CaF2 Nanocrystals Research Articles

Role of Li + ions on the luminescence properties in water dispersible mesoporous CaF 2 : Eu 3 + nanocrystals

Polyvinyl pyrrolidone- (PVP) and ethylene glycol- (EG) coated water dispersible mesoporous CaF2:Eu3+,Li+ nanocrystals were synthesized by a simple one-pot hydrothermal method. The role of Li+ ions on the morphology and spectra in the CaF2 nanocrystals have been reported. The synthesized nanocrystals were monodisperse with regular sphere-like shapes and exhibited obvious hollow mesoporous structure. The results from x-ray photoelectron spectroscopy determined the successful incorporation of Eu3+ ions and Li+ ions into the CaF2 host lattice. The excitation and emission intensity of Eu3+ enhanced with Li+ concentration until 7 mol.%. The nanocrystals could disperse in water for more than a month due to the EG and PVP molecules coated over the nanocrystals.

Infrared-to-Visible Up-conversion in Er-Pr-Yb Triply Doped Oxyfluoride Glass Ceramics

We synthesized Er3+-Yb3+, Pr3+-Yb3+, and Er3+-Pr3+-Yb3+-doped oxyfluoride glass ceramics containing CaF2 nanocrystals by proper thermal treatments. Spectral characteristics of down- and up-converted emissions in three kinds of glass ceramics under 365-nm and 980-nm excitations were compared and explained by cross relaxations, excited-state absorptions, and energy-transfer processes between different ions. The huge reduction of up-conversion emission in the triply doped glass ceramics under 980-nm excitation compared to the Er3+-Yb3+ codoped one was explained by the split pump power and the direct energy transfer from Er3+ to Pr3+ ions. Increasing Yb3+ concentration from 2% to 10% in the triply doped glass ceramics showed more than quadratic enhancement of the absorbed power, and we explained it by the enhanced energy-transfer efficiency from Yb3+ to Er3+ ions. We also observed enhanced up-converted emissions of Er3+ and Pr3+ ions in three kinds of glass ceramics under simultaneous excitation at 980 nm and 1550 nm, and suggested detailed up-conversion mechanisms.

Highly aqueous soluble CaF2:Ce/Tb nanocrystals: effect of surface functionalization on structural, optical band gap, and photoluminescence properties.

The design of nanostructured materials with highly stable water-dispersion and luminescence efficiency is an important concern in nanotechnology and nanomedicine. In this paper, we described the synthesis and distinct surface modification on the morphological structure and optical (optical absorption, band gap energy, excitation, emission, decay time, etc.) properties of highly crystalline water-dispersible CaF2:Ce/Tb nanocrystals (core-nanocrystals). The epitaxial growth of inert CaF2 and silica shell, respectively, on their surface forming as CaF2:Ce/Tb@CaF2 (core/shell) and CaF2:Ce/Tb@CaF2@SiO2 (core/shell/SiO2) nanoarchitecture. X-ray diffraction and transmission electron microscope image shows that the nanocrystals were in irregular spherical phase, highly crystalline (~20 nm) with narrow size distribution. The core/shell nanocrystals confirm that the surface coating is responsible in the change of symmetrical nanostructure, which was determined from the band gap energy and luminescent properties. It was found that an inert inorganic shell formation effectively enhances the luminescence efficiency and silica shell makes the nanocrystals highly water-dispersible. In addition, Ce3+/Tb3+-co-doped CaF2 nanocrystals show efficient energy transfer from Ce3+ to Tb3+ ion and strong green luminescence of Tb3+ ion at 541 nm(5D4→7F5). Luminescence decay curves of core and core/shell nanocrystals were fitted using mono and biexponential equations, and R 2 regression coefficient criteria were used to discriminate the goodness of the fitted model. The lifetime values for the core/shell nanocrystals are higher than core-nanocrystals. Considering the high stable water-dispersion and intensive luminescence emission in the visible region, these luminescent core/shell nanocrystals could be potential candidates for luminescent bio-imaging, optical bio-probe, displays, staining, and multianalyte optical sensing. A newly designed CaF2:Ce/Tb nanoparticles via metal complex decomposition rout shows high dispersibility in aqueous solvents with enhanced photoluminescence. The epitaxial growth of inert CaF2 shell and further amorphous silica, respectively, enhanced their optical and luminescence properties, which is highly usable for luminescent biolabeling, and optical bioprobe etc.

Development of Yb3+-doped oxyfluoride glass-ceramics with low OH−content containing CaF2nanocrystals for optical refrigeration

Oxyfluoride glasses and glass-ceramics (GCs) have some niche advantages over other oxide and fluoride glasses, as they possess combined properties. This paper reports the structural, thermal, and photoluminescence (PL) properties of Yb3+-doped SiO2−Al2O3−CaO−CaF2 oxyfluoride glasses and transparent GCs containing CaF2 nanocrystals. Special efforts were undertaken to minimize the hydroxyl (OH−) content in the prepared samples to improve their optical features. Differential scanning calorimetry analyses were performed to determine the characteristic temperatures of the base glasses. X-ray diffractometry studies have confirmed the fluorite CaF2 nanocrystals to be 10 nm in size. Reduced transparency in the ultraviolet (UV)–visible to the near-infrared (NIR) regions was observed for the GCs compared to the base glass with increasing thermal treatment temperature. A higher PL intensity upon 920-nm excitation was obtained in the GCs compared to that of the base glass. The absolute photoluminescence quantum yield upon 920-nm laser excitation was evaluated using an integrating sphere and an optical spectrum analyzer. It was observed that the lifetime of the F5/22 level of the Yb3+ ions decreases with increasing ceramization temperature. The potential advantages of using such oxyfluoride GCs over commonly studied single crystals for laser cooling applications are discussed.

Enhanced upconversion emission in crystallization-controllable glass-ceramic fiber containing Yb3+-Er3+ codoped CaF2 nanocrystals

Functional nanocrystal-containing materials have been a hot topic in recent years. However, few researches have focused on functional nanocrystals contained in optical glass fibers. In this research, transparent CaF2 glass-ceramic was prepared by a melt-quenching method. Greatly enhanced upconversion luminescence was observed after heat treatment. By applying a novel method called melt-in-tube, precursor fiber free of crystals was fabricated at the drawing temperature where the clad was softened while the core was melted. Glass-ceramic fiber with fiber core containing Yb3+-Er3+ codoped CaF2 nanocrystals was obtained after heat treatment at a relatively low temperature. Electron probe micro-analyzer measurement shows no obvious element diffusion between the core and clad. Greatly enhanced upconversion emission was detected in the glass-ceramic fiber excited by a 980 nm laser, suggesting the developed glass-ceramic fiber is a promising material for upconversion laser.

Recrystallization of Er3+:CaF2 in Transparent Fluorophosphate Glass‐Ceramics with the Co‐Firing Method

Transparent glass‐ceramics containing Er3+:CaF2 crystallites were prepared with the co‐firing method. The formation process of the glass‐ceramics was investigated by means of SEM, XRD, and DSC. The results reveal that the Er3+:CaF2 nanocrystals do not dissolve into the fluorophosphates (FP) glassmelt until the co‐firing temperature increase higher than about 920°C. Below this temperature, Er3+:CaF2 survives the co‐firing process and the nanocrystals just grow to spherical crystals of micrometers in size. Co‐firing temperature higher than this temperature leads to the dissolution of Er3+:CaF2 and the dissolved Er3+:CaF2 recrystallized during quenching process and takes the shape of dendrite.

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.

Near-infrared down-conversion in Er3+–Yb3+ co-doped transparent nanostructured glass ceramics for crystalline silicon solar cells

A two-step energy transfer was achieved in Er3+–Yb3+ co-doped transparent glass ceramics containing CaF2 nanocrystals, which involved down-conversion of an absorbed visible photon to two emitted near-infrared photons.

Energy transfer in color-tunable water-dispersible Tb–Eu codoped CaF2 nanocrystals

The energy transfer process between Tb3+ and Eu3+ ions in water-dispersible CaF2 nanocrystals prepared using a simple process is reported. Fine colour control and long excited state lifetimes make the nanosystem suitable for biosensing applications.

Ce3+ sensitized bright white light emission from colloidal Ln3+ doped CaF2 nanocrystals for the development of transparent nanocomposites

Intense white light emissions are observed from colloidal single component Ce3+/Ce3+/Ce3+/Ce3+-doped CaF2 nanocrystals and their transparent nanocomposites.

New insight into two-step near-infrared quantum cutting in Pr^3+ singly doped oxyfluoride glass-ceramics

Efficient near-infrared (NIR) quantum cutting (QC) in RE3+/Yb3+ requires the UV-blue photon excited RE3+ donor has intermediate energy level to separate the energy gaps (~10000 cm−1) resonant to Yb3+absorption. Thus the unique Pr3+/Yb3+ resonant QC at low Yb3+content (room temperature) essentially requires Pr3+-doping features distinctive NIR radiative transitions: in our prepared Pr3+-doped oxyfluoride glass-ceramics containing CaF2 nanocrystals, a two-photon NIR-QC from blue-photon excited 3Pj (j = 0, 1, 2) states takes place efficiently with 1G4 acting as an intermediate level. The underlying energy transfer mechanisms involving the two-step sequential transitions of 3P0→1G4 ~915 nm and 1G4→3H4 ~990 nm (crucial resonant routes for the further 1Pr3+→2Yb3+), as well as the 1D2→3F2 ~873 nm and 1D2→3F3,4 ~1040 nm, are rationally distinguished by means of photoemission, excitation, and time-resolved fluorescence spectra.

Structure and optical band gap study of transparent oxyfluoride glass-ceramics containing CaF2 nanocrystals

Oxyfluoride glass-ceramics containing CaF2 nanocrystals were prepared by one-step crystallization of SiO2–Al2O3–CaO–CaF2 glasses at different temperatures. X-ray diffraction (XRD) results have revealed that CaF2 was the only precipitated crystalline phase in glass-ceramic samples. Fourier transform infrared spectroscopy (FT-IR) study of the samples showed the effect of crystallization temperature on the structure. According to DTA and XRD results, suitable glass and its associated glass-ceramics were selected as the best sample in order to study the effect of crystallization on the optical properties. UV–Vis spectra of samples were recorded to determine optical properties. Fermi energy level decreased for glass-ceramics due to the increment of semiconducting behavior. Urbach energy, optical band of the specimens have been reduced by increasing of nanocrystals in glassy matrix which could be attributed to increasing of the order of the structural and formation of crystals and dangling bonds, respectively.

Highly matched spectrum needed for photosynthesis in Ce3+/Er3+/Yb3+ tri-doped oxyfluoride glass ceramics

A series of oxyfluoride glass ceramics containing CaF2 nano-crystals tri-doped with Ce3+/Er3+/Yb3+ ions were prepared by high temperature melting method and subsequent heat treatment. The structural properties were examined by X-ray diffraction measurements. The absorption, excitation, and emission spectra of the glass ceramics were investigated. Difference in erbium emission spectra between glass and glass ceramics had been studied. The emission bands originating from the 4F9/2 state of Er3+ were enhanced when the CaF2 nano-crystal created. By down-converting the ultraviolet wavelength region (280∼400 nm) light and up-converting the near-infrared wavelength region (900∼1100 nm) light, the glass ceramics can also emit strong reddish orange emission. The emission spectra consisting of bluish violet (400∼500 nm) and reddish orange (640∼680 nm) bands match well with the action spectrum of photosynthesis and absorption spectra of chlorophylls. Our materials will be favored to promote the development of glass greenhouses for green plant.

Influence of local phonon energy on quantum efficiency of Tb3+-Yb3+ co-doped glass ceramics containing fluoride nanocrystals

The Tb3+ single-doped and Tb3+-Yb3+ co-doped glass ceramics with the precipitation of CaF2, CaF2-SrF2 solid state solution and SrF2 nanocrystals were designed and prepared by taking different amounts of CaF2 and SrF2 as the starting fluorides to investigate the influence of the crystalline phase on the total quantum efficiency. The formation of the fluoride nanocrystals and the incorporation of the doped rare earth ions into the fluoride nanocrystals were proved by the XRD measurement. The energy transfer from Tb3+ to Yb3+ was studied by the steady and time resolved spectra. The total internal quantum efficiencies were calculated based on the measured Tb3+ lifetime, which was about 10.5% improved in the SrF2 nanocrystals precipitated glass ceramics compared with that in the CaF2 nanocrystals precipitated glass ceramics mainly due to the lower phonon energy environment. Meanwhile, the total external quantum efficiencies were evaluated with the integrating sphere measurement system, which were 18.6%, 19.3% and 24.4%, respectively, for the CaF2, CaF2-SrF2 and SrF2 nanocrystals precipitated glass ceramics. Additionally, obvious difference between the calculated total internal quantum efficiency and the measured total external quantum efficiency was also discussed.

Effect of substitution of SiO2 by CaO/CaF2 on structure and synthesis of transparent glass-ceramics containing CaF2 nanocrystals

Oxyfluoride glass ceramics with varied composition in system SiO2–Al2O3–CaO–CaF2 with fixed content of alumina were synthesized by isothermal heat-treatment of melt-quenched glasses. The as-prepared glasses and glass-ceramics were characterized in thermodynamics, microstructure, crystalline phase, and morphology and optical properties. The results show that substitution of SiO2 by CaO and/or CaF2 leads to rupture of Si–O network and hence escalating disorder of glass structure. Glass transition temperature increases with substituting SiO2 by CaO but decreases by CaF2. Depending on glass composition and treating temperature, the initial crystalline phase CaF2 and the secondary phase anorthite can successively crystallized. Substitution of SiO2 by CaO and/or CaF2 may facilitate the precipitation of CaF2 crystal but suppress the crystallization of anorthite. Transparent glass ceramic containing nanosized CaF2 can be synthesized using glass with a molar ratio of SiO2 to (CaO + CaF2) approaching 1.3 via heat-treatment close to the initial crystallization temperature. Elevating treating temperature improves the crystallization of CaF2 but in return decreases transparency of the glass ceramics (GC).

Microwave-assisted solvothermal synthesis and upconversion luminescence of CaF2:Yb3+/Er3+ nanocrystals

Water-dispersible CaF2 and Yb3+/Er3+ codoped CaF2 (CaF2:Yb3+/Er3+) nanocrystals with different sizes and different Yb3+ and Er3+ dopant concentrations were synthesized using ionic liquid 1-n-butyl-3-methyl imidazolium tetrafluoroborate as a fluorine source by the rapid microwave-assisted solvothermal method. It was found that the morphology, size and crystallinity of CaF2:Yb3+/Er3+ nanocrystals could be adjusted by using adenosine 5′-triphosphate disodium salt (ATP). Yb3+ and Er3+ ions were doped into CaF2 nanocrystals to enable upconversion luminescence emission, and the as-prepared CaF2:Yb3+/Er3+ samples exhibited upconversion luminescence upon excitation at 980nm. Confocal laser scanning microscopy images showed that the CaF2:Yb3+/Er3+ nanocrystals could be used for efficient labeling of human gastric carcinoma cells. Moreover, in vitro cytotoxicity experiments indicated that the as-prepared CaF2:Yb3+/Er3+ nanocrystals had essentially little cytotoxicity. These results indicate that the as-prepared CaF2:Yb3+/Er3+ nanocrystals are promising for the application as a luminescent label material in biological imaging.

Improved multiphoton ultraviolet upconversion photoluminescence in ultrasmall core-shell nanocrystals.

Near-infrared to ultraviolet multiphoton upconversion photoluminescence in ultrasmall Tm3+/Yb3+-codoped CaF2 nanocrystals (∼6.7 nm in size) was observed and further significantly enhanced by growing an active shell of NaYF4:Yb3+. Owing to the active shell, the lanthanide emitters inside the core are effectively prevented from the surface quenchers, and the excitation energy is absorbed more efficiently via the additional luminescence sensitizer Yb3+ embedded in the shell. The details of underlying physics were investigated and discussed. The results present a good ultrasmall luminescent material system for achieving efficient multiphoton upconversion, which shows great potential in versatile industrial and biological applications.

Structure investigations of nanocrystalline glass ceramics by ASAXS

Glass ceramics containing fluoride crystals such as BaF2 or CaF2 with crystallite sizes in the range from 5 to 100 nm are potential candidates for numerous photonic applications. Glass ceramics containing rare-earth-doped fluoride crystals are candidates for laser materials. The size distribution plays an important role and often a narrow size distribution is required for photonic applications. In the last years a hindered growth effect leading to a more narrow size distribution was observed during the crystallization of BaF2 as well as CaF2 nanocrystals in oxy-fluoride glasses. The aim of this study is a detailed quantitative structural and nano-chemical analysis of the formation of BaF2 or CaF2 in two glass ceramics by Anomalous Small-Angle X-ray Scattering (ASAXS) to reveal and understand the mechanism of hindered growth. Nanocrystals of BaF2 precipitate during heat treatment of a silicate glass of composition 69.6SiO2-7.52Al2O3-15.04K2O-1.88Na2O-4BaF2-2BaO. X-ray diffraction measurement proved the formation of BaF2 crystals in the glass matrix. High resolution TEM showed the formation of spherical particles of sizes in range from 10-40 nm surrounded by a layer enriched with SiO2. SAXS reveal the growth of nanocrystals with increasing annealing time and temperatures. ASAXS experiments are done at four energies close to the Ba-L3 X-ray absorption edge (5247eV). The ASAXS curves for the sample annealed at 5400C for 20h revealed a spherical core-shell model. It turned out that the layer surrounding the BaF2 crystals is enriched with SiO2. Sizes and compositions of these layers are analyzed quantitatively. Furthermore, the ASAXS analysis reveals the presence of very small nucleates of size of about 3 nm in the as melted glass sample already [1]. A precipitation of CaF2 nanoparticles takes place during heat treatment of glasses of composition 7.65Na2O–7.69K2O–10.58CaO–12.5CaF2–5.77Al2O3–55.8SiO2 up to 40 hours. SAXS experiments and especially ASAXS near the Ca-K edge proves the formation of CaF2 nanoparticle surrounded with SiO2 enriched layers, quantitatively. The ASAXS effect is very pronounced at this untypical low energy for ASAXS studies at the Ca-K edge. The ASAXS result reveals crystal sizes between 10-20 nm surrounded by a shell of lower electron density. Additional very small heterogeneities are found after long annealing with diameters of about 1.6 nm [2].

Preparation and Characterization of Transparent Oxyfluoride Tellurite Glass-Ceramics Containing CaF<sub>2</sub> Nanocrystals

Transparent oxyfluoride tellurite glass-ceramics containing CaF2 nanocrystals were prepared by high temperature melting method. The glass-ceramics were characterized by Differential scanning calorimetry, X-ray diffraction and scanning electron microscope, respectively. The results showed that the CaF2 nanocrystals were spherical and the average size was found to be 60 nm.

Samarium‐Doped Oxyfluoride Glass‐Ceramic as a New Fast Erasable Dosimetric Detector Material for Microbeam Radiation Cancer Therapy Applications at the Canadian Synchrotron

There is a special need to develop a dosimetry technique with a large‐dynamic range and high‐spatial resolution to characterize the microstructured X‐ray beams used in microbeam radiation therapy (MRT) for cancer. We report the synthesis and characterization of oxyfluoride glass‐ceramic (SiO2–Al2O3–CaF2–CaO–SmF3) plates, which contain trivalent‐samarium‐doped calcium fluoride (CaF2:Sm3+) nanocrystals, for use as a dosimetric detector material, particularly for MRT applications. Our approach utilizes the extent of Sm3+→Sm2+ valence reduction caused by X‐ray irradiation as a probe of the X‐ray dose delivered; and confocal fluorescent microscopy is used to read out the distribution of valence reduction through the photoluminescence (PL) signal. Our study showed that the Sm3+→Sm2+ valence reduction takes place in CaF2 nanocrystals, but not in the glass matrix. The Sm2+ shows PL emission predominantly due to the fast 4f55d1→7F0 transition, which allows us to read out the detector plate at a high scanning speed. Further, our experiments showed that the detection dose range reaches several thousands of grays, and X‐ray dose distribution is detected at a micrometer scale. In addition, the Sm2+ signal can be erased either by heating the irradiated sample at a suitable high temperature or by exposing it to UV light; and the erased glass‐ceramic plate is reusable. The new Sm‐doped oxyfluoride glass‐ceramic with CaF2 nanocrystals reported in this work shows potential for practical use in high‐dose and high‐resolution dosimetry for MRT.