Exploring the Potential of Tb and Bi Doped LaF3 as a UV Absorber

doi:10.21203/rs.3.rs-4984775/v1

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Exploring the Potential of Tb and Bi Doped LaF3 as a UV Absorber

https://doi.org/10.21203/rs.3.rs-4984775/v1

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Lanthanide-based nanophosphors are finding significant applicability in various fields such as LED, solar spectral convertors, lasers, biological sensors etc, owing to their superior stability and luminescence properties. However, their function as a UV protector hasn't evolved, possibly because of their limited absorption cross-section in the UV spectral region. Hence, to overcome this limitation, in this work, a strategy to utilize Bi³⁺ ion as a co-dopant in LaF₃:Tb³⁺ nanophosphor has been devised. These nanophosphors with uniform morphology and narrow particle size distribution were synthesized using hydrothermal method. Luminescence mechanism involving energy transfer from Bi³⁺ to Tb³⁺ were investigated. Excitation spectrum of LaF₃ nanophosphor, co-doped with Bi and Tb, reveals distinct absorption bands. Absorption ascribed to Bi³⁺ characterizes the UV range between 220–275 nm, whereas Tb³⁺ is associated with absorption in another UV range spanning 280 to 380 nm. This shows the potential applicability of the Bi and Tb co-doped LaF₃ nanophosphor as UV absorber. The cytotoxicity of these nanophosphors was tested on HaCaT cells, which indicates their potential applicability in health care field.

Nanophosphor

Biocompatible

UV-Blocker

Cytotoxicity

Sunscreen

Photoluminescence

LaF3:Tb3+Bi3+

Energy-Transfer

The solar radiation reaching Earth encompasses a broad range of wavelengths, extending from ultraviolet (UV) to infrared (IR) frequencies. Notably, UV radiation plays a pivotal role in photochemical reactions that lead to the formation of the ozone layer within the Earth's stratosphere.[1, 2] The depletion of this ozone layer has brought about amplified intensities of UV light reaching the Earth's surface, subsequently giving rise to a multitude of concerns.[3, 4]

One of the primary repercussions of heightened UV radiation exposure is the exacerbation of various health issues. Prolonged exposure to UV radiation has been linked to adverse effects such as sunburn, skin cancer, vitamin A deficiency and DNA damage.[4–7]

Thus, to protect skin from UV radiation, two approaches are mainly used termed as chemical filters and physical filters. The chemical filters are the organic compound, which includes Oxybenzone, Homosalate and Octocrylene.[8] They absorb UV radiation and transform it into heat. However, long term application of it leads to allergic reaction or burning sensation. Also, on prolong exposure to sunlight, chemical breakdown, termed as photochemical degradation were observed in it.[9] Hence, its effectivity decreases with exposure duration.

Physical filters on the other hand are inorganic materials such as Zinc Oxide and Titanium Oxide. They absorb as well as scatter incident UV rays and thus act as a physical barrier to them. Nevertheless, upon absorbing UV radiation, they initiate the production of reactive oxygen species (ROS) through a photocatalytic process.[10–12] ROS is associated with cytotoxic and carcinogenic effects. Thus, their utilization would cause serious health concerns. [9, 12] In order to address these issues, we propose a biocompatible LaF₃ based nanophosphor which absorbs incident harmful UV radiation and transform it into eye-safe visible light.

In phosphor, host materials play crucial role in determining the luminescence efficiency. Thus, amongst various host materials like sulphide, oxide, aluminate etc., fluorides are preferred because of low phonon energy of its lattice.[13–15] The low phonon energy enhances the probability of radiative relaxation of excited ion.[16]

Lanthanide based phosphor (Ln: phosphor) is one of the class of luminescent material, which has attracted much interest of researchers because of its unique optical properties. This uniqueness originates from intra band 4f-4f transition in Ln³⁺ ion. [17–19] The intra band transitions result in its improved luminescence features like sharp emission band, long luminescence lifetime, resistant to photobleaching and so on.[18, 20] Further, the biomedical significance arises from its desirable physical and chemical properties such as high chemical and thermal stability and low cytotoxicity, compared to its conventionally used semiconductor quantum dots.[21–23] Low cytotoxicity is a crucial feature determining the biological application of nanoparticles, since it involves intentional contact of these nanoparticles with biomolecules.[22, 24–26]

Despite the various advantages that Ln: phosphor offer for luminescence applications, lower quantum yield compared to its counterparts because of forbidden nature of 4f-4f transitions can be a roadblock for its applicability.[27] While the increase in dopant concentration of Ln³⁺ improves the absorbance, its emission is not enhanced due to the concentration-dependent luminescence quenching phenomenon.[28–30]Hence, in order to overcome this problem, one of the crucial strategy is to use an additional dopant. This dopant can act as a sensitizer and absorb incident light and transfer it to an activator. Herein, one of the prerequisites is that the absorption cross section of the sensitizer must be higher as compared to that of activator. In this respect, it is well known that, for Ln³⁺( Sm³⁺, Eu³⁺, Er³⁺ etc.) activators, Bi³⁺ is utilized as a sensitizer, which has comparable ionic radii and substantial absorption in the near-UV region.[31–34]

In the current investigation, we present the synthesis of Tb³⁺ doped and Bi³⁺, Tb³⁺ codoped LaF₃ nanoparticles. Structural, particle size distribution and morphological characterization were carried out to assess quality of synthesized nanoparticles. The photoluminescence (PL) and absorption spectra with and without Bi³⁺ as co-dopant in Tb³⁺:LaF₃ nanophosphor are discussed in detail.

2.1 Materials

Following chemicals, as received from their respective sources were utilized. Lanthanum oxide (La₂O₃, Alfa Aesar, 99.99%), Terbium oxide (Tb₂O_3, Alfa Aesar, 99.99%), Bismuth oxide Bi₂O₃, Acros Organics, 99.999%), Ammonium Fluoride (NH₄F, Merck, 98%), Ammonia solution (NH₃, CDH, 25%) and Nitric acid (HNO₃, Merck, 69%).

2.2 Synthesis procedure

The LaF₃ nanoparticles doped with Tb³⁺ and Tb³⁺,Bi³⁺ were prepared by following the synthesizes procedure reported previously for LaF₃:Tb,Yb nanoparticles.[35] However, minor modifications were carried to attain further control over nanoparticle size and their agglomeration.

In a typical synthesis, an aqueous solution was prepared by dissolving Bi₂O₃, Tb₂O₃ and La₂O₃ as per the required stoichiometry (e. g. Tb1mol%, Bi1mol%: LaF₃) in dilute nitric acid (2M). Then the second solution, termed as fluoride solution was prepared after dissolving NH₄F in deionized water. Its pH value is increased to 8 by adding few drops of ammonia in it. Finally, precipitation was performed by addition (5 ml/min) of nitrate solution dropwise into fluoride solution at 50 ^oC temperature. After, the formation of a milky white dispersion, the solution was transferred into an autoclave and heated at ~ 150^oC for 1 h. Then, the solution was allowed to cool down to room temperature.

Further steps, which involves washing by methanol solvent and drying at 80^oC were carried out. The above procedure was repeated for preparing LaF₃ nanoparticles with varying dopant concentration of Tb³⁺ and Bi³⁺ ions.

2.3 Characterizations:

The X-ray diffraction measurement of prepared samples for examining their crystal structure were carried out on Rigaku diffractometer under Cu K_α (λ = 1.5406 A^o) source. The Malvern Zetasizer Nano ZS90 nanoparticle analyser were utilized to characterize particle size distribution and their respective zetapotential in aqueous medium. The particle size and morphology were analysed using transmission electron microscopy (FEI Tecnai G² F20, 200 kV). XPS analysis of the synthesized nanophosphor was carried out utilizing the BL-14 XPES beamline at the Indus-2 synchrotron radiation source, RRCAT, Indore, India. The beamline is equipped with a double-crystal Si (111) monochromator for photon energy selection and measurements was performed under ultrahigh vacuum conditions. Data acquisition was performed with SPECS PHOIBOS 225 HV hemispherical electron energy analyzer using 4085 eV photon energy. Subsequent data analysis was conducted with CasaXPS software. FTIR was carried out on Bruker ALPHA II spectrometer. The PL characterization, which involves measurement of excitation and emission spectra, fluorescence lifetime and absolute quantum yield of nanophosphors were carried out on FLS920-s fluorescence spectrometer (Edinburgh Instruments Ltd.) equipped with a 450 W Xenon lamp source.

The cytotoxicity assessment of the synthesized samples was carried out using MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay on HaCaT (Human keratinocytes) cells .[36] At first ~ 5000 cells per well seeded in 96-well plate and incubated at 37^oC with 5% CO₂ for 24 h. Then, these cells were treated with medium, containing LaF₃ NPs which was prepared by adding LaF₃:Tb³⁺,Bi³⁺ sample of various concentration (0.1, 0.2, 0.3, 0.4 and 0.5 mg/mL) in DMEM (Dulbecco's Modified Eagle's Medium) for 24 h at 37^oC with 5% CO₂. Following this incubation, MTT (0.5 mg/ml) assay was added and cells were incubated for 4 h under same environmental condition. The insoluble formazan salts formed, which were dissolved in dimethyl sulfoxide (DMSO). Finally, absorbance of this DMSO solutions were measured at 590 nm source using a micro well plate reader while monitoring emission at 630 nm. The production of formazan salt quantifies the presence of living cells. Hence, cell viability percentage were estimated from the absorbance of respective DMSO solutions.

The X-ray diffraction (XRD) patterns of synthesized Tb³⁺ and Tb³⁺&Bi³⁺ co-doped LaF₃ samples are shown in Fig. 1a and 1b with their respective dopant concentrations. The total number of peaks and their position are observed to match well with that of ICDD File no.: 32–0483, which indicate that all the samples formed are in hexagonal phase of LaF₃ (space group P3c1). However, peak shifting towards higher 2θ values were observed with increase in concentration of Tb³⁺ (Fig. 1a) as compared to that of Bi³⁺( Fig. 1b) in LaF₃. This may be due to the variation in ionic radius of Tb³⁺( 92.3 pm, VI coordination) which is lesser compared to La³⁺( 103.2 pm, VI coordination), whereas the ionic radius of Bi³⁺( 103 pm, VI coordination) is similar to that of La³⁺.[37] Thus, it shows the effective replacement of La³⁺ with dopants Tb³⁺ and Bi³⁺ in LaF₃ unit cell. Furthermore, no variation in width of diffraction peaks were observed irrespective of dopants (Tb³⁺& Bi³⁺) concentration or their combination used in samples. This proves all the samples are formed with nearly similar crystallite size. By utilizing Scherrer formula, the calculated crystallite size from FWHM of (111) reflection is ~ 11 nm.[38]

Particle size distribution or hydrodynamic diameter of samples were measured using dynamic light scattering (DLS) technique. Figure 2 shows the particle size distribution of all samples. As shown in them, size distribution of particles is spread over 10–100 nm range with most of them are in 20–40 nm range. This resultant narrow particle size distribution is one of the advantages of synthesis technique utilized for sample preparation. Further, measured zeta potential value is ~ + 45 mV, which indicate that particles are well dispersed in deionised water (at pH = 7) due to electrostatic repulsion occurring between them.

Since, the hydrodynamic diameter is observed to be similar for all samples, particle size analysis using transmission electron microscope (TEM) were carried out on one of it. Figure 3 shows the TEM image of LaF₃:1%Tb,1%Bi sample. The nanoparticles are formed in spherical shape with average particle size of ~ 20 nm. However, this particle size value is much larger than that of crystallite size calculated using XRD data. This discrepancy may be due the polycrystalline nature of nanoparticles as evident in Fig. 3, which is caused by self-assembling of subunits to shape the resultant nanoparticle.[39]

X-ray Photoelectron Spectroscopy (XPS) was performed to determine the elemental presence in the LaF₃:1%Tb,1%Bi nanophosphor. A wide-scan spectrum covering a binding energy range of 0-1300 eV was acquired. After carbon correction, the survey spectrum exhibited peaks attributed to C 1s, O 1s, F 1s, La (3p, 3d, 4p, and 4d), Bi (3d and 4f), and Tb 3d as shown in Fig. 4. The presence of Bi and Tb peaks in the spectrum confirms the successful doping of these ions into the LaF₃ host lattice.

Fourier Transform Infrared (FTIR) spectroscopy was used to identify the functional groups present on the surface of LaF₃:1%Tb,1%Bi nanophosphor. As shown in Fig. 5, a broad band centered at 3415 cm⁻¹indicates the O-H stretching vibration of adsorbed water, while the peak at 1630 cm⁻¹ is attributed to the bending vibrations of its O-H groups.[40] The characteristic peak at 1452 cm⁻¹ is assigned to the asymmetric and symmetric bending vibrations of the O-H group from methanol. Additionally, the peak at 1352 cm⁻¹ arises from the bending vibrations of the C-H bonds in the methyl group of methanol.[41] The spectra also show absorption of atmospheric CO₂, evident by a small peak at ~ 2350 cm⁻¹.[40] These findings suggest that the surface of the nanophosphor is contaminated with adsorbed water and methanol, likely introduced during the sample preparation process.

To assess the light absorption properties relevant to sunscreen applications, we investigated the UV-visible absorption spectra of LaF₃:Tb,Bi nanophosphors (such as LaF₃:1.5%Tb and LaF₃:1.5%Tb,10%Bi), as shown in Fig. 6.The spectra revealed a broad peak around 250 nm, corresponding to the electronic transition (¹S₀ → ¹P₁) in Bi³⁺ ions.[42, 43] This peak suggests efficient absorption of UV radiation by the nanophosphors. The broadness of the peak might indicate the presence of localized defect states within the material.[44]

Next, we explored the luminescence properties using photoluminescence (PL) excitation spectroscopy. These spectra assess how effectively different wavelengths of light can excite the material and cause it to emit light. The spectra were recorded in the 200–400 nm range while monitoring the characteristic Tb³⁺ emission at 543 nm (due to the ⁵D₄ → ⁷F₅ transition) as shown in Fig. 7.[45] This allows to observe how different light wavelengths influenced the emission from the Tb³⁺ ions.

The PL excitation spectra confirmed excitation of both LaF₃:Tb and LaF₃:Tb,Bi nanophosphors across various wavelengths in the near-UV region. These excitations correspond to different energy level transitions within Tb³⁺ ions (f-d and f-f transitions). Notably, LaF₃:Tb,Bi displayed an additional strong excitation band between 225–275 nm, which was absent in LaF₃:Tb alone. This new band aligns with the previously observed Bi³⁺ absorption peak (around 250 nm) and is attributed to the ¹S₀ → ¹P₁ transition in Bi³⁺ ions. The presence of this Bi³⁺ absorption band in the Tb³⁺ excitation spectrum indicates that energy transfer between the dopant ions might be possible.

Since, the intense excitation peak in the case of LaF₃:1%Tb was observed at ~ 350 nm which corresponds to ⁵F₆ → ⁵G₄ transition of Tb³⁺.[46] Thus, this excitation source was used for optimizing the concentration of Tb³⁺ in LaF₃ host. Figure 8a shows the emission spectra of LaF₃:x%Tb (x = 0.5, 1, 1.5 & 2) in visible spectral range upon 350 nm excitation. Mainly four distinct emission peaks corresponding to ⁵D₄→⁷F₆ (488 nm), ⁵D₄→⁷F₅ (543 nm), ⁵D₄→⁷F₄ (583 nm) and ⁵D₄→⁷F₃(620 nm) electronic transitions in Tb³⁺ ion were observed. [47–49]

Initially, with increase in concentration of Tb³⁺, the PL emission intensity also increases. This increase may be due to the increase in absorption cross section of Tb³⁺ ion at 350 nm. However, emission intensity decreases for higher concentration of Tb³⁺ (> 1.5 mol%). This may be occurred due to the concentration quenching effects.[30, 50]

Similarly, Bi³⁺ excitation peak centred at ~ 250 nm, was used for optimising its concentration in LaF₃:1.5%Tb nanophosphor. Figure 8b shows the emission spectra of LaF₃:1.5%Tb,y%Bi (y = 0, 5, 10, 15 & 20) in visible spectral range upon 250 nm excitation. The observed emission spectra are identical to those of Tb³⁺ singly doped in LaF₃ nanophosphors (Fig. 8a). Hence, all the emission peaks are characteristics of electronic transitions in Tb³⁺ ion. Since, on Bi³⁺ excitation (i.e., at 250 nm), the emission intensity of Tb³⁺ increases without any variation in number or position of peaks observed in PL emission spectrum demonstrates applicability of Bi³⁺ as an efficient sensitizer for Tb³⁺ ion. Figure 8c shows photographs of LaF₃:1.5%Tb,10%Bi nanophosphors dispersed in water under daylight and a 250 nm UV light source. This comparison allows one to correlate the observed emission spectrum with the human eye's perception of colour.

To explore the luminescence capabilities and assess their suitability as phosphor applications, it's crucial to ascertain the material's quantum efficiency. This efficiency is defined as the proportion of emitted photons to that of absorbed photons. The quantum efficiency of LaF₃:1.5%Tb³⁺,x%Bi³⁺ nanophosphors were assessed by monitoring the excitation at 250 nm and emission within 400 to 800 nm wavelength range as shown in table 1.

The energy transfer from sensitizer to activator ion occurs mainly by two ways, termed as radiative and nonradiative energy transfer process. The radiative energy transfer involves, reabsorption of sensitizer emission by an activator ion. Thus, to evaluate its contribution in energy transfer, overlap between the emission spectrum of LaF₃:1%Bi³⁺ (upon 250 nm excitation) and excitation spectrum of LaF₃:1%Tb³⁺ (for 543 nm emission) plotted in Fig. 9. A very small overlap between them were observed, which demonstrates that dominant contribution of non-radiative energy transfer process.[51]

Further, to investigate the type of non-radiative energy transfer process, occurring between Bi³⁺ and Tb³⁺ co-doped in LaF_3, luminescence decay analysis was carried out. Figure 10a shows luminescence decay curves obtained upon excitation at 250 nm (corresponding to ¹S₀$\:\to\:$¹P₁ transition of Bi³⁺) and monitoring emission at 550 nm (corresponding to ³P₁$\:\to\:$¹S₀ transition of Bi³⁺). These decay curves are well fitted with double exponential function: [52]

$$\:I\left(t\right)={I}_{0}+\:{A}_{1}{e}^{-\frac{t}{{\tau\:}_{1}}}+{A}_{2}{e}^{-\frac{t}{{\tau\:}_{2}}}$$

here, I(t) and I₀ represents the photoluminescence intensity at time t and at t = 0, $\:{\tau\:}_{1}$ and $\:{\tau\:}_{2}$ denote decay lifetimes, with $\:{A}_{1}$ and $\:{A}_{2}$ as constants. Utilizing this, we can determine the average lifetime ($\:<\tau\:>)$ through [52, 53]

$$\:<\tau\:>=\frac{{A}_{1}{\tau\:}_{1}^{2}+{A}_{2}{\tau\:}_{2}^{2}}{{A}_{1}{\tau\:}_{1}+{A}_{2}{\tau\:}_{2}}$$

The decay times corresponding to different dopant concentrations are illustrated in inset of Fig. 10a. Notably, the analysis reveals a sequential decrease in < τ > of the excited Bi³⁺ state from 993 ns to 805 ns with increasing Tb³⁺ concentration in LaF₃. This observation substantiates the existence of energy transfer from Bi³⁺ to Tb³⁺ via a non-radiative mechanism. Efficiency ($\:{\eta\:}_{ET}$) of this energy transfer can be determined using:[53]

$$\:{\eta\:}_{ET}=1-\frac{{\tau\:}_{sa}}{{\tau\:}_{s}}$$

here $\:{\tau\:}_{s}\:\text{a}\text{n}\text{d}\:{\tau\:}_{sa}\:$represents the lifetime of Bi³⁺ singly doped and codoped with Tb³⁺ in LaF₃, respectively. A consistent increase in $\:{\eta\:}_{ET}$ is observed with rising Tb³⁺ concentration as shown in Fig. 10b. However, Tb³⁺ concentration is constrained to 2 mol% due to the onset of concentration quenching beyond it, as evidenced in PL emission spectra (Fig. 8a).

Adhering to the Dexter and Reisfeld approximation for multipolar interaction type for energy transfer, we evaluate the relation[54, 55]

$$\:\frac{{\tau\:}_{sa}}{{\tau\:}_{s}}\alpha\:\:{C}^{n/3}$$

here, $\:C$ denotes the total concentration (i.e. sum of Tb³⁺ an Bi³⁺ concentrations in LaF₃), and $\:n=\:$6, 8 and 10 correspond to dipole-dipole, dipole-quadrupole and quadrupole-quadrupole interaction, respectively. Figure 11 presents the result following linear fitting of the above equation. The optimal fitting outcome for n = 8, suggests the energy transfer is predominantly a consequence of dipole-quadrupole interaction.

The schematic diagram depicting the involved energy transfer between Bi³⁺ and Tb³⁺ is shown in Fig. 12. The incident 250 nm photon excites electron of Bi³⁺ from ground state ¹S₀ to ¹P₁ level via electric dipole transition.[33, 56] Then it relaxes via multiphoton process to lower energy level ³P₁.[31] In the absence of activator ion (Tb³⁺) emission peak centred at ~ 550 nm occurs due to radiative emission caused by electronic transition from ³P₁ excited state to ground state ¹S₀ of Bi³⁺. However, in the presence of Tb³⁺ as a co-dopant in LaF₃:Bi³⁺ nanophosphor, resonant type energy transfer from ³P₁ (of Bi³⁺) to ⁵D₄ (of Tb³⁺) occurs via electric multipolar interaction.[56]

Cytotoxicity evaluation of LaF₃:1.5%Tb,10%Bi was carried out using MTT assay on HaCaT cells. Cells treated with nanoparticles of different concentration (0, 0.1, 0.2, 0.3, 0.4, and 0.5 mg/mL). The cell viability percentage on treatment with respective dispersion concentrations are as shown in Fig. 13. The relative cell viability was estimated by taking the ratio of observed cell viability percentage after treatment of cells with and without nanoparticles. These are > 80% till 0.4 mg/mL concentration, which indicates negligible toxicity effect on HaCaT cell. Thus, obtained result display potential applicability of synthesized nanophosphors as an ingredient in sunscreen lotion to protect skin from UV rays.

Nanophosphors of Tb, Bi doped LaF₃ with spherical morphology and narrow particles size distribution were successfully synthesized using hydrothermal method. The structural analysis of series of these synthesized nanophosphors (i.e., LaF₃: x%Tb (x = 0.5, 1, 1.5 & 2) and LaF₃: 1.5%Tb y%Bi (y = 5, 10, 15, 20)) revealed the hexagonal phase of LaF₃(space group P3c1). The optimization of Tb³⁺ dopant concentration in LaF_3, were carried out using 350 nm excitation source, which corresponds to ⁵F₆ → ⁵G₄ transition in Tb³⁺ ion. The emission intensity increases with increase in Tb³⁺ concentration till 1.5 mol% and above this concentration the emission intensity decreases due to concentration quenching. The absolute quantum efficiency of 48.54% was achieved in LaF₃: 1.5%Tb 10%Bi nanophosphor under 250 nm exciation. Energy transfer from Bi³⁺ to Tb³⁺ in LaF₃ were evident from its PL excitation/emission spectra, which shows distinct excitation peak at ~ 250 nm (corresponding to ¹S₀ → ¹P₁ transition in Bi³⁺) and emission peak at 488 nm, 543 nm, 583 nm and 620 nm corresponding to electronic transitions in Tb³⁺ ion. Further, the sequential decrease in lifetime of Bi³⁺ excited state with increase in Tb³⁺ dopant also validates dominance of non-radiative energy transfer probability between them. To summarize, we have comprehensively examined the mechanism involved in Tb/Bi co-doped LaF₃ nanophosphor and optimized their respective dopant concentration to achieve intense emission at 543 nm. Tb/Bi co-doped LaF₃ nanophosphor shows broad absorption peak in UV which confirms its potential as UV absorber. Further, the negligible cytotoxic effect of these nanophosphor tested on HaCaT cells and broad absorbance in UV range paves the way for its potential applicability as an active sunscreen ingredient.

CONFLICTS OF INTEREST

The authors declare no conflict of interest.

No funding was received for the submitted work.

Author Contribution

A. : conceptualization, data curation, methodology, visualization, writing – original draft.B: formal analysisC: conceptualization, methodology, investigation.D: supervision, validation, writing –review & editing.E: supervision, resource, writing –review & editing

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Table I: The quantum efficiency values of LaF₃: 1.5 %Tb³⁺, x%Bi³⁺ under 250 nm excitation.

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Exploring the Potential of Tb and Bi Doped LaF3 as a UV Absorber

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