Photo- and radio-luminescence of porphyrin functionalized ZnO/SiO2 nanoparticles.

The development of hybrid nanoscintillators is hunted for the implementation of modern detection technologies, like in high energy physics, homeland security, radioactive gas sensing, and medical imaging, as well as of the established therapies in radiation oncology, such as in X-ray activated photodynamic therapy. Engineering of the physico-chemical properties of nanoparticles (NPs) enables the manufacture of hybrids in which the conjugation of inorganic/organic components leads to increased multifunctionality and performance. However, the optimization of the properties of nanoparticles in combination with the use of ionizing radiation is not trivial: a complete knowledge on the structure, composition, physico-chemical features, and scintillation property relationships in hybrid nanomaterials is pivotal for any applications exploiting X-rays. In this paper, the design of hybrid nanoscintillators based on ZnO grown onto porous SiO2 substrates (ZnO/SiO2) has been performed in the view to create nanosystems potentially suitable in X-ray activated photodynamic therapy. Indeed, cytotoxic porphyrin dyes with increasing concentrations have been anchored on ZnO/SiO2 nanoparticles through amino-silane moieties. Chemical and structural analyses correlated with photoluminescence reveal that radiative energy transfer between ZnO and porphyrins is the principal mechanism prompting the excitation of photosensitizers. The use of soft X-ray excitation results in a further sensitization of the porphyrin emission, due to augmented energy deposition promoted by ZnO in the surroundings of the chemically bound porphyrin. This finding unveils the cruciality of the design of hybrid nanoparticles in ruling the efficacy of the interaction between ionizing radiation and inorganic/organic moieties, and thus of the final nanomaterial performances towards the foreseen application.

Morphology Related Defectiveness in ZnO Luminescence: From Bulk to Nano-Size.

This study addresses the relationship between material morphology (size, growth parameters and interfaces) and optical emissions in ZnO through an experimental approach, including the effect of different material dimensions from bulk to nano-size, and different excitations, from optical sources to ionizing radiation. Silica supported ZnO nanoparticles and ligand capped ZnO nanoparticles are synthesized through a sol-gel process and hot injection method, respectively. Their optical properties are investigated by radioluminescence, steady-state and time-resolved photoluminescence, and compared to those of commercial micrometric powders and of a bulk single crystal. The Gaussian spectral reconstruction of all emission spectra highlights the occurrence of the same emission bands for all samples, comprising one ultraviolet excitonic peak and four visible defect-related components, whose relative intensities and time dynamics vary with the material parameters and the measurement conditions. The results demonstrate that a wide range of color outputs can be obtained by tuning synthesis conditions and size of pure ZnO nanoparticles, with favorable consequences for the engineering of optical devices based on this material.

Efficient, fast and reabsorption-free perovskite nanocrystal-based sensitized plastic scintillators.

The urgency for affordable and reliable detectors for ionizing radiation in medical diagnostics, nuclear control and particle physics is generating growing demand for scintillator devices combining efficient scintillation, fast emission lifetime, high interaction probability with ionizing radiation and mitigated reabsorption losses in large-volume/high-density detectors. To date, the simultaneous achievement of all such features is still an open challenge. Here we realize this regime with poly(methyl methacrylate) nanocomposites embedding CsPbBr3 perovskite nanocrystals as sensitizers for a conjugated organic dye featuring a large Stokes shift and a fast emission lifetime in the red spectral region. Complete energy transfer from the nanocrystals to the dye under both X-rays and α-particle excitation results in highly stable radioluminescence with an efficiency comparable to that of commercial-grade inorganic and plastic scintillators; an ~3.4 ns emission lifetime, competitive with fast lanthanide scintillators; and reabsorption-free waveguiding for long optical distances.

Bright Blue Emitting Cu-Doped Cs2ZnCl4 Colloidal Nanocrystals.

We report here the synthesis of undoped and Cu-doped Cs2ZnCl4 nanocrystals (NCs) in which we could tune the concentration of Cu from 0.7 to 7.5%. Cs2ZnCl4 has a wide band gap (4.8 eV), and its crystal structure is composed of isolated ZnCl4 tetrahedra surrounded by Cs+ cations. According to our electron paramagnetic resonance analysis, in 0.7 and 2.1% Cu-doped NCs the Cu ions were present in the +1 oxidation state only, while in NCs at higher Cu concentrations we could detect Cu(II) ions (isovalently substituting the Zn(II) ions). The undoped Cs2ZnCl4 NCs were non emissive, while the Cu-doped samples had a bright intragap photoluminescence (PL) at ∼2.6 eV mediated by band-edge absorption. Interestingly, the PL quantum yield was maximum (∼55%) for the samples with a low Cu concentration ([Cu] ≤ 2.1%), and it systematically decreased when further increasing the concentration of Cu, reaching 15% for the NCs with the highest doping level ([Cu] = 7.5%). The same (∼2.55 eV) emission band was detected under X-ray excitation. Our density functional theory calculations indicated that the PL emission could be ascribed only to Cu(I) ions: these ions promote the formation of trapped excitons, through which an efficient emission takes place. Overall, these Cu-doped Cs2ZnCl4 NCs, with their high photo- and radio-luminescence emission in the blue spectral region that is free from reabsorption, are particularly suitable for applications in ionizing radiation detection.

Radiation hardness of Ce-doped sol-gel silica fibers for high energy physics applications.

The results of irradiation tests on Ce-doped sol-gel silica using x- and γ-rays up to 10 kGy are reported in order to investigate the radiation hardness of this material for high-energy physics applications. Sol-gel silica fibers with Ce concentrations of 0.0125 and 0.05 mol. % are characterized by means of optical absorption and attenuation length measurements before and after irradiation. The two different techniques give comparable results, evidencing the formation of a main broad radiation-induced absorption band, peaking at about 2.2 eV, related to radiation-induced color centers. The results are compared with those obtained on bulk silica. This study reveals that an improvement of the radiation hardness of Ce-doped silica fibers can be achieved by reducing Ce content inside the fiber core, paving the way for further material development.

Excitonic pathway to photoinduced magnetism in colloidal nanocrystals with nonmagnetic dopants.

Electronic doping of colloidal semiconductor nanostructures holds promise for future device concepts in optoelectronic and spin-based technologies. Ag+ is an emerging electronic dopant in III-V and II-VI nanostructures, introducing intragap electronic states optically coupled to the host conduction band. With its full 4d shell Ag+ is nonmagnetic, and the dopant-related luminescence is ascribed to decay of the conduction-band electron following transfer of the photoexcited hole to Ag+. This optical activation process and the associated modification of the electronic configuration of Ag+ remain unclear. Here, we trace a comprehensive picture of the excitonic process in Ag-doped CdSe nanocrystals and demonstrate that, in contrast to expectations, capture of the photohole leads to conversion of Ag+ to paramagnetic Ag2+. The process of exciton recombination is thus inextricably tied to photoinduced magnetism. Accordingly, we observe strong optically activated magnetism and diluted magnetic semiconductor behaviour, demonstrating that optically switchable magnetic nanomaterials can be obtained by exploiting excitonic processes involving nonmagnetic impurities.

Real-time dosimetry with Yb-doped silica optical fibres.

Over the years, many efforts have been made to develop radiation detectors to handle the complex issues of small field dosimetry and achieve the increasing accuracy, precision and in vivo dose monitoring required by the new advanced treatment modalities. In this context, interest has surged in the development of sensors based on scintillating optical fibres. In this paper, the near-infrared radioluminescence and dosimetric properties of Yb-doped silica optical fibres, coupled with a laboratory prototype based on an avalanche photodiode, were studied by irradiating the fibres with photons and electron beams generated by a Varian Trilogy accelerator. The performance of the system in standard and small field sizes has also been investigated, comparing the output factor, percentage depth dose and off-axis ratio measurements of the prototypal detector with other commercial sensors, including the Exradin W1 scintillator. The results of this study demonstrate that the drawback due to the stem effect in Yb-doped silica optical fibres can be managed in a simple but effective way by optical filtering. The robustness of the system in complex dosimetric scenarios and the accuracy and precision achieved by Yb-doped fibres in relative dose assessments suggest an effective use of the system for real-time in vivo dosimetry applications.

NIR persistent luminescence of lanthanide ion-doped rare-earth oxycarbonates: the effect of dopants.

A series of luminescent rare-earth ion-doped hexagonal II-type Gd oxycarbonate phosphors Gd2-xRExO2CO3 (RE = Eu(3+), Yb(3+), Dy(3+)) have been successfully synthesized by thermal decomposition of the corresponding mixed oxalates. The Yb(3+) doped Gd-oxycarbonate has evidenced a high persistent luminescence in the NIR region, that is independent from the temperature and makes this materials particular attractive as optical probes for bioimaging.

Multifunctional role of rare earth doping in optical materials: nonaqueous sol-gel synthesis of stabilized cubic HfO2 luminescent nanoparticles.

In this work a strategy for the control of structure and optical properties of inorganic luminescent oxide-based nanoparticles is presented. The nonaqueous sol-gel route is found to be suitable for the synthesis of hafnia nanoparticles and their doping with rare earths (RE) ions, which gives rise to their luminescence either under UV and X-ray irradiation. Moreover, we have revealed the capability of the technique to achieve the low-temperature stabilization of the cubic phase through the effective incorporation of trivalent RE ions into the crystal lattice. Particular attention has been paid to doping with europium, causing a red luminescence, and with lutetium. Structure and morphology characterization by XRD, TEM/SEM, elemental analysis, and Raman/IR vibrational spectroscopies have confirmed the occurrence of the HfO2 cubic polymorph for dopant concentrations exceeding a threshold value of nominal 5 mol %, for either Lu(3+) or Eu(3+). The optical properties of the nanopowders were investigated by room temperature radio- and photoluminescence experiments. Specific features of Eu(3+) luminescence sensitive to the local crystal field were employed for probing the lattice modifications at the atomic scale. Moreover, we detected an intrinsic blue emission, allowing for a luminescence color switch depending on excitation wavelength in the UV region. We also demonstrate the possibility of changing the emission spectrum by multiple RE doping in minor concentration, while deputing the cubic phase stabilization to a larger concentration of optically inactive Lu(3+) ions. The peculiar properties arising from the solvothermal nonaqueous synthesis here used are described through the comparison with thermally treated powders.

Thermo-stimulated luminescence of x-ray- and alpha-irradiated yttria-stabilized zirconia.

Yttria-stabilized zirconia (ZrO2 : Y3+) single crystals (with 9.5 mol% Y2O3) were irradiated with x-rays and α particles. Thermally stimulated luminescence (TSL) data show a main broad peak centred at ∼500-550 K in the glow curves of all irradiated samples. The TSL peak maximum temperature is consistent with the characteristic recovery temperature (∼450 K) of colour centres (T centres) deduced from isochronal annealing curves measured by electron paramagnetic resonance (EPR) spectroscopy. However, the trap-depth energies (ranging between 0.8 and 1.2 eV) deduced from the initial rise of partially cleaned TSL peaks (and from a rough approximation using Urbach's formula) are much larger than the activation energies for defect recovery of 0.3 eV deduced from the EPR data. A second TSL peak centred at ∼350-450 K found in freshly irradiated samples is seen to decay substantially in aged samples. The processes involved in TSL are discussed in relation to the defect annealing processes, and available defect-level energy and TSL data.

Thermo-stimulated luminescence of ion-irradiated yttria-stabilized zirconia.

Yttria-stabilized zirconia (ZrO(2):Y(3+)) single crystals (with 9.5 mol% Y(2)O(3)) were irradiated with ions (from 1 MeV He to 2.6 GeV U). Electron paramagnetic resonance (EPR) data show that two kinds of colour centres (F(+)-type and T centres) are produced. Thermo-stimulated luminescence (TSL) data exhibit a quite strong peak at ∼ 500-550 K in the glow curves of all irradiated samples regardless of the ion species and energy. Moreover, the 3D-TSL measurements reveal that this peak is correlated with a light emission at a wavelength of ∼ 620 nm (i.e. photon energy ∼ 2 eV). The TSL peak maximum temperatures are consistent with characteristic temperatures of about 500 K of annealing stages of colour centres. However, the trap-depth energies (ranging between 0.7 and 1.4 eV) deduced from the initial rise of partially cleaned TSL peaks, or from a rough approximation using Urbach's formula, are rather larger than the activation energies for defect recovery, ranging between 0.3 and 0.7 eV, as deduced from the EPR data. The processes involved in TSL are discussed in relation to available photoluminescence and defect energy-level data.