CaF2 Nanoparticles Research Articles

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.

Nd3+ activated CaF2 NPs as colloidal nanothermometers in the biological window

CaF2 nanoparticles activated with Nd3+ ions have been investigated as potential optical nanothermometers in the biological windows. The variation of the Nd3+ emission around 850 and 1060 nm as a function of the temperature has been measured. A ratiometric approach, based on the relative changes in the intensities of different emission bands of the Nd3+ has been considered to investigate the thermometric properties of the NPs. The evaluated thermal sensitivity is around 0.12% K−1, a value that is similar to those found for other water dispersible nanothermometers based on the near-infrared luminescence of the Nd3+ lanthanide ion. The temperature uncertainty is around 1.8 °C, small enough to monitor the local temperature during a photothermal treatment.

Double-jet precipitation synthesis of CaF2 nanoparticles: The effect of temperature, solvent, and stabilizer on size and morphology

In this study, we report the double-jet precipitation synthesis of CaF2 nanoparticles under different reaction conditions and the effect of these conditions on the morphology, size and size distribution of CaF2 nanoparticles. We varied temperature in the range of 20–95 °C and utilized water, methanol, ethanol, 2-propanol, acetone, and acetonitrile as solvents. Citrate ions were used as a stabilizer and a surface modifier in some experiments. The products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), surface measurements by BET method, elemental analysis, and IR spectroscopy. The results indicate that we synthesized cubic phase CaF2 nanoparticles. We demonstrated that the double-jet precipitation technique can be successfully used for the synthesis of CaF2 nanoparticles with a narrow size distribution, spherical or cubic shape, and an average size of 9–180 nm depending on the reaction conditions. It is shown that the nature of the solvent and a stabilizer additive influence the morphology and size of CaF2 nanoparticles and play a significant role in the particle formation.

Structural and optical study of CaF2 nanoparticles produced by a microwave-assisted hydrothermal method

CaF2 nanoparticles were synthesized by the microwave-assisted hydrothermal method. With the addition of the ethylenediamine (EDA) as chelating agent, the size of the particles was reduced. The CaF2 exhibit single phase identified for X-ray diffraction (XRD) and confirmed by Rietveld refinement. Scanning electron microscopy (SEM) images showed nanoparticles with non-uniform morphology and statistical analysis of collections of particles reviewed that the EDA decreases both the average particle size and average aspect ratio of the particles. The chemical composition of the surface of the particles was investigated by X-ray Photoelectron Spectroscopy (XPS) and the results indicated the presence of reasonable amounts of hydroxyl groups and oxygen ions in the samples produced with EDA. Radioluminescence (RL) measurements showed that both types of nanoparticles presented intrinsic scintillation emission formed by two main bands and that the CaF2 samples produced without EDA presented higher emission intensity. The broad RL band centered at approximately 293nm is related to self-trapped exciton (STE) emission of calcium fluoride, while the band at 428nm can be due to the presence of F centers. The STE excitation and optical band gap were measured through photoluminescent excitation spectra in the VUV range.

Characterizations of a hot-pressed Er and Y codoped CaF2 transparent ceramic

In this work, Er:CaF2 transparent ceramic codoped with non-active ions (Y3+ ions) was fabricated by hot-pressed (HP) sintering method. Er and Er/Y codoped CaF2 nanoparticles were synthesized by a simple coprecipitation method, the phase composition and the shape of obtained nanoparticles were analysed by X-ray diffraction patterns and transmission electron microscopy, respectively. Fluorite transparent ceramics were fabricated by HP sintering method at a temperature of 800°C. For 2mm thickness ceramic samples, the optical transmission of Er, Y:CaF2 transparent ceramic in the near-infrared range reached about 90%, and the optical transmission of Er, Y:CaF2 transparent ceramic was much higher than Er:CaF2 ceramic sample. Microstructures were characterized using SEM analysis, the two ceramic samples exhibited nearly pore-free microstructure and the average grain sizes were less than 1.0μm. The absorption, upconversion and infrared emission spectras of ceramic samples were also measured and discussed.

Near-infrared Quantum Cutting for Solar Cells in Ce3+-Yb3+ Co-Doped CaF2 Nanoparticles.

A series of Ce3+-Yb3+ co-doped CaF2 nanoparticles were prepared by a hydrothermal synthesis method. The structure, morphology, photoluminescence properties, and fluorescence dynamics were studied systemically. Under the excitation of the 5d level of Ce3+, an efficient energy transfer from Ce3+ to Yb3+ occurred, and the efficient near-infrared emission at 900-1050 nm matching to the energy of Si band gap of Si-based solar cells was observed. The lifetime of Ce3+ decreased, and the inner quantum efficiency increased with increasing Yb3+ concentration. The quantum efficiency dependent on Yb3+ concentration was calculated, and the maximum efficiency approached 186%. As a down-conversion luminescent convertor, this kind of material can be used in front of Si-based solar cells to reduce thermalization loss and enhance conversion efficiency of solar cells.

Eu3+-doped CaF2 nanoparticles functionalized by salicylic acid: synthesis, structural, optical and morphological studies

Luminescent CaF2:Eu3+ nanoparticles sensitized by organic ligands have shown a great potential in the areas of optoelectronics. The present study reports the synthesis of salicylic acid functionalized CaF2:Eu3+ nanoparticles and their structural, optical and morphological properties have been discussed. The effect on photoluminescent properties with different doping concentrations of salicylic acid was explained by the energy transfer from salicylic acid to Eu3+ ion which resulted in the sensitization of luminescence with increase in the amount of salicylic acid up to at 2.5 mmol and then decreases with the addition of more salicylic acid due to concentration quenching. The change in the PL intensity has also been studied at different excitation wavelengths 248, 298 and 350 nm. The UV–Vis absorption spectra of the synthesized nanoparticles also show a slight red shift relative to the salicylic acid, suggests the coordination of salicylic acid to the surface of CaF2:Eu3+ nanoparticles. The FTIR spectra indicates the coordination of salicylic acid to the CaF2:Eu3+ nanoparticles and the transmission electron microscopy (TEM) depicts the morphology of the synthesized nanoparticles, indicating that the Eu3+ ion is surrounded by the dispersed salicylic acid functionalized CaF2:Eu3+ nanoparticles. The results indicate that salicylic acid can efficiently sensitize the luminescence of Eu3+ ion embedded CaF2 nanoparticles and these photoluminescent properties can be efficiently utilized for use in optoelectronic devices.

Elaboration, Structure and Luminescence of Sphere-Like CaF2:RE Sub-Microparticles by Ionic Liquids Based Hydrothermal Process.

A new and simple method for the synthesis of rare earth ion doped CaF₂ (CaF₂:RE³⁺) sub- microparticles is presented, using an ionic liquids based hydrothermal process. The structural properties of the CaF₂ nanoparticles were investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The CaF₂ nanoparticles exhibited a sphere-like morphology with a diameter of about 150 nm. During the synthesis, the ionic liquid [bmim]BF₄(1-butyl, 2-methylimidazolium tetrafluoroborate) acts as both a co-solvent and reactant. The crucial effect of EDTA-2Na (ethylene diamine tetra acetic acid disodium salt) on the formation of CaF₂:RE sub-microparticles was explored and discussed. The strong green (513-569 nm) and strong red (636-685 nm) upconversion emissions of the CaF₂:Er³⁺, Yb³⁺ nanoparticles (λex = 980 nm) were also investigated. The luminescent properties of CaF₂:Eu³⁺ and CaF₂:Ce³⁺,Tb³⁺ were also evaluated. This work may represent a new step in synthesizing fluoride sub-nanocrystals using ionic liquids.

Transparent bulk-size nanocomposites with high inorganic loading

With relatively high nanoparticle loading in polymer matrices, hybrid nanocomposites made by colloidal dispersion routes suffer from severe inhomogeneous agglomeration, a phenomenon that deteriorates light transmission even when the refractive indices of the inorganic and organic phases are closely matched. The dispersion of particles in a matrix is of paramount importance to obtain composites of high optical quality. Here, we describe an innovative, yet straightforward method to fabricate monolithic transparent hybrid nanocomposites with very high particle loading and high refractive index mismatch tolerance between the inorganic and organic constituents. We demonstrate 77% transmission at 800 nm in a 2 mm-thick acrylate polymer nanocomposite containing 61 vol. % CaF2 nanoparticles. Modeling shows that similar performance could easily be obtained with various inorganic phases relevant to a number of photonic applications.

Nd<sub>2</sub>Fe<sub>14</sub>B/CaF<sub>2</sub> Composite Magnet Synthesized by Liquid-Phase Coating

The electrical resistance is one of the important properties for Nd2Fe14B magnets with low eddy-current loss. We have coated Nd–Fe–B melt-spun particles with CaF2 by the liquid-phase coating technique and then performed hot-pressing and hot-deformation to make Nd2Fe14B/CaF2 composite magnets. The morphology and distribution of CaF2 in the magnets and its influence on the coercivity mechanism and the electrical properties of the Nd2Fe14B/CaF2 magnets have been investigated by scanning electron microscope, hysteresisgraph, and four-probe resistivity meter. The morphology of CaF2 depended on the liquid-phase synthesis parameters, especially the concentrations of the reactants. On the surface of Nd–Fe–B particles, there formed only a few CaF2 nanoparticles and nanoflakes when 0.5 and 1 mol/l reactant aqueous solutions were used as precursors. With the increase of the concentrations of the reactants, there obtained a flake-shaped thin CaF2 coating on the Nd–Fe–B particles. In addition, the CaF2 coating became thicker, $\sim 150$ –200 nm, with further increasing the concentrations of the reactants to 2 mol/l. After compaction and hot-deformation, the electrical resistances of the Nd2Fe14B/CaF2 magnets fabricated using the concentrations of the reactants of 2 mol/l reached approximately 580 $\mu \Omega $ cm, which increased about 348% comparing with the corresponding pure Nd–Fe–B magnet. It is interesting to find that the coercivity of the Nd2Fe14B/CaF2 composite magnets increased from 12.42 to 13.5 kOe when the concentrations of the reactants increased from 0.5 to 1.5 mol/l and then decreased to 10.2 kOe with further increasing the concentrations of the reactants to 2 mol/l. The reason for the increment of coercivity may be that CaF2 nanoparticles and nanoflakes acted like lubricants that were beneficial to the rotation and the orientation of Nd2Fe14B particles during the hot-deformation process.

Removal of fluoride and turbidity from semiconductor industry wastewater by combined coagulation and electroflotation

Reduction of toxic dissolved fluoride and CaF2 nanoparticle pollution is a critical environmental problem for the semiconductor industry. In this study, suspended matter and fluoride are simultaneously eliminated by combining coagulation and electroflotation. The EF cell was equipped with DSA titanium coated with ruthenium oxide (Ti/RuO2) as anode and stainless steel as cathode. High turbidity removal efficiency is achieved by using EF as a separation technique. The effect of the following parameters: electrolysis time, coagulant concentration, initial pH, nature of neutralizing salt and current intensity was studied. Removal efficiencies of both fluoride and CaF2 nanoparticles are satisfactory. Under optimum conditions, the solid–liquid separation efficiency is about 97% in terms of turbidity removal which corresponds to a residual turbidity of 4.4 NTU complying with the standard limit (5 NTU), while fluoride efficiency removal may reach 73% corresponding to 10 mg/L, which is below the environmental recommendations.

Influence of Yb Concentration on the Optical Properties of CaF2 Transparent Ceramics Codoped with Er and Yb

Er, Yb:CaF2 nanoparticles with different Yb concentrations were synthesized by a coprecipitation method using nitrates as raw materials. X‐ray powder diffraction and transmission electron microscopy analysis showed that the nanoparticles were single fluorite phase and the nanoparticle size was found to decrease with increasing Yb concentrations. The obtained nanoparticles were hot‐pressed at 800°C under 30 MPa under vacuum environment to fabricate Er, Yb:CaF2 transparent ceramics. The influence of Yb ion concentrations on the optical transmission, microstructure, and luminescence properties of Er, Yb:CaF2 transparent ceramics were investigated. The addition of Yb ions was found effectively to reduce grain size and has a positive effect on improving the optical transmission of Er, Yb:CaF2 transparent ceramics. The highest transmittance in the near‐infrared spectral region of the Er, Yb:CaF2 transparent ceramic reached about 90%. The green, red, and near‐infrared emission intensities were found to increase with increasing Yb concentration.

Defect luminescence in CaF2 nanoparticles

The stationary X-ray excited luminescence (XEL) spectra at 80 and 294K as well as the thermally stimulated luminescence (TSL) in the 80–320К range of the CaF2 nanoparticles with various grain sizes (20–60nm) were studied. Similarly to the earlier studied single CaF2 crystals, specific bands were revealed both for XEL and for TSL of the radiation defects. XEL spectra of the CaF2 nanoparticles contain emission bands in the 240–500nm region caused by excitonic-like excitations localized at the radiation defects of the CaF2 lattice as well as recombination luminescence band at 560nm. TSL integral curves of CaF2 nanoparticles contain peaks that correspond to the delocalization of the hole centers and to their recombination with F-centers and with electronic centers of the cationic sublattice. Activation energies of delocalization processes of the hole centers were determined.It was established that the light output of both XEL and TSL of the CaF2 nanoparticles increases with an increase of size of nanoparticles in the region of 20–50nm.

Microstructure and optical properties of hot-pressed Er:CaF2 transparent ceramics

CaF2 nanoparticles doped with different Er concentrations were synthesized by a co-precipitation method. The phase compositions and morphology of the obtained Er:CaF2 nanoparticles were characterized by X-ray diffraction and transmission electron microscopy analysis. It was found that the size and shape of obtained CaF2 nanoparticles were modified through Er3+ doping. High quality Er:CaF2 transparent ceramics were fabricated by hot-pressed sintering method in a vacuum environment. The influence of Er3+ doping concentrations and sintering temperatures on optical transmission and microstructure of Er:CaF2 transparent ceramics were investigated. When doped with 5 mol% Er3+, the CaF2 ceramic sintered at 800 °C showed the best optical transmission, and for the 2 mm thickness sample, the optical transmission reached about 90% in the near-infrared spectral region.

Effect of doping of calcium fluoride nanoparticles on the photoluminescence properties of europium complexes with benzoic acid derivatives as secondary ligands and 2-aminopyridine as primary ligand

The present article reports the synthesis of three Eu(III) complexes [Eu(BA)3(2-ap)] (1), [Eu(HBA)3(2-ap)] (2) and [Eu(ABA)3(2-ap)] (3) (BA=benzoic acid, HBA=2-hydroxy benzoic acid, ABA=2-amino benzoic acid and 2-ap=2-aminopyridine) carried out in ethanol solution. The complexes were further doped with CaF2 nanoparticles and a change in the photoluminescence properties was observed. The compositions and structural investigation of the complexes were determined by elemental analysis and Fourier transform infrared spectroscopy (FTIR) which suggest the coordination of ligands with the central Eu(III) ion. The optical properties of the complexes were studied by Ultraviolet Visible absorption spectroscopy (UV–Vis) and photoluminescence studies (PL). The relative PL intensity was enhanced in the Eu(III) complexes doped with CaF2 nanoparticles as compared to the pure Eu(III) complexes, however the increase in intensity varied in the order of ligands ABA>HBA>BA. The photoluminescence lifetime decay curves also revealed the longer lifetime (τ) and higher quantum efficiency (η) for europium complexes with ABA ligands suggesting the efficient energy transfer and better sensitizing ability of the ligand to europium ion. The morphology of the synthesized compounds were studied by Scanning Electron Microscopy (SEM) revealing spherical morphology with agglomeration of the nanoparticles.

Formation of nanoscopic CaF2via a fluorolytic sol–gel process for antireflective coatings

For the first time transparent antireflective CaF2-coatings were prepared from clear CaF2-sols obtained via the fluorolytic sol–gel synthesis and containing homo-dispersed CaF2 nano-particles.

Spatially selective Er/Yb-doped CaF2 crystal formation by CO2 laser exposure

We report the glass–ceramic precipitation on the oxyfluoride glass surface by spatially selective annealing with a CO2 laser and a heat gun exposure. X-ray diffraction analysis showed the formation of major CaF2 and miner Ca2SiO4 nanoparticles. We observed ∼100nm nanoparticle aggregation by tunneling electron microscopy and element distribution in glass and crystal phases. Spatial distribution of glass ceramics near the glass surface was probed by confocal fluorescence microscope by using much enhanced emission from the Er ions in the laser-treated area. Strong emissions at 365nm excitation and visible up-conversion emissions at 980nm excitation also indicated well incorporation of Er and Yb ions into a crystalline environment.

Structural plasticity of calmodulin on the surface of CaF2 nanoparticles preserves its biological function.

Nanoparticles are increasingly used in biomedical applications and are especially attractive as biocompatible and biodegradable protein delivery systems. Herein, the interaction between biocompatible 25 nm CaF2 nanoparticles and the ubiquitous calcium sensor calmodulin has been investigated in order to assess the potential of these particles to serve as suitable surface protein carriers. Calmodulin is a multifunctional messenger protein that activates a wide variety of signaling pathways in eukaryotic cells by changing its conformation in a calcium-dependent manner. Isothermal titration calorimetry and circular dichroism studies have shown that the interaction between calmodulin and CaF2 nanoparticles occurs with physiologically relevant affinity and that the binding process is fully reversible, occurring without significant alterations in protein secondary and tertiary structures. Experiments performed with a mutant form of calmodulin having an impaired Ca(2+)-binding ability in the C-terminal lobe suggest that the EF-hand Ca(2+)-binding motifs are directly involved in the binding of calmodulin to the CaF2 matrix. The residual capability of nanoparticle-bound calmodulin to function as a calcium sensor protein, binding to and altering the activity of a target protein, was successfully probed by biochemical assays. Even if efficiently carried by CaF2 nanoparticles, calmodulin may dissociate, thus retaining the ability to bind the peptide encompassing the putative C-terminal calmodulin-binding domain of glutamate decarboxylase and activate the enzyme. We conclude that the high flexibility and structural plasticity of calmodulin are responsible for the preservation of its function when bound in high amounts to a nanoparticle surface.

Fabrication of highly-transparent Er:CaF2 ceramics by hot-pressing technique

Abstract Highly-transparent trivalent erbium ion doped calcium fluoride (5 mol % Er:CaF2) ceramics were fabricated by a hotpressing (HP) method using high-purity Er:CaF2 nanoparticles, which were synthesized by co-precipitation method. The mean grain size of the nanoparticles was about 24.7 nm. The nanoparticles were sintered at 600 °C, 700 °C, 800 °C and 900 °C, respectively, for 30 min under a uniaxial pressure of 30 MPa and vacuum of 10−3 Pa with 1 mol % lithium fluoride (LiF) as sintering additive. The 5 mol % Er:CaF2 ceramics sintered at 800 °C exhibits high density and pore-free microstructure with an average grain size of about 8 μm. The optical transmittance of the transparent ceramics is close to 85 % at visible and nearinfrared wavelengths. The strong and broad absorptions peaks corresponding to characteristic absorption of trivalent erbium ions make the ceramics a potential candidate for infrared and upconversion laser operating.

Spatially selective Er/Yb-doped CaF2 crystal formation by CO2 laser exposure

We report the glass–ceramic precipitation on the oxyfluoride glass surface by spatially selective annealing with a CO2 laser and a heat gun exposure. X-ray diffraction analysis showed the formation of major CaF2 and miner Ca2SiO4 nanoparticles. We observed ∼100nm nanoparticle aggregation by tunneling electron microscopy and element distribution in glass and crystal phases. Spatial distribution of glass ceramics near the glass surface was probed by confocal fluorescence microscope by using much enhanced emission from the Er ions in the laser-treated area. Strong emissions at 365nm excitation and visible up-conversion emissions at 980nm excitation also indicated well incorporation of Er and Yb ions into a crystalline environment.