English

• 1. INTRODUCTION

  Lanthanide (Ln³⁺)-doped upconversion (UC) nanocrystals (UCNCs) are a unique kind of nanomaterials that can nonlinearly turn low energy irradiation into high energy emission^[1-4]. Thanks to their ladder-like 4f-4f transitions^[5-7], the doping of Ln³⁺ ion into the host matrix generally endows these nanomaterials with exceptional optical properties like sharp emission bandwidth, tunable wavelength and long emission lifetime^[7-18]. Therefore, Ln³⁺-doped UCNCs are considered as a promising alternative to the traditional fluorescent probes like quantum dots and organic dyes, and have shown great potential in a wide range of areas including full-color displays, therapeutics, and biological imaging^{[17, 19-30]}.

  Over the past decade, the selection of host materials to deliver intense upconversion luminescence (UCL) was mainly restricted to the inorganic fluorides, especially in the category of NaREF₄ (RE = rare earth)^{[9, 16, 31-35]} due to their perfect chemical stability and low phonon energy less than 500 cm^{-1[36, 37]}. However, all currently available UCNCs may cause unintended acute or chronic toxicity to the living organisms owing to their rich Ln³⁺ components^{[38, 39]}. Moreover, when compared with green light, red light possesses a better tissue penetration ability (the penetration depth can reach 10~15 nm) that makes it applicable in various areas such as biological imaging, clinical medicine, plant cultivation and cell research^[25]. Therefore, it is highly desirable to explore a new class of non-toxic Ln³⁺-doped UCNCs that can emit large red-to-green (R/G) ratio UC emission for various applications.

  Herein, orthorhombic-phase NaMgF₃ that displays good biocompatibility^[40-44] was chosen as the host material for the synthesis of Ln³⁺-doped UCNCs. The symmetry of doping site of Ln³⁺ ion in the NaMgF₃ host lattice was demonstrated for the first time by means of the high-resolution photoluminescence spectroscopy of Eu³⁺ at low temperature (10 K). More importantly, intense UCL emissions of Ln³⁺-doped NaMgF₃ UCNCs ranging from UV to visible and to NIR spectral regions were readily achieved after the doping of typical UCL couples of Yb³⁺/Er³⁺, Yb³⁺/Tm³⁺ and Yb³⁺/Ho³⁺. Among the as-synthesized Ln³⁺-doped NaMgF₃ UCNCs, large UCL R/G ratio of Er³⁺ was achieved in the NaMgF₃: Yb³⁺/Er³⁺ UCNCs, thereby enabling them highly promising for various applications such as bioimaging, biodetection, and biotherapy.

  2. EXPERIMENTAL

  2.1 Chemicals and materials

  Mg(CH₃CO₂)₃·4H₂O (99%), Yb(CH₃CO₂)₃·4H₂O (99.999%), Er(CH₃CO₂)₃·4H₂O (99.99%), Ho(CH₃CO₂)₃·4H₂O (99.99%), Tm(CH₃CO₂)₃·4H₂O (99.99%), Eu(CH₃CO₂)₃·4H₂O (99.99%), oleic acid (OA, technical grade 90%) and trioctylamine (TOA, 98%) were purchased from Sigma-Aldrich (China). NaOH (96%) and NH₄F (98%)were purchased from Aladdin (China). Cyclohexane, methanol, and ethanol were purchased from Sinopharm Chemical Reagent Co., China.

  2.2 Preparation of orthorhombic-phase NaMgF₃: Ln³⁺ NCs

  The orthorhombic-phase NaMgF₃: Ln³⁺ (Ln³⁺ = Yb³⁺, Er³⁺, Ho³⁺, Tm³⁺, and Eu³⁺) NCs were synthesized via a modified co-precipitation method at high temperature (310 ℃). In a typical procedure for the synthesis of NaMgF₃: 4%Yb³⁺/1%Er³⁺ NCs, a mixture of 0.475 mmol Mg(CH₃CO₂)₃·4H₂O, 0.02 mmol Yb(CH₃CO₂)₃·4H₂O, 0.005 mmol Er(CH₃CO₂)₃·4H₂O, 5 mL OA and 15 mL TOA was added into a 100 mL three-neck flask. The solution was heated to 155 ℃ under N₂ flow with constant stirring for 30 min to form a clear solution, and then cooled down to room temperature. Thereafter, 10 mL of methanol solution containing NH₄F (1.5 mmol) and NaOH (1.5 mmol) was added, and the resultant solution was stirred at 75 ℃ for 30 min. After the methanol was evaporated, the solution was heated to 310 ℃ under N₂ flow with vigorous stirring for 60 min, and then cooled down to room temperature naturally. The resulting NaMgF₃: Yb/Er UCNCs were precipitated by the addition of acetone, collected by centrifugation at 12000 rpm for 5 min, and washed with cyclohexane and ethanol for several times. Finally, the powder samples were made after being dried at 50 ℃ for 8 hours.

  2.3 Characterization

  The crystallization phase of as-synthesized samples was analyzed by powder X-ray diffraction (XRD) on MiniFlex2 (Rigaku, Japan) from 20° to 80° with Cu-Κα radiation (λ = 0.154187 nm). The sample images of morphology, size and lattice fringes were obtained by transmission electron microscopy (TEM) and high-resolution TEM (HTEM) on Tecnai F20 (USA, 200kV) equipped with energy-dispersive X-ray spectroscopy (EDS). Photoluminescence (PL) emission, excitation spectra and PL decay curve for samples were recorded with UV/V/NIR Fluorescence Spectrometer (FLS980, Edinburgh Instruments) equipped with both continuous (450 W) xenon and pulsed flash lamps. The upconversion luminescence spectra were carried out upon 980 nm excitation provided by a continuous-wave semiconductor laser diode. The effective lifetime (τ_eff) was determined by:

  $ {\tau _{eff}} = \frac{1}{{{I_0}}}\int_0^\infty {I(t)dt} $

  where I₀ and I(t) represent the maximum luminescence intensity and the luminescence intensity at time t after cutoff of the excitation light, respectively.

  3. RESULTS AND DISCUSSION

  3.1 Structure and morphology of NaMgF₃: Ln³⁺ NCs

  Fig. 1a presents the unit cell of orthorhombic-phase NaMgF₃ crystal (space group Pbnm (62), a = 5.365, b = 5.492, c = 7.674 Å, Z = 4). The detailed atomic parameters including atom sites and orientations of NaMgF₃ are shown in Table 1, where Na⁺ and Mg²⁺ ions have only one site, but F^- ions have two sites (F1 and F2). According to previous reports in the literature^[45], the interatomic distance between Mg–F(1) (1.9779(4) Å) and Mg–F(2) (1.9790(7) and 1.9802(7) Å) are nearly identical in the NaMgF₃ lattice, indicating that Mg²⁺ ions occupy a crystallographic site with symmetry of O_h, the center of octahedra built by six F^- ions. Theoretically, Ln³⁺ ions are easier to replace the Mg²⁺ ions in the NaMgF₃ lattice, and magnesium or sodium vacancies are formed to maintain the charge balance (Fig. 1b)^{[41, 43]}.

  Figure 1

  

  Figure 1.  (a) Crystal structure of orthorhombic-phase NaMgF₃, (b) The most possible doping lattice site of Ln³⁺ ion and magnesium (sodium) vacancies formed in NaMgF₃ host lattice to maintain the charge balance

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  Table 1

  

  Table 1.  Atomic Parameters in the NaMgF₃ Crystal

  DownLoad: CSV

  Phase	Atom	Site	x/a	y/b	z/c
  Monoclinic	Na	4c	0.9896(9)	0.0452(8)	0.25
  	Mg	4b	0	0.5	0
  	F1	4c	0.0872(5)	0.4741(5)	0.25
  	F2	8d	0.7030(3)	0.2953(3)	0.0464(3)

  Monodisperse NaMgF₃: Ln³⁺ (Ln³⁺ = Yb³⁺/Er³⁺, Yb³⁺/Ho³⁺, Yb³⁺/Tm³⁺) UCNCs were synthesized via a modified high temperature co-precipitation method^{[46, 47]} by using oleic acid (OA) and trioctylamine (TOA) as surfactant and solvent. As shown in Fig. 2a~2c, representative transmission elec-tron microscopy (TEM) images for the NaMgF₃ UCNCs doped with Yb³⁺/Er³⁺ (4/1 mol%), Yb³⁺/Ho³⁺ (4.5/0.5 mol%) and Yb³⁺/Tm³⁺ (4.8/0.2 mol%) demonstrate their nearly square morphologies with average sizes of 12.6 ± 1.5, 8.6 ± 0.74, and 9.3 ± 0.9 nm, respectively. Clear lattice fringes with a d-spacing of 0.38 nm are observed in the high-resolution TEM (HR-TEM) image of NaMgF₃: Yb³⁺/Er³⁺ UCNCs, in good agreement with the lattice spacing of (020) plane of orthorhombic-phase NaMgF₃ (JCPDS No. 82-1224) (inset of Fig. 2a). Componential analysis by EDX reveals the presence of host elements of Na, Mg and F and the dopants of Yb³⁺ and Er³⁺ in the as-synthe- sized Ln³⁺-doped UCNCs (Fig. 2d). All as-synthesized samples can be indexed in accordance with the standard pattern of orthorhombic-phase NaMgF₃ crystal (JCPDS No. 82-1224) (Fig. 2e). These results clearly indicate that the category of Ln³⁺ dopants has negligibly impact on the size, morphology and crystal phase of the resulting NaMgF₃: Ln³⁺ NCs.

  Figure 2

  

  Figure 2.  Representative low-resolution TEM images for the as-synthesized NaMgF₃: Ln³⁺ UCNCs doped with (a) Yb³⁺/Er³⁺, (b) Yb³⁺/Ho³⁺ and (c) Yb³⁺/Tm³⁺, respectively, and the inset of (a) shows the high-resolution TEM image of a randomly selected NaMgF₃: Yb³⁺/Er³⁺ nanocrystal. Note that their corresponding nanocrystal sizes and size distribution histograms are inserted in the bottoms of (a)~(c). (d) EDX pattern for the as-synthesized NaMgF₃: Yb³⁺/Er³⁺ UCNCs. (e) XRD patterns for the as-synthesized NaMgF₃: Ln³⁺ (Ln³⁺ = Yb³⁺/Er³⁺, Yb³⁺/Ho³⁺ and Yb³⁺/Tm³⁺) UCNCs

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  However, it is found that different Na precursors have a great influence on the size of the resulting NaMgF₃: Ln³⁺ NCs. As shown in Fig. 3a and 3b, it is obvious that the NaMgF₃: Ln³⁺ NCs synthesized with NaOH precursor possessed a smaller size with a narrower size distribution than those synthesized with NaF precursor. This can be further substantiated by the XRD analysis. Although both XRD patterns match the standard orthorhombic-phase NaMgF₃ crystal in terms of the line positions, the full width at half maximum of the samples synthesized with NaOH precursor is wider than that synthesized with NaF precursor (Fig. 3c). The variation in the nanocrystal size is quite understandable owing to the controlled growth of NCs associated with OA^- concentration in the solvents. It is well established that a high concentration of OA^- is able to limit the increase of nanocrystal size^[13]. In comparison with NaF precursor that hardly reacted with OA, NaOH precursor is easily able to react with OA to release a higher concentration of OA^-, thus resulting in a smaller nanocrystal size of NaMgF₃: Ln³⁺ NCs.

  Figure 3

  

  Figure 3.  Typical low-resolution TEM image for NaMgF₃: Yb³⁺/Er³⁺ UCNCs synthesized with (a) NaOH and (b) NaF precursors, respectively. Insets of (a) and (b) show their corresponding nanocrystal sizes and size distribution histograms. (c) XRD patterns for the as-synthesized NaMgF₃: Yb³⁺/Er³⁺ UCNCs with NaOH and NaF precursors

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  3.2 Optical properties

  It is well known that the Eu³⁺ ion was usually employed as a sensitive optical/structural probe to study the crystal structure of host material due to its sensitive response to the crystal-field (CF) environment. By comparing the CF splitting number of the transitions of ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4, 5 and 6) with theoretical predictions^{[47, 48]}, multiple spectroscopic sites of Eu³⁺ within different CF surroundings can be easily identified in Ln³⁺-doped NCs, especially for the case of ⁵D₀ → ⁷F_{1, 2} transitions. Generally, the ⁵D₀ → ⁷F₁ transition of Eu³⁺ exhibits a magnetic dipole transition nature that is insensitive to the change of CF surroundings. On the contrary, the ⁵D₀ → ⁷F₂ transition of Eu³⁺ possessing an electric dipole transition nature is hypersensitive to different local crystal structures. Moreover, according to the electric dipole selection rule, the ⁵D₀ → ⁷F₀ transition of Eu³⁺ is only allowed in 10 site symmetries including C₁, C_s, C₂, C_2v, C₄, C_4v, C₃, C_3v, C₆ and C_6v (Table S1). On this basis, the high-resolution excitation, emission spectra and decays of NaMgF₃: 5%Eu³⁺ NCs were comprehensively surveyed at low temperature (10 K) to confirm the symmetry of the doping site of Ln³⁺.

  Fig. 4a compares the PL excitation spectra of NaMgF₃: Eu³⁺ NCs by monitoring the ⁵D₀ → ⁷F₂ transition of Eu³⁺ at 609 and 612 nm, respectively. The characteristic PL bands arising from the ⁷F₀ → ⁵D₄ (361.2 nm), ⁷F₀ → ⁵G₂ (375 nm) and ⁷F₀ → ⁵L₆ (393 nm) transitions were readily detected for both cases. However, a marked difference for these two cases was observed, where an extra peak centered at 398.2 nm assigned to the ⁷F₀ → ⁵L₆ transition appeared for the second case, clearly demonstrating that Eu³⁺ occupies two different crystalline sites in the NaMgF₃ lattice (hereafter defined as sites A and B).

  To further determine the specific doping sites of Eu³⁺ in NaMgF₃ NCs, we then mearsured the high-resolution site-selective emission spectra under the excitation at 393 and 398.2 nm originating from the ⁷F₀ → ⁵L₆ transition of Eu³⁺ at 10 K. As shown in Fig. 4b (upper), five well-resolved emission peaks centered at 578, 591, 612, 648 and 699 nm assigned to the intra-4f transitions from ⁵D₀ to its lower states of ⁷F_J (J = 0~4) of Eu³⁺ were observed under the excitation at 393 nm (site A). Theoretically, if Eu³⁺ occupies the site of ideal O_h symmetry in NaMgF₃: Eu³⁺ NCs, the ⁵D₀ → ⁷F_{0, 2, 4} transitions would be strictly forbidden and only magnetic dipole transition of ⁵D₀ → ⁷F₁ can be detected (Table S1). However, the site symmetry of Eu³⁺ in NaMgF₃: Eu³⁺ NCs would degrade due to the mismatch of valences and ion radiuses between Eu³⁺ and Mg²⁺. Specifically, 1, 2, 2 and 4 well-resolved emission lines assigned to the ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₄ transitions were observed (Fig. 4b, upper), indicative of a C_4v site symmetry for site A (Table S1). In contrast, 3 well-resolved emission lines assigned to the ⁵D₀ → ⁷F₁ transition were detected for site B (Fig. 4b, lower), suggesting that this site occupies a lower symmetry than site A. Based on branching rules of the 32 point groups (Fig. S1), C_4v can theoratically be degraded to C₄, C_2v, C₂, C_s and C₁. Therefore, site B of Eu³⁺ in NaMgF₃: Eu³⁺ NCs occupies a C_2v site symmetry, as a result of 5 emission lines for the ⁵D₀ → ⁷F₄ transition. These results can be further confirmed by their corresponding PL decay lifetime (Fig. 4c). The lifetime of ⁵D₀ → ⁷F₂ transition for site A was determined to be 9.5 ms, much longer than that of site B (2.7 ms). In a word, Eu³⁺ ions of site A are incorporated in the inner lattice sites of a high symmetry in NaMgF₃ NCs, while Eu³⁺ ions of site B occupy the distorted lattice sites near the surface or surface sites.

  Figure 4

  

  Figure 4.  (a) High-resolution PL excitation spectra for the NaMgF₃: 5%Eu³⁺ NCs by monitoring the ⁵D₀ → ⁷F₂ transition of Eu³⁺ at 609 (red) and 612 nm (black). (b) Site-selective PL emission spectra for the NaMgF₃: 5%Eu³⁺ NCs under excitation at 393 nm (red) and 398.3 nm (black), respectively. (c) PL decays for the ⁵D₀ → ⁷F₂ transition of Eu³⁺ in NaMgF₃: 5%Eu³⁺ NCs under site-selective excitation at 393 (red) and 398.2 nm (black) by monitoring the emission at 612 and 609 nm, respectively. Noted that all the PL spectra and decays were measured at low temperature (10 K)

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  After demonstrating the successful hetero-valence doping of Ln³⁺ ions, we then doped the typical Ln³⁺ couple of Yb³⁺/Er³⁺ into the lattice of NaMgF₃ NCs via a modified high temperature co-precipitation method. To determine the optimal doping amount of Ln³⁺, different amounts of Ln³⁺ were doped into the NaMgF₃ lattice firstly, which was clearly revealed by the XRD analysis (Fig. 5a). Notably, the orthorhombic crystal phase of NaMgF₃: Ln³⁺ NCs kept nearly unchanged as the doping amount of Ln³⁺ gradually increased (Fig 5a). The magnified XRD pattern shown in Fig. 5a (right) suggested a heterogeneous phase that can be assigned to the α-NaYbF₄ crystal (JCPDS No. 77-2043) when a high doping amount of 20.5% Ln³⁺ (Yb³⁺/Er³⁺) was introduced into the NaMgF₃ lattice. To avoid the possible impact of impurity phase in NaMgF₃: Ln³⁺ (Ln³⁺ = Yb³⁺/Er³⁺, Yb³⁺/Ho³⁺ and Yb³⁺/Tm³⁺) UCNCs, we then selected 5% as a total doping concentration for the subsequent UCL measurements.

  Under excitation at 980-nm diode laser with a power density of 50 W·cm^-2, the as-synthesized NaMgF₃: Yb³⁺/Er³⁺ UCNCs exhibit intense red and extremely weak green UC emissions, which are assigned to the ⁴F_9/2 → ⁴I_15/2 (654 nm) and ²H_11/2/⁴S_3/2 → ⁴I_15/2 (515~565 nm) transitions of Er³⁺, respectively (Fig. 5b). Moreover, the integrated UC emission intensity was found to increase at first and then decrease as the concentration of Er³⁺ gradually increased, thus obtaining the optimal Yb³⁺/Er³⁺ doping concentration of 4%/1% mol. It should be noted that the NaMgF₃: Yb³⁺/Er³⁺ UCNCs exhibit high UCL R/G ratios of Er³⁺ regardless of the doping concentration of Yb³⁺/Er³⁺. Such high UCL R/G ratios of Er³⁺ can be explained when looking at the crystal structure of NaMgF₃ lattice that features a high-symmetry substituted site of Mg²⁺ by Ln^3+[2].

  Figure 5

  

  Figure 5.  (a) XRD patterns for the as-synthesized NaMgF₃: Yb³⁺/Er³⁺ UCNCs with different amounts of Yb³⁺/Er³⁺ from 2.5% to 20.5%. The specific XRD pattern for NaMgF₃: 20.5%Ln³⁺ UCNCs is shown in the right side. UCL spectra for NaMgF₃: Ln³⁺ UCNCs with different amounts of (b) Yb³⁺/Er³⁺, (c) Yb³⁺/Ho³⁺, and (d) Yb³⁺/Tm³⁺ under excitation of a 980-nm diode laser at a power density of 50 W/cm²

  DownLoad: Full-Size Img PowerPoint

  In addition, efficient UCL was also achieved when the typical UCL couples of Yb³⁺/Ho³⁺ and Yb³⁺/Tm³⁺ were introduced into the NaMgF₃ host lattice. As shown in Fig. 5c, the NaMgF₃: Yb³⁺/Ho³⁺ UCNCs exhibit green and red UC emissions, which are assigned to the ⁵F₄ → ⁵I₈ (542 nm) and ⁵F₅ → ⁵I₈ (655 nm) transitions of Ho³⁺, respectively. As for the case of Yb³⁺/Tm³⁺, the NaMgF₃: Yb³⁺/Tm³⁺ UCNCs exhibit green, red and NIR UC emissions, which is assigned to ¹G₄ → ³H₆, ¹G₄ → ³F₄, and ³H₄ → ³H₆ transitions of Tm³⁺, respectively (Fig. 5d). The optimal doping concentration of Yb³⁺/Ho³⁺ and Yb³⁺/Tm³⁺ in the NaMgF₃ lattice is determined to be 4.5%/0.5% and 4.8%/0.2% mol. Taken together, the as-synthesized ultrasmall NaMgF₃: Ln³⁺ UCNCs are a new class of Ln³⁺-doped UCNCs that exhibit efficient UCL properties, thereby making them highly promising for various applications.

  4. CONCLUSION

  In this paper, we described a new class of orthorhombic-phase NaMgF₃ ultrasmall (~10 nm) NCs doped with different Ln³⁺ ions that were synthesized via a modified high-temperature co-precipitation method. Monodisperse and uniform Ln³⁺-doped NaMgF₃ NCs with tunable nanocrystal size were obtained by varying the category of Na precursors including NaOH and NaF. By taking advantage of high-resolution photoluminescence spectro-scopy of Eu³⁺ at low temperature (10 K), the symmetry of doping site of Ln³⁺ was demonstrated for the first time in NaMgF₃ NCs. More importantly, owing to the successful Ln³⁺ doping, intense UCL in the UV, visible and NIR spectral regions for these Ln³⁺-doped NaMgF₃ NCs were obtained by changing the type of Ln³⁺ dopant couple of Yb³⁺/Er³⁺, Yb³⁺/Tm³⁺ and Yb³⁺/Ho³⁺. These findings reported here unambiguously demonstrate that Ln³⁺-doped NaMgF₃ NCs without containing toxic elements hold great promise as one of the possible candidates for diverse biological applications both in vitro and in vivo.

  
  
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  Figure 1  (a) Crystal structure of orthorhombic-phase NaMgF₃, (b) The most possible doping lattice site of Ln³⁺ ion and magnesium (sodium) vacancies formed in NaMgF₃ host lattice to maintain the charge balance

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  Figure 2  Representative low-resolution TEM images for the as-synthesized NaMgF₃: Ln³⁺ UCNCs doped with (a) Yb³⁺/Er³⁺, (b) Yb³⁺/Ho³⁺ and (c) Yb³⁺/Tm³⁺, respectively, and the inset of (a) shows the high-resolution TEM image of a randomly selected NaMgF₃: Yb³⁺/Er³⁺ nanocrystal. Note that their corresponding nanocrystal sizes and size distribution histograms are inserted in the bottoms of (a)~(c). (d) EDX pattern for the as-synthesized NaMgF₃: Yb³⁺/Er³⁺ UCNCs. (e) XRD patterns for the as-synthesized NaMgF₃: Ln³⁺ (Ln³⁺ = Yb³⁺/Er³⁺, Yb³⁺/Ho³⁺ and Yb³⁺/Tm³⁺) UCNCs

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  Figure 3  Typical low-resolution TEM image for NaMgF₃: Yb³⁺/Er³⁺ UCNCs synthesized with (a) NaOH and (b) NaF precursors, respectively. Insets of (a) and (b) show their corresponding nanocrystal sizes and size distribution histograms. (c) XRD patterns for the as-synthesized NaMgF₃: Yb³⁺/Er³⁺ UCNCs with NaOH and NaF precursors

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  Figure 4  (a) High-resolution PL excitation spectra for the NaMgF₃: 5%Eu³⁺ NCs by monitoring the ⁵D₀ → ⁷F₂ transition of Eu³⁺ at 609 (red) and 612 nm (black). (b) Site-selective PL emission spectra for the NaMgF₃: 5%Eu³⁺ NCs under excitation at 393 nm (red) and 398.3 nm (black), respectively. (c) PL decays for the ⁵D₀ → ⁷F₂ transition of Eu³⁺ in NaMgF₃: 5%Eu³⁺ NCs under site-selective excitation at 393 (red) and 398.2 nm (black) by monitoring the emission at 612 and 609 nm, respectively. Noted that all the PL spectra and decays were measured at low temperature (10 K)

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  Figure 5  (a) XRD patterns for the as-synthesized NaMgF₃: Yb³⁺/Er³⁺ UCNCs with different amounts of Yb³⁺/Er³⁺ from 2.5% to 20.5%. The specific XRD pattern for NaMgF₃: 20.5%Ln³⁺ UCNCs is shown in the right side. UCL spectra for NaMgF₃: Ln³⁺ UCNCs with different amounts of (b) Yb³⁺/Er³⁺, (c) Yb³⁺/Ho³⁺, and (d) Yb³⁺/Tm³⁺ under excitation of a 980-nm diode laser at a power density of 50 W/cm²

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  Table 1.  Atomic Parameters in the NaMgF₃ Crystal

  Phase	Atom	Site	x/a	y/b	z/c
  Monoclinic	Na	4c	0.9896(9)	0.0452(8)	0.25
  	Mg	4b	0	0.5	0
  	F1	4c	0.0872(5)	0.4741(5)	0.25
  	F2	8d	0.7030(3)	0.2953(3)	0.0464(3)

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