CaF2 Nanocrystals Research Articles

Photon avalanche upconversion in Ho3+–Yb3+ co-doped transparent oxyfluoride glass–ceramics

The Ho3+–Yb3+ co-doped transparent glass and glass–ceramics containing CaF2 nanocrystals have been prepared. Differential thermal analysis and X-ray diffraction measurements have been made to characterize thermal properties of glass and structural changes in glass–ceramics, respectively. Photon avalanche upconversion has been achieved by exciting the samples at 745nm at room temperature. An intense green and a weak red upconverted emissions corresponding to the 5S2:5F4→5I8 and 5F5→5I8 transitions, respectively, have been observed. The upconversion intensity has been found to increase with the increase in the size of the fluoride nanocrystals in glass–ceramics. Experimental evidences confirm that the mechanism of upconversion is photon avalanche.

Up-conversion luminescence properties and energy transfer of Er3+/Yb3+ co-doped oxyfluoride glass ceramic containing CaF2 nano-crystals

The Er3+/Yb3+ co-doped transparent oxyfluoride glass-ceramics containing CaF2 nano-crystals were successfully prepared. After heat treatments, transmission electron microscopy (TEM) images showed that CaF2 nano-crystals of 20–30 nm in diameter precipitated uniformly in the glass matrix. Comparing with the host glass, high efficiency upconversion luminescence of Er3+ at 540 nm and 658 nm was observed in the glass ceramics under the excitation of 980 nm. Moreover, the size of the precipitated nano-crystals can be controlled by heat-treatment temperature and time. With the increase of the nano-crystal size, the intensity of the red emission increased more rapidly than that of the green emission. The energy transfer process of Er3+ and Yb3+ was convinced and the possible mechanism of Er3+ up-conversion was discussed.

An efficient near infrared photocatalyst of Er3+/Tm3+/Yb3+ tridoped (CaWO4@(TiO2/CaF2)) with multi-stage CaF2 nanocrystal formation

A near-infrared photocatalyst, Er3+/Tm3+/Yb3+ tridoped (CaWO4@(TiO2/CaF2) is presented, in which the CaF2 nanocrystals are converted from CaWO4 precursors in a multi-stage process, and the remaining CaWO4 are wrapped in CaF2 and TiO2 nanocrystals to form a heterostructure.

Influence of silver nanoparticles on Er3+ up-conversion in CaF2 precipitated oxyfluoride glass-ceramics

The influence of silver nanoparticles on Er3+ up-conversion in CaF2 precipitated oxyfluoride glass-ceramics is investigated. After heat-treatments, transmission electron microscopy images show that CaF2 nanocrystals precipitate in the glass matrix uniformity, and sliver nanoparticles are spread around the CaF2 nano-crystals simultaneously. Comparing with the samples without Ag doped, high efficiency up-conversion luminescences of Er3+ at 540 and 658 nm are distinctly observed in the silver nanoparticles containing glass-ceramics by the 980-nm excitation. Moreover, since the intensity ratio of green and red emissions changes after silver nanoparticles precipitation, the up-conversion mechanism of Er3+ is discussed.

Upconversion in Nd–Tm–Yb triply doped oxyfluoride glass–ceramics containing CaF2 nanocrystals

Upconversion and energy transfer processes in Nd3+–Tm3+–Yb3+ triply doped oxyfluoride glass–ceramics containing CaF2 nanocrystals have been studied. Under 792nm excitation, we investigated the strong blue emission from Tm3+ ions in Tm3+–Yb3+, Tm3+–Nd3+, and Nd3+–Tm3+–Yb3+ doped glass–ceramics. The Nd3+–Tm3+–Yb3+ doped glass–ceramics has shown the strongest blue emission via a multiple-step energy transfer processes following the ground state absorption of Tm3+ and Nd3+ ions. In the Nd3+–Yb3+ doped glass–ceramic, much enhanced visible emission was observed due to the back transfer from Yb3+ ions. However, under 980nm excitation of Yb3+ ions, the triply doped glass–ceramics showed slightly weaker visible upconversion emission than the Tm3+–Yb3+ and Nd3+–Yb3+ co-doped glass–ceramics. Detailed upconversion mechanisms for observed emissions were suggested.

Fabrication and properties of highly transparent Nd-doped CaF2 ceramics

The synthesis of Nd-doped CaF2 nanocrystals using a direct precipitation method was reported. X-ray powder diffraction and transmission electron microscopy analysis showed that the products were single phased with an average particles size around 18nm. The transparent ceramic was fabricated by hot pressing (HP) the powders at a temperature of 900°C under a pressure of 30MPa. The micrograph of the Nd:CaF2 transparent ceramic exhibited a pore-free structure and the average grain size was about 0.5μm. For a 2mm thickness sample, the highest transmittance of the sample reached 90% in the wavelength from 250nm to 1400nm, which made it hopeful to be a kind of laser media for miniaturization of laser devices.

27 μm emission in Er^3+:CaF_2 nanocrystals embedded oxyfluoride glass ceramics

Using conventional melt-quenching and subsequent thermal treatment, Er(3+) doped CaF(2) transparent glass ceramic (GC) was prepared. X-ray diffraction and high-resolution transmission electron microscopy confirmed the formation and microstructure of CaF(2) nanocrystals in glass. An energy-dispersive spectrometer was used to investigate the distribution of Er(3+) ions and CaF(2) nanocrystals in glass. It was found that Er(3+) ions prefer to concentrate in the CaF(2) nanocrystals rather than in a glass matrix, and the amount of Er(3+) ions plays a key role in the formation of CaF(2) nanocrystals in a glass matrix with the Er(3+) ions as nucleating agent. An intense 2.7 μm emission due to Er(3+): I(11/2)4 → I(13/2)4 was achieved upon excitation at 980 nm with a laser diode, while the 2.7 μm emission can be neglected in the as-prepared glass counterpart, which confirmed the incorporation of Er(3+) ions into CaF(2) nanocrystals. An obvious enhancement of 2.7 μm emerged in the GC doped with 3% Er(3+) and heat-treated at 620 °C.

Near-infrared quantum cutting in Pr3+-Yb3+ Co-doped oxyfluoride glass ceramics containing CaF2 nanocrystals

The Pr3+-Yb3+ co-doped oxyfluoride glass-ceramics containing CaF2 nanocrystals were obtained by thermal treatment on the as-made glasses. The structure of fluoride nanocrystals was investigated. The light-emitting mechanism of Pr3+-Yb3+ in the near infrared region was proposed and the fluorescence lifetime and quantum efficiency was calculated. The results indicate that the main phase in the oxyfluoride glassceramics is CaF2 nanocrystal sized at 30 nm. Energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) have proved the incorporation of Pr3+ and Yb3+ into CaF2 nanocrystal lattice. Near-infrared quantum cutting involving Yb3+ 980 nm and 1 015 nm (2F5/2→2 F 7/2) emission has been achieved upon the excitation of the 3 P 0 energy level of Pr3+ at 482 nm. The fluorescence lifetime decreases sharply and quantum efficiency increases with increasing Yb3+ concentration, and the optimal quantum efficiency reaches 191%.

Ultraviolet and visible up-conversion luminescence of Er3+/Yb3+ co-doped CaF2 nanocrystals in sol–gel derived glass-ceramics

Glass-ceramics containing Er3+/Yb3+ codoped CaF2 nanocrystals have been prepared by using the controlled crystallization above 650°C of the (Yb3+/Er3+) codoped CaF2–SiO2 xerogels. X-ray diffraction and scanning electron microscopy data have revealed the formation of CaF2 nanocrystals of about 16nm size. For pumping at 973nm, this glass-ceramics shows, besides green and red up-conversion luminescence, violet (at ∼408nm) and near-ultraviolet emission (at ∼380nm). The mechanisms responsible for the upconversion luminescence are presented and discussed. The fluorescence lifetimes of the metastable levels responsible for violet, green and red luminescence are measured and their quantum efficiencies are estimated.

Optical Active Nano‐Glass‐Ceramics

This paper reviews the synthesis and characterization of several transparent glass‐ceramics with optical active nanocrystals. Glass‐ceramics containing ferroelectric SrxBa1‐xNb2O6 nanocrystals with an ellipsoidal shape show optical phase modulations in the presence of alternative electric fields. In the glass‐ceramics with Ba2TiSi2O8 (BTS) nanocrystals, BTS crystalline layers with a thickness of approximately 120 nm are formed at the surface and ellipsoidal‐shaped crystallites with a diameter of 100–200 nm are dispersed in the glass matrix. Some TeO2‐based and GeO2‐based glasses show a prominent nanocrystallization. RE‐doped CaF2 nanocrystals are patterned in a spatially selected region by laser irradiations. The size, morphology, and dispersion state of nanocrystals should be carefully checked in each glass system and composition. The basic concept for the design of glass system and composition is also discussed. Some data on optical active performances in transparent glass‐ceramics with nanocrystals were introduced.

Enhancement of photoluminescence and upconversion in Er–Yb codoped nanocrystalline glass–ceramics

Oxyfluoride glass ceramics containing CaF2 nanocrystals were prepared with dopants of Er2O3 and Yb2O3 through melt-quenching and proper thermal treatments. Emission spectra, emission decay and energy dispersive X-ray spectroscopy analysis of the glass ceramics indicated the incorporation of rare earth ions into CaF2 nanocrystals. Photoluminescence intensity of the glass–ceramics has been increased more than a hundred times compared with that of the precursor glass. We also obtained enormous enhancement in upconversion emission and relative population increase of the 4F9/2 level by energy transfer process under excitation at 980nm in resonance with the 2F5/2 of Yb3+ and 4I11/2 of Er3+ ions.

Preparation and luminescence properties of Eu3+ doped oxyfluoride borosilicate glass ceramics

Oxyfluoride borosilicate glass with the molar composition of 60SiO2-15B2O3-15Na2O-8CaF2-2NaF-0.25Eu2O3 was synthesized by a traditional glass melting method. Glass ceramics containing CaF2 nanocrystals were prepared by heat treating the glass samples at a temperature in the range of 620–680 °C. The results of X-ray diffraction (XRD) indicated that the average crystallite size and the lattice constant of CaF2 nanocrystals increased with the heat treatment temperature increasing. The luminescence spectra showed that the emission intensity of Eu3+ doped glass ceramics was stronger than that of the glass matrix, and increased with the heat treatment temperature increasing. The left edge of excitation band shifted to shorter wavelength in the glass ceramics. The local environments of Eu3+ ions in the glass and glass ceramics were different.

Effect of alumina content and heat treatment on microstructure and upconversion emission of Er3+ ions in oxyfluoride glass-ceramics

The effect of alumina content and heat treatment temperature and time, on microstructure and Er3+ (0.5 mol.%) emission of oxyfluoride glass-ceramics were investigated in this research. Two values of 1.8 (SA1.8Er0.5) and 2.18 (SA2.18Er0.5) were selected in this research for SiO2/Al2O3 ratio. According to DTA results, precursor glasses were heat treated at 630, 660 and 690 °C for 4 h and some glasses were also heat treated at 630 °C for 48 and 72 h. The results indicated that alumina content had significant effect on phase separation and viscosity of the glasses. Therefore the size, size distribution, and volume concentration of nano CaF2 crystals which precipitated during the heat treatment depended on alumina content of the glass. Due to the much smaller size of the precipitated CaF2 crystals in the glasses of low alumina content, these samples maintained excellent transparency and had narrower crystal size distribution than the high alumina glasses. The crystal size was increased markedly with the temperature increasing from 630 to 690 °C. On the other hand a slight increase was observed in the crystal size by raising the heat treatment time in both glasses. Results indicated that in low alumina content glass (SA2.18Er0.5) the size of CaF2 nanocrystals was controlled in one order of magnitude. The increase of heat treatment time and temperature led to the incorporation of Er3+ ions into CaF2 crystalline phase, increasing significantly the upconversion intensity. After heat treatment at 690 °C for 4 h, atomic force microscope (AFM) revealed the development of small crystals with an average size of 80 and 30 nm in SA1.8Er0.5 and SA2.18Er0.5 samples, respectively.

Morphology and photoluminescence properties of Er3+-doped CaF2 nanocrystals patterned by laser irradiation in oxyfluoride glasses

Lines with Er3+-doped CaF2 nanocrystals were patterned by laser irradiations in aluminoborosilicate-based oxyfluoride glasses. The morphology and dispersion state of CaF2 nanocrystals were examined from transmission electron microscope (TEM) observations combined with focused ion beam sample preparations. The size of CaF2 nanocrystals present in the region close to the center of lines was larger than that in the region close to the edge of lines. Micro-photoluminescence (PL) spectra for the line part showed clear emissions corresponding to the f–f transitions of Er3+ ions, and the correlation between the morphology and dispersion state of CaF2 nanocrystals and PL properties (intensity and degree of the Stark splitting) of Er3+ ions incorporated into in CaF2 nanocrystals was clarified. The formation mechanism of CaF2 nanocrystals in the laser irradiated part was discussed from the temperature distribution model.

Spectroscopic ellipsometry investigations of Eu-doped oxy-fluoride glass and glass-ceramics

Oxyfluoride glass-ceramics in the system SiO2–Al2O3–CaF2–EuF3 containing Eu3+-doped CaF2 nanocrystals were produced by using controlled crystallization of melt-quenched glass. X-ray diffraction and scanning electron microscopy data have revealed the formation of CaF2 nanocrystals of about 50nm size. The Eu3+-dopant ions act as the nucleating agent necessary to initiate the crystallization process. The refractive index is higher in the glass ceramics than in the initial glass and varies as the annealing time increases. Two competitive processes are responsible for this behavior, the crystallization of the CaF2 phase and the decrease of the glass ceramic mass density.

Morphology and optical properties of Mg and Sr doped CaF2 nanocrystals

Magnesium (Mg) and Strontium (Sr) doped Calcium fluoride nanocrystals were synthesized by co-precipitation method. The cubic structure of the samples was confirmed by Powder X-ray diffraction. The average crystallite size of Mg doped samples was found to be ~25nm whereas in Sr doped one it was ~35nm. The morphological features revealed that the nanocrystals were agglomerated, crispy and porous. The as-prepared samples showed the presence of hydroxyl groups. The optical absorption spectrum of as-prepared Mg doped samples showed a strong absorption band peaked at ~233nm whereas the Sr doped one showed a prominent absorption peak at 248nm. A strong PL emission was observed at ~300nm in Mg doped samples. However, the Sr doped samples showed two prominent emissions at ~345 and 615nm.

Fluorine deficient layer at the surface of transparent glass-ceramics with CaF2 nanocrystals

Optically transparent glass-ceramics with CaF2 nanocrystals (diameter: ∼30nm) for 25CaF2–5CaO–20Al2O3–50SiO2 were fabricated using a conventional glass crystallization method in an electric furnace in air and the dispersion state of CaF2 nanocrystals at the surface was examined from transmission electron microscope observations combined with focused ion beam sample preparations. It was demonstrated that a fluorine deficient layer of about 150–200nm is formed during the heating at temperatures above the glass transition temperature in the surface region and thus CaF2 nanocrystals are not present in the surface region. The reason for the formation of such fluorine deficient layers is due to the removal of some fluorine from the surface of bulk glass during heating. The present study gives new information on the crystal growth behavior of fluoride nanocrystals in oxyfluoride glass-ceramics.

New transparent glass–ceramics containing large grain Eu3 +:CaF2 nanocrystals

Transparent glass–ceramics containing Eu3+ doped CaF2 nanocrystals were prepared. The microstructural evolution of the samples was studied with DTA, X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). After heat-treatment at 560°C for 10h, the precipitated CaF2 crystallites in the sample were 60–90nm in size and dispersed equally in the amorphous phase. The high transparency of glass–ceramics was mainly ascribed to the small difference of refractivity index between CaF2 and the parent glass (⊿nD≈0.05). The luminescent properties of parent glass and glass–ceramics were also estimated to confirm the incorporation of Eu3+ into the precipitated CaF2 phase.

Structural Evolution During Precipitation of Alkaline‐Earth Fluoride Nanocrystals in Oxyfluoride Glasses: A Multinuclear Nuclear Magnetic Resonance Spectroscopic Study

The fluorine and aluminum coordination environments and their evolution during precipitation of CaF2, SrF2, and (CaxSr1−x)F2 nanocrystals in oxyfluoride glasses and glass–ceramics are investigated using 19F and 27Al magic‐angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. These structural aspects of the parent glasses and of the resulting glass–ceramics are found to be strongly dependent on the nature of the modifier cation. In the calcium‐modified glass the preexisting F–Ca(n) sites act as major precursors of F–Ca(4) sites in CaF2 nanocrystals that are precipitated upon ceramming. In contrast, for the strontium‐modified glass, besides F–Sr(n) sites a large fraction of the Al–F–Sr(n) sites in the parent glass are converted into F–Sr(4) sites in the glass–ceramic during precipitation of SrF2 nanocrystals. In the case of a glass containing both calcium and strontium as modifying cations precipitation of (CaxSr1−x)F2 solid solution nanocrystals is achieved upon ceramming. However, Ca2+ ions preferentially partition into the crystal phase resulting in a Ca:Sr atomic ratio that is significantly higher in the nanocrystal compared with that in the parent glass. These observations are consistent with the higher field strength of Ca2+ compared with Sr2+. Incorporation of Yb3+ ions into the lattices of these fluoride nanocrystals is also evidenced in the 19F NMR spectra.

Pressure‐induced structural transition in CaF2 nanocrystals

AbstractThe structural transition of CaF2 nanocrystals was studied by in situ high pressure synchrotron radiation X‐ray diffraction measurements up to about 46.5 GPa at ambient temperature. A pressure‐induced fluorite structure (Fm3m) to orthorhombic PbCl2‐type structure (Pnma) transition starts at 14.0 GPa, and phase transition is sluggish. The orthorhombic phase of nanocrystalline CaF2 is stable up to 46.5 GPa. The enhancement of transition pressure in CaF2 nanocrystals as compared with the corresponding bulk material is mainly caused by the surface energy difference between the phases involved.