Introduce Ce³⁺ Ions to Realize Enhancement of C+L Band Luminescence of KMnF₃: Yb, Er Nanoparticles

Abstract

Polymer-based waveguide amplifiers are essential components in integrated optical systems, as their gain bandwidths directly determine the operating wavelength of optical circuits. However, development of the wideband gain media has been challenging, making it difficult to fabricate devices with broadband amplification capability. Rare earth ion-doped nanoparticles (NPs) are a key component in the gain media, and their full width at half maximum (FWHM) of the emission peak decides the final gain bandwidth of the gain media. Here, KMnF₃: Yb, Er, Ce@KMnF₃: Yb NPs with the broad full width at half maximum (FWHM) of the emission peak covering the S+C band was prepared. The NPs were synthesized using a hydrothermal method, and the FWHM of the emission peak of NPs reached 76 nm under the excitation of a 980 nm laser. The introduction of Ce³⁺ ions and a core-shell structure coating greatly enhanced the emission intensity of NPs at C band. Since KMnF₃: Yb, Er, Ce@KMnF₃: Yb NPs have exceptional broadband luminescence properties at C band, KMnF₃: Yb, Er, Ce@KMnF₃: Yb NPs can be the potential gain medium in the future polymer-based waveguide amplifiers.

1. Introduction

Rare earth ion-doped nanoparticles (NPs) have attracted much attention since their excellent optical properties. The fluoride nanoparticles can be used in solar cells, biomarkers, in vivo drug delivery, photocatalysis, biological imaging, cancer therapy, optical waveguide amplifiers, anti-counterfeiting, and other fields [1,2,3,4,5,6,7,8,9,10,11,12]. Rare earth ion-doped nanoparticles (NPs) have attracted much attention due to their excellent optical properties compared with the conventional metal nanoparticles [13,14,15]. Recently, the rare earth ions-doped NPs have gained widespread acceptance as a key component in the gain medium for the fabrication of waveguide amplifiers [16,17]. The gain bandwidth of a waveguide amplifier was determined by the emission bandwidth of the NPs doped in the gain medium. Due to the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ions located at 1.53 μm, the luminescence peaks of Er³⁺ ions coincide well with the C band [18]. Therefore, Er³⁺ ions-doped NPs can be utilized in C-band waveguide amplifiers. Specially, the waveguide amplifiers based on KMnF₃: Yb, Er NPs have been reported recently because of the broadband luminescence properties in C band of the NPs [19]. However, the low luminescence intensity of KMnF₃: Yb, Er NPs makes the NPs difficult for practical requirements. In order to obtain Er³⁺-doped optical waveguide amplifier with high-gain, it is necessary to obtain a gain medium with intensive luminescence characteristics. The improvement of the luminescence properties of KMnF₃: Yb, Er NPs becomes a challenge. So far, several methods have been proposed to enhance the luminescence intensity of Er³⁺-doped fluoride at 1.53 μm. The first strategy is to increase the transition rate from the highly excited state energy level of Er³⁺ to the ⁴I_13/2 energy level [20,21], thereby increasing the population of Er³⁺ at the ⁴I_13/2 energy level. And finally, the luminescence intensity at 1.53 μm can be enhanced due to the increase of radiative transition ⁴I_13/2 → ⁴I_15/2 energy level. The second strategy is to coat the surface of nanometer-sized core particles with a shell, which can repair the surface defect of the cores and effectively shield the emissive lanthanide ions near the surface from quenchers in the surroundings. The core-shell structure enhances the luminescence intensity at 1.53 μm of Er³⁺, thereby improving the luminescence properties of KMnF₃: Yb, Er NPs [22,23,24,25,26].

In this paper, we choose KMnF₃ as the matrix material since the FWHM of emission peak of the Er³⁺ ions-doped KMnF₃ NPs is broader than that of the typical NaYF₄ [27]. We prepared KMnF₃: Yb, Er NPs via a hydrothermal method, and the influence of Yb³⁺ and Er³⁺ concentrations on the luminescence intensity at 1.53 μm of the NPs was investigated. And the doping concentrations of Yb³⁺ and Er³⁺ ions were found when Er³⁺ ion had the strongest luminescence at 1.5 μm. In order to further enhance the luminescence intensity at 1.53 μm, Ce³⁺ ions were introduced due to the existence of the energy transfer process between Ce³⁺ and Er³⁺: ⁴I_11/2 (Er³⁺) + ²F_5/2 (Ce³⁺) → ⁴I_13/2 (Er³⁺) + ²F_7/2 (Ce³⁺) [28]. We also investigated the relationship between the Ce³⁺ concentration and the luminescence intensity at 1.53 μm of KMnF₃: Yb, Er, Ce NPs. And the doping concentration of Ce³⁺ ion was found when Er³⁺ ion had the strongest luminescence at 1.5 μm. In order to reduce surface quenching, a core-shell structure was also introduced. Finally, we obtained core-shell NPs (KMnF₃: Yb, Er, Ce@KMnF₃: Yb) with strong luminescence at 1.53 μm.

2. Experiment Details

YCl₃·6H₂O (99.99%), YbCl₃·6H₂O (99.99%), ErCl₃·6H₂O (99.99%), CeCl₃·6H₂O(99.99%) were purchased from Yutai Chemical Reagent, Shandong, China. Oleic Acid (OA, 500 mL) was purchased from Alpha Aesop Company, Shanghai, China. Ethanol (98%) and cyclohexane (99%) was obtained from Beijing Fine Chemical Company, Beijing, China. KOH (98%), KF·2H₂O (98%), MnCl₂·2H₂O (98%) were purchased from Aladdin, Shanghai, China. All chemicals were used without further purification.

2.1. Preparation of KMnF₃: Yb, Er, Ce NPs

First, 12 mmol KOH was dropped in a 50 mL beaker containing 10 mL oleic acid, 5 mL ethanol and 3 mL deionized water under stirring. About 30 min later, another water solution of 0.4 mmol chloride (the four chlorides of YCl₃·6H₂O, YbCl₃·6H₂O, ErCl₃·6H₂O, CeCl₃·6H₂O, and MnCl₂·2H₂O are 0.4 mmol in total) was also added drop by drop. About 30 min later, a 1 mL water solution of 3.5 mmol KF·2H₂O was added dropwise. After stirring thoroughly, the solution was transferred to a hydrothermal reactor and heated to 200 °C for 3 h. The reaction solution was naturally cooled to room temperature, and the obtained KMnF₃: Yb, Er, Ce nanoparticles were washed three times with ethanol and cyclohexane and dried to obtain powdery KMnF₃: Yb, Er, Ce nanoparticles.

Next, 0.2 mmol of the as-prepared KMnF₃: Yb, Er, Ce nanoparticles were dissolved in cyclohexane as core to induce the subsequent shell.

2.2. Preparation of KMnF₃: Yb, Er, Ce@KMnF₃: Yb Core-Shell NPs

Next, 12 mmol KOH was dropped in a 50 mL beaker containing 10 mL oleic acid, 5 mL ethanol, and 3 mL deionized water under stirring. About 30 min later, another water solution of 0.4 mmol chloride (the four chlorides of YCl₃·6H₂O, YbCl₃·6H₂O, ErCl₃·6H₂O, CeCl₃·6H₂O, and MnCl₂·2H₂O are 0.4 mmol in total) was also added drop by drop. About 30 min later, a 1 mL water solution of 3.5 mmol KF·2H₂O was added dropwise. At last, a 5 mL solution containing core NPs was dropped into the beaker. After stirring thoroughly, the solution was transferred to a hydrothermal reactor and heated to 200 °C for 12 h. The reaction solution was naturally cooled to room temperature, and the obtained KMnF₃: Yb, Er, Ce@KMnF₃: Yb nanoparticles were washed three times with ethanol and cyclohexane and then dried to obtain powdery KMnF₃: Yb, Er, Ce@KMnF₃: Yb nanoparticles.

2.3. Characterization

The phase of the NPs was characterized by X-ray powder diffraction (XRD) (Model Rigaku Ru-200b), using a nickel-filtered Cu-Kα radiation (λ = 1.5406 Å), and the scan ranged from 10° to 70°. The morphology of the particles was characterized by JEM-2100F electron microscope (Tokyo, Japan) at 200 KV. Under the excitation of a 980 nm laser diode, the emission spectrum at 1.53 μm of the sample was collected with a SPEX1000M spectrometer (HORIBA Group, Kyoto, Japan) at room temperature, and the fixed power density was 70 W·cm² (the slit width was 0.2 mm).

3. Results and Discussion

3.1. Crystal Structure and Morphology

The KMnF₃: Yb, Er and NaYF₄: Yb, Er NPs were prepared, and their normalized up-conversion luminescence spectrum and spectrum at 1.53 μm excited by 980 nm laser were shown in Figure 1. It can be seen the luminescence intensity at the 522 nm (²H_11/2 → ⁴I_15/2) and 540 nm (⁴S_3/2 → ⁴I_15/2) of Er³⁺ was reduced due to the energy transfer between Mn²⁺ and Er³⁺ in the KMnF₃: Yb, Er NPs. Therefore, the green up-conversion luminescence of KMnF₃: Yb, Er NPs was weakened, and the red up-conversion luminescence was highlighted. Thus, KMnF₃: Yb, Er NPs exhibits red up-conversion luminescence at 980 nm excitation. It can be seen in the spectrum of Figure 1b that KMnF₃: Yb, Er NPs has two luminescence peaks at 1490 nm and 1530 nm, and its FWHM is about 10 nm wider than that of NaYF₄: Yb, Er NPs. We believe that difference of emission spectrum at 1.53 μm was due to the different symmetry of the crystal fields of the two materials.

In order to enhance the luminescence intensity at 1.53 μm of KMnF₃: Yb, Er NPs, we introduced Ce³⁺ ions and prepared an active core-shell structure. The phase structure of the resulting product was analyzed on a Model Rigaku Ru-200b X-ray powder diffractometer (XRD) with nickel-filtered Cu K_α radiation (λ = 1.5406 Å). We successfully synthesized KMnF₃: 18Yb, 2Er, KMnF₃: 18Yb, 2Er, 4Ce and KMnF₃: 18Yb, 2Er, 4Ce@KMnF₃: 20Yb NPs by hydrothermal method. The diffraction peak was good agreement with the literature value (JCPDS: 82-1334). The diffraction peaks at 21.2°, 30.1°, 37.2°, 43.1°, 48.48°, 53.5°, 62.5° can be ascribed to the (100), (110), (111), (200), (210), (211), and (220) planes. The diffraction peaks of the samples can be indexed to the cubic phase KMnF₃. The X-ray diffraction pattern of the sample is shown in Figure 2. In KMnF₃ crystals, Mn²⁺ ions have an ionic radius of 80 Å. The ionic radii of doped ions are as follows: Er³⁺ (88.1 Å), Yb³⁺ (85.8 Å), Ce³⁺ (103.4 Å). The ionic radius of the Yb³⁺ ion and the Er³⁺ ion is similar to that of the Mn²⁺ ion, so the Yb³⁺ ion and Er³⁺ ion can occupy the lattice site of the Mn²⁺ ion well. The ionic radius of the Ce³⁺ ion is slightly larger than that of the Mn ion, but the doped Ce³⁺ ion has little effect on the crystal phase of the KMnF₃ crystal due to the low doping concentration of the Ce³⁺ ion.

The schematic diagram and TEM of KMnF₃:18Yb, 2Er, 4Ce@KMnF₃: 20Yb core-shell structured nanoparticles were shown in the Figure 3 and Figure 4. The cyan pellets represent the core nanoparticles, and the red pellets represent the shell material.

3.2. Optical Properties

3.2.1. Effect of the Concentration of Yb³⁺ and Er³⁺ on the Luminescence Properties of KMnF₃: Yb, Er NPs

Most materials exhibit Stokes-shifted, also known as down-conversion emission, where each emitted photon has lower energy than the absorbed photon. However, there are also materials that have the ability to generate anti-Stokes shift luminescence, where the emitted photons have higher energy than the photons used for excitation. Two-photon absorption-based luminescence and second harmonic generation are two examples of anti-Stokes processes that require high-energy pulsed lasers as excitation sources. Depending on the lifetime of the excited state, the two-photon or multi-photon processes require simultaneous or nearly simultaneous absorption of two coherent near-infrared (NIR) photons at high excitation power densities (~10⁶ W cm⁻²), due to the small two-photon absorption cross-section. From the research on macroscopic inorganic crystals, three major up-conversion mechanisms have been elucidated: (i) ground-state absorption combined with excited-state absorption (GSA/ESA); (ii) energy transfer up-conversion (ETU); and (iii) photon avalanche. Among these categories, ETU is considered to be the most effective up-conversion (UC) mechanism. When a macroscopic crystal is simply doped with a low concentration of a trivalent rare-earth (RE) element, the interactions between ions can be neglected, and GSA/ESA is responsible for the UC process. As the doping concentration increases, the interactions between ions become significant, and the probability of energy transfer between ions in the excited state and the ETU mechanism increases. One approach to improve up-conversion (UC) efficiency is to use sensitizers with a simple energy scheme and high absorption cross-section in the near-infrared (NIR) region. These sensitizers absorb photon energy and transfer it to the up-conversion activators. As we all know, Yb³⁺ was often used as a sensitizer due its large absorption cross section at 980 nm in the Er³⁺ and Yb³⁺ co-doped system, due to the large energy overlap between the ²F_5/2 → ²F_7/2 energy level transition of Yb³⁺ and the transition of many energy levels of Er³⁺. Therefore, Yb³⁺ continuously absorbs 980 nm photons and transfers energy to Er³⁺ to populate the high-energy level of Er³⁺ under the excitation of a 980 nm laser diode. And the high-energy level of Er³⁺ was radiated to the low-energy level through transition, thereby emitting up-conversion luminescence and luminescence at 1.53 μm. In the KMnF₃: Yb, Er NPs, the Mn²⁺ will undergo four energy transfer processes with Er³⁺: ⁶A₁ (Mn²⁺) + ²H_9/2 (Er³⁺) → ⁴T₁ (Mn²⁺) + ⁴I_13/2 (Er³⁺); ⁶A₁ (Mn²⁺) + ²H₁₁ (Er³⁺) → ⁴T₁ (Mn²⁺) + ⁴I_13/2 (Er³⁺); ⁶A₁ (Mn²⁺) + ⁴S_3/2 (Er³⁺) → ⁴T₁ (Mn²⁺) + ⁴I_13/2 (Er³⁺); ⁴T₁ (Mn²⁺) + ⁴I_15/2 (Er³⁺) → ⁶A₁ (Mn²⁺) + ⁴F_9/2 (Er³⁺) [29]. The energy transfer process in KMnF₃: Yb, Er NPs was shown in Figure 5.

The concentration of Yb³⁺ and Er³⁺ could greatly affect the luminescence intensity of the up-conversion luminescence and luminescence at 1.53 μm of the KMnF₃: Yb, Er NPs. In order to explore the best concentration of Yb³⁺ and Er³⁺ in KMnF₃: Yb, Er NPs, we performed a series of experiments. First, the concentration of Er³⁺ was doped at 1 mmol% and the concentration of Yb³⁺ (12%, 14%, 16%, 18%, 20%, 22%) was changed, then their up-conversion luminescence emission spectrum and luminescence at 1.53 μm emission spectrum was tested under the excitation of a 980 nm laser diode. Figure 6 shows the up-conversion luminescence emission spectrum and luminescence at 1.53 μm emission spectrum with varying Yb³⁺ concentration under the excitation of a 980 nm laser. We found that the intensity of the luminescence at 1.53 μm gradually increased with increasing Yb³⁺ concentration from 12% to 18% (as shown in Figure 6b). This was due to the fact that increasing the concentration of Yb³⁺ doping can enhance the absorption of pump energy. However, it is important to note that at higher concentrations of Yb³⁺ ions (>18%), the luminescence intensity at 1.53 μm starts to decrease due to concentration quenching caused by the high Yb³⁺ concentration. Second, the best concentration of Er³⁺ was determined. The concentration of Yb³⁺ was doped at 18 mmol% and the Er³⁺ concentration (0.5%, 1%, 2%, 3%, 4%) was changed, then their up-conversion luminescence emission spectrum and luminescence at 1.53 μm emission spectrum was tested under the excitation of a 980 nm laser diode. Figure 7 shows the up-conversion luminescence emission spectrum and luminescence at 1.53 μm emission spectrum with varying Er³⁺ concentration under the excitation of a 980 nm laser. We found that the intensity of the luminescence at 1.53 μm gradually increased with increasing Er³⁺ concentration from 0.5% to 2% (as shown in Figure 7b). This was due to the fact that the Er³⁺ ion is the luminescence center ion, and the increase of luminescence center increases the luminescence intensity. However, it is important to note that at higher concentrations of Er³⁺ ions (>2%), the luminescence intensity at 1.53 μm starts to decrease due to concentration quenching caused by the high Er³⁺ concentration.

Through the above experiments, we determined that the best doping concentration of Yb³⁺ and Er³⁺ in the KMnF₃ matrix were 18% and 2%.

3.2.2. Effect of Ce³⁺ Concentration on the Luminescence Properties of KMnF₃: Yb, Er NPs

After the introduction of Ce³⁺ ions, the luminescence intensity at 1.53 μm of Er³⁺ ions was significantly increased. In KMnF₃: Yb, Er, Ce NPs, all the energy transfer processes were shown in Figure 8. The Yb³⁺ ions were excited from the ²F_7/2 to the ²F_5/2 energy level under the excitation of the 980 nm laser diode, and the energy was transferred to the Er³⁺ ions to accumulate the higher energy level of the Er³⁺ ions: ²H_9/2, ⁴F_7/2, ²H_11/2, ⁴S_3/2, ⁴F_9/2. Among them, 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 give up-conversion emission. The ⁴I_13/2 → ⁴I_15/2 transition gives luminescence at 1.53 μm. Interestingly, the energy transfer occurs between Ce³⁺ ions and Er³⁺ ions with the introduction of Ce³⁺ ions: ⁴I_11/2 (Er³⁺) + ²F_5/2 (Ce³⁺) → ⁴I_13/2 (Er³⁺) + ²F_7/2 (Ce³⁺). As a result, the ⁴I_11/2 energy levels of the Er³⁺ ions were populated at the ⁴I_13/2 energy level [30,31,32]. The intensity of up-conversion luminescence was reduced, while the luminescence intensity at 1.53 μm was improved after the introduction of Ce³⁺. As shown in the energy transfer schematic in Figure 8. The energy of the excitation laser at 980 nm is first transferred to the Yb³⁺ ions, and then the energy is transferred to the ⁴I_11/2 energy level of the Er³⁺ ions through the energy transfer between the Yb³⁺ ions and the Er³⁺ ions. The ⁴I_11/2 energy level of the Er³⁺ ions can continue to receive energy transferred from the Yb³⁺ ions, thereby increasing the population of the other excited state energy levels of the Er³⁺ ions. After the introduction of Ce³⁺ ions, the energy transfer process occurs between Ce³⁺ ions and Er³⁺ ions. The population of the ⁴I_11/2 energy level of Er³⁺ ions decreases and the population of the ⁴I_13/2 energy level of Er³⁺ ions increases. The decrease of the population of the ⁴I_11/2 energy level of Er³⁺ ions leads to the decrease of the population of all other excited state energy levels of Er³⁺ ions and the weakening of the up-conversion luminescence intensity. The increase of the population of the ⁴I_13/2 energy level of Er³⁺ ions leads to the increase of the luminescence intensity at 1.5 μm (⁴I_13/2 → ⁴I_15/2). As shown in the Figure 9, after the introduction of Ce³⁺ ion, the intensity of luminescence of the sample at 1.5 μm was significantly increased, while the up-conversion luminescence intensity of the sample was significantly decreased. With the increasing Ce³⁺ ion concentration, the luminescence intensity of the sample at 1.5 μm has been increased, and the up-conversion luminescence intensity has been decreased. When the Ce³⁺ ion reaches the optimal doping concentration, the luminescence intensity of the sample reaches the maximum at 1.5 μm. As the Ce³⁺ ion concentration continued to increase, the luminescence intensity at 1.5 μm of the sample began to weaken.

In order to investigate the best concentration of Ce³⁺ ions, we measured the up-conversion luminescence spectrum of KMnF₃:18Yb, 2Er, xCe (x = 1%, 2%, 3%, 4%, 5%) under the excitation of a 980 nm laser diode, the data was shown in Figure 9a. The luminescence peaks were attributed to the ²H_11/2 → ⁴I_15/2 (525 nm), ⁴S_3/2 → ⁴I_15/2 (545 nm), ⁴F_9/2 → ⁴I_15/2 (655 nm).

The luminescence at 1.53 μm of the NPs under the excitation of a 980 nm laser diode gradually increased with the increase of Ce³⁺ concentration (as shown in Figure 9b). This is due to the energy transfer between Ce³⁺ and Er³⁺: ⁴I_11/2 (Er³⁺) + ²F_5/2 (Ce³⁺) → ⁴I_13/2 (Er³⁺) + ²F_7/2 (Ce³⁺). This results in an increase in the population of the ⁴I_13/2 level of the Er³⁺ ion and a decrease in the population of the ⁴I_11/2 level. The increase of the population of the ⁴I_13/2 energy level greatly increases the luminescence intensity of the Er³⁺ ion at 1.5 μm. When the Ce³⁺ concentration was 4 mmol%, the luminescence intensity at 1.53 μm of KMnF₃: Yb, Er, Ce NPs reaches the maximum under excitation of the 980 nm laser diode (as shown in Figure 9d). As the Ce³⁺ concentration continues to increase, the luminescence intensity at 1.53 μm begins to weaken, which was due the concentration quenching. Through the above experiments, we determined that the best Ce³⁺ doping concentration in KMnF₃: Yb, Er NPs was 4%. This Ce³⁺ doping concentration will be used in the subsequent core-shell coating experiments.

3.2.3. Up-Conversion Luminescence and Luminescence at 1.53 μm Characteristics of Core-Shell KMnF₃: Yb, Er, Ce@KMnF₃: Yb NPs

We compared the up-conversion luminescence spectrum and luminescence at 1.53 μm of KMnF₃:18Yb, 2Er, 4Ce core NPs with KMnF₃:18Yb, 2Er, 4Ce@KMnF₃:20Yb core-shell NPs under excitation of 980 nm laser diode (as shown in Figure 10). It can be seen that when the active shell layer was coated on the surface of the core NPs, the intensity of up-conversion luminescence and luminescence intensity at 1.53 μm was significantly enhanced. Due to this, the insertion of the shell can inhibit the non-radiative transition [23,24,33], and the Yb³⁺ ions in the shell can transfer energy from the pump source to the core. Thereby, the active shell contributes to the enhancement of the luminescence intensity of the up-conversion and luminescence intensity at 1.53 μm [34].

In addition, the lifetime of the ⁴I_13/2 energy level of Er³⁺ in KMnF₃: 18Yb, 2Er, KMnF₃:18Yb, 2Er, xCe (x = 1%, 2%, 3%, 4%, 5%) and KMnF₃:18Yb, 2Er, 4%Ce@KMnF₃: 20Yb NPs was measured under excitation of 980 nm pulsed laser with a pulse width of 400 µs and a frequency of 50 HZ as the excitation source. The result was shown in Figure 11. Each of the delay curves can be fitted well with a single-exponential function as $\mathrm{I}={\mathrm{I}}_0\exp \left(-\mathrm{t}/\tau \right)$, where ${\mathrm{I}}_0$ is the initial emission intensity at $\mathrm{t}=0$ and $\tau$ is the lifetime of the monitored level. We found that after the introduction of Ce³⁺ ions, the lifetime of the ⁴I_13/2 energy level was increased. This was because after the introduction of Ce³⁺ ions, the energy transfer occurs between Ce³⁺ ions and Er³⁺ ions with the introduction of Ce³⁺ ions: ⁴I_11/2 (Er³⁺) + ²F_5/2 (Ce³⁺) → ⁴I_13/2 (Er³⁺) + ²F_7/2 (Ce³⁺). After energy transfer occurs, the population of ⁴I_13/2 level of Er ion increases, which leads to the increase of the lifetime of ⁴I_13/2 level. The lifetime of the ⁴I_13/2 level increases with the increase of Ce ion concentration. When the doping concentration of Ce ion reaches 4%, the luminescence of the Er ion at 1.5 μm and the lifetime of the ⁴I_13/2 energy level reach the maximum. With the continuous increase of Ce ion doping concentration, the luminescence of the Er ion at 1.5 μm and the lifetime of ⁴I_13/2 level will decrease, which is caused by concentration quenching. In a series of lifetime curves, the core-shell structure sample has the longest lifetime. This is due to growing an active shell on the core NPs, and the increase in lifetime is due to the surface passivation effect, which leads to a reduction in nonradiative relaxation rate.

4. Conclusions

In summary, we prepared KMnF₃ NPs by a hydrothermal method. Through multiple sets of control experiments, we found the best concentration of Yb³⁺ and Er³⁺ when the KMnF₃: Yb, Er has the strongest luminescence at 1.53 μm under excitation of 980 nm laser diode. The introduction of Ce³⁺ ions enhanced the luminescence intensity at 1.53 μm of KMnF₃: Yb, Er NPs, and the optimal concentration of Ce³⁺ was found. The preparation of the core-shell structure further enhanced luminescence intensity at 1.53 μm of KMnF₃: Yb, Er NPs. It can be seen from the spectrum that after the active shell was coated, the intensity of up-conversion luminescence and luminescence intensity at 1.53 μm of the NPs under the excitation of 980 nm laser diode was greatly enhanced; this also shows that the core-shell structure can well inhibit the surface quenching effect. Since KMnF₃: Yb, Er, Ce@KMnF₃: Yb NPs have good broadband luminescence properties at 1530 nm, they can be the potential gain medium in the future polymer-based waveguide amplifiers.