32 research outputs found
SILICATE NANOPARTICLES PRODUCED BY LABORATORY SIMULATED SPACE WEATHERING OF OLIVINE SINGLE CRYSTALS
Silicate nanoparticles, otherwise
referred to as very small grains (VSGs) [1], occur in
the interstellar medium. These grains experience a
strong structural modification during their lifetime in
the diffuse interstellar medium, due to events such as
grain-grain collisions and irradiation. Grain
amorphization is one of the major effects, transforming
crystalline dust concentrated in star envelopes into
amorphous silicate grains populating the interstellar
medium [2]. Moreover, several studies have pointed
out that the main building blocks of these silicates are
O, Si, Fe, Mg, Al and Ca, all elements that are among
the principal constituents of the Earth’s surface [3],
thus leading to the name “astronomical silicates”.
However, the structure and chemical evolution
together with the origin of these grains are still poorly
understood and intensively debated [4,5].
The aim of this study is the simulation of space
weathering processes by liquid phase pulsed laser
ablation (LP-PLA) on olivine single crystals. We adopt
a multiple technique characterization, taking advantage
of optical spectroscopy analyses and high- resolution
transmission electron microscopy (HR-TEM), to shed
light on the structure and chemical evolution of the
ablated material
Astronomical silicate nanoparticle analogues produced by pulsed laser ablation on olivine single crystals
Silicate nanoparticles, otherwise referred to as very small grains (VSGs) [1], occur in various
astrophysical environments. These grains experience substantial processing (e.g., amorphization)
during their lifetime in the diffuse interstellar medium due to events such as grain-grain collisions
and irradiation [2]. Moreover, several studies have pointed out that the main building blocks of
these silicates are O, Si, Fe, Mg, Al and Ca, all elements that are among the principal constituents of
the Earth’s surface [3], thus leading to the name “astronomical silicates”. However, the structure
and chemical evolution together with the origin of these grains are still poorly understood and
intensively debated [4,5].
The aim of this study is the simulation of space weathering processes on olivine single
crystals by liquid phase pulsed laser ablation (LP-PLA). The study of the resulting structure of both
the target and the ablated material together with their chemical evolution has been carried out by a
multiple technique characterization. In particular, spectroscopy and dynamic light scattering
measurements, analyses of the electrostatic properties and reactivity to acids and bases on the
obtained colloidal solutions of the ablated nanoproducts have been performed and coupled with highresolution transmission electron microscopy (HR-TEM).
Selected olivine target crystals (Fo87) from the SĂŁo Miguel island (Azores) were analyzed
by Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray spectroscopy (EDX). LP-PLA
experiments were performed with a Nd:YAG laser focused via a singlet lens onto the surface of the
target, which was fixed at the bottom of a polystyrene box filled with 4 ml of deionized water (type
1) to immerge it completely. Laser pulses of 5 ns and 100 mJ simulate the timeframe and energy
exchange occurring during grain-grain interstellar collisions [6] and they generate a plasma plume
at the crystal/liquid interface. The rapid cooling induced by the confining liquid layer brings about
the condensation of the chemical vapor it contains with production of a colloidal solution of
nanoparticles. These solutions were analyzed by dynamic light scattering techniques and optical
absorption spectroscopy in the range from 200 nm to 1100 nm (6.20 eV - 1.13 eV). Absorption
measurements on the colloidal solutions have been compared against reference colloidal solutions
dispersed in deionized water (i.e. mesoporous silica [SiO2] nanoparticles, brucite [Mg(OH)2]
nanoparticles, aluminum hydroxide [Al(OH)3] nanoparticles, chrysotile [Mg3Si2O5(OH)4] nanotubes,
and synthetic forsterite [Mg2SiO4] nanoparticles). Moreover, additional absorption analyses have
been carried out as a function of the addition of known aliquots of sulfuric acid and sodium
hydroxide solutions. TEM/EDS analyses were then performed on the ablated nanoparticles deposited
via electrophoresis on C-coated Cu grids and compositional variations of the ablated target were
determined by X-ray photo-emission spectroscopy analyses.
The size distribution of LP-PLA synthesized nanoparticles is typically multimodal due to
aggregation phenomena. Aggregation is consistent with the measured ζ-potential, which is negative
with a relatively low absolute value, within the range 30-50 mV. Nonetheless, a recurrent mode is
centered at about 2 nm (hydrodynamic diameter) and it is consistent with the measured size
distribution obtained by transmission electron microscopy analysis (average nanoparticles diameter
around 3-5 nm). Optical absorption measurements on the ejected material show a main band
around 215 nm. This feature is very similar to the “B2 band” reported in several studies on silica
glass [7] and ascribed to oxygen vacancies, but its nature is still far to be fully understood. We also
found that this feature at 215 nm is very common among both Si and Mg compounds (e.g., Sioxide, Mg-hydroxide, chrysotile). Moreover, additional absorption bands in the range 240-350nm are
observed suggesting the formation of new space weathering products as result of the ablation
process.
Therefore, these results suggest that substantial chemical processing might be expected
during space weathering of “typical” interstellar grains into VSGs. Moreover, coupling these
experimental results with remote sensing datasets will provide fundamental information about the
origin and evolution of these silicate grains
Bypassing the statistical limit of singlet generation in sensitized upconversion using fluorinated conjugated systems
The photon upconversion based on triplet–triplet annihilation (TTA) is a mechanism that converts the absorbed low-energy electromagnetic radiation into higher energy photons also at extremely low excitation intensities, but its use in actual technologies is still hindered by the limited availability of efficient annihilator moieties. We present here the results obtained by the synthesis and application of two new fluorinated chromophores based on phenazine and acridine structures, respectively. Both compounds show upconverted emission demonstrating their ability as TTA annihilator. More interesting, the acridine-based chromophore shows an excellent TTA yield that overcomes the one of some of best model systems. By correlating the experimental data and the quantum mechanical modeling of the investigated compound, we propose an alternative efficient pathway for the generation of the upconverted emissive states involving the peculiar high-energy triplet levels of the dye, thus suggesting a new development strategy for TTA annihilators based on the fine tuning of their high-energy excited states properties
Fast emitting nanocomposites for high-resolution ToF-PET imaging based on multicomponent scintillators
Time-of-Flight Positron Emission Tomography is a medical imaging technique,
based on the detection of two back-to-back {\gamma}-photons generated from
radiotracers injected in the body. Its limit is the ability of employed
scintillation detectors to discriminate in time the arrival of {\gamma}-pairs,
i.e. the coincidence time resolution (CTR). A CTR < 50 ps that would enable
fast imaging with ultralow radiotracer dose. Monolithic materials do not have
simultaneously the required high light output and fast emission
characteristics, thus the concept of scintillating heterostructure is proposed,
where the device is made of a dense scintillator coupled to a fast-emitting
light material. Here we present a composite polymeric scintillator, whose
density has been increased upon addition of hafnium oxide nanoparticles. This
enhanced by +300% its scintillation yield, surpassing commercial plastic
scintillators. The nanocomposite is coupled to bismuth germanate oxide (BGO)
realizing a multilayer scintillator. We observed the energy sharing between its
components, which activate the nanocomposite fast emission enabling a net CTR
improvement of 25% with respect to monolithic BGO. These results demonstrate
that a controlled loading with dense nanomaterials is an excellent strategy to
enhance the performance of polymeric scintillators for their use in advanced
radiation detection and imaging technologies
Design Principles of Hybrid Nanomaterials for Radiotherapy Enhanced by Photodynamic Therapy
Radiation (RT) remains the most frequently used treatment against cancer. The main limitation of RT is its lack of specificity for cancer tissues and the limited maximum radiation dose that can be safely delivered without damaging the surrounding healthy tissues. A step forward in the development of better RT is achieved by coupling it with other treatments, such as photodynamic therapy (PDT). PDT is an anti-cancer therapy that relies on the light activation of non-toxic molecules—called photosensitizers—to generate ROS such as singlet oxygen. By conjugating photosensitizers to dense nanoscintillators in hybrid architectures, the PDT could be activated during RT, leading to cell death through an additional pathway with respect to the one activated by RT alone. Therefore, combining RT and PDT can lead to a synergistic enhancement of the overall efficacy of RT. However, the involvement of hybrids in combination with ionizing radiation is not trivial: the comprehension of the relationship among RT, scintillation emission of the nanoscintillator, and therapeutic effects of the locally excited photosensitizers is desirable to optimize the design of the hybrid nanoparticles for improved effects in radio-oncology. Here, we discuss the working principles of the PDT-activated RT methods, pointing out the guidelines for the development of effective coadjutants to be tested in clinics
Second-Order Photochemical Upconversion in Organic Systems
In order to extend the photon energy
shift of sensitized upconversion
processes based on triplet–triplet annihilation in multicomponent
organic systems, we have demonstrated that it is possible to exploit
a sequence of consecutive upconversion steps. We have therefore realized
an all-optical device for double upconversion: a light blue-shift
of more than 0.9 eV was obtained at an excitation irradiance of a
few tens of milliwatts per square centimeter
Nanodroplet-Containing Polymers for Efficient Low-Power Light Upconversion
Sensitized triplet-triplet-annihilation-based photon upconversion (TTA-UC) permits the conversion of light into radiation of higher energy and involves a sequence of photophysical processes between two dyes. In contrast to other upconversion schemes, TTA-UC allows the frequency shifting of low-intensity light, which makes it particularly suitable for solar-energy harvesting technologies. High upconversion yields can be observed for low viscosity solutions of dyes; but, in solid materials, which are better suited for integration in devices, the process is usually less efficient. Here, it is shown that this problem can be solved by using transparent nanodroplet-containing polymers that consist of a continuous polymer matrix and a dispersed liquid phase containing the upconverting dyes. These materials can be accessed by a simple one-step procedure that involves the free-radical polymerization of a microemulsion of hydrophilic monomers, a lipophilic solvent, the upconverting dyes, and a surfactant. Several glassy and rubbery materials are explored and a range of dyes that enable TTA-UC in different spectral regions are utilized. The materials display upconversion efficiencies of up to approximate to 15%, approaching the performance of optimized oxygen-free reference solutions. The data suggest that the matrix not only serves as mechanically coherent carrier for the upconverting liquid phase, but also provides good protection from atmospheric oxygen
Spectral converters for photovoltaics – what’s ahead
The effective incorporation of photovoltaic (PV) elements into the urban environment, specifically in buildings, is a challenging process. Apart from the aesthetic limitations of having black and opaque PV cells when installed in building façades, they are not optimally positioned for high-efficiency electric energy generation because of shadowing effects due to neighboring trees and buildings. Emphasis is given to luminescent solar concentrators (LSCs) that have emerged as an appealing solution for concentrating a large area of sunlight into a small beam of high field intensity. The coupling of PV cells to the edges of LSCs also presents an exciting strategy to PV urban integration. Here, we outline the mechanistic framework for LSCs, review the current experimental state of the art involving optically active centers in various geometrical device configurations, and discuss the performance quantification of LSCs currently in development. For the sake of completeness, a brief discussion of the other spectral converters suitable for PV applications is also reported.publishe