CaF2 Nanocrystals Research Articles

Yb3+-doped oxyfluoroborosilicate glasses and glass ceramics containing CaF2 nanocrystals for 981 nm near infrared fiber lasers

Yb3+-doped oxyfluoroborosilicate glasses and glass ceramics containing CaF2 nanocrystals for 981 nm near infrared fiber lasers

Photoinduced electron transfer in Eu2+ and Sm3+ co-doped CaF2 nanocrystals prepared by co-precipitation

We report on the photoinduced electron transfer in Eu2+ and Sm3+ co-doped CaF2 nanocrystals prepared by a facile co-precipitation method using CaCl2, EuCl2, SmCl3 and NH4F. The doping of divalent Eu2+ was realized by employing granular zinc in the CaCl2/EuCl2 solution under nitrogen to reduce Eu3+→ Eu2+. The CaF2:xEu,0.2%Sm nanocrystals were prepared with different Eu concentrations of x = 0, 0.2, 0.5, 1.5, and 2.5%, resulting in an average crystallite size between 54 and 36 ± 1 nm. The maximum luminescence intensities of Eu2+ and Sm2+ were obtained for the x = 0.5% sample. Photoinduced electron transfer and back transfer in the CaF2:0.5%Eu,0.2%Sm nanocrystals were measured by detecting the Eu2+ and Sm2+ luminescence, upon exposure to 340 and 310 nm UV LED light. Interestingly, the photoionization of Eu2+ occurred 6 times faster for the sample co-doped with 0.2% Sm. Upon exposure to 340 nm light, a reduction and increase of the Eu2+ and the Sm2+ luminescence were observed, respectively. Subsequent exposure to 310 nm light decreased the Sm2+ luminescence, but the luminescence of Eu2+ kept on being bleached, albeit at a lower rate. Whilst the Sm valence state could be reversibly changed between the trivalent and divalent state by 340 and 310 nm light, the observation that the Eu2+ luminescence did not recover may indicate that a third centre is involved in the photoinduced electron transfer mechanisms.

Surface modification of a biomass-derived self-supported carbon nano network as an emerging platform for advanced field emitter devices and supercapacitor applications.

Herein, a self-supported carbon network is designed through the sole pyrolysis of Carica papaya seeds (biomass) without any activation agent, demonstrating their field emission and supercapacitor applications. The pyrolysis of seeds in an argon atmosphere leads to the formation of interconnected, rod-like structures. Furthermore, the hydrofluoric acid treatment not only removed impurities, but also resulted in the formation of CaF2 nanocrystals with the addition of F-doping. From the field emission studies, the turn-on field values defined at an emission current density of ∼10 μA cm-2 were found to be ∼2.16 and 1.21 V μm-1 for the as-prepared carbon and F-doped carbon, respectively. Notably, F-doped carbon exhibits a high emission current density of ∼9.49 mA cm-2 and has been drawn at an applied electric field of ∼2.29 V μm-1. Supercapacitor studies were carried out to demonstrate the multi-functionality of the prepared materials. The F-doped carbon electrode material exhibits the highest specific capacitance of 234 F g-1 at 0.5 A g-1. To demonstrate the actual supercapacitor application, the HFC//HFC symmetric coin cell supercapacitor device was assembled. The overall multifunctional applicability of the fabricated hybrid structures provides a futuristic approach to field emission and energy storage applications.

A membrane biosensor based on colour differences encoded upconversion silica nanoparticles for multiplex nucleic acids detection

Upconversion luminescent nanomaterials provide a powerful tool for multiplex nucleic acids detection simultaneously. However, the conventional encoding methods based on emission spectrum or intensity have limitations due to spectral overlap and cross-relaxation quenching among various activators. Herein, we present a facile method for the preparation of high-capacity upconversion luminescent coding nanoparticles by multiple regulating the synthesis kinetics of lanthanide ions doped CaF2 nanocrystals in mesoporous silica nanotemplets (named C@S). The secondary growth of nanocrystals with different lanthanide ions inside mesopores has a similar performance to that of core/multi-shell-structured upconversion nanoparticles prepared through complicated processes. The textural properties and chemical conformations are maintained after repeated recombination, which is beneficial for reproducible multiplex bioassays. A membrane biosensor is constructed using various probe-modified C@S and a flexible graphene oxide-coated film to detect and identify multiple molecular species in one sample. The colour differences of encoded C@S derived from CIE-L*a*b* colour coordinate system are applied to quantify the divergence of upconversion luminescence for the first time, which allows the upconversion nanoparticles with similar spectra and luminescence colours to be used in the same multiplex detection platform and avoids subjective judgement. This work opens new opportunities for upconversion luminescent materials in the field of automatic high-throughput detection.

Lanthanide-doped upconversion glass–ceramic photocatalyst fabricated from fluorine-containing waste for the degradation of organic pollutants

Fluorine-containing waste is one kind of hazardous waste characteristic by hard disposal and utilization, it is an attractive way to prepare for fluoride-based luminescent matrix. In this work, to realize the high value-added utilization of fluorine-containing waste and reduce cost of the raw materials for preparation near-infrared (NIR) glass–ceramic (GC) photocatalyst, the pure fluoride of luminescent matrix was replaced by introducing fluorine-containing waste. The waste contained NIR GC photocatalyst was synthesis by the method of facile in-situ etching of an upconversion GC with HCl, which possesses core–shell structure, where the GC micro-powder including optically active centers lanthanides doped CaF2 nanocrystals are displayed as the core, and the BiOCl is as the superficial coating. The upconversion emission performance of CaF2 based luminescent matrix in photocatalyst is not weakened with HCl etching. NIR GC photocatalyst has high methyl orange and enrofloxacin degradation rate of 86 % and 82 % over 180 min after NIR light irradiation, respectively. The UV–Vis–NIR photocatalytic activity was enhanced degradation rate (93 % in 15 min) of enrofloxacin compared with those of commercial P25 and BiOCl. In addition, the photocatalyst had stable photocatalytic activity and it also can be regenerated. The study provided references for high value-added utilization fluorine-containing waste.

The role of Li+ interstitial ions in up-conversion intensity of CaF2:Yb3+, Tb3+ phosphors

CaF2 nanocrystals doped with Yb3+, Tb3+ and Li+ ions were investigated to study the correlation between their structural, electronic and up-conversion emission properties. The role of interstitial and substitutional Li+ ions in enhancing the up-conversion intensities were discussed. Evidence of Li+ at both interstitial and substitutional sites was provided by X-ray diffraction and X-ray photoelectron spectroscopy. The diffuse reflection spectra confirmed that the dopants were successfully incorporated. Up-conversion emission spectra under 980 nm excitation wavelength revealed the vital role of Li+ ions in enhancing both blue and green emission. The results reported during this investigation are expected to provide better understanding of the mechanism involved in enhancing the up-conversion emission in many Yb3+-Tb3+ co-doped fluoride materials due to Li+ ions doping.

A Calcium Fluoride Nanozyme for Ultrasound‐Amplified and Ca2+‐Overload‐Enhanced Catalytic Tumor Nanotherapy

The anticancer mechanism of nanozymes is dominantly associated with the capacity for generation of reactive oxygen species (ROS) caused by the valence change of metal elements. However, very little research is focused on and has achieved the exploration and development of enzyme-mimicking activities of valence-invariable metal compounds. Herein, a distinct valence-invariable calcium fluoride (CaF2 ) nanozyme with ultrasound (US)-enhanced peroxidase (POD)-mimicking activity is rationally designed and engineered for efficient calcium (Ca2+ )-overload-enhanced catalytic tumor nanotherapy, which is the first paradigm of Ca-based nanozymes for catalytic cancer treatment. The release of exogenous Ca2+ ions from CaF2 nanocrystals and deleterious ROS generation derived from US-amplified POD-mimicking properties facilitate intracellular Ca2+ accumulation and achieve Ca2+ -overload-induced mitochondrial dysfunction through introducing exogenous Ca2+ ions and regulating calcium-pumping channels of neoplastic cells. Especially, US as an exogenous energy input is capable of substantially amplifying POD-mimicking catalytic activities of CaF2 nanozyme, ultimately achieving efficient anti-neoplastic outcome on both 4T1 breast tumor and H22 hepatic carcinoma animal models. Such a discovery of enzyme-like activity of valence-invariable metal compounds can broaden the cognition scope of nanozymes and effectively serves the field of catalytic and chemoreactive nanomedicine.

Europium (II)-Doped CaF2 Nanocrystals in Sol-Gel Derived Glass-Ceramic: Luminescence and EPR Spectroscopy Investigations.

The remarkable properties of Eu2+-activated phosphors, related to the broad and intense luminescence of Eu2+ ions, showed a high potential for a wide range of optical-related applications. Oxy-fluoride glass-ceramic containing Europium (II)-doped CaF2 nanocrystals embedded in silica matrix were produced in two steps: glass-ceramization in air at 800° with Eu3+-doped CaF2 nanocrystals embedded followed by Eu3+ to Eu2+ reduction during annealing in reducing atmosphere. The broad, blue luminescence band at 425 nm and with the long, weak tail in the visible range is assigned to the d → f type transition of the Eu2+ located inside the CaF2 nanocrystals in substitutional and perturbed sites, respectively; the photoluminescence quantum yield was about 0.76. The X-ray photoelectron spectroscopy and Electron paramagnetic spectroscopy confirmed the presence of Eu2+ inside the CaF2 nanocrystals. Thermoluminescence curves recorded after X-ray irradiation of un-doped and Eu2+-doped glass-ceramics showed a single dominant glow peak at 85 °C related to the recombination between F centers and Eu2+ related hole within the CaF2 nanocrystals. The applicability of the procedure can be tested to obtain an oxy-fluoride glass-ceramic doped with other divalent ions such as Sm2+, Yb2+, as nanophosphors for radiation detector or photonics-related applications.

Optical properties of glasses and glass-ceramics for optical amplifiers, photovoltaic devices, color displays, optical limiters, and Random Lasers

A review of recent works with glasses and glass-ceramics having characteristics appropriate to be used in various types of optical devices is presented. Germanate and tellurite glasses, doped with rare-earth ions and heat-treated in proper ways were studied. The improved performance of neodymium ions (Nd3+) doped GeO2-PbO glasses as optical amplifiers due to the addition of Al2O3 in the original glass composition is presented. Proof-of-concept experiments with europium ions (Eu3+) doped GeO2-PbO glasses containing TiO2 nanoparticles, used as cover layer to enhance silicon solar-cells efficiency, are discussed. Experiments with powders of Nd3+ doped TeO2–ZnO glasses allowed to characterize this system as promising medium for operation of Random Lasers. Characterization of B2O3–CaF2 transparent glass-ceramics with CaF2 nanocrystals, praseodymium ions (Pr3+), and silver nanoparticles, revealed enhanced processes of energy-transfer favored by the presence of the metal-nanoparticles. This material has potential to be used as tunable color display. Finally, the synthesis and spectroscopic properties of TeO2–Ba(PO3)2 glass and glass-ceramics and their application as optical limiters is considered.

Charge State Effects in Swift-Heavy-Ion-Irradiated Nanomaterials

The aim of this experimental work was to investigate the influence of the ion beam charge state on damage production in nanomaterials. To achieve this, we employed Raman spectroscopy, atomic force microscopy, and transmission electron microscopy to investigate nanomaterials irradiated by a 23 MeV I beam. We found a significant influence of the ion charge state on damage production in monolayer graphene, but found no evidence of this effect in bilayer and trilayer graphene, nor in graphite. Furthermore, we found no evidence of this effect in CaF2 and SiO2 nanocrystals irradiated with the same ion beam.

Cation-Ligand Complexation Mediates the Temporal Evolution of Colloidal Fluoride Nanocrystals through Transient Aggregation.

Colloidal inorganic nanofluorides have aroused great interest for various applications with their development greatly accelerated thanks to advanced synthetic approaches. Nevertheless, understanding their colloidal evolution and the factors that affect their dispersion could improve the ability to rationally design them. Here, using a multimodal in situ approach that combines DLS, NMR, and cryogenic-TEM, we elucidate the formation dynamics of nanofluorides in water through a transient aggregative phase. Specifically, we demonstrate that ligand-cation interactions mediate a transient aggregation of as-formed CaF2 nanocrystals (NCs) which governs the kinetics of the colloids’ evolution. These observations shed light on key stages through which CaF2 NCs are dispersed in water, highlighting fundamental aspects of nanofluorides formation mechanisms. Our findings emphasize the roles of ligands in NCs’ synthesis beyond their function as surfactants, including their ability to mediate colloidal evolution by complexing cationic precursors, and should be considered in the design of other types of NCs.

Assessment of Bond Integrity, Durability, and Degree of Conversion of a Calcium Fluoride Reinforced Dentin Adhesive

Our study aimed to synthesize and compare the mechanical properties and dentin interaction of two adhesives; experimental adhesive (EA) and EA containing 5 wt.% calcium fluoride (CaF2) nano-crystals (CaF2 adhesive-CAFA). CaF2 nano-crystals were synthesized by reacting two solutions (containing calcium and fluoride) in a glass chamber using a heated air system. The EA was produced using a mix of monomers, photo-initiators, camphorquinone, and electron initiators. The synthesized CaF2 nano-crystals were centrifuged to guarantee that inside the adhesive there is homogenized dispersion of the filler particles. Their integration in the EA yielded two groups; Gp-1: EA (without CaF2, control) and Gp-2: (5 wt.% CaF2 containing adhesive, CAFA). Sixty teeth were prepared and set to form bonded specimens using the two adhesives. The CaF2 nano-crystals were irregularly shaped with an average particle size of 30–200 nm. The highest μTBS values were obtained for CAFA-non-thermocycled (NTC) samples (32.63 ± 3.15), followed by EA-NTC (31.80 ± 3.75) specimens. On thermocycling (TC), both adhesive groups presented lower μTBS values (CAFA-TC: 29.47 ± 3.33 and EA-TC: 24.04 ± 3.69). Hybrid layer (HL) formation and resin tags of varying depths were perceived for both adhesive groups. The EDX analysis demonstrated the presence of carbon (C), silica (Si), calcium (Ca), and fluoride (F) for CAFA group. Micro-Raman spectroscopy revealed distinct peaks for CaF2 nano-crystals. The CAFA group presented the greatest DC. The addition of CaF2 nano-crystals in the adhesive caused improved bond μTBS and DC. The incorporation also demonstrated suitable dentin interaction, depicted by appropriate HL and resin tag development.

Optical properties of B2O3–CaF2 glass-ceramics doped with silver nanoparticles and praseodymium ions

We report on the synthesis and optical experiments with borate glass-ceramics (B2O3–CaF2 glass enclosing CaF2 nanocrystals), either undoped or doped with praseodymium ions (Pr3+) and containing silver nanoparticles (Ag-NPs). The techniques of transmission electron microscopy, X-rays diffraction, linear optical absorption, and photoluminescence (PL), were used to characterize the samples prepared. The glass samples, prepared by using the diffusion method, were heat-treated for nucleation of CaF2 nanocrystals (CaF2–NCs) with cubic structure and dimensions of ~20 nm, and Ag-NPs (with sizes between ~2 and ~50 nm). Optical absorption and PL experiments were performed. The glass-ceramic with Ag-NPs, but without Pr3+, exhibited tenfold enhancement (~1000%) of the blue PL from the CaF2–NCs, in comparison with the samples without Ag-NPs. In the samples with Pr3+ the presence of Ag-NPs induced small enhancement of the absorption spectra in the blue region [Pr3+ transitions: 3H4→3PJ (J = 0, 1, 2), 1I6] and 75% increase of the emitted intensity in the red region (Pr3+ transitions: 1D2→3H4). On the other hand, the CaF2–NCs luminescence was totally quenched due to the presence of Pr3+. This effect was attributed to resonant energy-transfer from the CaF2–NCs to Pr3+. The phenomena observed in the experiments were understood considering the contributions of the local electric field in the vicinities of the Ag-NPs and processes of energy-transfer among the CaF2–NCs, Ag-NPs and Pr3+.

Effect of CaF2 substitution by CaO on spectroscopic properties of oxyfluoride glasses

The main purpose of the present study was to investigate the optical properties and structural changes of oxyfluoride glasses containing various amounts of CaF2 (CaO). Four nominal chemical compositions, i. e., 45SiO2-20Al2O3-(30-x)CaF2-(5+x)CaO (x = 0, 5, 10, 15) (mole ratio), were chosen and glass samples were obtained by convenient melt- quenching process. On the basis of DSC curves, the peak temperature of CaF2 crystallization was increased from 673 °C to 805 °C by replacing CaF2 with CaO. This result was related to the reduction of the number of non-bridging fluorines (NBFs) and non-bridging oxygens (NBOs) in presence of higher CaO amounts. Increment of density (from 2.74 to 2.92 g/cm3) and decrement of molar volume (from 27.24 to 24.48 cm3) of samples approved this claim to some extent. In addition, according to XRD patterns, the size of CaF2 nanocrystals got smaller (from 29.21 to 10.13 nm) by increasing the quantity of CaO. This was ascribed to the difficulty of the penetration of ions through the diffusion barriers around fluoride nanocrystals in glasses with more continuous structure. Fluorine loss measurements and Structural studies showed that more CaO addition decreased NBFs and NBOs and more Al-F bonds were increased compare to Si-O-Al bonds. UV–vis spectra were used to determine the optical properties comprising Fermi energy level, optical band gaps and Urbach energy. Calculations of Fermi energy and optical band gaps demonstrated the reduction of weak and dangling bonds in samples with more CaO. Moreover, Urbach energy, the indicator of the disorderliness of the network, was decreased from 0.275 to 0.245 eV. The sample containing 25 and 10 mol ratio of CaF2 and CaO respectively, showed higher transmittance in IR region and its cut-off was red-shifted to higher wavelengths in comparison to other glasses.

Structural and Photoluminescence Investigations of Tb3+/Eu3+ Co-Doped Silicate Sol-Gel Glass-Ceramics Containing CaF2 Nanocrystals.

In this work, the series of Tb3+/Eu3+ co-doped xerogels and derivative glass-ceramics containing CaF2 nanocrystals were prepared and characterized. The in situ formation of fluoride crystals was verified by an X-ray diffraction technique (XRD) and transmission electron microscopy (TEM). The studies of the Tb3+/Eu3+ energy transfer (ET) process were performed based on excitation and emission spectra along with luminescence decay analysis. According to emission spectra recorded under near-ultraviolet (NUV) excitation (351 nm, 7F6 → 5L9 transition of Tb3+), the mutual coexistence of the 5D4 → 7FJ (J = 6–3) (Tb3+) and the 5D0 → 7FJ (J = 0–4) (Eu3+) luminescence bands was clearly observed. The co-doping also resulted in gradual shortening of a lifetime from the 5D4 state of Tb3+ ions, and the ET efficiencies were varied from ηET = 11.9% (Tb3+:Eu3+ = 1:0.5) to ηET = 22.9% (Tb3+:Eu3+ = 1:2) for xerogels, and from ηET = 25.7% (Tb3+:Eu3+ = 1:0.5) up to ηET = 67.4% (Tb3+:Eu3+ = 1:2) for glass-ceramics. Performed decay analysis from the 5D0 (Eu3+) and the 5D4 (Tb3+) state revealed a correlation with the change in Tb3+–Eu3+ and Eu3+–Eu3+ interionic distances resulting from both the variable Tb3+:Eu3+ molar ratio and their partial segregation in CaF2 nanophase.

Luminescence and photoionization of X-ray generated Sm2+ in coprecipitated CaF2 nanocrystals.

We report photoluminescence and photoionization properties of Sm2+ ions generated by X-irradiation of nanocrystalline CaF2:Sm3+ prepared by coprecipitation. The nanocrystals were of 46 nm average crystallite size with a distribution of ±20 nm and they were characterised by XRD, TEM and SEM-EDS. At room temperature, the X-irradiated sample displayed broad electric dipole allowed Sm2+ 4f55d (A1u) → 4f6 7F1 (T1g) luminescence at 725 nm that narrowed to an intense peak at 708 nm on cooling to ∼30 K. The narrow f-f transitions of Sm3+ were also observed. The X-irradiation-induced reduction of Sm3+ + e- → Sm2+ as a function of X-ray dose was investigated over a very wide dynamic range from 0.01 mGy to 850 Gy by monitoring the photoluminescence intensities of both Sm2+ and Sm3+ ions. The reverse Sm2+ → Sm3+ + e- photoionization can be modelled by employing dispersive first-order kinetics and using a standard gamma distribution function, yielding an average separation of 13 Å between the Sm2+ ions and the hole traps (e.g. oxide ion impurities). The present results point towards potential applications of Sm doped CaF2 nanocrystals in the fields of dosimetry and X-ray imaging.

Lattice distortion of CaF2 nanocrystals for shortening their 19F longitude relaxation time

Shortened interplanar spacing of CaF2 nanocrystals led to significant improvement of their 19F imaging capability.

Investigating the Kinetic Parameters of SiO2-Al2O3-CaO-CaF2-K2O Oxyfluoride Glass

Oxyfluoride glass-ceramics containing CaF2 nanocrystals are kind of attractive materials for the optical applications due to their low phonon energy and high transparency. Moreover, the crystallization control and consequently, the kinetic properties are important for oxyfluoride glasses. Therefore, in the present research, the crystallization kinetics of isochronal transformation of the 37.26SiO2-28.11Al2O3-7.73CaO-26.89CaF2-4.5 K2O (wt%) glass have been determined upon the basis of maximum transformation rate using Differential Thermal Analysis (DTA) technique. Hence, it is concluded that the crystallization of the mentioned glass is a process controlled by Avrami nucleation, three-dimensional diffusion-controlled growth, and anisotropic growth impingement mode. The effective activation energy Qp =181 kJ.mol-1, growth exponent n=2.272, nucleation activation energy QN= 123, and growth activation energy QG=211 have been determined.

Preparation and upconversion luminescent properties of Yb3+/Er3+ doped transparent glass-ceramics containing CaF2 nanocrystals

Yb3+/Er3+ doped high-fluoride glass-ceramics were prepared by the high-temperature melting method. The crystal morphology and structure of the rare-earth doped glass-ceramics were characterized by X-ray diffraction, infrared spectra, and high-resolution transmission electron microscope. The influence of crystal size and distribution on the transmittance of glass-ceramics was analyzed. The upconversion emission intensity of the rare earth doped glass-ceramics at different heat treatment temperatures was evaluated. It is shown that the precipitation of CaF2 crystals greatly improves the upconversion emission efficiency of rare earth ions. By analyzing different high-power pump lasers and corresponding up-conversion luminous intensities, the mechanism of up-conversion luminescence of this material is further elaborated.

Quantum cutting and near-infrared emissions in Ho3+/Yb3+ codoped transparent glass-ceramics

Visible to near-infrared quantum cutting and 2.0 μm near-infrared emissions have been obtained in Ho3+/Yb3+ co-doped oxyfluoride transparent glass-ceramic, containing cubic CaF2 nanocrystals, under 473 nm excitation. The formation of CaF2 nanocrystals and the incorporation of Ho3+/Yb3+ ions into these nanocrystals have been confirmed from x-ray diffraction and photoluminescence studies, respectively. Lifetime of 1.2 μm emission has been enhanced by three orders of magnitude in the glass-ceramic compared to that of the precursor glass. Time-resolved near-infrared quantum cutting emission of Yb3+ ions at different delay times under 532 nm laser excitation has also been obtained. Studies reveal that the infrared quantum cutting emission of Yb3+ and Ho3+ ions in the CaF2 nanocrystalline glass-ceramic could improve the efficiency of crystalline-Si solar cells, and that the 1.2 μm emission could be useful for further enhancing transmission windows using optical amplifiers.