S. Agnello

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.

Combined High Dose and Temperature Radiation Effects on Multimode Silica-Based Optical Fibers

We investigate the response of Ge-doped, P-doped, pure-silica, or Fluorine-doped fibers to extreme environments combining doses up to MGy(SiO 2) level of 10 keV X-rays and temperatures between 25 °C and 300 °C. First, we evaluate their potential to serve either as parts of radiation tolerant optical or optoelectronic systems or at the opposite, for the most sensitive ones, as punctual or distributed dosimeters. Second, we improve our knowledge on combined ionizing radiations and temperature (R&T) effects on radiation-induced attenuation (RIA) by measuring the RIA spectra in the ultraviolet and visible domains varying the R&T conditions. Our results reveal the complex response of the tested fibers in such mixed environments. Increasing the temperature of irradiation increases or decreases the RIA values measured at 25 °C or sometimes has no impact at all. Furthermore, R&T effects are time dependent giving an impact of the temperature on RIA that evolves with the time of irradiation. The two observed transient and stationary regimes of temperature influence will make it very difficult to evaluate sensor vulnerability or the efficiency of hardening approaches without extensive test campaigns.

Atomic force microscopy and Raman investigation on the sintering process of amorphous SiO2 nanoparticles

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 quantitative analysis of the AFM images shows that the grain growth process takes place approximately at constant rate for thermal treatment durations up to 290 h. Nevertheless, the value of the grain growth rate is sensitive to the system properties. In fact, it is found to increase with decreasing the particle mean diameter,...

Luminescence of γ-radiation-induced defects in α-quartz

Optical transitions associated with γ-radiation-induced defects in crystalline α-quartz were investigated by photoluminescence excited by both pulsed synchrotron radiation and steady-state light. After a 10 MGy γ-dose we observed two emissions at 4.9 eV (ultraviolet band) and 2.7 eV (blue band) excitable in the range of the induced absorption band at 7.6 eV. These two luminescence bands show a different temperature dependence: the ultraviolet band becomes bright below 80 K; the blue band increases below 180 K, but drops down below 80 K. Both emissions decay in a timescale of a few ns under pulsed excitation, however the blue band could also be observed in slow recombination processes and it afterglows in about 100 s at the end of steady-state excitation. The origin of the observed luminescence bands and the comparison with optical features of oxygen-deficient centres in silica glass are discussed in the framework of different models proposed in the literature.

Chapter 1 – Optical absorption, luminescence, and ESR spectral properties of point defects in silica

Publisher Summary This chapter is divided into two parts: (1) In the introductory part, it describes the problems of point defects in a-SiO2, and (2) in the second part it discusses the experimental results. This chapter focuses on the Oxygen-Deficient Centers (ODCs) species in silica. This chapter investigates the ODC defects in a-SiO2 through their optical absorption, photoluminescence, and electron spin resonance activities. The effects of γ-ray irradiation are also investigated to evidence their ability to generate or transform structural defects. The aim of this chapter is to understand the optical activity of such defects to help in the characterization of their structure. The properties of point defects in a wide variety of both natural and synthetic silica types of commercial origin are investigated in the chapter. This chapter outlines the role of structural and dynamic properties of the vitreous matrix in determining the observed spectral properties of different centers. It deals with the theoretical aspects of the mechanism that are able to influence the fine structure of the spectral band profiles of point defects in interaction with the glassy matrix.

Sol-Gel GeO2-Doped SiO2 Glasses for Optical Applications

Optical and structural properties of silica materials with controlled Ge-content were investigated in aerogels samples with Ge concentration up to 100000 molar ppm prepared by a sol-gel method and densified at 1150°C. An optical absorption band centered at 242 nm, commonly ascribed to an under-coordinated germanium point defect, was observed in all doped samples, and its amplitude was found to be almost linearly correlated with the Ge-content. This feature may be ascribed to the new preparation technique so that this is potentially useful to produce materials with controlled defect content for specific optical applications.

Fluorescent nitrogen-rich carbon nanodots with an unexpected β-C3N4 nanocrystalline structure

Carbon nanodots are a class of nanoparticles with variable structures and compositions which exhibit a range of useful optical and photochemical properties. Since nitrogen doping is commonly used to enhance the fluorescence properties of carbon nanodots, understanding how nitrogen affects their structure, electronic properties and fluorescence mechanism is important to fully unravel their potential. Here we use a multi-technique approach to study heavily nitrogen-doped carbon dots synthesized by a simple bottom-up approach and capable of bright and color-tunable fluorescence in the visible region. These experiments reveal a new variant of optically active carbonaceous dots, that is a nanocrystal of beta carbon nitride (β-C3N4) capped by a disordered surface shell hosting a variety of polar functional groups. Because β-C3N4 is a network of sp3 carbon and sp2 nitrogen atoms, such a structure markedly contrast with the prevailing view of carbon nanodots as sp2-carbon materials. The fluorescence mechanism of these nanoparticles is thoroughly analyzed and attributed to electronic transitions within a manifold of surface states associated with nitrogen-related groups. The sizeable bandgap of the β-C3N4 nanocrystalline core has an indirect, albeit important role in favoring an efficient emission. These results have deep implications on our current understanding of optically active carbon-based nanoparticles and reveal the role of nitrogen in controlling their properties.

Interstitial O2 distribution in amorphous SiO2 nanoparticles determined by Raman and photoluminescence spectroscopy

The O2 content and emission properties in silica nanoparticles after thermal treatments in oxygen rich atmosphere have been investigated by Raman and photoluminescence measurements. The nanoparticles have different sizes with average diameter ranging from 7 up to 40 nm. It is found that O2 concentration in nanoparticles monotonically increases with nanoparticles size. This finding is independent on the measurement technique and evidences that oxygen molecules are not present in all the nanoparticles volume. This dependence is interpreted on the basis of a structural model for nanoparticles consisting of a core region able to host the oxygen molecules and a surface shell of fixed size and free from O2.

Weak hyperfine interaction of E′ centers in gamma and beta irradiated silica

We report on the effects of photon (γ) and electron (β) irradiation in a dose range extending from 100 to 5×109 Gy in a variety of silica samples studied by electron paramagnetic resonance. The E′ centers and a weak intensity satellite signal of their resonance line were generated both in γ- and in β-irradiated samples. We investigated the dependence of their intensity on the irradiation dose. Evidence of the existence of a common generation mechanism for the related paramagnetic point defects is found. These defects are induced mainly through the conversion of precursors except at very high doses, where the direct activation from the unperturbed matrix is concurrent. Our data support the model attributing the satellite signal to the weak hyperfine structure of the E′ center arising from interaction with a second nearest neighbor nuclear spin.

Ambipolar MoS2 Transistors by Nanoscale Tailoring of Schottky Barrier Using Oxygen Plasma Functionalization

One of the main challenges to exploit molybdenum disulfide (MoS2) potentialities for the next-generation complementary metal oxide semiconductor (CMOS) technology is the realization of p-type or ambipolar field-effect transistors (FETs). Hole transport in MoS2 FETs is typically hampered by the high Schottky barrier height (SBH) for holes at source/drain contacts, due to the Fermi level pinning close to the conduction band. In this work, we show that the SBH of multilayer MoS2 surface can be tailored at nanoscale using soft O2 plasma treatments. The morphological, chemical, and electrical modifications of MoS2 surface under different plasma conditions were investigated by several microscopic and spectroscopic characterization techniques, including X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), conductive AFM (CAFM), aberration-corrected scanning transmission electron microscopy (STEM), and electron energy loss spectroscopy (EELS). Nanoscale current-voltage mapping by CAFM showed that the SBH maps can be conveniently tuned starting from a narrow SBH distribution (from 0.2 to 0.3 eV) in the case of pristine MoS2 to a broader distribution (from 0.2 to 0.8 eV) after 600 s O2 plasma treatment, which allows both electron and hole injection. This lateral inhomogeneity in the electrical properties was associated with variations of the incorporated oxygen concentration in the MoS2 multilayer surface, as shown by STEM/EELS analyses and confirmed by ab initio density functional theory (DFT) calculations. Back-gated multilayer MoS2 FETs, fabricated by self-aligned deposition of source/drain contacts in the O2 plasma functionalized areas, exhibit ambipolar current transport with on/off current ratio Ion/Ioff ≈ 103 and field-effect mobilities of 11.5 and 7.2 cm2 V-1 s-1 for electrons and holes, respectively. The electrical behavior of these novel ambipolar devices is discussed in terms of the peculiar current injection mechanisms in the O2 plasma functionalized MoS2 surface.