absorption and Electron Paramagnetic Resonance measurements on the effects of exposure of Ge-doped amorphous SiO2 to gamma ray radiation at room temperature. We have evidenced that irradiation at doses of the order of 1 MGy is able to generate Ge-related defects, recognizable from their optical properties as twofold coordinated Ge centers. Until now, such centers, responsible for photosensitivity of Ge-doped SiO2, have been induced only in synthesis procedures of materials. The found result evidences a role played by gamma radiation in generating photosensitive defects and could furnish a novel basis for photosensitive pattern writing through ionizing radiation.

Annealing of radiation induced oxygen deficient point defects in amorphous silicon dioxide: evidence for a distribution of the reaction activation energies.

The selective annealing of point defects with different activation energies is studied, by performing sequences of thermal treatments on gamma irradiated silica samples in the temperature range 300-450 °C. Our experiments show that the dependence on time of the concentration of two irradiation induced point defects in silica, named ODC(II) (standing for oxygen deficient centre II) and the E(γ)(') centre, at a given temperature depends on the thermal history of the sample for both of the centres studied; moreover in the long time limit this concentration reaches an asymptotic value that depends on the treatment temperature alone. These results suggest the existence of a distribution of the activation energies of the reaction process responsible for the annealing of the defects investigated, intimately related to the intrinsic disorder of the amorphous lattice. Furthermore, our data show that the thermal treatment can modify this distribution of activation energies and as a consequence the thermal properties of the centre itself.

(29)Si hyperfine structure of the E(')(alpha) center in amorphous silicon dioxide.

We report a study by electron paramagnetic resonance on the E'(alpha) point defect in amorphous silicon dioxide (a-SiO(2)). Our experiments were performed on gamma-ray irradiated oxygen-deficient materials and pointed out that the (29)Si hyperfine structure of the E'(alpha) consists of a pair of lines split by approximately 49 mT. On the basis of the experimental results, a microscopic model is proposed for the E'(alpha) center, consisting of a hole trapped in an oxygen vacancy with the unpaired electron sp(3) orbital pointing away from the vacancy in a back-projected configuration and interacting with an extra oxygen atom of the a-SiO(2) matrix.

Delocalized nature of the E'delta center in amorphous silicon dioxide.

We report an experimental study by electron paramagnetic resonance (EPR) of E(')(delta) point defect induced by gamma-ray irradiation in amorphous SiO2. We obtained an estimation of the intensity of the 10 mT doublet characterizing the EPR spectrum of such a defect arising from hyperfine interaction of the unpaired electron with a 29Si (I=1/2) nucleus. Moreover, determining the intensity ratio between this hyperfine doublet and the main resonance line of E(')(delta) center, we pointed out that the unpaired electron wave function of this center is actually delocalized over four nearly equivalent silicon atoms.

Epidemiological surveillance of bluetongue in Sicily.

The authors describe the status of bluetongue (BT) since 13 October 2000, when the first outbreak was reported in Sicily. The results of the epidemiological surveillance programme, based on sentinel animals distributed over the entire region, are also given. In Sicily, the incidence of the disease is relatively low compared to some other areas in the Mediterranean Basin. Seventy-five outbreaks of the disease were recorded in the first three epidemics (October 2000 to May 2003). Overall morbidity was 13.25%, mortality 5.36% and the case fatality rate 41.49%. The Province of Catania seems to have been the worst affected; the incidence rate in August 2002 was 0.8%. The monthly incidence rate was calculated for sentinel animals of which the estimated total was 3 654, distributed in 63 areas. It is important to underline that in the period under consideration, a total of 2 382 animals was examined. During the surveillance period, which extended from September 2001 to May 2003, the incidence of BT peaked in September 2002, at 5.91% -/+ 0.979. The cumulative incidence rate from September 2001 to August 2002 and September 2002 to March 2003 was 4.53% -/+ 0.76 and 20.03% -/+ 1.85, respectively. The circulation of BT virus serotypes 2, 4, 9 and 16 is described, as revealed by seroconversion in sentinel animals.