Doped CaF2 Research Articles

Luminescence and scintillation properties of Eu2+ doped CaF2 glass ceramics for radiation spectroscopy

A new scintillation material including CaF2: Eu nanocrystals contained in SiO2–Al2O3–CaO–CaF2–LiF system was successfully prepared via melting method with additional heat treatment. Structural and luminescence properties were investigated by characterization techniques including XRD, SEM, photoluminescence (PL) excitation and emission spectra and absorption profile. The emission intensity of the glass ceramics containing CaF2: Eu nanocrystals under illumination at 356 nm were significantly more than the as-made glass and enhanced with increasing the temperature and duration of the heat treatment. Spectral analysis of gamma rays showed a full-energy peak in comparison with the similar chemical compositions that do not show such a response. Also an energy continuum with specific reaction-product peak was detected for thermal neutrons. The scintillation efficiency was enhanced with increasing crystallization. The fabricated Eu-doped transparent CaF2 glass ceramics has the potential to be a desired composite scintillator for detection of low energy gamma rays and thermal neutrons.

Fluorine losses in Er3+ oxyfluoride phosphate glasses and glass-ceramics

Er3+ doped phosphate glasses with the composition 75NaPO3-25CaF2 (mol%) were prepared at different melting temperatures to demonstrate the importance to quantify the fluorine content when preparing oxyfluoride glasses. Indeed, increasing the melting temperature from 900 to 1000 °C leads to a small reduction in the fluorine content from 9.4 at % to 8.8 at % as quantified using EPMA. Whereas this loss of fluorine can be suspected from small changes in the thermal properties of the glass, it increases significantly the glass crystallization tendency in this glass system. This means that a heat treatment of the as-prepared glass should be performed when evaporation of fluorine during the glass melting is suspected. Sample preparation for the characterization of the spectroscopic properties of the glasses is discussed here as well; bulk glasses should be used when measuring the spectroscopic properties of oxyfluoride glasses, which are known to be hygroscopic. It is shown, in this work, that a heat treatment of the glass within the investigated glass system leads to transparent glass-ceramics with volume precipitation of Er3+ doped CaF2 crystals with strong upconversion.

Luminescent properties of Eu3+ doped CaF2, SrF2, BaF2 and PbF2 powders prepared by high-energy ball milling

Eu3+ doped metal fluorides in fluorite structure were prepared on a solvent-free mechanochemical synthesis route. Beside structural characterization by XRD and 19F MAS NMR, their luminescence behavior was followed in dependence on the Eu3+ content and in comparison with other synthesis strategies. High luminescence life times were measured for as-prepared nanoparticles. Additionally, pure EuF3 was mechanochemically prepared and characterized for comparison.

Application of Judd–Ofelt theory in analyzing Nd3+ doped SrF2 and CaF2 transparent ceramics

Nd3+ doped SrF2 and CaF2 transparent ceramics were fabricated by vacuum hot-press sintering and the absorption spectra, emission spectra as well as luminescence decays of the samples were measured. Judd-Ofelt (J–O) theory was used to analyze the optical performance of Nd3+ in these two isostructural hosts. The Nd: SrF2 transparent ceramic was found to have smaller line strength, larger radiative lifetime and smaller Ω2 value (corresponding to more ionic Nd3+-ligand bonding and more symmetry of Nd3+ environment). These features made it easier for Nd: SrF2 to realize population inversion and strong emission, thus doing good to laser performance. The strong emission of 4F3/2 → 4I9/2 transition in Nd: SrF2, which was predicted by J–O theory and demonstrated by luminescence spectrum, made it possible to achieve effective laser output around 900 nm. The intensity parameters and radiative lifetimes of ceramics were found comparable with their corresponding single crystals.

Study of femtosecond laser writing in the bulk of Nd3+, Y3+ co-doped CaF2 crystals

We reported on the femtosecond (fs) laser induced permanent refractive index changes, stress-induced birefringence, and micro-photoluminescence properties of Nd3+, Y3+ codoped CaF2 crystals. Permanent changes in the linear optical properties were studied for various pulse energies (0.01 - 5 µJ) at two different repetition rates (10 kHz and 500 kHz). Optimum quantitative phase changes are close to - 1.7π radians within an irradiated area, surrounded by positive phase changes due to the stress field that can reach + 0.7π. Provided that, we avoid heat-accumulation at 500 kHz, written laser lines offer higher linear birefringence on the order of λ/4 at 200 nm. From the spectroscopic point of view, we observed the Raman changes and micro-photoluminescence spectra of irradiated fs-laser Nd3+, Y3+ doped CaF2.

Gamma-rays induced color centers in Pb2+ doped CaF2 crystals

Pure and PbF2 doped CaF2 crystals were obtained in our crystal research laboratory. The influence of PbF2 concentration on the color centers induced by γ-and x-rays irradiation were studied. Two broad absorption bands with maxima peaked at 410 and 730 nm appear in the optical absorption spectra; the spectra are quite similar in band structure for all samples and for the both radiation. Different bands showed different responses to the irradiation and to the long-term recovery. The most stable color centers are (Fi0) and the dimer P-P (Pb+(1)-Pb2+). These absorption bands remain stable even for trace amount of Pb2+ ions, so CaF2:PbF2 is not a good material for applications in which the new optical absorption bands are undesirable.

Monodisperse, colloidal and luminescent calcium fluoride nanoparticles via a citrate-assisted hydrothermal route

Luminescent calcium fluoride (CaF2) nanoparticles, because of their excellent biocompatibility, excellent photostability and strong fluorescence, have received increasing attention as drug carriers and bioprobes in cell imaging. Inspired by the role of citrate in the growth of apatite crystals during natural bio-mineralization, uniform and nearly monodisperse Eu3+-doped CaF2 nanoparticles with excellent colloidal stability and high fluorescence in aqueous media have been successfully synthesized in the presence of sodium citrate using a hydrothermal method. X-ray diffraction and transmission electron microscopy show that CaF2 nanoparticles grown in the presence of sodium citrate are cubes of relatively uniform size (15 nm), and that the Eu3+ doping level has little effect on size and morphology. Zeta potentials and dynamic light scattering demonstrate that in the synthesis with sodium citrate, the colloidal stability of CaF2 nanoparticles is greatly improved upon the increase of Eu3+ doping level. Moreover, aqueous dispersions of these nanoparticles are colloidally stable and can be maintained over a wide range of pH from 5.0 to 11.0 for more than a month. Fluorescence spectra demonstrate that the doped CaF2 nanoparticles display strong red fluorescence. Fourier transform infrared spectra and thermogravimetric analyses demonstrate the adsorption of substantial quantities of sodium citrate on the surfaces of the CaF2 nanoparticles. Taken together, such colloidal behavior should be related to strong crystal inhibition of citrate ions and Eu3+ doping induced promotion thermal-decomplexing between citrate ions and calcium ions. The luminescent CaF2 nanoparticles obtained using this protocol should be promising candidates for use in many bio-related applications.

Tunable multicolor and enhanced red emission of monodisperse CaF2:Yb3+/Ho3+ microspheres via Mn2+ doping

Transition metal ions (e.g. Mn2+) and lanthanide co-doped upconversion (UC) materials have attracted wide attention in recent years due to their promising application in multicolor display. Here, we report the hydrothermal synthesis and characterization of Mn2+ doped monodisperse CaF2:Yb3+/Ho3+ microspheres. The results of X-ray diffraction (XRD) revealed that Mn2+ doping does not change the cubic phase of CaF2 material but will lead to diffraction peaks shifting slightly towards higher angle due to the substitution of larger Ca2+ by the relatively smaller Mn2+. Under the excitation of 980 nm continuous wave (CW) laser, these microspheres exhibit green-yellow-red tuning colors and remarkable enhancement of both red to green ratio (R/G) and red to blue ratio (R/B) when increasing Mn2+ concentration from 0 to 30 mol%. The energy migration process between Ho3+ and Mn2+ was proposed and supported by time-decay and power dependence measurements of Ho3+ UC emission. These upconversion materials may have potential applications in optical devices, color display, nanoscale lasers and biomedical imaging.

Luminescence of Eu3+ Activated CaF2 and SrF2 Nanoparticles: Effect of the Particle Size and Codoping with Alkaline Ions

Eu3+ doped CaF2 and SrF2 nanoparticles were synthesized through a facile hydrothermal technique, using citrate ions as capping agents and Na+ or K+ as charge compensator ions. A proper tuning of the reaction time can modulate the nanoparticle size, from few to several tens of nanometers. Analysis of EXAFS spectra indicate that the Eu3+ ions enter into the fluorite CaF2 and SrF2 structure as substitutional defects on the metal site. Laser site selective spectroscopy demonstrates that the Eu3+ ions are mainly accommodated in two sites with different symmetries. The relative site distribution for lanthanide ions depends on the nanoparticle size, and higher symmetry Eu3+ sites are prevalent for bigger nanoparticles. Eu3+ ions in high symmetry sites present lifetimes of the 5D0 level around 27 ms, among the longest lifetimes found in the literature for Eu3+ activated materials. As a proof of concept of possible use of the Eu3+ activated alkaline-earth fluoride nanoparticles in nanomedicine, the red luminescenc...

Alleviating luminescence concentration quenching in lanthanide doped CaF2 based nanoparticles through Na+ ion doping.

Luminescence concentration quenching, mainly due to a cross relaxation (CR) process between lanthanide ions (Ln3+), widely occurs in Ln3+ doped luminescent materials, setting a limit in the dopant content of Ln3+ emitters to withhold the brightness. Here, we introduced Na+ ions into the CaF2 host lattice codoped with Nd3+ emitters that alleviates concentration quenching greatly. And we show that the optimal dopant concentration of Nd3+ in colloidal CaF2:Nd nanoparticles increased from 10 to 30 mol%, resulting in an ∼32 times near-infrared (NIR) (1052 nm) brightness under 800 nm laser irradiation. Our mechanistic investigation suggests that the enhancement of NIR photo-luminescence (PL) could be attributed to not only the increasing crystallinity of nanoparticles but also the reducing concentration quenching of Nd3+ by improving the dopant distribution of Nd3+ ions in the CaF2 lattice, as evidenced by the high angle annular dark field images. These result in the optimal concentration increase to produce brightness enhancement greatly. This strategy can be utilized for other Ln3+ doped CaF2 based nanomaterials for bio-imaging.

Spatio‐Temporal Characterization of Pump‐Induced Wavefront Aberrations in Yb3 + ‐Doped Materials

AbstractA comprehensive spatio‐temporal characterization is presented describing the pump‐induced wavefront aberrations in Yb3 + ‐doped YAG, CaF2, and fluorophosphate glass. Time‐resolved interferometric measurements were performed to reveal the profiles of the total optical path differences (OPDs), which are described by the spatio‐temporal superposition of thermal as well as electronic contributions, across the free aperture of the considered diode‐pumped active materials. These contributions were individually determined by a COMSOL‐based thermal profile model along with a detailed characterization of the electronic changes by measuring the single‐pass gain and the spatial fluorescence profile. Due to the low quantum defect, the amplitude of the electronic component becomes comparable for all three materials and, in the case of Yb:CaF2, almost completely compensates the thermal component resulting from a pump pulse during the time frame of laser pulse amplification. Finally, all relevant material constants – such as the photoelastic constant and the polarizability difference – could be determined during this investigation, allowing the accurate modeling of the total pump‐induced wavefront aberrations and subsequent optimization for laser systems worldwide employing these Yb3 + ‐doped materials.

F−-Eu3+ charge transfer energy and local crystal environment in Eu3+ doped calcium fluoride

A series of CaF2:xEu3+ (x = 0.01–0.05) phosphors were synthesized by coprecipitation and high temperature solid-state method. Three different F--Eu3+ charge transfer (CT) bands in CaF2:xEu3+ (x = 0.01–0.05) were detected through the PL spectra. Three local crystal environments (deformed Oh octahedron, C3V, C4V) were proposed in Eu3+ doped CaF2. The broad peak at 277 nm is F- → Eu3+ CT peak, in which Eu3+ ions located deformed Oh octahedron sites, the broad peak at 312 nm is due to Eu3+ ions situating at C4v sites, also, Eu3+ ions with C3v sites can make the F- → Eu3+ CT produce a 38-nm redshift to 315 nm. Density functional theory (DFT) was performed to calculate the energy band gap(Eg) according to the three models. The calculation results consist with the experiment. The excitation peaks can be tuned in the same host through changing the local structure of Eu3+.

Plasmon-enhanced fluorescence of CaF2:Eu2+ nanocrystals by Ag nanoparticles

We report plasmon-enhanced fluorescence in Eu2+ doped CaF2 (CaF2:Eu2+) nanocrystals assembled with Ag nanoparticles (NPs) using a simple hydrothermal method. The structure, morphology and composition of the nanocrystals are confirmed by X-ray diffraction (XRD), transmission electron Microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and UV–vis absorption spectrum. Ag nanoparticles are ideal building blocks for plasmonic materials that can exhibit a wide range of unique and potentially useful optical phenomena. Surface plasmon enhancement makes significant contribution to enhance the blue fluorescence emission of CaF2:Eu2+. The enhancement on the Eu emission in visible regions is up to a factor of 12.3-fold. As shown in time-resolved measurements, surface Plasmon Resonance (SPR) can reduce the fluorescence lifetime and increase the quantum yield. Furthermore, with increasing Ag concentration, the fluorescence intensity increases to its maximum and then decreases.

Novel and easy access to highly luminescent Eu and Tb doped ultra-small CaF2, SrF2 and BaF2 nanoparticles - structure and luminescence.

A universal fast and easy access at room temperature to transparent sols of nanoscopic Eu3+ and Tb3+ doped CaF2, SrF2 and BaF2 particles via the fluorolytic sol-gel synthesis route is presented. Monodisperse quasi-spherical nanoparticles with sizes of 3-20 nm are obtained with up to 40% rare earth doping showing red or green luminescence. In the beginning luminescence quenching effects are only observed for the highest content, which demonstrates the unique and outstanding properties of these materials. From CaF2:Eu10 via SrF2:Eu10 to BaF2:Eu10 a steady increase of the luminescence intensity and lifetime occurs by a factor of ≈2; the photoluminescence quantum yield increases by 29 to 35% due to the lower phonon energy of the matrix. The fast formation process of the particles within fractions of seconds is clearly visualized by exploiting appropriate luminescence processes during the synthesis. Multiply doped particles are also available by this method. Fine tuning of the luminescence properties is achieved by variation of the Ca-to-Sr ratio. Co-doping with Ce3+ and Tb3+ results in a huge increase (>50 times) of the green luminescence intensity due to energy transfer Ce3+ → Tb3+. In this case, the luminescence intensity is higher for CaF2 than for SrF2, due to a lower spatial distance of the rare earth ions.

Quenching cooperative transitions by destroying Yb3+-clusters.

In our previous study, we have reported the cooperative luminescence of Yb3+-trimers and cooperatively sensitized Gd3+ luminescence by Yb3+-tetramers in a doped CaF2 host. In this study, we experimentally observed an unusual luminescent phenomenon of Gd3+ in CaF2:Yb3+/Gd3+. Upon excitation with a 980 nm laser, the upconversion luminescence of Gd3+ first increases and then decreases in the Gd3+ concentration range of 0-0.9 mol%; this is different from the monotonic increase of Gd3+ luminescence observed in the downconversion spectra via the direct excitation of Gd3+. This special luminescent behavior was indicated to be related to the energy transfer from the Yb3+-tetramers to Gd3+ and the destruction of Yb3+-clusters. Herein, we proposed a new luminescence quenching mechanism, Yb3+-cluster destructive quenching, which was verified by fluorescence dynamic analysis and an optically inactive rare-earth ion-doping experiment.

Structure and optical properties of transparent Er3+-doped CaF2–silica glass ceramic prepared by controllable sol–gel method

Transparent Er3+-doped CaF2–silica glass ceramics were prepared by the direct physical introduction of Er3+ doped CaF2 nanocrystals into acid-catalyzed sol–gel silica glass. The physical methods of ball milling, ultrasonic baths, and stirring were investigated to disperse Er3+ doped CaF2 nanocrystals in the silica sols. The CaF2–silica sol mixture went through gelation and heat-treatment to form Er3+-doped CaF2–silica glass ceramics. The morphology of Er3+ doped CaF2 in silica glass did not change after heat-treatment at 600°C for 10h. The experimental results showed that Er3+ doped CaF2 in the glass ceramic prepared with the assistance of ball milling possesses the best dispersity and homogeneity. The highest in-line transmittance of the glass ceramic reached up to 85% in visible region. Glass ceramic exhibits efficient up-conversion emissions corresponding to the Er3+:4F9/2→4I15/2 transition and long lifetime of 4F9/2 level (1.73ms) under 980nm excitation.

Investigation of the mechanisms of upconversion luminescence in Ho3+ doped CaF2 crystals and ceramics upon excitation of 5I7 level

The mechanisms of upconversion luminescence of CaF2:Ho crystals and ceramics from 5F3, 5S2(5F4), 5F5 and 5I6 levels upon excitation of 5I7 level of Ho3+ ions were investigated. Different mechanisms are responsible for the populating and depletion of the energy levels of Ho3+ ion in CaF2:Ho crystals and ceramics upon excitation of 5I7 level. The upconversion luminescence from 5F3, 5F5, and 5I6 levels in CaF2:Ho crystals and ceramics is explained by energy transfer upconversion processes. Our results also confirmed that both excited-state absorption and energy transfer upconversion are responsible for the populating of 5S2(5F4) level.

Scintillation and optical stimulated luminescence of Ce-doped CaF2

Scintillation and optical stimulated luminescence of Ce 0.1–20% doped CaF2 crystals prepared by Tokuyama Corp. were investigated. In X-ray induced scintillation spectra, luminescence due to Ce3+ 5d–4f transition appeared around 320 nm with typically 40 ns decay time. By 241Am 5.5 MeV α-ray irradiation, 0.1% doped one showed the highest scintillation light yield and the light yield monotonically decreased with Ce concentrations. Optically stimulated luminescence after X-ray irradiation was observed around 320 nm under 550 or 830 nm stimulation in all samples. As a result, intensities of optically stimulated luminescence were proportional to Ce concentrations. Consequently, scintillation and optically stimulated luminescence resulted to have a complementary relation in Ce-doped CaF2 system.

Water (H2O and D2O) Dispersible NIR-to-NIR Upconverting Yb3+/Tm3+ Doped MF2 (M = Ca, Sr) Colloids: Influence of the Host Crystal

Tm3+/Yb3+ doped CaF2, SrF2, and cubic phase NaYF4 nanoparticles dispersed as colloids in water (H2O and D2O) or saline solutions have been directly prepared by a one-step hydrothermal technique, using citrate anions as capping agents, without the need for any postsynthesis reaction. The size monodispersed nanoparticles are directly dispersed in water. Comparison of the upconversion emissions at 800 nm (Tm3+ ions) among the CaF2, SrF2, and NaYF4 hosts indicates that the SrF2 host leads to the highest emission intensity, 2 orders of magnitude higher than the NaYF4 one. Alkali ions (Na+ or K+) counter cations of the citrate salts used as precursors can enter the fluoride host crystals as charge compensators and strongly influence the spectroscopic properties of the lanthanide ions. The Tm3+/Yb3+ doped SrF2 nanoparticles dispersed in a 0.4 g/L concentration solution show detectable upconversion with laser excitation intensities as low as 1 W/cm2.

Doping concentration dependence on VUV luminescence of Tm:CaF2

Luminescence and scintillation properties of Tm3+ 0.1–15% doped CaF2 crystals were systematically investigated. The crystals were grown by a simple solidification method which allowed us to obtain single crystals conveniently. In X-ray induced radio luminescence spectra, the luminosity of the self-trapped-exciton (STE) emission of CaF2 at 270nm decreased with increasing of Tm3+ concentration. The Tm3+ 5d-4f transition based emission lines appeared at 175nm (4f115d high spin state (HS)→3H6) and at 165nm (low spin state (LS) to 3H6). Pulse height spectra were measured under α-ray irradiation by using vacuum ultra-violet (VUV) sensitive photomultiplier tube (PMT) R8778 and Tm3+ 0.1% doped specimen exhibited the highest light yield.