Synthesis of Small Ce³⁺-Er³⁺-Yb³⁺ Tri-Doped BaLuF₅ Active-Core-Active-Shell-Active-Shell Nanoparticles with Strong Down Conversion Luminescence at 1.5 μm

Abstract

Small fluoride nanoparticles (NPs) with strong down-conversion (DC) luminescence at 1.5 μm are quite desirable for optical fiber communication systems. Nevertheless, a problem exists regarding how to synthesize small fluoride NPs with strong DC emission at 1.5 μm. Herein, we propose an approach to improve 1.5 μm emission of BaLuF₅:Yb³⁺,Er³⁺ NPs by way of combining doping Ce³⁺ ions and coating multiple BaLuF₅: Yb³⁺ active-shells. We prepared the BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺ NPs through a high-boiling solvent method. The effect of Ce³⁺ concentration on the DC luminescence was systematically investigated in the BaLuF₅:Yb³⁺,Er³⁺ NPs. Under a 980 nm laser excitation, the intensities of 1.53 μm emission of BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺ NPs was enhanced by 2.6 times comparing to that of BaLuF₅:18%Yb³⁺,2%Er³⁺ NPs since the energy transfer between Er³⁺ and Ce³⁺ ions: Er³⁺:⁴I_11/2 (Er³⁺) + ²F_5/2 (Ce³⁺) → ⁴I_13/2 (Er³⁺) + ²F_7/2 (Ce³⁺). Then, we synthesized BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺@BaLuF₅:5%Yb³⁺@BaLuF₅:5%Yb³⁺ core-active-shell-active-shell NPs via a layer-by-layer strategy. After coating two BaLuF₅:Yb³⁺ active-shell around BaLuF₅:Yb³⁺,Er³⁺,Ce³⁺ NPs, the intensities of the 1.53 μm emission was enhanced by 44 times compared to that of BaLuF₅:Yb³⁺,Er³⁺ core NPs, since the active-shells could be used to not only suppress surface quenching but also to transfer the pump light to the core region efficiently through Yb³⁺ ions inside the active-shells.

1. Introduction

Recently, trivalent rare-earth (RE³⁺) ions doped fluoride nanoparticles (NPs) have been applied widely in many fields of high technology, such as bioimaging, drug delivery, photodynamic therapy, solar cells [1,2,3,4,5,6,7,8,9,10,11,12], etc. In particular, Er³⁺-doped fluoride NPs have been applied in waveguide amplifiers [13,14,15,16] since intra-4f-shell transitions of Er³⁺ ions not only cause visible light emissions but also send an emission at 1.5 μm (the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ions), which is located in low loss windows of optical communication networks. In order to get high-gain Er³⁺-doped waveguide amplifiers, Er³⁺-doped fluoride NPs should not only have a strong down-conversion luminescence at 1.5 μm, but also have a small size. So far, various strategies have been developed to improve luminescence intensity of Er³⁺-doped fluoride NPs at 1.5 μm. One is to increase the nonradiative decay rate that the high energy levels of Er³⁺ ions relax nonradiatively to the ⁴I_13/2 level [17,18,19]. Zhai et al. synthesized NaYF₄:Er³⁺,Yb³⁺,Ce³⁺ NPs, and found the 1.53 μm emission band of Er³⁺ ions in the NPs was enhanced by 6 times after co-doping Ce³⁺ ions owing to the efficient energy transfer between Ce³⁺ and Er³⁺:⁴I_11/2 (Er³⁺) + ²F_5/2 (Ce³⁺) → ⁴I_13/2 (Er³⁺) + ²F_7/2 (Ce³⁺) [20]. The other strategy is to decrease the defects on the surface of NPs through growing an inert shell (the shell and the core NPs have similar lattice constants) around the core NPs [21,22,23,24,25]. Bo et al. reported that the intensity of the 1540 nm emission of LaF₃:Yb³⁺,Er³⁺ core NPs was enhanced after coating a LaF₃ inert shell since the coating inert shell method can suppress the surface quenching effect. [26]. The last strategy is to increase the rate of the pump light through coating an active shell (e.g., the shell containing Yb³⁺ icons) on the core NPs [27,28,29,30,31]. Zhai et al. reported a method to improve the intensity BaYF₅:Yb³⁺,Er³⁺ NPs at 1.53 μm through doping Yb³⁺ ions into the BaYF₅ shell. BaYF₅:Yb³⁺,Er³⁺@BaYF₅:Yb³⁺ inert-core-active-shell NPs were obtained and the intensity of the 1.53 μm was enhanced when compared to the BaYF₅:Yb³⁺,Er³⁺ core NPs [32]. Despite, recent progress in this field, it is necessary to explore new approaches to achieve small NPs with strong down-conversion luminescence at 1.5 μm for applications regarding near infrared optical communication networks.

In this paper, we choose BaLuF₅ as the matrix since its phase is a single crystalline phase [33]. We prepared BaLuF₅:Yb³⁺,Er³⁺,Ce³⁺ NPs by a high-boiling solvent method and studied the effect of the Ce³⁺ concentration on the up-conversion (UC) emission and down-conversion (DC) emission (at 1.5 μm) of BaLuF₅:Yb³⁺,Er³⁺ NPs. We synthesized BaLuF₅:Yb³⁺,Er³⁺,Ce³⁺@BaLuF₅:Yb³⁺ core-shell NPs via growing a BaLuF₅:Yb³⁺ shell and investigated the effect of the Yb³⁺ concentration of the shell on the 1.5 μm emission of the BaLuF₅ core-shell NPs. Finally, we compounded multi-layer BaLuF₅ core-shell NPs via a layer-by-layer strategy, and obtained BaLuF₅:Yb³⁺,Er³⁺,Ce³⁺@BaLuF₅:Yb³⁺@BaLuF₅:Yb³⁺ core-active-shell active-shell NPs with a strong down-conversion luminescence at 1.5 μm.

2. Materials and Methods

All chemicals were used directly without further purification. Lu(NO₃)₃·6H₂O, Yb(NO₃)₃·6H₂O, Er(NO₃)₃·6H₂O, and Ce(NO₃)₃·6H₂O were purchased from Sigma-Aldrich Chemicals (Shanghai, China). Oleic acid (OA), 1-octadecene (ODE) and Barium stearate were obtained by Alfa Aesar Company (Shanghai, China). NaOH, NH₄F and stearic acid (C₁₇H₃₅COOH) were obtained from China National Pharmaceutical Group Corporation (Beijing, China).

2.1. Preparation of BaLuF₅:Yb³⁺,Er³⁺ NPs and BaLuF₅:Yb³⁺,Er³⁺ Core-Shell NPs

Synthesis of rare-earth stearate: 10 mmol rare-earth nitrate (Lu(NO₃)₃·6H₂O, Yb(NO₃)₃·6H₂O, Er(NO₃)₃·6H₂O, or Ce(NO₃)₃·6H₂O) and 10 mmol stearic acid were dissolved in 120 mL ethanol, and the system was kept at 80 °C for 30 min. Then, a 20 mL NaOH solution (containing 1.2 g NaOH) was added dropwise into the system. The resulting mixture was refluxed at 80 °C for another 10 h. The reaction product was washed with water and ethanol [34].

Synthesis of BaLuF₅ nanoparticles: 0.5 mmol barium stearates, 0.5 mmol pre-prepared rare-earth stearate (Re(C₁₇H₃₅COO)₃), 15 mL ODE, and 15 mL OA were added to a four-neck round-bottom reaction vessel. After the reaction, the mixture was heated to 150 °C for 30 min under an argon (Ar) flow. A 10 mmol methanol solution containing 0.12g NH₄F was added dropwise into the reaction mixture, and the reaction mixture was heated to 50 °C for 30 min. Then, the reaction mixture was rapidly heated to 300 °C for 1 h and cooled to room temperature (RT) under an Ar flow. The reaction product was washed with cyclohexane and ethanol [31]. The finally obtained nanoparticles were dispersed into cyclohexane.

Synthesis of BaLuF₅ core-shell nanoparticles: 0.5 mmol barium stearates, 0.5 mmol pre-prepared rare-earth stearate (Re(C₁₇H₃₅COO)₃), 15 mL ODE, and 15 mL OA were added to a four-neck round-bottom reaction vessel. The reaction system was heated to 150 °C for 30 min under an Ar flow. After the reaction system was cooled down to 60 °C. The core nanoparticles cyclohexane solution was added into the reaction system with vigorous stirring. A 10 mmol methanol solution containing 0.12 g NH₄F was added dropwise into the reaction system, and the system was heated to 50 °C for 30 min. Then the reaction mixture was rapidly heated to 300 °C for 1 h and cooled to RT under an Ar flow. The reaction product was washed with cyclohexane and ethanol.

Synthesis of BaLuF₅ core-shell-shell nanoparticles: To coat the second layer of the shell, these as-synthesized core-shell NPs were used as seeds. The same coating process was repeated. BaLuF₅ core-shell-shell nanoparticles were obtained.

2.2. Characterization

The X-ray powder diffraction (XRD, Rigaku, Japan) were collected by Model Rigaku Ru-200b with Cu K_α (40 kV, 40 Ma) irradiation (λ = 1.5406 Å). The scan range was set from 10° to 70°. The morphology of the particles was characterized by a JEM-2100F electron microscope (Tokyo, Japan) at 200 kV. The up-conversion spectra of the samples were recorded by a Hitachi F-4500 fluorescence spectrophotometer (Tokyo, Japan) at room temperature under the excitation of a 980 nm laser diode with a fixed power density of 70 W·cm⁻² (1.0 nm for slit resolution and 700 V for PMT voltage). The DC spectra of the samples were collected by a SPEX 1000M spectrometer (HORIBA Group, Kyoto, Japan) at room temperature under the excitation of a 980 nm laser diode with a fixed power density of 70 W·cm⁻² (2 mm for slit width).

3. Results and Discussion

3.1. Crystal Structure and Morphology

The XRD patterns of the BaLuF₅:18%Yb³⁺,2%Er³⁺ NPs, BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺ NPs, BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺@BaLuF₅ core-inert-shell NPs, BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺@BaLuF₅:5%Yb³⁺ core-active-shell NPs and BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺@BaLuF₅:5%Yb³⁺@BaLuF₅:5%Yb³⁺ core-active-shell active-shell NPs are shown in Figure 1. It shows that all the diffraction peaks of the samples were well-assigned to the tetragonal phase BaGdF₅ (JCPDS No. 24-0098), which indicates that the samples are BaLuF₅ nanoparticles. To characterize the morphology of the samples, we also measured the TEM images of the above samples, and the results are shown in Figure 2. From the TEM image (Figure 2), it is easily seen that the samples are round without agglomeration. The average sizes of BaLuF₅:Yb³⁺,Er³⁺ core-only NPs and BaLuF₅:Yb³⁺,Er³⁺,Ce³⁺ core-only NPs were both about 6 nm, and the above results show that the doping Ce³⁺ ions have not changed the size of BaLuF₅ core-only NPs. The average size of the BaLuF₅ core-active-shell NPs was about 8 nm after the epitaxial growth of a shell layer. The size of the BaLuF₅ core-active-shell-active-shell NPs was further increased to about 10 nm after the growth of two shell layers. The particle diameter of nanoparticles was calculated from the XRD pattern, according to the Scherrer equation, and the samples were calculated by the particle size ranges of the nanoparticles at 6 nm, 6 nm, 8 nm, and 10.3 nm. The calculated sizes coincided with the TEM results. The above results indicated that the NPs had a uniform morphology and the average sizes of the shells had not changed.

3.2. Optical Properties

3.2.1. Effect of Ce³⁺ Concentration on the Luminescence Properties of BaLuF₅:Yb³⁺,Er³⁺ NPs

In BaLuF₅:Yb³⁺,Er³⁺,Ce³⁺ systems, all the energy transfer processes are shown in the Figure 3. With the excitation of a 980 nm laser diode, the Yb³⁺ ions are excited from the ²F_7/2 level to the ²F_5/2 level and then transfer the energy to Er³⁺ to populate higher energy levels of the Er³⁺ ions:⁴H_11/2 level, ⁴F_9/2, and ⁴F_7/2. The ⁴H_11/2 → ⁴I_15/2 (≈525 nm), ⁴S_3/2 → ⁴I_15/2 (≈545 nm), and ⁴F_9/2 → ⁴I_15/2 (≈655 nm) transitions gives the UC emission, and the ⁴I_13/2 → ⁴I_15/2 transition gives the DC emission at 1.53 μm. Interestingly, with the addition of Ce³⁺ ions, the energy transfer occurs between Ce³⁺ and Er³⁺:⁴I_11/2 (Er³⁺) + ²F_5/2 (Ce³⁺) → ⁴I_13/2 (Er³⁺) + ²F_7/2 (Ce³⁺). Furthermore, the ⁴I_11/2 state of Er³⁺ ions populate the ⁴I_13/2 state [17,35,36,37]. The intensity of the DC emission is enhanced and that of the UC emission are suppressed by the addition of Ce³⁺ ions.

In order to investigate the effect of Ce³⁺ ion on the UC emission, we measured the UC emission spectra of BaLuF₅:18%Yb³⁺,2%Er³⁺,x%Ce³⁺ NPs with different Ce³⁺ concentrations (x = 0, 1, 2, 3, and 4) under the excitation of a 980 nm laser diode, and the data is shown in Figure 4a. All samples exhibit several UC emission peaks, which are attributed to the ⁴H_11/2 → ⁴I_15/2 (≈525 nm), ⁴S_3/2 → ⁴I_15/2 (≈545 nm), and ⁴F_9/2 → ⁴I_15/2 (≈655 nm) transitions of Er³⁺ ions, respectively. When the Ce³⁺ concentration was 0%, the UC luminescence of BaLuF₅:Yb³⁺,Er³⁺ NPs was the strongest one. It is clear that the intensity of the UC emissions decreased gradually with the increase of Ce³⁺ concentration from 0% to 2% (as shown in Figure 4b). This is due to the following energy transfer occurring between Ce³⁺ and Er³⁺:⁴I_11/2 (Er³⁺) + ²F_5/2 (Ce³⁺) → ⁴I_13/2 (Er³⁺) + ²F_7/2 (Ce³⁺) [17,35,36,37]. However, when the concentration of Ce³⁺ ions reached 4%, the intensity of the UC emissions increased monotonically (as shown in Figure 4b). The above results show that doping Ce³⁺ ions led to the suppression of the UC emissions of Er³⁺ ions.

In addition, we also studied the influence of the Ce³⁺ concentration on the DC luminescence of BaLuF₅:Yb³⁺,Er³⁺ NPs. BaLuF₅:18%Yb³⁺,2%Er³⁺,x%Ce³⁺ (x = 0, 1, 2, 3, and 4) NPs were synthesized using a high-boiling solvent method. Figure 4c shows the DC emission of the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ions with varying Ce³⁺ concentration under the excitation of a 980 nm laser. We found that the intensity of the DC luminescence gradually increased with increasing Ce³⁺ concentration from 0% to 2% (as shown in Figure 4d). This may be due to the branching ratio of the Er³⁺:⁴I_11/2 → ⁴I_13/2 transition, which can be increased by doping with Ce³⁺ ions, and the energy transfer process can increase the population of ⁴I_13/2 state of Er³⁺ ions through the following energy transfer process: ⁴I_11/2 (Er³⁺) + ²F_5/2 (Ce³⁺) → ⁴I_13/2 (Er³⁺) + ²F_7/2 (Ce³⁺) [17,35,36,37]. The results led to the enhancement of the DC emission of Er³⁺ ions. Meanwhile, the intensity of the DC emissions reduced monotonically with increasing Ce³⁺ concentration from 2% to 4% (as shown in Figure 4d), since the cross relaxation: Er³⁺:⁴I_13/2 + Ce³⁺: ²F_5/2 → Er³⁺:⁴I_15/2 + Ce³⁺: ²F_7/2 happened. These results indicate that when the concentration of Ce³⁺ ions was 2%, the intensity of the DC luminescence reached its maximum. The DC emissions of BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺ NPs were about 2.6 times compared to that of BaLuF₅:18%Yb³⁺,2%Er³⁺ NPs, which means that doping Ce³⁺ ions led to the enhancement of the DC emissions of Er³⁺ ions. Thus, the optimum concentration of Er³⁺ was about 2% for tri-doped BaLuF₅ NPs.

3.2.2. Effect of Yb³⁺ Concentration of the Shell on the DC Luminescence Properties of BaLuF₅:Yb³⁺,Er³⁺,Ce³⁺@BaLuF₅:Yb³⁺ Core-Shell NPs

Here, we choose BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺ NPs as the core, and prepared BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺@BaLuF₅:x%Yb³⁺ (x = 0, 2.5, 5, 7.5 and 10) core-shell NPs. To clarify the effects of Yb³⁺ concentration on the shell on the DC luminescence properties of BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺@BaLuF₅:Yb³⁺ core-shell NPs, we measured the DC emission spectra of the core-shell NPs with different Yb³⁺ concentrations (0%, 2.5%, 5%, 7.5%, and 10%) under a 980 nm laser excitation, and the measured data is shown in Figure 5a. We can see from the Figure 5a that BaLuF₅:Yb³⁺,Er³⁺ core NPs and BaLuF₅:Yb³⁺,Er³⁺,Ce³⁺ core NPs show the weakest emission peak at 1530 nm. The main reason is that the surface area-to-volume ratio of the core-only NPs was very high and a large portion of the dopants should be located at the surface. Hence, the energy from the pump light will be easily quenched by the surface defects of the core-only NPs. The luminescence intensity of BaLuF₅ core-inert-shell NPs was obviously increased after the BaLuF₅ insert shell. The luminescence intensity of BaLuF₅ core-inert-shell NPs was increased by 3.7 times compared to that of the BaLuF₅:Yb³⁺,Er³⁺ core NPs with doping Ce³⁺ ions. This is because the insert shell can suppress the nonradiative transitions [23,24]. Interestingly, when the shell was doped with Yb³⁺ ions, the luminescence intensity of the BaLuF₅:Yb³⁺,Er³⁺,Ce³⁺ core-active-shell NPs could be increased further compared to that of the BaLuF₅:Yb³⁺,Er³⁺,Ce³⁺ core-insert-shell NPs. The luminescence intensity of the BaLuF₅:Yb³⁺,Er³⁺,Ce³⁺ core-active-shell NPs monotonically enhanced with increasing of Yb³⁺ concentration in the shell from 0% to 5% (as shown in Figure 5b). However, when the Yb³⁺ concentration in the shell was 5%, the luminescence intensity of the BaLuF₅ core-shell NPs reached its maximum value. This was due to the Yb³⁺ ions in the shell could transfer energy from the pump source to the core and make a contribution to the DC emissions [31]. When the Yb³⁺ concentration in the shell continuously increased from 5% to 10%, the luminescence intensity of the BaLuF₅:Yb³⁺,Er³⁺,Ce³⁺ core-active-shell NPs gradually decreased since the concentration quenching effect occurred [31,38]. These results indicate that the optimum concentration of Yb³⁺ in the shell was about 5% for BaLuF₅:Yb³⁺,Er³⁺,Ce³⁺ core-active-shell NPs, the intensity of the UC emissions of BaLuF₅:Yb³⁺,Er³⁺,Ce³⁺@BaLuF₅:Yb³⁺ core-active-shell NPs was increased by 9.4 times compared that of the BaLuF₅:Yb³⁺,Er³⁺,Ce³⁺ core NPs, and was increased by 24.6 times compared to that of the BaLuF₅:Yb³⁺,Er³⁺ core NPs without doping Ce³⁺ ions.

3.2.3. Synthesis of BaLuF₅:Yb³⁺,Er³⁺,Ce³⁺@BaLuF₅:Yb³⁺@BaLuF₅:Yb³⁺ Core-Shell-Shell NPs with Strong Down-Conversion Luminescence

In order to get the tri-doped BaLuF₅:Yb³⁺,Er³⁺,Ce³⁺ core-shell NPs with strong DC luminescence, we synthesized BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺@BaLuF₅:5%Yb³⁺@BaLuF₅:5%Yb³⁺ core-shell-shell NPs via a high boiling solvent process through a layer-by-layer strategy. Figure 6c shows the DC emission of BaLuF₅:18%Yb³⁺,2%Er³⁺ core NPs, BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺ core NPs, BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺@BaLuF₅:5%Yb³⁺ core-active-shell NPs and BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺@BaLuF₅:5%Yb³⁺@BaLuF₅:5%Yb³⁺ core-active-shell-active-shell NPs under the excitation of a 980 nm laser diode. The DC emission intensity of BaLuF₅ core NPs without doping Ce³⁺ ions shows the weakest DC emission. By doping with Ce³⁺ ions, the luminescence intensity of the BaLuF₅:Yb³⁺,Er³⁺,Ce³⁺ core NPs was 2.6 times more than that of BaLuF₅:Yb³⁺,Er³⁺ core NPs due to the energy transfer between Er³⁺ and Ce³⁺. After, by growing an BaLuF₅:5%Yb³⁺active shell, the DC emission intensity of the BaLuF₅:Yb³⁺,Er³⁺,Ce³⁺ core-active-shell NPs was further enhanced, since the active shell could be used to not only suppress surface quenching but also transfer energy from the pump light to the core region efficiently through Yb³⁺ ions inside the active shells. When the number of the active shell layers reaches two, the DC emission intensity of the BaLuF₅:Yb³⁺,Er³⁺,Ce³⁺ core-active-shell-active-shell NPs was 16.8 times more than that of BaLuF₅:Yb³⁺,Er³⁺,Ce³⁺ core NPs. This was because the size of the active shell was too small to completely suppress surface quenching, and coating two active shell layers could effectively suppress surface quenching [31]. In the last, we got BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺@BaLuF₅:5%Yb³⁺@BaLuF₅:5%Yb³⁺ core-active-shell-active-shell NPs with the strong DC luminescence at 1.5 μm. The DC emission intensity of the BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺@BaLuF₅:5%Yb³⁺@BaLuF₅:5%Yb³⁺ core-active-shell-active-shell NPs was 44 times more than that of BaLuF₅:Yb³⁺,Er³⁺ core NPs without the dopant Ce³⁺ ions. Besides, we measured the photo stability of the BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺@BaLuF₅:5%Yb³⁺@BaLuF₅:5%Yb³⁺ core-active-shell-active-shell NPs (as shown in Figure S1). Those results the show that the core-active-shell-active-shell NPs have optical stability.

In addition, we measured the lifetime of the ⁴I_13/2 level of Er³⁺ in BaLuF₅:18%Yb³⁺,2%Er³⁺ NPs, BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺ NPs, BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺@BaLuF₅:5%Yb³⁺ NPs, and BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺@BaLuF₅:5%Yb³⁺@BaLuF₅:5%Yb³⁺ NPs by using a 980 nm pulsed laser with a pulse width of 100 μs and a repetition rate of 20 Hz as the excitation source. The result is shown in Figure 7. Each of the decay curves could be fitted well with a single-exponential function as I = I₀ exp(−t/τ), where I₀ is the initial emission intensity at t = 0 and τ is the lifetime of the monitored level. Obviously, the lifetime of the ⁴I_13/2 level of Er³⁺ was extended from 517 μs to 579 μs by introducing Ce³⁺ ions into the BaLuF₅: Yb³⁺,Er³⁺ core NPs. This was because the quenching of Er³⁺ ions from the ⁴I_11/2 state to the ⁴I_13/2 state by the energy transfer occurs between Ce³⁺ and Er³⁺: ⁴I_11/2 (Er³⁺) + ²F_5/2 (Ce³⁺) → ⁴I_13/2 (Er³⁺) + ²F_7/2 (Ce³⁺). Interestingly, by growing a BaLuF₅: 5% Yb³⁺ shell on the core NP, the lifetime of the ⁴I_13/2 level was extended from 579 μs to 818 μs owing to the reduction of the nonradiative relaxation rate caused by the surface passivation. When the number of the shell layer reached two, the lifetime of the ⁴I_13/2 level further increased to 936 μs since the thickness of each shell layer was about 2 nm, and therefore one shell layer was not enough for suppressing surface quenching completely. The result agreed well with the tendency toward the dependence of the measured DC emissions (shown in Figure 6c).

4. Conclusions

In summary, we synthesized BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺ core NPs by introducing Ce³⁺ ions via a high boiling solvent process. In the case of BaLuF₅:18%Yb³⁺,2%Er³⁺ core NPs, the UC emission intensity of BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺ core NPs significantly decreased and the DC emission intensity obviously increased due to the energy transfer between Er³⁺ and Ce³⁺ ions according to: ⁴I_11/2 (Er³⁺) + ²F_5/2 (Ce³⁺) → ⁴I_13/2 (Er³⁺) + ²F_7/2 (Ce³⁺). We prepared BaLuF₅:Yb³⁺,Er³⁺,Ce³⁺ core-active-shell-active-shell NPs via a layer-by-layer strategy. In comparison with the optical properties of BaLuF₅:Yb³⁺,Er³⁺ core NPs, the DC emission intensity of BaLuF₅:Yb³⁺,Er³⁺,Ce³⁺ core-active-shell-active-shell NPs were enhanced by 44 times after coating with two-layer BaLuF₅:Yb³⁺ active shells. We effectively enhanced the DC emission intensity of Yb³⁺-Er³⁺ co-doping BaLuF₅ NPs through introducing Ce³⁺ ions into BaLuF₅ NPs and multiple BaLuF₅:5%Yb³⁺ active-shell coatings.