The emission in solid phase of Carbon Dots (CDs) deposited by drop-casting technique is investigated by means of micro-photoluminescence. Graphene and SiO2 are used as substrates, and the influence ...of their different nature – conductive or insulating – on the emission of CDs is highlighed. In particular, a systematic loss of efficiency in the emission of CDs on graphene is found, suggesting a CD-graphene interaction possibly due to a photoinduced electron transfer between the surface states of CDs and the conduction band of graphene. Finally, thanks to the negligible influence on CDs emission, SiO2 substrate is used as support to perform thermal processing of CDs in solid phase, showing the possibility to modulate the CDs surface composition, and therefore, their emission.
•Graphene – Carbon Dots interaction is driven by charge transfer.•Carbon Dots are dispersed on Graphene substrate by drop casting.•Carbon Dots are modified by temperature increase with minor influence of atmosphere.
We report an experimental investigation on the effects of Ce-codoping in determining the radiation response of germanosilicate and phosphosilicate Optical Fibers (OFs) in the UV-Visible domain and up ...to doses of 1 MGy(SiO 2 ). We show that the addition of Ce strongly impacts the Radiation Induced Attenuation (RIA) of both types of fibers. In the first case the radiation induced losses increase, whereas in the second one decrease. By combining the online RIA measurements with the Electron Paramagnetic Resonance (EPR) ones, we are able to infer the basic microscopic mechanisms taking place under irradiation, which involve the cerium codopant and some of the known Ge-related or P-related defects. More precisely, we found that part of the Ce atoms are incorporated in the glass matrix as Ce 3+ ions by the production process and act as electron donor centers under irradiation. Consequently, the concentrations of radiation induced hole centers of Ge and P are drastically reduced. The reported results give an insight into possible ways of exploiting Ce codoping to control the radiation sensitivity of the OFs. Moreover, the OFs doped with cerium and phosphorous show a strongly reduced saturation effect at high radiation doses that make them a potential candidate for RIA-based dosimetry applications in a wide range of radiation doses.
An experimental study of the molecular O
2
diffusion process in high purity non-porous silica nanoparticles having 50 m
2
/g BET specific surface and 20 nm average radius was carried out in the ...temperature range from 127 to 177 °C at O
2
pressure in the range from 0.2 to 66 bar. The study was performed by measuring the volume average interstitial O
2
concentration by a Raman and photoluminescence technique using a 1,064 nm excitation laser to detect the singlet to triplet emission at 1,272 nm of the molecular oxygen in silica. A dependence of the diffusion kinetics on the O
2
absolute pressure, in addition to temperature dependence, was found. The kinetics can be fit by the solution of Fick’s diffusion equation using an effective diffusion coefficient related to temperature and O
2
external pressure. The fit results have evidenced that the temperature and pressure dependencies can be disentangled and that the pressure effects are more pronounced at lower temperatures. An Arrhenius temperature law is determined for the effective diffusion coefficient and the activation energy and pre-exponential factor are found in the explored experimental range. The reported findings have not been evidenced previously in the studies in bulk silica and could probably be originated by the reduced spatial extension of the considered system.
We report an experimental investigation by electron paramagnetic resonance (EPR) on methyl radical (CH3) observed in γ-ray irradiated high-purity amorphous silicon dioxide (a-SiO2) and in a ...polycrystalline sample of Melanophlogite, a rare natural form of SiO2-clathrate. From the analysis of the EPR spectra we estimate the correlation time of the hindered rotational motion of CH3 molecules at T=77K in the two different materials. This physical quantity gives a quantitative measure of the freedom of motion of CH3 molecules trapped in the two solid systems, putting forward relevant information on the properties of the cavities/interstices in which the radicals are confined. In particular, our data suggest that in a-SiO2 the CH3 molecules are trapped in interstices with diameter significantly lower than about 5.7Å, which is the size of the smaller cavity involved in the crystalline structure of Melanophlogite.
► CH3 is observed in irradiated amorphous SiO2 and in Melanophlogite. ► The rotational correlation time of CH3 molecules at T=77K is measured by EPR. ► Relevant information on the interstices in which the radical is confined is obtained. ► The comparison between amorphous SiO2 and Melanophlogite is found to be fruitful.
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 by the main Raman line peaked at about 440 cm−1, is modified by thermal treatments above 400 °C and tends toward typical values of bulk silica materials, whereas the three-membered ring population, probed by the D2 line peaked at about 600 cm−1, changes but does not reach bulk silica features. The surface adsorbed water content, estimated by IR measurements, gradually decreases, starting from 100 °C. The peculiar properties of fumed silica, which suggest a strained atomic network structure, together with the investigation of its modifications induced by thermal treatments, are interpreted in terms of a shell-like model of the constituting particles. In particular, the model assumes that each particle comprises a surface shell characterized by a network structure highly strained and a core shell with a less strained structure more similar to that of bulk silica. This model, discussed on the basis of our experimental results, suggests that the structural property modifications induced by thermal treatments into the surface shell are related to the dehydroxylation process and to the buildup of particle-to-particle linking (sintering effects), whereas the structural modifications into the core shell arise from the network relaxation activated by thermal treatments.
We present an experimental investigation focused on the effects of alpha and deuteron irradiation on different silica nanoparticles. The study has been devoted also to characterize the induced point ...defects and the eventual structural modifications to evaluate the effects of the different irradiation source in comparison with the bulk materials. After irradiation up to about 10
16
ions cm
−2
, we performed electron paramagnetic resonance (EPR), photoluminescence (PL), infrared (IR) absorption, Raman, and atomic force microscopy (AFM) measurements. We found that the two types of irradiation qualitatively induce comparable effects. Furthermore, irradiation generates the so-called twofold coordinated Si and the H(I) point defects, originating from the reaction of the former with hydrogen atoms. The occurrence of these defects is a not trivial and interesting finding due to its connection to irradiation-induced oxygen deficiency, not yet evidenced by other irradiation of silica nanoparticles. We also detected the E′Si paramagnetic centers and found that their lineshape at the highest fluence, independently from the nanoparticles size and irradiation source, is different from the one observed in the bulk. Furthermore, the integral of the E′Si signal does not depend significantly on the nanoparticles size differing from previous irradiation with
β
-ray of the same nanoparticles. AFM images indicate the absence of significant radiation induced sintering between the nanoparticles in the plan orthogonal to the irradiation direction suggesting the absence of morphological changes, whereas IR measurements suggest the occurrence of some structural modifications in all the nanoparticles, which consist in the decrease of the peak value of the Si–O–Si angle distribution. Finally, irradiation effects are induced not uniformly along the irradiation direction, as supported also by micro-Raman investigation of an irradiated bulk silica material and simulations of ions penetration profiles. These results suggest the occurrence of some irradiation effects due to light ion bombardment specific of nanoparticles.
We report an experimental investigation on the effects of γ-ray irradiation in three types of fumed silica previously loaded with O2 molecules. Our data indicate that the main effect of irradiation ...in these systems is to generate a very large concentration of HO2 interstitial radicals (about 1018molecules/cm3). Furthermore, the number of generated HO2 was found to be larger in the samples with higher O2 contents before irradiation. This correlation suggests that HO2 radicals are induced by reaction of interstitial O2 molecules with radiolytic H atoms, as previously suggested for O2-loaded bulk amorphous silicon dioxide (a-SiO2 or silica) samples. However, at variance with respect to bulk materials, in fumed silica the radiolytic H does not arise from SiOH or SiOOH groups, as no EPR signal due to non-bridging oxygen hole centers (NBOHC) or to peroxy radicals (POR) is detected in the spectra of irradiated samples. As a reasonable alternative we propose that radiolytic hydrogen atoms could arise from a radiation induced breaking of interstitial H2O molecules, indicating that fumed silica in its pristine form could possess a very large concentration of interstitial water molecules.
► The effects of γ-ray irradiation in O2-loaded fumed silica is studied by EPR. ► Irradiation generates a very large concentration of HO2 interstitial radicals. ► HO2 radicals are generated by a reaction of interstitial O2 with radiolytic H atoms. ► Relevant information on the origin of the radiolytic H atoms are found and discussed.
We studied the emission of the O2 molecules embedded in fumed silica (amorphous silicon dioxide) nanoparticles differing for diameters and specific surface. By using a 1064 nm laser as a source we ...recorded both the O2 emission and the Raman signal of silica. Our experimental data show that the O2 emission/Raman signal (at 800 cm–1) ratio decreases with increasing the specific surface for both the as-received and the loaded samples. By performing a thermal treatment (600 °C for 2 h) we modified the structure and the water content of the smallest nanoparticles without observing any significant change in the O2 emission/Raman signal ratio. Our data are explained by a shell model showing that the O2 emission is essentially due to the molecules entrapped in the core of the nanoparticles, whereas the contribution due to the surface shell, having a thickness of about 1 nm, is negligible because of its high content of Si–OH groups that introduce nonradiative relaxation channels or because of the very low content of molecules trapped in this thin region.
The O2 diffusion process in silica nanoparticles is experimentally studied in samples of average radius of primary particles ranging from 3.5 to 20 nm and specific surface ranging from 50 to 380 ...(m2/g). The investigation is done in the temperature range from 98 to 177 °C at O2 pressure ranging from 0.2 to 66 bar by measuring the interstitial O2 concentration by Raman and photoluminescence techniques. The kinetics of diffusion can be described by the Fick’s equation with an effective diffusion coefficient depending on the temperature, O2 pressure, and particles size. In particular, the dependence of the diffusion coefficient on the pressure and nanoparticles size is more pronounced at lower temperatures and is connected to morphological and physical factors.
