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.

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+:⁴I11/2 (Er3+) + ²F5/2 (Ce3+) → ⁴I13/2 (Er3+) + ²F7/2 (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 4I13/2 → 4I15/2 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 4I13/2 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+:4I11/2 (Er3+) + 2F5/2 (Ce3+) → 4I13/2 (Er3+) + 2F7/2 (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 Kα (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.

Figure 1

X-ray powder diffraction (XRD) patterns of (A) standard BaGdF5 NPs (the vertical bars denote the standard data for tetragonal structure of bulk BaGdF5 NPs (JCPDS-24-0098)); (B) BaLuF5:18%Yb3+,2%Er3+ core NPs; (C) BaLuF5:18%Yb3+,2%Er3+,2%Ce3+ core NPs; (D) BaLuF5:18%Yb3+,2%Er3+,2%Ce3+@BaLuF5:5%Yb3+ core-active-shell NPs; and (E) BaLuF5:18%Yb3+,2%Er3+,2%Ce3+@BaLuF5:5%Yb3+@BaLuF5:5%Yb3+ core-active-shell-active-shell NPs.

Figure 2

(a) Schematic illustration of BaLuF5 core NPs, BaLuF5 core-active-shell NPs and BaLuF5 core-active-shell-active-shell NPs. TEM images of (b) BaLuF5:18%Yb3+,2%Er3+ core NPs; (c) BaLuF5:18%Yb3+,2%Er3+,2%Ce3+ core NPs; (d) BaLuF5:18%Yb3+,2%Er3+,2%Ce3+@BaLuF5:5%Yb3+ core-active-shell NPs; and (e) BaLuF5:18%Yb3+,2%Er3+,2%Ce3+@BaLuF5:5%Yb3+@BaLuF5:5%Yb3+ core-active-shell-active-shell NPs.

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 2F7/2 level to the 2F5/2 level and then transfer the energy to Er3+ to populate higher energy levels of the Er3+ ions:4H11/2 level, 4F9/2, and 4F7/2. The 4H11/2 → 4I15/2 (≈525 nm), 4S3/2 → 4I15/2 (≈545 nm), and 4F9/2 → 4I15/2 (≈655 nm) transitions gives the UC emission, and the 4I13/2 → 4I15/2 transition gives the DC emission at 1.53 μm. Interestingly, with the addition of Ce3+ ions, the energy transfer occurs between Ce3+ and Er3+:4I11/2 (Er3+) + 2F5/2 (Ce3+) → 4I13/2 (Er3+) + 2F7/2 (Ce3+). Furthermore, the 4I11/2 state of Er3+ ions populate the 4I13/2 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.

Figure 3

Diagram of energy levels of Yb3+-Er3+-Ce3+ and up-conversion (UC) emission and down-conversion (DC) emission processes in the BaLuF5:Yb3+,Er3+,Ce3+ systems under 980 nm excitation.

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 4H11/2 → 4I15/2 (≈525 nm), 4S3/2 → 4I15/2 (≈545 nm), and 4F9/2 → 4I15/2 (≈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+:4I11/2 (Er3+) + 2F5/2 (Ce3+) → 4I13/2 (Er3+) + 2F7/2 (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.

Figure 4

(a) UC and (c) DC 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. Intensity enhancement of (b) UC and (d) DC emission depending on the Ce3+ concentrations in BaLuF5:18%Yb3+,2%Er3+,x%Ce3+ NPs.

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 4I13/2 → 4I15/2 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+:4I11/2 → 4I13/2 transition, which can be increased by doping with Ce3+ ions, and the energy transfer process can increase the population of 4I13/2 state of Er3+ ions through the following energy transfer process: 4I11/2 (Er3+) + 2F5/2 (Ce3+) → 4I13/2 (Er3+) + 2F7/2 (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+:4I13/2 + Ce3+: 2F5/2 → Er3+:4I15/2 + Ce3+: 2F7/2 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.

Figure 5

(a) DC emission spectra of BaLuF5:18%Yb3+,2%Er3+ core NPs, BaLuF5:18%Yb3+,2%Er3+,2%Ce3+ core NPs and BaLuF5:18%Yb3+,2%Er3+,2%Ce3+@BaLuF5:x%Yb3+ NPs (x = 0, 2.5, 5, 7.5 and 10) core-active-shell NPs under the excitation of a 980 nm laser diode. (b) Intensity enhancement of DC emission depending on the Yb3+ concentrations in BaLuF5 core-active-shell NPs.

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.

Figure 6

(a) Schematic illustration and (b) energy transfer mechanisms of BaLuF5:18%Yb3+,2%Er3+,2%Ce3+@BaLuF5:5%Yb3+@BaLuF5:5%Yb3+ core-active-shell-active-shell NPs. (c) DC emission spectra 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. (d) Intensity enhancement of DC emission in all the above NPs.

In addition, we measured the lifetime of the 4I13/2 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 I is the initial emission intensity at t = 0 and τ is the lifetime of the monitored level. Obviously, the lifetime of the 4I13/2 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 4I11/2 state to the 4I13/2 state by the energy transfer occurs between Ce3+ and Er3+: 4I11/2 (Er3+) + 2F5/2 (Ce3+) → 4I13/2 (Er3+) + 2F7/2 (Ce3+). Interestingly, by growing a BaLuF5: 5% Yb3+ shell on the core NP, the lifetime of the 4I13/2 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 4I13/2 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).

Figure 7

The lifetime of the 4I13/2 level of Er3+ (monitored at 1530 nm corresponding to the 4I13/2 → 4I13/2) 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.

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: 4I11/2 (Er3+) + 2F5/2 (Ce3+) → 4I13/2 (Er3+) + 2F7/2 (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.