SrF2 Nanocrystals Research Articles

Effect of adding Er3+ on the precipitated crystalline phase of SrF2–ZnO–B2O3 glass and upconversion luminescence

Elucidating the relationship between nucleation and its structure in glasses is essential for the efficient development of new glass-ceramics. In this study, we developed new oxyfluoride nanocrystallized glass-ceramics with potential for upconversion luminescence, phosphor for LEDs, and fiber laser hosts. We investigated the glass-forming region in the SrF2–ZnO–B2O3 system and the crystallization behavior, glass structure, and optical properties of the xSrF2–(50−x)ZnO–50B2O3 system with the lowest quantity of glass-forming oxides. Whereas both the x = 10 and x = 20 samples precipitated more than 100 nm diameter of Zn4B6O13 by heat treatment, the Er3+-doped samples with x = 10 and x = 20 precipitated SrF2 nanocrystals of less than 20 nm. The resulting SrF2 nanocrystallized glass was transparent, and upconversion luminescence was observed by irradiating with a 980-nm LD. We found that the addition of Er3+ ions has a significant impact on crystallization, and the proposed mechanism for this is that the bond selectivity arising from the polarizability forms fluoride-rich domains, and Er3+ further assists the nucleation of SrF2 by serving as a crystal nucleation site. This study is expected to provide a guideline for the development of new transparent nanocrystallized glasses.

Simultaneous Observation of Visible Upconversion and Near-Infrared Downconversion in SrF2:Nd3+/Yb3+/Er3+ Nanocrystals and Their Application for Detecting Metal Ions under Dual-Wavelength Excitation.

In this work, a sequence of Nd3+, Yb3+, and Er3+ tridoped SrF2 nanocrystals (NCs) is synthesized by a hydrothermal method. Both the efficient near-infrared downconversion luminescence (DCL) and visible upconversion luminescence (UCL) of the Er3+ and Nd3+ ions are simultaneously observed and systematically demonstrated under dual-wavelength excitation (808 and 980 nm continuous-wave lasers). Subsequently, the SrF2:Nd3+/Yb3+/Er3+ (15/4/0.2 mol %) NCs with the strongest luminescence were utilized for detecting the metal ion concentrations under 808 nm excitation. The results reveal that both the UCL and DCL gradually decrease as the metal ion concentrations increase, and high sensitivity is obtained for Cu2+ ions with a detection limit of 0.22 nM (∼650 nm) and 0.63 nM (∼976 nm). In addition, these SrF2:Nd3+/Yb3+/Er3+ NCs are further demonstrated to achieve a solid-state display under 980 nm excitation, exhibiting obvious “red” and “green” patterns by varying the doping rare earth ion concentrations.

Determination of near-infrared downconversion emission of Yb3+ and optical temperature sensing performances in Nd3+-sensitized SrF2 nanocrystals

In this work, a series of Nd3+ and Yb3+ codoped SrF2 nanocrystals (NCs) were synthesized by the hydrothermal method. The TEM results show that the average size of these NCs is approximately ∼50 nm. The structure of SrF2 NCs and its optical properties of near-infrared (NIR) downconversion luminescence (DCL) are systematically characterized. Under the excitation of 808 nm laser, intense NIR DCL centered at 976 nm from Yb3+ ions is experimentally determined. The mechanism of the DCL is further interpreted based on the efficient energy-transfer (ET) processes from Nd3+ to Yb3+. In addition, utilizing the fluorescence intensity ratio (FIR) technique, the temperature sensing behaviors of SrF2:Nd3+/Yb3+ (15/4 mol%) are also investigated through the heterogeneously coupled states between Nd3+ (4F3/2 → 4I11/2) and Yb3+ (2F5/2 → 2I7/2) in a temperature range of 298–573 K. The results confirm that the maximum thermal sensitivity is up to 0.00346 K−1 at 473K. The excellent thermal sensitivity of these NCs, particularly the DCL located at the NIR region, have the potential for biological applications and temperature measurement at the nanoscale.

From Structure to Luminescent Properties of B2O3-Bi2O3-SrF2 Glass and Glass-Ceramics Doped with Eu3+ Ions.

Glass-ceramics with the composition B2O3-Bi2O3-SrF2 were synthesized by the conventional melt-quenching technique and subsequent crystallization of the parental glasses. The temperature at which the ceramization was carried out was selected based on differential scanning calorimetry (DSC) analysis. The structure of the studied materials and the formation of SrF2 nanocrystals were confirmed by the Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) techniques. It was found that the amount of strontium fluoride introduced into the parental borate-bismuth glass has a significant impact on the growth of SrF2 nanocrystals. In particular, the influence of the crystalline SrF2 phase on luminescence intensity and kinetics was studied using Eu2O3-doped samples. An increase in luminescence intensity was observed in the samples in which SrF2 nanocrystals were formed. This is most likely related to the fact that some of the Eu3+ ions were (after annealing of the glass) located in the crystalline structure of strontium fluoride. This was confirmed both by the luminescence lifetime obtained based on the luminescence decay curves and the calculated Judd–Ofelt parameters, Ω2 and Ω4. The results achieved confirm that the glasses and glass-ceramics described in this work could be considered as a new phosphor for light-emitting diodes (LEDs).

Selective enrichment of Ln3+ (Ln = Yb; Er) and Cr3+ into SrF2 and ZnAl2O4 nanocrystals precipitated in fluorosilicate glass-ceramics: A dual mode optical temperature sensing study

The dual mode optical thermometry offers precise temperature sensing with self-calibration merit. To realise this, an oxy-fluoride precursor glass was transformed to a semi-transparent glass-ceramic having simultaneously precipitated SrF2:Yb3+/Er3+ and ZnAl2O4:Cr3+ phases. Under 980 nm excitation, intense green up-conversion (UC) luminescence of Er3+: 2H11/2, 4S3/2 → 4I15/2 and under 535 nm excitation, enhanced red down-conversion (DC) luminescence lifetime of Cr3+: 2E, 4T2 → 4A2 was realized. Both the UC and DC transitions were highly reliant upon temperature. The fluorescence intensity ratio (FIR) study of erbium's thermally coupled energy states (2H11/2 and 4S3/2) produced maximum temperature sensing sensitivity values of 1.11% K−1 at 334 K and 0.66% K−1 at 275 K, respectively. Likewise, temperature dependent luminescence lifetime of Cr3+ attributed to the transitions arising from the parity forbidden (2E → 4A2) and spin allowed (4T2 → 4A2) relaxations produced maximum temperature sensing sensitivity value equal to 0.19% K−1 at 435 K.

Efficient and regenerative near-infrared glass-ceramic photocatalyst fabricated by a facile in-situ etching method

Low cost and large-scale fabrication of high-efficiency near-infrared (NIR) photocatalysts are especially promising for the degradation of organic pollutants. Herein, a facile in-situ etching method by growing BiOCl nanosheets on self-crystallized upconversion SrO-Bi2O3-B2O3 (SBBO) glass-ceramic (GC) is developed to synthesize an efficient NIR GC photocatalyst. Through structure and morphology characterizations, it is found that the NIR GC photocatalyst possesses the core-shell structure, where the SBBO GC micro-powder including optically active centers (Er3+, Yb3+, and Y3+) doped SrF2 nanocrystals are displayed as the core, and the BiOCl is as the superficial coating. The NIR GC photocatalysts can efficiently harvest the NIR photons and emit strong UV (379 nm), violet (408 nm), green (523 and 540 nm), and red (656 nm) light, which are able to be utilized by themselves for photocatalysis. In the degradation of organic pollutants under NIR irradiation for 90 min, high removal rates of 66% (antibiotic norfloxacin) and 79% (methyl orange) are achieved. Furthermore, the NIR GC photocatalyst can be facile regenerated by using HCl etching again, and suitable for large-scale fabrication that will supply a new strategy for development of efficient photocatalyst.

Photoluminescence investigation of sol-gel glass-ceramic materials containing SrF2:Eu3+ nanocrystals

In the present paper, the nanocrystalline transparent oxyfluoride SrF2:Eu3+ glass-ceramic materials (nGCs) were synthesized via a low-temperature sol-gel method, in which trifluoroacetic acid (TFA) was used as a fluorination reagent. The thermal degradation of precursor xerogels was examined by TG/DSC analysis and the formation of SrF2 nanocrystals inside silicate xerogel host was proved by XRD measurements. Additionally, to verify the structural changes within silicate network during ceramization process, the IR-ATR measurements were also carried out. The optical properties of Eu3+ ions in precursor xerogels and fabricated nGCs were investigated based on photoluminescence excitation (PLE: λem = 611 nm) and emission (PL: λexc = 393 nm) spectra as well as luminescence decay analysis of the 5D0 excited level. It was observed, that for precursor silica xerogels the electric-dipole transition (ED) 5D0 → 7F2 was dominant, meanwhile the magnetic-dipole transition (MD) 5D0 → 7F1 had the greatest intensity for SrF2:Eu3+ nGCs. Thus, the R/O-ratio (R/O = I(5D0 → 7F2)/I(5D0 → 7F1)) was calculated to estimate the symmetry in the local framework around Eu3+ ions. Moreover, the reddish-orange photoluminescence is long-lived (about 30-fold longer) for SrF2:Eu3+ nGCs compared with xerogels. The changes in emission spectra as well as double-exponential character of luminescence decay curves recorded for obtained nGCs indicated the successful migration of optically active Eu3+ ions from amorphous silica framework to low phonon energy SrF2 nanocrystal phase.

From structure to luminescence investigation of oxyfluoride transparent glasses and glass-ceramics doped with Eu3+/Dy3+ ions

Glasses and glass-ceramics with nominal composition 73 TeO2– 4BaO– 3Bi2O3–18SrF2-2RE2O3 (where RE = Eu, Dy) have been synthesized by conventional melt-quenching technique and subsequent heat treatment at 370 °C for 24 h in air atmosphere. Various Eu3+ to Dy3+ molar ratio have been applied to investigate luminescence properties in both glass and glass-ceramic matrices. Especially, white light emission through simultaneous excitation of Eu3+ and Dy3+ has been studied in detail. Influence of crystalline SrF2 phase on luminescence kinetics has been determined by luminescence decay time measurements. Presence of crystalline SrF2 phase has been confirmed by X-ray diffraction technique XRD and transmission electron microscopy TEM. X-ray photoelectron spectroscopy XPS and Fourier-transform infrared spectroscopy FTIR have been applied to get further insight into structural properties of glass and glass-ceramic materials. Color tunable white light emission has been obtained using UV excitation. Influence of the SrF2 crystallization on luminescence properties of prepared materials have been described in detail. Moreover, luminescence properties and especially emission color dependence on the Eu3+ to Dy3+ molar ratio have been exhaustively studied. Color-tunable white light emission has been observed as a result of simultaneous radiative transition of both, Eu3+ and Dy3+ ions when applying UV excitation. Judd – Ofelt and other optical parameters have been calculated based on luminescence emission spectra. Achieved results confirm that tellurite glass-ceramics containing SrF2 nanocrystals are good hosts for RE3+ ions and they can be considered as new phosphors for white light emitting diodes WLEDs.

Stabilization of Fluorescent [Agm]n+ Quantum Clusters in Multiphase Inorganic Glass-Ceramics for White LEDs

Silver quantum clusters ([Agm]n+ QCs) are a type of efficient broadband fluorescence centers with m and n related quantum size effects but usually lack chemical and thermal stability. To solve such a problem and exploit [Agm]n+ QCs potential applications in white LED lighting, [Agm]n+ QCs and rare earth ions (RE3+) were designed to be selectively enriched into B2O3-rich spinodal nanophase separation and SrF2 nanocrystals in fluoroborosilicate multiphase glass-ceramics. In this work, Ag/RE3+-codoped glasses and glass-ceramics with a designed composition were prepared through a melt-quenching method and subsequent heat treatment. The B2O3-rich spinodal nanophase separation and SrF2 nanocrystals were stepwise formed in these glass-ceramics. Taking Ag/Er3+-codoped glass-ceramics for an example, the special microstructures were clearly revealed by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), and energy-disper...

Infrared-laser precipitation of Dy3+-Yb3+ codoped SrF2 nanocrystals in glass and upconversion luminescence

Oxyfluoride glass-ceramics containing SrF2nanocrystals have been prepared by a CO2 laser treatment on the glass prepared by high temperature melt quenching. The optimized scan speed and laser power produced nanocrystals with sizes of ~10 nm. Energy dispersive spectroscopy mapping analysis showed that Dy3+ and Yb3+ ions are highly populated inside the SrF2 nanocrystals. We also report a substantially enhanced down-converted visible emission from the 4F9/2 state of Dy3+ ions, including some violet emissions from the higher energy states under 365 nm excitation in the glass-ceramics, as compared to the as-made glass. Moreover, 980 nm laser diode excitation produced strong green and red emissions from the 4I15/2 state and blue and yellow emissions from the 4F9/2 state. A model has been proposed to interpret the upconversion emissions through the multistep-excitation of Dy3+ ions and energy transfers from, efficient sensitizers, Yb3+ ions to Dy3+ ions.

Enhanced visible emissions of Pr3+-doped oxyfluoride transparent glass-ceramics containing SrF2 nanocrystals

The Pr3+-doped oxyfluoride transparent glass and glass-ceramic (GC) with the composition of 41SiO2 + 10Al2O3 + 25.5LiF + 23SrF2 + 0.5Pr2O3 were prepared and investigated their optical and luminescence properties. The formation of SrF2 nanocrystals in GC has been confirmed by X-ray diffraction (XRD) and transmission electron micrographs (TEM). The Fourier transform infrared spectroscopy (FT-IR) studies were used to examine the network structure characteristics of silicates in the glass matrices. The XRD and TEM results suggest that the Pr3+ ions are progressively incorporated into the SrF2 nanocrystals in the GC with increase in time of thermal treatment at 650°C, corresponding to the first crystallization temperature of the glass. The obtained visible emissions of Pr3+-doped GC are several times enhanced than that in the glass and the lifetime of the 3P0 level of the Pr3+ ions in glass and GC are found to be 7 and 12μs, respectively. Therefore, the enhanced visible emission and lifetimes in GC are due to the incorporation of Pr3+ ions into the lower phonon energy of SrF2 nanocrystals in the GCs. Moreover, the smaller difference in ionic radius between the added trivalent ions (Pr3+) and Sr2+ induces the larger enhancement of luminescence intensity in the GC. Hence, these enhanced visible luminescence properties indicate that the present glass and GC could be useful for photonic device applications.

Electron paramagnetic resonance and photoluminescence investigation of europium local structure in oxyfluoride glass ceramics containing SrF2 nanocrystals

Different compositions of europium doped aluminosilicate oxyfluoride glass ceramics prepared in air atmosphere have been studied by electron paramagnetic resonance (EPR) and optical spectroscopy methods. X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements show presence of homogenously distributed SrF2 nanocrystals after the heat treatment of the precursor glass. Efficient Eu3+ incorporation in the high symmetry environment of glass ceramics is observed from the photoluminescence spectra. EPR spectra indicate Eu3+ → Eu2+ reduction upon precipitation of crystalline phases in the glass matrix. For composition abundant with Eu2+ in the glassy state such behaviour is not detected. Local structure around europium ions is discussed based on differences in chemical compositions.

A white phosphor based on oxyfluoride nano-glass-ceramics co-doped with Eu3+ and Tb3+: Energy transfer study

Strontium fluoride nanocrystals doped separately with Ln3+ = Eu3+ or Tb3+ ions (95SrF2·5LnF2), as well as co-doped with both the ions (95SrF2·Eu4Tb) were synthesized for comparison. Then, glass-ceramics consisting of the mono-doped or di-doped SrF2 nanocrystals incorporated into amorphous silica (90SiO2·9.5SrF2·0.5Ln or 90SiO2·9.5SrF2·0.1EuF3·0.4Tb) were prepared by sol-gel procedure. The studied materials were characterized by XRD, while the glass-ceramics additionally by SEM and BSE (backscattered electrons) techniques. Luminescence emission and excitation spectra of the doubly doped materials exhibited presence of such effects as simultaneous independent excitation and energy transfer Tb3+ → Eu3+ as well as cross-relaxation in the materials with Tb3+ dopant. Tb3+ lifetime in 90SiO2·9.5SrF2·0.5EuF3·0.4TbF3 was distinctly reduced compared to 90SiO2·9.5SrF2·0.5TbF3 probably by the Tb3+ → Eu3+ energy transfer. Owing to blue emission of SiO2, green light emitted by Tb3+ and red one by Eu3+ the 90SiO2·9.5SrF2·0.5EuF3·0.4TbF3 nano-glass-ceramics can play a white phosphor role.

High pressure impedance spectroscopy of SrF2 nanocrystals

ABSTRACTThe charge transport behavior of strontium fluoride nanocrystals has been investigated by in situ impedance measurement up to 41 GPa. It was found that the parameters changed discontinuously at each phase transition. The charge carriers in SrF2 nanocrystals include both F− ions and electrons. Pressure makes the electronic transport more difficult. The defects at grains dominate the electronic transport process. Pressure could make the charge–discharge processes in the Fm3m and Pnma phases more difficult.

High pressure electrical transport behavior in SrF2 nanoplates**Project supported by the National Natural Science Foundation of China (Grant Nos. 11374131, 11674404, 11404137, and 61378085), the Program for New Century Excellent Talents in University, China (Grant No. NCET-13-0824), the Program for the Development of Science and Technology of Jilin Province, China (Grant Nos. 201201079 and 20150204085GX), and the Twentieth Five-Year Program

The charge transport behavior of strontium fluoride nanocrystals has been investigated by in situ impedance measurement up to 35 GPa. It was found that the parameters changed discontinuously at each phase transition. The charge carriers in SrF2 nanocrystals include both F ions and electrons. In the phase, pressure makes the electronic transport easier, while makes it more difficult in the Pnma phase. The defects at grain boundaries dominate the electronic transport process. Pressure could make the charge–discharge processes in the phase much easier, but make it more difficult in the Pnma phase.

Tb3+掺杂含SrF2纳米晶硅酸盐微晶玻璃的光谱性质

Tb³⁺掺杂含SrF₂纳米晶硅酸盐微晶玻璃的光谱性质

Eu3+ doped tellurite glass ceramics containing SrF2 nanocrystals: Preparation, structure and luminescence properties

Eu3+ doped tellurite glass ceramics containing SrF2 nanocrystals were prepared using melt quenching technique and subsequent heat treatment of glass in 370 °C for different time periods. Thermal properties of glass matrix have been determined based on DSC measurements. XRD and XPS results confirmed formation of SrF2 nanocrystals in glass matrices after annealing at 370 °C. FTIR studies revealed absorption bands in the range of 500–850 cm−1 characteristic to TeO2 glasses. Obtained materials exhibited emission originated from the 5DJ (J = 0–2) to the 7FJ (J = 0–4) transitions of Eu3+ ions under 395 and 465 nm excitations. TRES measurements indicated presence of two different surroundings of the Eu3+ ions. In glass ceramics samples longer lifetimes of 615 nm emission of Eu3+ were achieved in comparison to the precursor glass.

Luminescence and energy transfer in Dy3+/Tb3+ co-doped transparent oxyfluorosilicate glass-ceramics for green emitting applications

Transparent oxyfluorosilicate glass-ceramics containing cubic SrF2 nanocrystals doped with Dy3+, Tb3+, and Dy3+/Tb3+ were successfully synthesized by melt quenching followed by heat treatment process. The crystalline phase of SrF2 nanocrystals during heat treatment was confirmed by X-ray diffraction. The emission intensity of glass-ceramics is much stronger than that of their parent glasses, on the one hand due to progressive incorporation of Dy3+ and Tb3+ ions within the SrF2 nanocrystals, on the other hand due to an efficient energy transfer from Dy3+ to Tb3+. Luminescence quantum efficiency of the samples was measured by using an integrating sphere. Luminescence decay curves of the 4F9/2 level were fitted to Inokuti-Hirayama (IH) model for analyzing the energy transfer rates between Dy3+ and Tb3+ ions. The chromaticity coordinates were determined from the measured emission spectra and they are located at the green and white light regions.

Highly Selective and Sensitive Detection of Cu(2+) Ions Using Ce(III)/Tb(III)-Doped SrF2 Nanocrystals as Fluorescent Probe.

We report a green synthetic approach to the synthesis of water dispersible Ce(3+)/Tb(3+)-doped SrF2 nanocrystals, carried out using environment friendly microwave irradiation with water as solvent. The nanocrystals display strong green emission due to energy transfer from Ce(3+) to Tb(3+) ions. This strong green emission from Tb(3+) ions is selectively quenched upon addition of Cu(2+) ions, thus making the nanocrystals a potential Cu(2+) ions sensing material. There is barely any interference by other metal ions on the detection of Cu(2+) ions and the detection limit is as low as 2 nM. This sensing ability is highly reversible by the addition of ethylenediaminetetraacetic acid (EDTA) with the recovery of almost 90% of the original luminescence. The luminescence quenching and recovery cycle was repeated multiple times without much effect on the sensitivity. The study was extended to real world water samples and obtained similar results. In addition to the sensing, we strongly predict the small size and high luminescence of the Ce(3+)/Tb(3+)-doped SrF2 nanocrystals can be used for bioimaging applications.

Sr2+ and Tb3+ doping tuning the size, morphology, and photoluminescence of NaCeF4 nanorods via solvothermal method

Doping for nanocrystalline materials is of fundamental importance in tuning the size, phase, morphology and relational properties, such as fluorescence of rare earth fluoride nanocrystals. Here, we report hexagonal NaCeF4 nanorods with controlled size, morphology and photoluminescence were synthesized via solvothermal method by Sr2+ doping as well as Tb3+ doping. Transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) results illustrated Sr2+ which occupies Na+ site partially is crucial in controlling nanorods’ morphology, size, and emission intensity. The doping of Tb3+ as activation center can also play key role to tune the morphology of NaCeF4 nanorods containing Sr2+ from long nanobars to hexagonal prisms. Growth process revealed that ultrasmall SrF2 nanocrystals with cubic shape were fabricated firstly, and then Sr2+ doped NaCeF4 nanorods were obtained by oriented attachment and Ostwald ripening. The emission intensity was not only decided by Tb3+ doping concentration, but also morphology-dependent.