0000000000087941
AUTHOR
G. Vaccaro
showing 12 related works from this author
Sintering process of amorphous SiO2 nanoparticles investigated by AFM, IR and Raman techniques
2011
We report an experimental investigation on the effects of thermal treatments at different temperatures (room-1270 K) and for different duration (0-75 h) on amorphous silica nanoparticles (fumed silica) in powder tablet form. Three types of fumed silica are considered, comprising nearly spherical particles of 40 nm, 14 nm and 7 nm mean diameter. The experimental techniques used here are Raman and infrared absorption (IR) spectroscopy together with atomic force microscopy (AFM). Raman and IR spectra indicate that the structure of nanometer silica particles is significantly different with respect to that of a bulk silica glass. In particular, the main differences regard the positions of the IR…
Structure of amorphous SiO 2 nanoparticles probed through the E′ γ centers
2011
We report an experimental investigation by electron paramagnetic resonance (EPR) spectroscopy on the properties of the E′ γ centers induced by β-ray irradiation in nanoparticles of amorphous SiO 2 (fumed silica) with mean diameters from 7 up to 40 nm. We found that the E′ γ centers are induced in all the fumed silica types in the dose range 4-400 kGy. They are characterized by an EPR line shape similar to that observed in common bulk silica materials independently on the particle diameter. Moreover, the E′ γ center concentration decreases on decreasing of the particle size for each given dose. Our findings are interpreted in terms of a shell-like model of nanoparticles in which it is assume…
Thermally Induced Structural Modification of Silica Nanoparticles Investigated by Raman and Infrared Absorption Spectroscopies
2010
We report an experimental investigation by Raman and infrared (IR) absorption spectroscopies on the structural modifications induced by isochronal thermal treatments on amorphous SiO2 nanoparticles (fumed silica). In particular, three different commercial types of this material, characterized by particle mean diameters of 7, 14, and 40 nm, were subjected to thermal treatments from 100 up to 1000 °C. We found that some properties of fumed silica, such as the SiOSi mean bond angle, ring size distribution, and surface adsorbed water content, are drastically different from those of common bulk silica materials and intimately related to the particles' dimension. The SiOSi mean bond angle, probed…
Atomic force microscopy and Raman investigation on the sintering process of amorphous SiO2 nanoparticles
2010
We report an experimental investigation on the sintering process induced in fumed silica powders by isochronal thermal treatments at T=1270 K. Three types of fumed silica are considered, consisting of amorphous SiO2 (a-SiO2) particles with mean diameters 7, 14, and 40 nm. The study is performed by atomic force microscopy (AFM), to follow the morphological changes, and by Raman scattering, to obtain information on the concomitant structural modifications. The former method indicates that the sintering process proceeds by aggregation of single particles into larger grains, whose sizes increase with the thermal treatment duration. Furthermore, for each fumed silica type considered, the quantit…
Near-Infrared Emission of O2 Embedded in Amorphous SiO2 Nanoparticles
2011
We report an experimental study on the emission properties of O2 molecules loaded by a thermal diffusion process at 200 °C into high-purity silica nanoparticles with mean diameters of 7 and 40 nm. The embedded O 2 features a singlet to triplet emission band peaked at 1272 nm in agreement with the band observed for bulk silica materials. The photoluminescence excitation spectra have been determined in the visible and in the infrared range and are characterized by narrow bands peaked at 691, 764, and 1069 nm, respectively. By comparison of the transition energies, the vibrational quanta have been determined for the ground and for both the excited states; the values found are lower than the co…
Structural and luminescence properties of amorphous SiO2 nanoparticles
2011
We report an experimental study on the photoluminescence band peaked at 2.7 eV (blue band) induced by thermal treatments in nanometric amorphous SiO 2. In particular the emission dependence on the nanometric particles size as a function of their mean diameter from 7 nm up to 40 nm is investigated. We found that the emission amplitude increases on decreasing the particle diameter, showing a strong correlation between the blue band and the nanometric nature of the particles. By Raman spectroscopy measurements it is evidenced that the SiO2 nanoparticles matrix is significantly affected by the reduction of size. Basing on the shell-like model, these findings are interpreted assuming that the de…
Structural properties of the range-II- and range-III order in amorphous-SiO2 probed by electron paramagnetic resonance and Raman spectroscopy
2010
In the present work we report an experimental investigation by electron paramagnetic resonance spectroscopy on the hyperfine structure of the E. point defect, probing the local arrangement of the network (range-II order), and by Raman spectroscopy on the D 1 and D 2 lines, probing mean features of the network (range-III order). Our studies, performed on a-SiO 2 samples thermally treated at 1000 °C in air for different time durations, show that changes of the hyperfine structure and of the D 1 and D 2 lines occur in a correlated way. These results give strong evidence that the range-II and range-III order properties are intimately related to each other and that these properties are determine…
Structural organization of silanol and silicon hydride groups in the amorphous silicon dioxide network
2011
We present a study on the effects of an isothermal annealing treatment on a-SiO 2 having a significant content of silanol hydride groups (Si-H). We examined the properties of the IR absorption bands of silanol (Si-OH) and silicon hydride groups as a function of the duration of the thermal treatment. We showed that the Si-OH and Si-H groups contents decrease in a linearly correlated way. The annealing dynamics suggest that the two species are close to each other in the amorphous network. We showed that the profile of the silanol groups absorption band is the same as that observed in other commercial a-SiO 2 materials, irrespectively of the concomitant presence of nearby Si-H groups, and, mor…
MR imaging of the brain: findings in asymptomatic patients with thalassemia intermedia and sickle cell-thalassemia disease.
1999
The purpose of this study was to evaluate the spectrum of MR findings of the brain in asymptomatic patients affected with thalassemia intermedia or sickle cell-thalassemia disease to prevent brain damage by identifying patients at risk for stroke so that transfusional or pharmacologic treatment could be implemented.Forty-one asymptomatic patients who were younger than 50 years and were affected by minor hemoglobinopathies underwent MR imaging of the brain. Ischemic lesions were classified as small, medium, or large and as single or multifocal. Atrophic changes were graded subjectively as mild, moderate, or severe. A grade of brain damage was assigned to every patient. The frequency and seve…
Wide range excitation of visible luminescence in nanosilica
2010
The visible luminescence of nanometer-sized silica particles (7 nm mean diameter) was investigated using time resolved spectroscopy. This luminescence is characterized by a wide excitation in the visible and ultraviolet range. The emission spectrum is centred at 2.72 eV with a full width at half maximum of 0.70 eV when excited above 3.5 eV, whereas it progressively empties on the high energy side when excited below 3.5 eV. Moreover, the lifetime falls in the ns timescale and decreases on increasing the emission energy. These features are due to the exceptionally broad inhomogeneous distribution of the emitting centres peculiar to the silica nanoparticles. © 2010 Elsevier B.V. All rights res…
S29i attribution of the 1.3 mT hyperfine structure of the E′γ centers in amorphous SiO2
2009
We report an experimental investigation by electron paramagnetic resonance of the doublet of lines split by ∼1.3 mT and centered on the E′γ; center resonance line in the spectrum of irradiated amorphous SiO2 . Commercial and sol-gel materials, some of which subjected to hydrogen-deuterium exchange, were investigated. Exposure to γ or Β rays at room temperature of the samples and subsequent thermal treatments were carried out to induce the defects and to study their thermal stability. In all the materials used the ratio between the signal of the E′γ centers and that of the 1.3 mT doublet is constant and independent of the OH and OD contents. Furthermore, the 1.3 mT doublet and the E′γ center…
Variability of the Si-O-Si angle in amorphous-SiO2 probed by electron paramagnetic resonance and Raman spectroscopy
2009
We report an experimental investigation by electron paramagnetic resonance (EPR) and Raman spectroscopy on a variety of amorphous silicon dioxide materials. Our study by EPR have permitted us to point out that the splitting of the primary hyperfine doublet of the Eγ′ center shows a relevant sample-to-sample variability, changing from ∼41.8 to ∼42.6 mT in the set of materials we considered. The parallel study by Raman spectroscopy has enabled us to state that this variability is attributable to the different Si-O-Si angle characterizing the matrices of the different materials. © 2009 Elsevier B.V. All rights reserved.