Search

Radiation detection is of utmost importance in fundamental scientific research, as well as medical diagnostics, homeland security, environmental monitoring and industrial control. Lead halide perovskites (LHPs) are attracting growing attention as high-atomic-number materials for next-generation scintillators and photoconductors for ionizing radiation detection. To unlock their full potential as reliable and cost-effective alternatives to conventional materials, it is necessary for LHPs to conjugate high scintillation yields with emission stability under high doses of ionizing radiation. To date, no definitive solution has been devised to optimize the scintillation efficiency and kinetics of LHPs and nothing is known of their radiation hardness for doses above a few kilograys, to the best of our knowledge. Here we demonstrate that CsPbBr3nanocrystals exhibit exceptional radiation hardness for γ-radiation doses as high as 1 MGy. Spectroscopic and radiometric experiments highlight that despite their defect tolerance, standard CsPbBr3nanocrystals suffer from electron trapping in dense surface defects that are eliminated by post-synthesis fluorination. This results in >500% enhancement in scintillation efficiency, which becomes comparable to commercial scintillators, and still retaining exceptional levels of radiation hardness. These results have important implications for the widespread use of LHPs in ultrastable and efficient radiation detectors.

A prompt transport of radiation induced free carriers towards luminescence centers is a key factor for an efficient conversion of high energy radiation into light in scintillator materials. However the transport stage of the scintillation process can be hampered by the presence of lattice imperfections. In this study, we investigated the increase of radio-luminescence efficiency after prolonged material irradiation. The general character of the phenomenon was revealed by its occurrence in several scintillator crystals like Lu2Si2O7:Pr, Bi3Ge4O12, Lu3Al5O12:Ce, and Lu0.3Y0.7AlO3:Ce. A detailed investigation was carried out for Lu2Si2O7:Pr. We demonstrated that the RL efficiency increase is due to the progressive filling of traps responsible for the thermoluminescence glow peaks at 460 and 515 K, which compete with Pr3+ centers in free carrier trapping during irradiation. Correspondingly, their emptying results in the recovery of the initial lowest RL efficiency. Spatial correlation between traps and Pr3+ ions was evidenced by the detection of an a-thermal tunneling afterglow. Such hysteresis phenomenon represents a memory of previous irradiations which remains stored in the crystal. It is a further effect caused by traps, which deserves attention like other manifestations of defects in scintillators like the reduction of light yield and the occurrence of slow tails in the scintillation time decay.

This paper presents a detailed study of the influence of yttrium (Y) concentration on thermally stimulated luminescence (TSL) of LYSO:Ce crystals and establishes the relationship between electron traps and scintillation properties. With the increase of Y concentration, the glow curve shifts toward higher temperatures and the trap depth increases gradually. Our results revealed two groups of electron traps with one shallow (Et≈ 0.10–0.26 eV) and one deep (Et≈ 1.00–1.21 eV). Our TSL results suggest that the amount of shallow traps varied with Y content has a positive correlation with energy resolution, while the amount of deep traps is inversely proportional to light output. In order to analyze the variation trend of trap depths and their mechanism in detail, we calculated the band gap with the PBE functional and VUV excitation spectra. The results show that the increase of band gap possibly leads to the increase of deep electron trap depth.

We report here the synthesis of undoped and Cu-doped Cs2ZnCl4nanocrystals (NCs) in which we could tune the concentration of Cu from 0.7 to 7.5%. Cs2ZnCl4has a wide band gap (4.8 eV), and its crystal structure is composed of isolated ZnCl4tetrahedra 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 Cs2ZnCl4NCs 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 Cs2ZnCl4NCs, 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.

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 CsPbBr3perovskite 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.

The present study aims at supplying a more in-depth picture of the generation of environmentally persistent free radicals (EPFRs) from phenol (PhOH) on ZnO/SiO2systems, by exploring the properties of ZnO nanoparticles (NPs) with different intrinsic defectivity grown on highly porous silica with a spherical (ZnO/SiO2_S) and wormlike morphology (ZnO/SiO2_W). In detail, besides an extensive structural, morphological, and surface investigation, the occurrence of inequivalent defect centers in the samples was tracked by photoluminescence (PL) experiments, which unveiled, for ZnO/SiO2_W, intense blue emissions possibly involving radiative recombination from Zniexcited levels to the valence band or to VZnlevels. Electron spin resonance (ESR) spectra corroborated these results and revealed a remarkably different behavior of the samples in the EPFR formation model reaction. In fact, upon PhOH contact, the ESR spectrum of ZnO/SiO2_S showed the exclusive presence of a weak isotropic signal ascribable to a PhenO•EPFR. Instead, for ZnO/SiO2_W, intense features associated with oxygen species in proximity of VO+, VZn–, and (VZn–)2–centers dominate the spectra, while a minor contribution of the PhenO•radical can be discovered only by signal simulation. These outcomes definitively envisage a role of the intrinsic defectivity of ZnO NPs on the final yield and stability of EPFR generation, with VO+and VZn–defects possibly involved in dissociative adsorption or oxidation processes at the oxide surface. Although this work focuses on ZnO, it is expected to foster a critical re-examination and integration of important results on other metal oxide/silica systems already reported in the literature, offering the chance to better evaluate the dependence of EPFR generation on the oxide defects chemistry.

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.

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–vand ii–vinanostructures, introducing intragap electronic states optically coupled to the host conduction band. With its full 4dshell 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. A comprehensive study of excitonic processes in Ag-doped CdSe nanocrystals shows the conversion of nonmagnetic Ag+dopants into paramagnetic Ag2+.

Point defects in the host lattice of a scintillator material can trap carriers, slowing down their migration or even preventing their transfer to luminescent centers. Such competition schemes between defects and luminescent centers may explain also the hysteresis effect, which consists of a progressive enhancement of scintillation efficiency with accumulated dose. We propose a comparison between the scintillation hysteresis effect of Yb-doped sol–gel silica glasses in bulk and fiber forms, and we correlate them with traps monitored by wavelength-resolved thermally stimulated luminescence in both materials. The results demonstrate that the fiber-drawing process is responsible for modifications of the defectiveness of the glass network, with a change of the local distribution of the traps surrounding the luminescent center. The consequence of such modifications is the removal, in the fiber samples, of the thermally stimulated luminescence peak ascribed to traps closer to Yb ions and unstable at room temperature. We highlight that suitable postdensification thermal treatments can significantly modify the concentration and spatial distribution of defects around a luminescent center and can therefore be used as a tool for the engineering of scintillating glasses.

A series of luminescent rare-earth ion-doped hexagonal II-type Gd oxycarbonate phosphors Gd2–xRExO2CO3(RE = Eu3+, Yb3+, Dy3+) have been successfully synthesized by thermal decomposition of the corresponding mixed oxalates. The Yb3+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.

The role of charge carrier trapping in determining radioluminescence (RL) efficiency increase during prolonged irradiation of scintillators has been studied by using YPO4:Ce,Nd as a model material. The Nd3+ions act as efficient electron traps minimizing the role of intrinsic defects. Different Nd contents were considered in order to point out the correlation between the trap concentration and the detected RL efficiency dose dependence. RL measurements as a function of temperature clarified the role of the trap thermal stability in determining the shape and the magnitude of such effect. We propose also a model based on trap filling which is able to describe accurately the complex processes which are involved.

The problem of Eu incorporation into silica as dispersed dopants, clusters, separate-phase nanoparticles, or nanocrystals, which is of key importance for applications in the fields of lasers and scintillators, is faced by applying to sol–gel silica doped with nine different Eu3+concentrations (0.001–10 mol % range) various spectroscopic techniques, including crystal field and vibrational mode analysis by means of Fourier transform absorption and microreflectivity (in the 200–6000 cm–1and 9–300 K ranges), radioluminescence, and Raman scattering studies at 300 K. The variety of methods revealed the following concordant results: (1) amorphous Eu clusters grow when the Eu concentration is increased up to 3 mol % and (2) Si–OH groups are completely removed and ordered phase separation occurs at 10 mol % doping, as suggested by the remarkable narrowing of the spectral lines. Comparison with polycrystalline Eu oxide, Eu silicates, and α-quartz spectra allowed the unequivocal identification of Eu2Si2O7pyrosilicate and α-quartz as the main components of nanocrystals in 10 mol % Eu-doped silica. Such conclusions were brilliantly confirmed by transmission electron microscopy and electron diffraction analysis. Phonon coupling and anharmonicity were analyzed and are discussed for a few vibrational modes of nanocrystals.

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 HfO2cubic polymorph for dopant concentrations exceeding a threshold value of nominal 5 mol %, for either Lu3+or Eu3+. The optical properties of the nanopowders were investigated by room temperature radio- and photoluminescence experiments. Specific features of Eu3+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 Lu3+ions. The peculiar properties arising from the solvothermal nonaqueous synthesis here used are described through the comparison with thermally treated powders.

Oxygen plasmas, commonly used in typical C-MOS process flow, cause silicon oxide structure to be significantly modified: as a consequence wet processes, that are sensitive to oxide defectivity, need to be retargeted. In this paper structure modifications, induced in boron implanted SiO2 by RF and TCP plasma have been studied by thermally stimulated luminescence, charging measurements and Electron Paramagnetic Resonance (EPR). Different kind and quantity of point defects have been identified by thermally stimulated luminescence technique as a function of plasma generator. These results match with EPR analysis, charging measurements and HF etches rate profile.

Lutetium aluminum garnet, commonly known as LuAG, is a versatile material finding numerous optical applications, and whose scintillation properties have been widely investigated. In this study, experimental evidences of optically stimulated luminescence (OSL) phenomena occurring in LuAG:Ce crystals are shown. After demonstrating the possibility to optically bleach an electron trap responsible for a thermally stimulated luminescence (TSL) peak at 280 °C (heating rate 1.7 °C s−1) using red light, a direct measurement of the OSL decay curve and of the resulting characteristic Ce3+5d‐4f emission spectrum is carried out. The assessment of the bleaching spectrum of the TSL peak at 280 °C reveals that light with wavelength in the 600–650 nm interval is the most efficient in depleting the TSL peak, making He‐Ne lasers suitable sources of optical stimulation. The finding of OSL features in LuAG:Ce paves the way to new application perspectives. In particular, the achievement of a good balance between scintillation and energy storage properties of LuAG.Ce can drive a constructive integration between X‐rays imaging approaches based on photo‐stimulable phosphors and those exploiting scintillating materials. Evidence of the optically stimulated luminescencein Lu3Al5O12:Ce (LuAG:Ce) crystals under red light illumination. An electron trap responsible for a thermally stimulated luminescence peak at 280 °C is efficiently depleted by light exposure, resulting in the characteristic Ce3+emission.

Oxygen plasmas, commonly used in typical C-MOS process flow, cause silicon oxide structure to be significantly modified: as a consequence wet processes, that are sensitive to oxide defectivity, need to be retargeted. In this paper structure modifications, induced in boron implanted SiO2 by RF and TCP plasma have been studied by thermally stimulated luminescence, charging measurements and Electron Paramagnetic Resonance (EPR). Different kind and quantity of point defects have been identified by thermally stimulated luminescence technique as a function of plasma generator. These results match with EPR analysis, charging measurements and HF etches rate profile.