Scintillation Properties of Lanthanide Doped Pb₄Lu₃F₁₇ Nanoparticles

Abstract

Inorganic scintillators are of great significance in the fields of medical CT, high-energy physics and industrial nondestructive testing. In this work, we confirm that the Pb₄Lu₃F₁₇: Re (Re = Tb, Eu, Sm, Dy, Ho) crystals are promising candidates for a new kind of scintillator. Detailed crystal structure information is obtained by the Rietveld refinement analysis. Upon X-ray irradiation, all these scintillators exhibited characteristic 4f-4f transitions. The Ce and Gd ions were verified to be useful for enhancing the scintillation intensity via introducing energy transfer processes. The integrated scintillation intensity of the Pb₄Lu₃F₁₇: Tb/Ce is about 16.8% of the commercial CsI (Tl) single crystal. Our results manifested that Pb₄Lu₃F₁₇: Re has potential application in X-ray detection and imaging.

1. Introduction

Scintillator is a kind of luminescent material that can effectively absorb ionizing radiation and convert the absorbed radiation energy into visible light [1,2,3,4,5]. Inorganic scintillating materials have been broadly used in many fields, such as medical CT [6,7], high-energy physics [8,9] and industrial nondestructive testing [10,11]. Conventional oxide scintillators, such as Bi₄Ge₃O₁₂ (BGO) [12], PbWO₄ (PWO) [13] and Lu₂SiO₅: Ce (LSO: Ce) [14] show fast response and good chemical stability, while the halide scintillators, such as NaI (TI), CsI (TI) [15], LaCl₃: Ce [16] and CaF₂: Ce [17], have high luminous efficiency. For example, BaY₂F₈: Tb exhibited strong scintillation luminescence centered at 545 nm, and the intensity was about twice than that of CsI (Tl) [18]. The KY₃F₁₀: Pr showed strong scintillation luminescence centered at 260 nm, 490 nm and 610 nm, and the spectral integral intensity was ~2.5 times than that of Bi₄Ge₃O₁₂ [19]. Although many achievements have been made in these systems, the development of new fluoride scintillators with high performances is still of great significance.

Recently, it was reported that the lead halide perovskites showed strong scintillation intensity and high X-ray imaging quality, which is attributed to the existence of the heavy Pb atom. For example, the indirect X-ray imaging system based on CsPbBr₃ perovskite has faster response time (200 ns), better X-ray irradiation stability (>40 Gy_airs⁻¹ of X-ray exposure) and higher light output (177,000 photons/MeV) than the traditional GOS: Tb [20]. (CH₃NH₃) PbBr₃ crystals prepared by inverse temperature crystallization exhibited light yield up to 150,000 photons/MeV and sub-nanosecond response time at low temperature [21]. However, these kinds of scintillators exhibit poor environmental stability, which greatly restrict their practical applications. Lanthanide doped fluoride nanoparticles prepared with a low-temperature wet-chemical method possess the advantages of low toxicity, cheap fabrication cost, convenient device processability and adjustable emission wavelengths, which have been studied for high-performance X-ray detection and imaging very recently.

In this work, a series of novel rare-earth (Re = Tb, Eu, Sm, Dy, Ho) doped Pb₄Lu₃F₁₇ scintillators were developed by a simple hydrothermal method for the first time. The Rietveld refinement results were used to study the host structure. Upon X-ray irradiation, all these scintillators exhibited characteristic 4f-4f transitions. The optimal Tb³⁺ doping concentration was determined to be 30%, which is much higher than other hosts. Moreover, the Ce and Gd ions were verified to be useful for enhancing the scintillation intensity via introducing energy transfer.

2. Materials and Methods

Materials. The chemical reagents used in the experiments, including Pb(NO₃)₂ (99%), Gd(NO₃)₃ (99.9%), Lu(NO₃)₃ (99.99%), Ce(NO₃)₃ (99.95%), Tb(NO₃)₃ (99.9%), NH₄F, ethylene diamine tetraacetic acid (EDTA), and citric acid (CA), were all analytically pure and used directly without further purification. The above reagents, except Pb(NO₃)₂, were purchased from Shanghai Aladdin Company. Pb(NO₃)₂ was purchased from Tianjin Kemio Chemical Reagent Company (Tianjin, China), anhydrous ethanol was purchased from Hangzhou Shuanglin Chemical Reagent Company (Hangzhou, China), and deionized water was self-produced in the laboratory.

Synthesis of Pb₄Lu₃F₁₇: 30Tb/10Ce/5Gd NCs. Take the hydrothermal synthesis of Pb₄Lu₃F₁₇: 30Tb/10Ce/5Gd as an example: first, 8.5 mmol of NH₄F was dissolved in 15 mL of deionized water and stirred vigorously for 15 min using a magnetic stirrer; then, 2 mmol Pb(NO₃)₂, 0.55 × 1.5 mmol Lu(NO₃)₃, 0.3 × 1.5 mmol Tb(NO₃)₃, 0.1 × 1.5 mmol Ce( NO₃)₃, 0.05 × 1.5 mmol Gd(NO₃)₃ and 0.18 mmol EDTA were dissolved in 15 mL of deionized water and stirred vigorously for 30 min. Subsequently, the solution with NH₄F was added to the above mixture and stirred vigorously for another 30 min. Finally, the resulting mixture was transferred to a PTFE liner, set in a stainless steel hydrothermal reactor and placed in an oven. The parameters were set to a heating temperature of 200 °C and a heating time of 12 hours. After the reaction, the product was collected by centrifugation at 1,0000 r/min for 5 min, washed with anhydrous ethanol and centrifuged several times, and finally dried at 60 °C for 3 h. The product powder was obtained after grinding. Other materials were synthesized in a similar way.

Sample characterizations. The X-ray diffraction (XRD) patterns of the samples were obtained by a powder diffractometer (Bruker D8 Advance, Saarbrücken, Germany) using Cu-Kα (λ = 1.5405 A) radiation. XRD patterns were recorded in the 2(θ) range 10° to 80° with a scanning rate of 0.02°. The morphology and size of the products were observed by a Scanning electron microscopy (SEM, FEI Tecnai G2 F20, Hillsboro, OR, USA) equipped with an energy dispersive X-ray spectroscope (EDX, Aztec X-Max 80T, Oxford, England). The luminescence and afterglow of the samples were recorded by a X-ray fluorescence spectrometer (Zolix Omni-λ300i, Beijing, China). The X-ray source is a mini MAGPRO X-ray (target material: tungsten, P_max = 12 W, V_max = 60 kV).

3. Results and Discussion

The X-ray diffraction (XRD) patterns of the as-prepared Pb₄Lu₃F₁₇: Re are shown in Figure 1a. All these products are well indexed with the standard data of pure rhombohedral Pb₄Lu₃F₁₇ phase (JCPDS No.44-1373). These results suggested that the Re ions were successfully incorporated into the host without the emergence of extra impurity. In order to acquire the crystal structure information of the prepared samples, Rietveld structure refinements of the Pb₄Lu₃F₁₇: Tb have been performed by the Fullprof program. In the refinements, the crystallographic data of rhombohedral phase Pb₄Lu₃F₁₇ was used as the initial structural model. The Rietveld refinement results, cell paraments and atomic position coordinates are illustrated in

Table 1. The Rietveld refinement results, cell paraments and atomic position coordinates for the Pb₄Lu₃F₁₇: Tb.

Formula			Pb4Lu3F17: Tb
Crystal system			rhombohedral
Density (g/cm3)			7.144
Space-group			R3 (148)
a (Å) = b (Å)			10.72442
c (Å)			19.86123
α = β (°)			90
γ (°)			120
Rwp (%)			11.7
chi2			1.84
Atoms	X	Y	Z	B	Occ.	Site
Pb (1)	0	0	0.2586	1.658		6
Pb (2)	0.2292	0.0369	0.0836	2.163		18
Lu	0.09	0.6127	0.0835	0.774		18
F (1)	0.036	0.767	0.0376	1.5		18
F (2)	0.426	0.291	0.1101	1.5		18
F (3)	0.475	0.082	0.321	1.5		18
F (4)	0.203	0.485	0.341	1.5		18
F (5)	0.267	0.392	0.1735	1..5		18
F (6)	0	0	0.145	1.5		6
F (7)	0	0	0	1.5		3
F (8)	0.02	0.057	0.502	1.5	0.167	18

. The as-obtained goodness of fit parameters were Rwp = 11.7, chi² = 1.84 (Figure 1b), indicating that all atom positions, fraction factors and temperature factors well satisfy the reflection condition [22,23]. Similar Rietveld refinement results were achieved based on the Pb₄Lu₃F₁₇: Eu product as well (Figure S1 and Table S1).

As shown in Figure 1c, the anions F(1)–F(5) are connected with Lu to form the LuF₈ (square antiprism) polyhedron. The Pb(1) and Pb(2) atoms are located on the triangular faces of the octahedron. The three-dimensional network of the Pb₄Lu₃F₁₇ is formed by sharing the external edge of the LuF₈ polyhedron [24]. The whole ion arrangement in the unit cell is given in Figure 1d. The local symmetry also has a great influence on the luminescence properties, and the reduction of symmetry is beneficial to enhance the luminescence intensity of materials. In the rhombohedral Pb₄Lu₃F₁₇, the Pb(1) shows C₃ symmetry, the Pb(2) shows C₁ symmetry, and the Lu shows C₁ symmetry [25]. This low local symmetry indicates that the Pb₄Lu₃F₁₇ is a good potential host for photoluminescence from rare earth ions.

Scanning electron microscopy (SEM) was used to study the morphology of the as-prepared Pb₄Lu₃F₁₇: Tb using different surfactants. As shown in Figure 2a, the average particle size of the Pb₄Lu₃F₁₇: Tb nanoparticles was about 32 nm when using EDTA as surfactant. The size was increased to 64 nm when using citric acid (CA) as surfactant (Figure S2). The energy dispersive X-ray (EDX) spectrum revealed the presence of Pb, Lu, F and Tb elements in the final product (Figure 2b), and the EDX mapping results suggested the uniform distribution of these elements in the particles. As shown in Figure S3, the scintillation intensity of the EDTA coated nanoparticles with smaller size was much stronger than that of CA coated nanoparticles, which was probably attributed to the EDTA coated nanoparticles having higher crystallinity than the CA coated nanoparticles [26].

The normalized X-ray luminescence spectra of the Pb₄Lu₃F₁₇: Re (Re = Tb, Eu, Sm, Dy, Ho) are presented in Figure 3a. These scintillating nanoparticles exhibited characteristic emission peaks corresponding to different energy level transitions of Re³⁺ ions. Taking the Pb₄Lu₃F₁₇: Tb as an example: under the X-ray irradiation at 50 KV, the sample showed typical emissions of Tb³⁺ centered at 487 nm, 545 nm, 587 nm and 620 nm corresponding to the ⁵D₄→⁷F_j (j = 3–6) transitions. The green emission at 545 nm (⁵D₄-⁷F₅) is a magnetic dipole transition with ΔJ = ±1, which is more intense than the other transitions [27]. As shown in Figure 3b, the luminescence intensity of the Pb₄Lu₃F₁₇: Tb was increased when the doping concentration was changed from 5% to 30%, and then significantly decreased with a further increase in the doping concentration to 40%. This can be attributed to the typical concentration quenching effect [28]. Similarly, upon X-ray irradiation, the typical emissions of Sm, Eu, Dy, Ho were recorded as well (Figure 3a). As shown in Figure 3c,d, the luminescence intensity of Tb could be further improved by incorporating Ce or Gd. The optimal doping concentrations of Ce and Gd were measured to be about 10% and 5%, respectively. It should be noted that the X-ray luminescence intensity was decreased when simultaneously cooping 30 Tb/10 Ce/5 Gd in the Pb₄Lu₃F₁₇ host (Figure S4), which might be attributed to the generation of TbF₃ impurity phase (Figure S5) followed by the reduced Tb³⁺ concentration in the Pb₄Lu₃F₁₇ host.

The proposed luminous mechanism is shown in Figure 4a. The interaction between X-ray photons and heavy atoms of Lu and Pb leads to the generation of hot electrons through the photoelectric effect. Then, massive secondary electrons are generated via electron–electron scattering and the Auger process. Finally, these low-energy electrons are transported through the conduction band to the luminescence center of the Tb³⁺ ion. Figure 4b,c show the proposed energy transfer mechanism for the above enhanced luminescence intensity. The Ce: 5d and Gd: ⁶P_j states could enhance the electrons’ population efficiency in the Tb: ⁵D₃ level, which leads to the improved X-ray luminescence intensity [29,30].

Compared with the conventional commercial inorganic scintillator CsI (TI), the Pb₄Lu₃F₁₇: Tb/Ce nanoparticles have a main emission peak of 545 nm, which is close to the conventional commercial inorganic scintillation of CsI (TI) (516 nm) and can be well matched with the silicon photodiode’s sensitive wavelength band (520 nm–580 nm). As shown in Figure 5, the integrated scintillation intensity of the Pb₄Lu₃F₁₇: Tb/Ce is about 16.8% of the commercial CsI (Tl) single crystal. Through designing crystal structure, such as core/shell, the scintillating intensity of this new kind of scintillator might be further improved, which will be used for X-ray detection and imaging.

4. Conclusions

In conclusion: a series of the Pb₄Lu₃F₁₇: Re (Re = Tb, Eu, Sm, Dy, Ho) were prepared by a simple hydrothermal method. Our results revealed that the EDTA is a better surfactant than CA for scintillation intensity of the Pb₄Lu₃F₁₇: Re. All the doped rare earth ions in the Pb₄Lu₃F₁₇ host show their corresponding characteristic emissions. The optimal Tb³⁺ doping concentration is verified to be 30 mol%, which is much higher than most hosts. The integrated scintillation intensity of the Pb₄Lu₃F₁₇: Tb/Ce is about 16.8% of the commercial CsI (Tl) single crystal.