Doped CaF2 Research Articles

Harnessing the Polarizability of Capping Ligands for Enhancing the Intensity of Hypersensitive Electric Dipole Transition in Eu3+ Doped CaF2 Nanoparticles

Harnessing the Polarizability of Capping Ligands for Enhancing the Intensity of Hypersensitive Electric Dipole Transition in Eu3+ Doped CaF2 Nanoparticles

Energy transfer and laser gain cross-section analysis in Nd-Yb-Gd doped CaF2 crystals

A detailed spectroscopic characterization of CaF2 co-doped with neodymium and ytterbium ions is presented. The higher Nd3+ absorption cross-section around 791 nm than Yb3+ around 980 nm enables an efficient excitation of Yb3+ ions by pumping Nd3+ with a subsequent energy transfer (ET) from Nd3+ to Yb3+ ions. By combining Nd3+ and Yb3+, along with Gd3+ buffer ions Nd3+,Yb3+,Gd3+:CaF2 crystals exhibit very large emission bands extending the usual Yb3+ emission spectrum by adding the Nd3+ emission contribution at longer wavelengths. The Nd3+ ⟶ Yb3+ energy transfer efficiency is estimated using two different approaches based on luminescence intensity measurements and lifetimes, giving consistent results. An energy transfer efficiency of 80 % is achieved in CaF2:0.5%Nd,3%Yb,2%Gd illustrating the ET strength within Nd3+-Yb3+ clusters. Finally, a detailed study of the gain cross-sections and laser emission spectra, and their dependence with Yb3+ doping concentration and population inversion is carried out, providing a detailed model of the laser spectral dynamics in such crystals. The gain cross-section is either dominated by one of the Nd3+ peaks at 1065 nm or 1048 nm or exhibits a flat profile covering the 1045–1067 nm range, in which case the laser oscillates randomly over this spectral range. The results highlight the specifics of the Nd3+ four level laser coupled by energy transfer to the Yb3+ quasi three level laser. Among others, this study shows how a large Nd3+ inversion ratio initiates the depletion of the Yb3+2F7/2 ground state through the ET. This depletion then has two effects as it saturates the energy transfer since the number of Yb3+ acceptors diminishes and it also shifts the laser wavelength towards shorter wavelength as the Yb3+ laser reabsorption is weakened.

Dual-Mode Nanoprobes Based on Lanthanide Doped Fluoride Nanoparticles Functionalized by Aryl Diazonium Salts for Fluorescence and SERS Bioimaging.

The design of dual-mode fluorescence and Raman tags stimulates a growing interest in biomedical imaging and sensing applications as they offer the possibility to synergistically combine the versatility and velocity of fluorescence imaging with the specificity of Raman spectroscopy. Although lanthanide-doped fluoride nanoparticles (NPs) are among the most studied fluorescent nanoprobes, their use for the development of bimodal fluorescent-Raman probes has never been reported yet, to the best of the authors knowledge, probably due to the difficulty to functionalize them with Raman reporter groups. This gap is filled herein by proposing a fast and straightforward approach based on aryl diazonium salt chemistry to functionalize Eu3+ or Tb3+ doped CaF2 and LaF3 NPs by Raman scatters. The resulting surface-enhanced Raman spectroscopy (SERS)-encoded lanthanide-doped fluoride NPs retain their fluorescence labeling capacity and display efficient SERS activity for cell bioimaging. The potential of this new generation of bimodal nanoprobes is assessed through cell viability assays and intracellular fluorescence and Raman imaging, opening up unprecedented opportunities for biomedical applications.

Cr3+ doped CaF2 transparent ceramic with effective broadband near-infrared emission

Cr3+ has attracted much attention as activator because of the merits such as, broadband near-infrared emission and tunable emission wavelength. However, the mixed valence of Cr3+ in some matrix materials greatly affected its luminescence. In this work, we synthesized Cr3+ doped CaF2(CaF2:Cr3+) precursor powders with a single valence of Cr3+ ion by coprecipitation and successfully prepared CaF2:Cr3+ transparent ceramics by vacuum hot-press sintering. The obtained ceramics showed high transmittance with 83% at 1200 nm and effective broadband near-infrared emission covering 680–980 nm with a full width at half-maximum (FWHM) of 130 nm. Moreover, the photoluminescence properties of ceramics were tuned by the concentration of Cr3+ ions.

Analysis of site symmetries of Er3+ doped CaF2 and BaF2 crystals by high resolution photoluminescence spectroscopy

The understanding of complex relationships between luminescent properties, local symmetry of an emitting center, and the host crystal structure provides a better insight into optical properties of materials. In this work, the alkaline earths CaF2 and BaF2 fluoride crystals doped with 0.1 mol% ErF3 were investigated. The crystals structure has been studied using a synchrotron and laboratory X-ray diffraction. The C3v and C4v sites symmetry were determined using erbium probed high resolution emission spectroscopy (HRPL) at low temperature (LT) of 10 K. The considerable difference in room temperature (RT) optical properties for CaF2 compared to BaF2 crystals was observed. Such difference in absorption intensity of 4.7 times of erbium 4G11/2 manifold in UV, and 7.5 times in green emission from 4S3/2 manifold, could be due to the distinction in the host crystals cationic radius (ΔrCa,Ba) and the dopant-host ionic radius (ΔrCa-Er, ΔrBa-Er). Those Δr differences influence the structure and lead to the following symmetry formation: In CaF2, the C4v and C3v isolated centers were identified, with the determined Er3+- F−i bond lengths of 2.734 Å and 4.735 Å respectively; In BaF2, only C3v isolated centers were identified with the determined Er3+- F−i bond lengths of 5.380 Å. The present work is the first study that takes into account correlations of optical properties, the local symmetry and the structure in mentioned fluorides crystals, and it could be a step forward in the lanthanide doped optical materials systematics.

One- and two-exciton states of pair centers of Kramers Nd3+ions in Nd-doped CaF2 and SrF2 crystals, and their possible use as qubits

Using two-color excitation with narrow laser lines of 0.2 cm−1 and time-resolved site-selective fluorescence spectroscopy, we measured the fluorescence excitation spectra of the one-exciton 4G5/2(1) x 4I9/2(1) state monitored in the near IR, and the two-exciton 4G5/2(1) x 4G5/2(1) state monitored in the near UV of the pair M centers in the Nd3+ doped CaF2 and SrF2 crystals at 6.5 K. We observed the splitting of the one-exciton state of the order of 2–3 cm−1 depending on the type of crystal matrix. A significant inhomogeneous splitting ∼1.5 cm−1 of the spectral lines of the one-exciton state of the pair M centers in the Nd3+: SrF2 crystals was found. The measured single spectral line of the two-exciton state of the pair M centers in these crystals has in one-photon scale the linewidth of ∼0.8 cm−1, which is mainly determined by inhomogeneous broadening.

Novel hybrid xerogel with CaF2: Tb3+ nanoparticles covalently embedded into silicon-oxygen network through sol-gel process

In this work, Tb3+ doped CaF2 (CaF2: Tb3+) nanoparticles were synthesized with the silylant agent synthesized from meta-aminobenzoic acid and 3-isocyanatopropyltriethoxysilane as the surface modifier. The as-synthesized nanoparticles were covalently bonded to the silicon-oxygen network through a sol-gel process with tetraethyl orthosilicate (TEOS) as the precursor, resulting in the chemically bonded hybrid xerogel. The chemical doping makes the nanoparticles evenly dispersed in the stable hybrid system. The combined techniques SEM and EDX have confirmed the hybrid xerogel formed Si–O–Si network structure and the nanoparticles were successfully embedded into the Si–O–Si network. The efficient energy transfers from silylant agent to Tb3+ ions were observed in both the nanoparticles and hybrid xerogel, showing an obvious characteristic emission of Tb3+ ions. Both decay curves of nanoparticles and the hybrid xerogel were fitted as a double exponential decay, and the hybrid xerogel exhibited longer luminescent lifetime.

Modelling and analyzing the glass-like heat transfer behavior of rare-earth doped alkaline earth fluoride crystals

Based on the “three-stage” heat transfer behavior of rare-earth doped CaF2 crystals, numerical models at different stages for the thermal conductivity of rare earth doped fluorides are proposed to study the mechanism of glass-like heat conduction.

Magnetic properties of doped CaF2 powders

Room-temperature “ferromagnetism” found last decades in nonmagnetic nanoparticles is a puzzle that is not solved yet. In this work we report about the observation of room-temperature magnetism in powders of CaF2 doped with CeO2 as well as in the undoped powder but containing small amount impurity. The studied powders were obtained by grinding of bulk crystals using a sapphire mortar with a pestle. Magnetization dependences on magnetic field for the studied powder samples resemble the dependences observed in nonmagnetic oxide nanoparticles like CeO2. The comparative study of bulk and powder fluorite samples unambiguously confirms the key role of surface defects in the observed features. XRD and TEM measurements show that powder samples have particles of different sizes from the micro down to the submicron scale. The undoped bulk fluorite as well as doped with 0.5at.% (two differently grown crystals), 2at.%, 10at.% of CeO2 revealed only diamagnetic or paramagnetic contribution to magnetization curve whereas the powders of the same samples have ferromagnetic-like contribution on M−H curve. This work may help to get new insight in room-temperature “ferromagnetism” phenomenon of nonmagnetic nanoparticles.

Controllable multi‐color upconversion in glass ceramics through engineering crystal lattice distortion

AbstractLanthanide‐doped up‐conversion (UC) materials with color tunable property show promising application in many fields, such as biological imaging, optical multiplexing, and information security. Although great efforts have been devoted to regulate UC emission color via modifying chemical composition, crystal structure, or external stimuli condition, it is remain a daunting challenge to develop a general strategy that probably suitable for various lanthanide‐doped nano‐systems. Herein, we verify the cross‐relaxation possibility between sensitizer Yb3+ and activator Er3+ as well as Er3+‐ Er3+ pairs can be well modified by engineering the crystal lattice distortion of matrix, leading to the evident UC emission color variations. Specifically, with incorporating Al3+ or Ca2+ ions into the Yb/Er co‐doped BaF2 nanocrystals (NCs) contained glass ceramics (GCs), the contraction of the crystal lattice lead to the change of UC emission color from green to red. On the contrary, the high laser excitation power results in the lattice expansion of the Al3+‐doped BaF2:Yb/Er GCs at different Yb3+ concentration as well as Al3+ doped CaF2 GCs, which leads to the greatly increased green‐to‐red ratio. These multi‐color UC GCs show potential application in anti‐counterfeiting filed. Our results open up a general avenue for the control of UC emission color via engineering crystal lattice distortion.

Preparation of glass-based composites with green upconversion and persistent luminescence using modified direct doping method

New oxyfluorophosphate glass-based composites which exhibit not only green upconversion under 980 nm pumping but also green persistent luminescence (PeL) after being UV charged were successfully prepared using the direct doping method. The composites are composed of a glass-ceramic with Er3+ doped CaF2 crystals and of the persistent luminescent particles with the SrAl2O4:Eu,Dy composition. In the standard direct doping method, the glass melt is quenched few minutes after adding the PeL particles in the melt held at a temperature lower than the melting temperature. It is demonstrated that the direct doping method should be modified when preparing oxyfluoride glasses with PeL particles to limit not only the decomposition of the PeL particles in the glass but also the fluorine evaporation occurring during the glass preparation. Here, the composites were prepared by quenching the melt right after adding the PeL particles. The modified direct doping method allows the preparation of glass-based composites with strong green upconversion and homogeneous green persistent luminescence.

Ultraviolet to near infrared down-conversion in CaF2:Nd3+/Yb3+/Li+ phosphors

Down conversion (DC) in rare-earth-doped optical materials is a process of great interest for the possibility of a substantial increase of the efficiency of silicon solar cells. Here we report the structural and photoluminescence properties of co-doped CaF2 phosphors obtained by hydrothermal synthesis. In particular, the DC photoluminescence characteristics for UV (353 nm) excitation of Nd3+/Yb3+ co-doped CaF2 phosphors are discussed, underlining the effects due to the co-doping with Li+. The photoluminescence emission is dominated by the near-infrared (NIR) Yb3+ emission peaked at 975 nm, although the excitation spectrum corresponds to the characteristic peaks associated with Nd3+ excitation at both the UV and visible wavelength ranges. The Nd3+ to Yb3+ energy transfer mechanisms were determined from a detailed analysis of the excitation spectra characteristics of Nd3+ and Nd3+/Yb3+/Li + doped CaF2 phosphors. DC photoluminescence for UV excitation was confirmed by both the analysis of DC quantum yield efficiency and effective quantum yield measurements. In the first case, an efficiency up to 150% was found, while the effective quantum yield measurements, carried out for the UV (DC) and visible (downshift) excitation and NIR photoluminescence emission, give values of 81 ± 10% and 37 ± 5% for excitation with 353 nm and 577 nm light, respectively.

Comparison of co-operative down-conversion luminescence in Pr3+, Yb3+ doped CaF2 and SrF2

An optical study of the down conversion luminescence in Pr (0.5 mol%), Yb (1 mol%) doped CaF2 and SrF2 synthesized by high temperature Solid state reaction method has been done. The down-conversion luminescence intensities of both the hosts have also been compared to identify which is better in terms of optical properties. The photoluminescence excitation and emission were investigated under visible excitation at 445 nm. Besides optical characterization, magnetic properties were also investigated to quantify their magnetic behavior and their comparison. In the present case, the luminescence intensity is higher for SrF2: Pr,Yb and the detailed mechanism behind this has also been discussed. The study highlights the potential of Pr,Yb doped SrF2 and CaF2 for various optical and optoelectronic applications.

Tailoring the Glass Composition to Increase the Thermal Stability without Impacting the Crystallization Behavior of Oxyfluorophosphate Glass

Even though the (75 NaPO3-25 CaF2) (in mol%) glass can be heat-treated into transparent glass-ceramic with Er3+ doped CaF2 crystals precipitating in the volume of the glass during heat-treatment, this glass was found to be a poor glass former, limiting its use as upconverter under 975 nm pumping. In this study, the impact of the glass composition on the thermal, optical and structural properties of the glass was investigated in order to understand how the glass composition can be tailored for the development of thermally stable upconverter glass-based material. The addition of MgO, Fe2O3 and Al2O3 in the NaPO3-CaF2 glass system increases the thermal stability of glass due to the depolymerization of the glass network. However, the changes in the glass composition also impacted on the nucleation and growth process. Indeed, CaF2 and other crystals were found in the newly developed glasses after heat-treatment leading to glass-ceramics with lower intensity of upconversion than the (75 NaPO3-25 CaF2) glass-ceramic used as a reference. Glasses were also prepared with different concentrations of Er2O3 and ErF3. These glasses were found to be promising as not only are they thermally stable, but they also exhibit green and red emission with high intensity under 975 nm pumping due to Er3+ clustering.

Tm3+ codoping for mid-infrared laser applications of Dy3+ doped CaF2 crystals

We report here on a spectroscopic study of Dy3+-doped and Tm3+-Dy3+ doped CaF2 as promising candidates to develop crystalline waveguide laser sources around 3 μm. The advantages of Tm3+ ions as sensitizers to improve the excitation of Dy3+ ions in CaF2 is demonstrated: an energy transfer efficiency from Tm3+ to Dy3+ ions of 99.6% has been reached for Dy3+ concentration as high as 2 at.%, by considering a Tm3+ ratio set at 5 at.%. Moreover, the behavior of such doped crystals in a laser waveguide configuration has been modeled and the modeling results show that it seems possible to achieve promising laser perspectives around 3 μm, with laser thresholds in the watt level for singly doped Dy3+:CaF2 and around 0.2–0.3 W for codoped Tm3+-Dy3+:CaF2, presenting both laser efficiencies in the order of 30%. Finally, the saturation of the absorption which is observed in the modeling for such codoped Tm3+-Dy3+:CaF2 waveguide at Dy3+ concentration below 1% is discussed and its origin is explained.

Luminescent behaviour of SrF2 and CaF2 crystals doped with Eu ions under different annealing temperatures

The photoluminescent (PL) properties of europium (Eu) doped SrF2 and CaF2 nanoparticles have been investigated after annealed in an Ar/H2 atmosphere at different temperatures for possible down-conversion applications in solar cells. PL investigations revealed that the Eu doped samples exhibited emission from both Eu2+ and Eu3+ ions. The emission spectrum’s shape of the Eu2+ doped SrF2 sample stayed unchanged, unlike the Eu2+ doped CaF2 sample. Temperature treatments significantly affected the Eu3+ emission. A complete reduction of Eu3+ to Eu2+ occurred during heating under H2 atmosphere. Degradation of the Eu2+ emission intensity occurred during prolonged laser irradiation of the annealed SrF2 as-prepared samples, but it was stable at 800 °C annealing temperature. These results demonstrated that temperature treatment destroyed the surface trap centers and enhanced the photoemission stability of the Eu2+.

The effect of Li+ incorporation in Yb3+-Nd3+ co-doped CaF2 phosphors over the NIR photoluminescence emission excited under visible light

The structural and optical characteristics of Nd3+-Yb3+ doped CaF2 phosphors with and without the addition of Li+ ions are described in this work. The phosphors synthetized by hydrothermal and co-precipitation methods showed near-infrared (NIR) luminescence emission associated with inter-electronic transition of the Yb3+ ion in the range of 900–1050 nm via energy transfer process from Nd3+ ions under visible light excitation. The addition of Li+ to these phosphors resulted in an improvement of the NIR luminescence intensity by a factor up to 5. The effect of the incorporation of Li+ ions into the CaF2 crystallite structure, the reduction of luminescence quenching states, as well as the energy transfer mechanism involved are discussed.

Judd-Ofelt analysis and spectroscopic study of Tb:CaF2 and Tb/Pr:CaF2 co-doped single crystals

Abstract High quality of Tb3+-single doped and Tb3+/Pr3+ co-doped CaF2 crystals were grown by the temperature gradient technique (TGT). Pr3+ ions were used as sensitizing ions of Tb3+. Optical properties including absorption spectrum, emission spectrum, and fluorescence decay curve were recorded. Through co-doping sensitizing ions of Pr3+, the absorption cross section around 480 nm increased from 0.007 × 10−20 cm2 to 0.3 × 10−20 cm2, the large value is beneficial for the absorption of LD pump source. Judd-Ofelt theory was used to analyze the spectroscopic parameters. Fluorescence branch ratio, transition probability, radiative and fluorescence lifetimes, and quantum efficiency were determined as well. The emission cross section of Tb,Pr:CaF2 for the green emission at 544 nm and yellow emission at 587 nm corresponding to the transition of 5D4→7F5 and 5D4→7F4 were calculated to be 5.56 × 10−22 cm2 and 8.78 × 10−23 cm2, respectively, which is comparable to the Tb3+ doped CaF2 crystal.

Up-conversion study of CaF2 based oxy-fluoride core-shell particulate nano-glass ceramics via sol-gel method: Effect of Yb3+ concentration and cell viability study

The current study delineates the synthesis and characterization of CaF2 based Oxy-fluoride glass-ceramics with Erbium (1 mol%) and with varying Ytterbium concentration (1 mol% and 5 mol%) doping of fluoride source. The influence of yb3+ concentration on thermal, structural, and luminescence properties were investigated and analyzed through a sol-gel process. The differential thermal analysis (DTA) specified that the glass transition (Tg) temperature decreases with the increase of Yb3+concentration in the prepared glass-ceramics. The microstructural study performed by transmission electron microscope (TEM), indicates the core-shell structure of glass-ceramics nanoparticles in between 10–50 nm range. The incorporation of Er3+ and Yb3+ into the crystalline matrix was confirmed and analyzed from X-ray diffraction (XRD) analysis. The increased crystal size according to increased temperatures shows the CaF2 crystal growth. The up-conversion luminescence properties of the prepared nano-glass ceramics were tested by using a 980 nm laser. The photoluminescence emission showed two peaks at green (548 nm and 528 nm) emission and one at red emission (626 nm). Additionally, the cell viability study of the Er3+-Yb3+ doped CaF2 nano glass-ceramics was performed using MG-63 osteoblast cells and it shows significant cell growth as compared to control over a period of 24 h. The result confirmed that the prepared nano-glass-ceramics can be a potential optical material for bio-imaging.

Infrared optically stimulated luminescence of rare earth doped CaF2 and co-doped with Al or Li produced by combustion synthesis

The aim of this paper is to characterize the IRSL response of rare earth doped CaF2 and co-doped with Al or Li, produced by the solution combustion synthesis (SCS). The results showed a high intensity of the infrared optically stimulated luminescence (IRSL) response for the samples of CaF2 doped with Tb and co-doped with Li. For the samples of CaF2 doped with Ce and co-doped with Li present better IRSL response than the samples co-doped with Al. It was also observed that the samples of CaF2:Ce,Li present a high fading with 50% reduction of the IRSL signal in a period of 60 min. In case of the CaF2:Tb,Li there is no significant fading during this period. For clinical dosimetry the fast fading of the dosimeter is not adequate. However, this feature can be exploited for other applications such as tamper-proof passive systems that detect and timestamp the movement of undeclared nuclear materials, by the estimation of the OSL response of materials with different fading rates.