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Synthetic procedures to obtain size and shape-controlled microparticles hold great promise to achieve structural control on the microscale of macroscopic ceramic- or composite-materials. Lutetium oxide is a material relevant for scintillation due to its high density and the possibility to dope with rare earth emitter ions. However, rare earth sesquioxides are challenging to synthesise using bottom-up methods. Therefore, calcination represents an interesting approach to transform lutetium-based particles to corresponding sesquioxides. Here, the controlled solvothermal synthesis of size-tuneable europium doped Lu(OH)(2)Cl microplatelets and their heat-induced transformation to Eu:Lu2O3 above 800 degrees C are described. The particles obtained in microwave solvothermal conditions, and their thermal evolution were studied using powder X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), optical microscopy, thermogravimetric analysis (TGA), luminescence spectroscopy (PL/PLE) and infrared spectroscopy (ATR-IR). The successful transformation of Eu:Lu(OH)(2)Cl particles into polycrystalline Eu:Lu2O3 microparticles is reported, together with the detailed analysis of their initial and final morphology., Crystals, 11 (8), ISSN:2073-4352

Luminescent nanoparticles are researched for their potential impact in medical science, but no materials approved for parenteral use have been available so far. To overcome this issue, we demonstrate that Eu3+-doped hafnium dioxide nanocrystals can be used as non-toxic, highly stable probes for cellular optical imaging and as radiosensitive materials for clinical treatment. Furthermore, viability and biocompatibility tests on artificially stressed cell cultures reveal their ability to buffer reactive oxygen species, proposing an anti-cytotoxic feature interesting for biomedical applications.

International audience; The recent trends in scintillator technologies stimulate research efforts toward the development of novel materials morphologies, such as nanoparticles, able to efficiently convert ionizing radiations into light. For example, scintillating nanoparticles attract great interest in medical oncological therapies. In this work, the structural and morphological properties of HfO2:Ti nanoparticles with Ti concentrations from 0.03 to 10 mol% and subjected to calcination up to 1000 °C are thoroughly characterized; moreover, X‐ray photoelectron spectroscopy reveals the incorporation of Ti in both Ti (III) and Ti (IV) chemical states in as prepared samples, while the exclusive presence of Ti(IV) is unambiguously identified in calcined nanoparticles. The optical emission under X‐ray excitation evidences an intense Ti (IV)‐related luminescence at 2.5 eV in high temperature calcined samples with a few microseconds scintillation lifetime, and efficiency comparable to that of Bi4Ge3O12 reference scintillator. Finally, the competitive role of defects in charge carriers capture is demonstrated by the monotonic increase of the 2.5 eV band during prolonged X‐ray irradiation, more evident for nanoparticles with titanium concentration below 1 mol%. HfO2:Ti may also find application in X‐ray triggered oncological therapies by using the Ti (IV)‐related bright radioluminescence to excite photosensitizer molecules for singlet oxygen generation.

Defect engineering operated on metal oxides by chemical and structural modifications may strongly affect properties suitable for various applications such as photoelectrochemical behavior, charge transport, and luminescence. In this work, we report the tunable optical features observed in undoped monoclinic HfO2 nanocrystals and their dependence on the structural properties of the material at the nanoscale. Transmission electron microscopy together with X-ray diffraction and surface area measurements were used to determine the fine structural modifications, in terms of crystal growth and coalescence of crystalline domains, occurring during a calcination process in the temperature range from 400 to 1000 °C. The fit of the broad optical emission into spectral components, together with time-resolved photoluminescence, allowed us to identify the dual nature of the emission at 2.5 eV, where an ultrafast defect-related intrinsic luminescence (with a decay time of a few nanoseconds) overlaps with a slower emission (decay of several microseconds) due to extrinsic Ti-impurity centers. Moreover, the evolution of intrinsic visible bands during the material transformation was monitored. The relationship between structural parameters uniquely occurring in nanosized materials and the optical properties was investigated and tentatively modeled. The blue emissions at 2.5 and 2.9 eV are clearly related to defects lying at crystal boundaries, while an unprecedented emission at 2.1 eV enables, at relatively low calcination temperatures, the white luminescence of HfO2 under near-UV excitation.

Here a sustainable approach for the bottom-up fabrication of luminescent carbon dots (CDs) through a green synthetic route by using polymeric waste as a single precursor material is developed. Polylactide has been selected as a carbon-containing model polymeric waste because of its remarkable presence in the packaging field and its associated “white-pollution”. Spherical CDs with diameters of 3 nm have been synthesized through a one-pot hydrothermal reaction of water-dissolved polylactide oligomers (Mn = 1.200–1.900 g mol−1) at mild temperatures. Chemical, structural, morphological and optical properties of the synthesized CDs have been studied. Results reveal that CDs consist of a carbonaceous nucleus having a sp2 conformation with carboxyl and hydroxyl surface groups, making it dispersible in polar media. The optical features of obtained CDs depend on reaction conditions (time and temperature) and on the insertion of nitrogen groups within the CD structure. A proof of concept study of the application of the proposed approach for the production of luminescent CDs from commercial PLA lids for paper hot cups is also provided. The route here reported provides novel cues for a catalyst-free and organic-solvent-free industrial-scale fabrication of photoluminescent nanomaterials from actual polymeric waste as a viable non-toxic and cheap alternative to quantum dots and rare earth based nanophosphors.

International audience; In this work, we investigate the optical properties of hafnium dioxide nanocrystals, upon X-ray irradiation, looking for spectral evolution following thermal treatments in air up to 1000 °C that modify the crystal size as well as their point defect concentrations. Radio-luminescence measurements from 10 K up to room temperature reveal a rich and evolving picture of the optical features. A complete spectral analysis of the broad luminescence spectra reveals the presence of several emission components in the visible and UV regions. The lower energy components peaking at 2.1, 2.5, and 2.9 eV are characterized by a thermal quenching energy of 0.08 eV, while the corresponding value for the UV bands at 4.1 and 4.7 eV is close to 0.23 eV. We tentatively assign the components ranging from 2 to 3 eV to the presence of optically active defects of an intrinsic nature, together with the occurrence of titanium impurities; conversely, the bands at higher energies are likely to be of an excitonic nature. The comparison with previous photo-luminescence studies allows evidencing characteristic differences between the features of luminescence emissions caused by intra-centre excitation and those occurring under ionizing irradiation. Finally, scintillation measurements in the visible range reveal the existence of a fast decay in the nanosecond time scale for the smallest hafnia nanocrystals. This study offers a clear description of HfO2 luminescence characteristics upon excitation by X-rays and can lead to a better comprehension of the structure–property relationship at the nanoscale in metal oxides.

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−1 and 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 Eu2Si2O7 pyrosilicate 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 HfO2 cubic 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. © 2013 American Chemical Society.

Structural and optical properties of sol–gel silica based glassceramics doped with 0.1 mol% Ce and codoped with Gd at high concentrations, from 5 mol% up to 40 mol%, are investigated and compared to those of analogous samples doped only with Ce. Raman scattering, transmission electron microscopy, and x-ray diffraction reveal the formation of Gd apatite-like silicate (Gd4.67O(SiO4)3) and of Gd pyrosilicate (Gd2Si2O7) nanophases whose morphology and crystallinity depend on the Gd concentration and thermal treatments. Optical absorption measurements demonstrate the role of the densification atmosphere in modifying the charge state of Ce ions. The incorporation of Ce3+ ions in the nanophases is put in evidence by photo- and radio-luminescence results.

We measured temperature dependences of emission spectra, prompt nanosecond decays and millisecond delayed recombination decays for the Ce3+ center in the SrHfO3 (SHO) host. We characterized two samples with 0.1% and 5Î prepared by the acetate and citrate combustion and solid state reaction, respectively. The decrease of nanosecond decay times above approximately 220 and 260 K, respectively, is due to thermal ionization of 5d1 excited state of the Ce3+ center. Energy barrier of 300 meV, governing this process, is calculated for both the 0.1% and 5Î:SHO samples. Thermoluminescence glow curve measurement after X and UV (310 nm) irradiation at RT provides another support for the 5d1 excited-state ionization of Ce3+ in the SHO host. © 2010 Elsevier B.V. All rights reserved.

A set of terbium-doped silicate glasses densified up to 1050 °C was prepared by the sol-gel method. A broad range of dopant concentrations (0.001-10 mol%) was considered together with an undoped sample as a reference. The effects of a high-temperature (1600-1800 °C) post-densification rapid thermal treatment (RTT) on the luminescence properties of the samples were investigated. RTT enhances the scintillating efficiency of the samples increasing the radio-luminescence intensity up to two orders of magnitude. The relevant effects induced by RTT on radio- and photo-luminescence spectra (RL, PL) and on PL decay curves indicate the occurrence of modifications of the surroundings of Tb3 + luminescence centres. © 2007 Elsevier Ltd. All rights reserved.

We investigate the effects of erbium doping on SnO(2) nanoclustering in Sn-doped silica. Vibrational spectroscopy data from Raman and infrared absorption measurements show nanostructuring effects on the SnO(2) nanophase. Ultraviolet absorption spectra evidence a gap shift ascribable to size-dependent quantum confinement, also suggesting a role of erbium doping in determining cluster sizes and the amount of localized states on the nanophase boundary. Transmission electron microscopy confirms and details the spectroscopic data. As a result of these measurements, we find that the nanocrystal size distribution becomes narrower, increasing the erbium concentration, while the density of localized states at the nanocrystal surface decreases. The distribution of erbium ions among the possible environments is then examined through simultaneous spectroscopy of luminescence excited by nanocrystal-to-erbium energy transfer and the absorption of nanocrystal luminescence by erbium ions. This analysis shows that erbium behaves as an extrinsic nucleation centre of the SnO(2) nanophase at low doping levels, whereas at high concentrations it modifies the matrix, hindering the growth of SnO(2) crystals and passivating the interface.

Sn-doped silica and SnO2:SiO2 glass-ceramics were produced by sol-gel synthesis. Charge trapping and radiative recombinations from ionizing irradiation were investigated in the 8-300 K range. Three radioluminescence (RL) emissions were observed at 1.9, 2.3 and 3.2eV. Dose-dependence of RL shows that the 3.2eV emission grows during the irradiation. 2.3eV RL is the main process at low temperature, whereas it is a minor decay at 300K, as well as in phosphorescence and thermoluminescence. Glow curves show peaks at 80 and 200K. The results suggest that radiative decays and trapping processes occur in the silica network. © 2004 Elsevier B.V. All rights reserved.

We study trapping states of Ce 3+ -doped SrHfO 3 by thermally stimulated luminescence in a wide temperature range (10–730 K). We determine characteristic parameters of the traps by the initial rise technique. We also determine the energy of thermal ionization of the excited state of Ce 3+ in SrHfO 3 host by purely optical method based on the study of UV illumination-induced thermoluminescence. The method provides a value of the thermal ionization energy of about 0.25 eV.

We study lutetium hafnate, Lu4Hf3O12, prepared by acetate and citrate combustion in the form of nanometric powders. Optical properties including X-ray excited luminescence, steady-state and time resolved photoluminescence as well as thermally stimulated luminescence are investigated for undoped as well as Eu3+ and Tb3+ doped samples. We identify recombination centers in the host matrix and we tentatively associate the principal emission with a radiative transition of the F + center. We also demonstrate that athermal tunneling of charge carriers between traps and recombination centers is the predominant mechanism of delayed radiative recombination following irradiation by ionizing radiation. © 2011 Elsevier B.V. All rights reserved.

To better understand the scintillation mechanism in SrHfO3-based scintillators we investigate in detail their x-ray excited radioluminescence (RL) emissions. The Gaussian decomposition of the undoped SrHfO3 RL spectra manifests the presence of four emission bands peaking at 3.0, 4.0, 4.6, and 5.1 eV. Based on corresponding photoluminescence emission and excitation spectra we tentatively associate the observed bands with excitonic or defect-related emissions. When doped by Ce3+ or Pb2+ luminescent activators, the RL spectra feature two or three new bands, respectively. The temperature dependence of all RL spectra demonstrates a competition between activators and intrinsic or defect centers in carrier capture. Around room temperature the emission process is completely dominated by recombination at activator centers. In the case of Pb2+ activator we also study in detail the concentration dependence of the RL spectra below room temperature exhibiting an optimal value around 0.3 mol %.

We use acetate–citrate combustion to prepare SrHfO3 powders both undoped and doped with Ce3+ and Pb2+ ions in several concentrations, from 0.01 up to 3 mol%. The evolution of their morphological and structural features during the sintering process is studied by scanning electron microscopy, X-ray diffraction, and Raman experiments. We discuss the dependence of the optical properties, investigated by radioluminescence and time resolved luminescence, on sintering conditions and material structure.

We bring into consideration a novel, potentially promising scintillating material, Pb-doped SrHfO3. The structural and optical properties of a series of Pb-doped SrHfO3 powders prepared by acetate and citrate combustion are investigated and compared to those of undoped and Ce-doped powders with the same composition. Room temperature radioluminescence studies as a function of sintering temperature show that samples sintered up to the highest final temperature (1150 degrees C) display the highest light output. Moreover, Pb-doped SrHfO shows a higher radioluminescence efficiency with respect to Ce-doped SrHfO3, which in turn features a very fast photoluminescence decay time of 15 ns. The presence of point defects and their role in carrier trapping are studied by thermally stimulated luminescence as well as by radioluminescence as a function of temperature.

Photoluminescence in fluorine-modified Sn-doped silica has been analyzed by means of synchrotron radiation in the UV and vacuum-UV, from 120 to 330 nm, looking at the optical activity of oxygen-deficient-centers ODC(II) in Sn-substituted cationic sites. The comparison between F-modified Sn-doped samples and previous data on F-free Sn-doped material evidences differences in the intensity of the 3.2 eV emission band excited at 3.7 eV, and in the thermal dependence of the intensity of this emission excited via intersystem crossing. The role of fluorine in modifying the optical activity of ODC(II) and in the SnO2 clustering is discussed, showing that an efficient excitation transfer may be activated from SnO2 to the Sn-variant of ODC(II). © 2009 Elsevier B.V. All rights reserved.

The optical properties of Yb-doped sol-gel silica glasses were studied by optical absorption and radio-luminescence (RL) measurements, that revealed the typical absorption and emission pattern of Yb3+ ions. Moreover, RL bands in the 1.5-3.5 eV interval were also observed, and related to defects of the silica matrix. The RL intensity temperature dependence, investigated in the 10-320 K interval, evidenced the presence of the SiO2 self-trapped exciton emission at 2.2 eV, whose intensity was rapidly quenched by temperature increasing. At variance, the Yb3+ emission intensity increased markedly by temperature increasing. This phenomenon is interpreted by considering a competitive role of point defects in free carrier trapping, evidenced by parallel wavelength resolved thermally stimulated luminescence measurements. © 2007 Elsevier B.V. All rights reserved.

We give the spectroscopic evidence of energy transfer to erbium ions provided by ${\mathrm{SnO}}_{2}$ nanocrystals in silica. The spectral and time resolved analysis of the nanocluster luminescence resonant with ${\mathrm{Er}}^{3+}$ transitions demonstrates that erbium ions take part to energy-transfer mechanisms with an efficiency of about $20%$. The spectral features of erbium emission excited by energy transfer allow to ascribe it to ${\mathrm{Er}}^{3+}$ sites in ordered environments, nearby crystalline tin-dioxide nanoclusters.

Silica glass with SnO2 nanocrystals and Er3+ ions are prepared by the sol-gel route and treatment above 1000 °C. Transmission electron microscopy evidences a homogeneous dispersion of nanoclusters 4-6 nm in size in the amorphous silica matrix. Photoluminescence spectra excited at 3.5 eV, outside erbium transitions, show an inhomogeneous spectral distribution of light emission from interface defects, in the range 1.9-2.4 eV, resonant with transitions of erbium ions. The analysis of kinetics and temperature dependence of luminescence allows to quantify the efficiency of the energy transfer channel between nanoclusters and erbium ions. © 2006 Elsevier Ltd. All rights reserved.

Scintillating fibres can be utilized as sensors in many fields such as medical diagnosis, industrial monitoring and other scientific areas. The main advantages of optical-fibre radiation-sensors are the remote sensing and the position-sensitive radiation detection. At present, plastic scintillating fibres present limitation for what concerns both luminescence efficiency and durability (radiation hardness). In this work, new rare-earth-doped high-grade silica glasses are synthesized by a modified sol-gel method. These glasses have an efficiency comparable with the state-of-the-art BGO-sensors, high radiation hardness and high compatibility with the silica-based photonic technology. Powder-in-tube and rod-in-tube techniques are used to fabricate low-loss optical fibres with 135, 220, and 660 microns diameter. Device prototypes are obtained by fusion-splicing these fibres to commercial high NA optical fibres.

SnO2 nanocrystals are grown in silica starting from a sol-gel method and using Er doping to passivate the cluster boundaries. As a result, emission at 3.8 eV from the decay of SnO2 free excitons is observed in nanostructured SnO2:SiO2, besides the extrinsic 2 eV luminescence of defects in SnO2 and ascribable to substoichiometric nanocluster boundaries. The analysis of the extrinsic emission competitive with the ultraviolet (UV) luminescence evidences the involvement of a phonon mode at 210 cm(-1) from a SnO-like phase. The feasibility of passivated wide-band-gap nanocrystals in silica gives interesting perspectives for UV-emitting optical devices. (c) 2006 American Institute of Physics.