Scintillation Properties of Lanthanide Doped Pb4Lu3F17 Nanoparticles

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.

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 CsPbBr 3 perovskite has faster response time (200 ns), better X-ray irradiation stability (>40 Gy air s −1 of X-ray exposure) and higher light output (177,000 photons/MeV) than the traditional GOS: Tb [20]. (CH 3 NH 3 ) PbBr 3 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

Results and Discussion
The X-ray diffraction (XRD) patterns of the as-prepared Pb 4 Lu 3 F 17 : Re are shown in Figure 1a. All these products are well indexed with the standard data of pure rhombohedral Pb 4 Lu 3 F 17 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 Pb 4 Lu 3 F 17 : Tb have been performed by the Fullprof program. In the refinements, the crystallographic data of rhombohedral phase Pb 4 Lu 3 F 17 was used as the initial structural model. The Rietveld refinement results, cell paraments and atomic position coordinates are illustrated in Table 1. The as-obtained goodness of fit parameters were Rwp = 11.7, chi 2 = 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 Pb 4 Lu 3 F 17 : Eu product as well ( Figure S1 and Table S1).
initial structural model. The Rietveld refinement results, cell paraments and atomic pos tion coordinates are illustrated in Table 1. The as-obtained goodness of fit parameters we Rwp = 11.7, chi 2 = 1.84 (Figure 1b), indicating that all atom positions, fraction factors an temperature factors well satisfy the reflection condition [22,23]. Similar Rietveld refin ment results were achieved based on the Pb4Lu3F17: Eu product as well ( Figure S1 an Table S1). As shown in Figure 1c, the anions F(1)-F(5) are connected with Lu to form the Lu (square antiprism) polyhedron. The Pb(1) and Pb(2) atoms are located on the triangul faces of the octahedron. The three-dimensional network of the Pb4Lu3F17 is formed b sharing the external edge of the LuF8 polyhedron [24]. The whole ion arrangement in th unit cell is given in Figure 1d. The local symmetry also has a great influence on the lum nescence properties, and the reduction of symmetry is beneficial to enhance the lumine cence intensity of materials. In the rhombohedral Pb4Lu3F17, the Pb(1) shows C3 symmetr the Pb(2) shows C1 symmetry, and the Lu shows C1 symmetry [25]. This low local sym metry indicates that the Pb4Lu3F17 is a good potential host for photoluminescence fro rare earth ions.

Formula
Pb4Lu3F17: Tb Crystal system rhombohedral Density (g/cm 3 ) 7.144 As shown in Figure 1c, the anions F(1)-F(5) are connected with Lu to form the LuF 8 (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 Pb 4 Lu 3 F 17 is formed by sharing the external edge of the LuF 8 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 Pb 4 Lu 3 F 17 , the Pb(1) shows C 3 symmetry, the Pb(2) shows C 1 symmetry, and the Lu shows C 1 symmetry [25]. This low local symmetry indicates that the Pb 4 Lu 3 F 17 is a good potential host for photoluminescence from rare earth ions.
Scanning electron microscopy (SEM) was used to study the morphology of the asprepared Pb 4 Lu 3 F 17 : Tb using different surfactants. As shown in Figure 2a, the average particle size of the Pb 4 Lu 3 F 17 : 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 Pb 4 Lu 3 F 17 : 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 Re 3+ ions. Taking the Pb 4 Lu 3 F 17 : Tb as an example: under the X-ray irradiation at 50 KV, the sample showed typical emissions of Tb 3+ centered at 487 nm, 545 nm, 587 nm and 620 nm corresponding to the 5 D 4 → 7 F j (j = 3-6) transitions. The green emission at 545 nm ( 5 D 4 -7 F 5 ) is a magnetic dipole transition with ∆J = ±1, which is more intense than the other transitions [27]. As shown in Figure 3b, the luminescence intensity of the Pb 4 Lu 3 F 17 : 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 Pb 4 Lu 3 F 17 host ( Figure S4), which might be attributed to the generation of TbF 3 impurity phase ( Figure S5) followed by the reduced Tb 3+ concentration in the Pb 4 Lu 3 F 17 host.
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 Re 3+ ions. Taking the Pb4Lu3F17: Tb as an example: under the X-ray irradiation at 50 KV, the sample showed typical emissions of Tb 3+ centered at 487 nm, 545 nm, 587 nm and 620 nm corresponding to the 5 D4→ 7 Fj (j = 3−6) transitions. The green emission at 545 nm ( 5 D4-7 F5) 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 Tb 3+ 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 The proposed luminous mechanism is shown in Figure 4a. The interaction between Xray 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 electronelectron scattering and the Auger process. Finally, these low-energy electrons are transported through the conduction band to the luminescence center of the Tb 3+ ion. Figure 4b,c show the proposed energy transfer mechanism for the above enhanced luminescence intensity. The Ce: 5d and Gd: 6 P j states could enhance the electrons' population efficiency in the Tb: 5 D 3 level, which leads to the improved X-ray luminescence intensity [29,30].
Materials 2023, 15, x FOR PEER REVIEW 6 of 8 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 Tb 3+ ion. Figure 4b,c show the proposed energy transfer mechanism for the above enhanced luminescence intensity. The Ce: 5d and Gd: 6 Pj states could enhance the electrons' population efficiency in the Tb: 5 D3 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. Compared with the conventional commercial inorganic scintillator CsI (TI), the Pb 4 Lu 3 F 17 : 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 Pb 4 Lu 3 F 17 : 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.

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 Tb 3+ 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.