Enhancing Infrared Photoluminescence Performance via Li⁺ Substitution and Yb³⁺ Codoping in Na₃ScSi₃O₉:Cr³⁺ Phosphor

Abstract

Cr³⁺-doped broadband infrared luminescent materials have attracted growing attention in consideration of their potential applications in phosphor-converted infrared light sources. However, discovering infrared-emitting luminescent materials with ultrabroadband emission and excellent thermal stability still remains a challenge. In this work, we report the significant improvement of infrared photoluminescence properties in Na₃ScSi₃O₉:Cr³⁺ phosphor via Li⁺ substitution and Yb³⁺ codoping. The prepared Na₃ScSi₃O₉:Cr³⁺ phosphor can produce broad infrared emission over 650–1350 nm with a peak maximum at 898 nm under the excitation of blue light. Through the substitution of Li⁺ for Na⁺, the maximum infrared emission peak can be tuned from 898 nm to 850 nm. When the Li⁺ content is 0.5, the integrated infrared luminescence intensity of the obtained Na_2.5Li_0.5ScSi₃O₉:Cr³⁺ phosphor increases by 4.2 times compared with that of the Na₃ScSi₃O₉:Cr³⁺ phosphor, and the luminescence thermal stability is also improved significantly (58.5%@100 °C). Moreover, Yb³⁺ codoping can simultaneously realize the characteristic infrared luminescence of Cr³⁺ and Yb³⁺, resulting in a broadened spectral width due to efficient energy transfer from Cr³⁺ to Yb³⁺. Finally, an ultrabroadband infrared light-emitting diode prototype is fabricated through a combination of the optimized Na_2.5Li_0.5ScSi₃O₉:2%Cr³⁺,0.5%Yb³⁺ phosphor with a commercial 490 nm LED chip, giving an infrared output power of 5.2 mW at 320 mA drive current. This work provides an effective way to optimize the infrared photoluminescence performance of Cr³⁺-doped Na₃ScSi₃O₉ infrared phosphors.

1. Introduction

Phosphor-converted infrared light-emitting diodes (LEDs) have aroused considerable attention owing to their appealing advantages over traditional infrared light sources, including their simple fabrication process, small size, low cost, and high luminescence efficiency [1,2,3,4,5]. These unique features give infrared LEDs broad application prospects in the fields of night vision, biomedical imaging, industrial nondestructive analysis, and so on [6,7,8,9]. As an indispensable component in phosphor-converted infrared LEDs, infrared-emitting phosphors play a crucial role in determining the overall luminescence performance of the final infrared LED devices [10]. In recent years, Cr³⁺-activated broadband inorganic phosphor materials have attracted extensive attention in view of their superior infrared luminescence performance, including strong blue light absorption capability and tunable broadband emission in a weak octahedral crystal field [11,12,13,14,15,16,17]. Among them, Cr³⁺-doped silicate-based broadband infrared phosphors usually show good physicochemical stability, low cost, and high luminescence efficiency [18,19,20], which enables them to be desirable light conversion materials for phosphor-converted infrared LEDs.

The infrared photoluminescence properties of Cr³⁺-doped phosphors greatly depend on the surrounding crystal field environment where Cr³⁺ ions are located, which can exhibit narrowband and broadband emissions ascribed to the Cr^{3+ 2}E → ⁴A₂ and ⁴T₂ → ⁴A₂ transitions, respectively [21,22,23]. Recently, extensive research work has been devoted to improving the photoluminescence properties of these Cr³⁺-doped infrared phosphors through ion substitution. For instance, Miao et al. successfully realized the tune of infrared photoluminescence of Na₃Sc₂(PO₄)₃:Cr³⁺ phosphor from a sharp-line emission at 695 nm to a broad emission band peaking at 750 nm by codoping Ga³⁺ ion [24]. Liu et al. synthesized Ga_2−xSc_xO₃:Cr³⁺ solid-solution phosphors, in which Ga_1.594Sc_0.4O₃:0.006Cr³⁺ phosphor showed the best infrared photoluminescence performance with a high internal quantum efficiency (99%) [25]. Zhao et al. reported A₂MSbO₆:Cr³⁺ phosphors (A = Ba, Sr, and Ca; M = Sc, In, Y, and Ga), whose maximum infrared emission peak could be tuned over 825–1010 nm through the precise modulation of composition [26]. Shao et al. presented the anionic F-substitution strategy to improve the infrared luminescence performance of the MgGa₂O₄:Cr³⁺ phosphor, and a maximum external quantum efficiency (EQE) value of 60.6% was realized for the prepared Mg_1.2Ga_1.72O_3.8F_0.2:0.08Cr³⁺ phosphor [27]. All the aforementioned research work gives great inspiration to design and develop Cr³⁺-doped infrared broadband phosphors with tunable emission peaks.

On the other hand, for Cr³⁺-doped infrared-emitting phosphors, the maximum emission peak is usually less than 900 nm and the full width at half maximum (FWHM) usually cannot exceed 200 nm, which greatly limits their practical applications for phosphor-converted superbroadband infrared LEDs [28,29]. To address these disadvantages, the Yb³⁺ codoping strategy has been reported to improve the spectral width and luminescence thermal stability of Cr³⁺ single-doped phosphors by making use of efficient energy transfer between Cr³⁺ and Yb³⁺ ions, which has been verified as an effective strategy to enhance infrared photoluminescence performance of these Cr³⁺-doped phosphors. The representative works include Ca₂LuZr₂Al₃O₁₂:Cr³⁺,Yb³⁺ [30], Gd₃Sc_1.5Al_0.5Ga₃O₁₂:Cr³⁺,Yb³⁺ [31], Lu_0.2Sc_0.8BO₃Cr³⁺,Yb³⁺ [32], CaTaO₄:Cr³⁺,Yb³⁺ [33], LiScP₂O₇:Cr³⁺,Yb³⁺ [34], and KYbP₂O₇:Cr³⁺ phosphors [35]. However, developing novel infrared-emitting phosphors with long-wavelength broadband emission and excellent luminescence thermal stability still remains a challenge.

In this work, we reported the significant enhancement of infrared photoluminescence performance via Li⁺ substitution and Yb³⁺ codoping in Na₃ScSi₃O₉:Cr³⁺ phosphor. Broadband infrared emission with a spectral width of 234 nm and good luminescence thermal stability (66.8%@100 °C) can be achieved in the prepared Na_2.5Li_0.5ScSi₃O₉:Cr³⁺,Yb³⁺ phosphor. Moreover, infrared phosphor-converted broadband LEDs are constructed by combining Na_2.5Li_0.5ScSi₃O₉:Cr³⁺,Yb³⁺ phosphors with 490 nm LED chips, presenting a maximum infrared output power of 5.2 mW with a photoelectric conversion efficiency of 0.5% at 320 mA drive current.

2. Experimental Section

2.1. Synthesis

The Na_3−xLi_xScSi₃O₉:Cr³⁺,Yb³⁺ phosphors were synthesized by the high-temperature solid-state reaction method [35,36]. The starting powder raw materials of Na₂CO₃ (99.99%, Aladdin), Li₂CO₃ (99.99%, Aladdin), Sc₂O₃ (99.99%, Aladdin), SiO₂ (99.99%, Aladdin), Cr₂O₃ (99.99%, Aladdin), Yb₂O₃ (99.99%, Aladdin) were mixed and ground thoroughly for 30 min. After that, the mixed powders were sintered at 1150 °C for 5 h in air to obtain the final infrared phosphors.

2.2. Fabrication of Phosphor-Converted Infrared LEDs

The phosphor-converted infrared light sources were fabricated by combining an LED chip with the prepared Na_2.5Li_0.5ScSi₃O₉:2%Cr³⁺,0.5%Yb³⁺ phosphor. The HAAS 2000 photoelectric measuring system (350–1650 nm, EVERFINE, Shenzhen, China) was used to measure the light output power of the fabricated LEDs.

2.3. Characterization

A powder X-ray diffractometer was used to measure the phase purity and composition of the prepared phosphors (DMAX-2500PC, Rigaku, Tokyo, Japan). Field-emission scanning electron microscope (FE-SEM, JSM-7800F) (Edinburgh Instruments Ltd, Kirkton Campus, UK) was used to testify the microstructure and EDS elemental maps of the phosphors. An Edinburgh FLS1000 spectrofluorometer (Edinburgh Instruments Ltd, Kirkton Campus, UK) equipped with both continuous and pulsed xenon lamps was used to measure the infrared luminescence properties of the prepared phosphors, including photoluminescence excitation spectra, emission spectra, and luminescence decay curves. The temperature-dependent emission spectra were measured using a TAP02 high-temperature fluorescence test attachment (Edinburgh Instruments Ltd, Kirkton Campus, UK).

3. Results and Discussion

3.1. Structural Characterization of the Na₃ScSi₃O₉:Cr³⁺ Phosphor

Figure 1a depicts the crystal structure of the Na₃ScSi₃O₉ compound, which belongs to the orthorhombic lattice system with a P2₁2₁2₁ (No. 19) space group. There exist three distinct crystal sites for the Na⁺ ions, which connect with four oxygen ions, five oxygen ions, and six oxygen ions to form [NaO_x] polyhedra, respectively. In the meanwhile, four different crystallographic sites of Sc³⁺ and one crystallographic site of Si⁴⁺ also exist, which are linked to six oxygen ions and four oxygen ions to form the [ScO₆] octahedron and [SiO₄] tetrahedron, respectively. The crystal structure of the Na₃ScSi₃O₉ matrix can be formed through the ordered connections of these polyhedra. Considering the same chemical valence states of Cr³⁺ and Sc³⁺ and the similar ionic radii (when the coordination number is 6, the radius of the Cr³⁺ ion is 0.615 Å and the radius of the Sc³⁺ ion is 0.745 Å), the Cr³⁺ emitters prefer to occupy the position of Sc³⁺ in the [ScO₆] octahedron in the Na₃ScSi₃O₉ host. The XRD patterns of the Na₃ScSi₃O₉:x%Cr³⁺ (x = 0.5, 1, 2, and 3) phosphors are given in Figure 1b. The XRD peaks of the obtained phosphors are consistent with the standard JCPDS card of the Na₃YSi₃O₉ crystal (No. 36-0127), which confirms that the as-synthesized phosphor has a pure phase and the doping of Cr³⁺ ions does not introduce any impurity phases.

The SEM and EDS elemental mapping images of the Na₃ScSi₃O₉:2%Cr³⁺ phosphor are displayed in Figure 1c. As shown in the SEM results, the sample is composed of irregular particles with uneven particle size distribution. The elemental mapping results clearly reflect that Na, Sc, Si, O, and Cr elements are uniformly distributed throughout the selected luminescent particle. The above results further reveal that the Cr³⁺-doped Na₃ScSi₃O₉ infrared phosphor has been successfully synthesized.

3.2. Photoluminescence Properties of the Na₃ScSi₃O₉:Cr³⁺ Infrared Phosphor

Figure 2a shows the temperature-dependent infrared photoluminescence spectra of the prepared Na₃ScSi₃O₉:2%Cr³⁺ phosphor. As the test temperature increases from 25 to 200 °C, the infrared photoluminescence intensity exhibits a decreasing trend ascribed to the thermal quenching mechanism [36]. Moreover, as shown in Figure 2a, the spectral shape and emission peak stay almost unchanged as the test temperature gradually increases over 25–200 °C. The dependence of the infrared emission intensity on the temperature is given in Figure 2b. As the temperature increases to 100 °C, the integral emission intensity declines to 35.3% of the room temperature emission intensity, and the integral emission intensity decreases to <10% when the temperature reaches 200 °C, indicating that the luminescence thermal stability of the prepared Na₃ScSi₃O₉:Cr³⁺ phosphor is very poor. Therefore, further optimization of the luminescence thermal stability of the Na₃ScSi₃O₉:Cr³⁺ phosphor is needed for practical applications. Figure 3 shows the Gaussian deconvolutions of the photoluminescence spectrum of the Na₃ScSi₃O₉:Cr³⁺ phosphor. The emission spectra can be decomposed into four Gaussian peaks in units of energy (cm⁻¹), which indicates that there exist four crystallographic sites for Cr³⁺-emitting centers in the Na₃ScSi₃O₉ lattice.

3.3. Structural Characterization and Photoluminescence Properties of the Na_3−yLi_yScSi₃O₉:Cr³⁺ Phosphors

To improve the thermal stability of the Na₃ScSi₃O₉:Cr³⁺ phosphor, a series of Na_3−yLi_yScSi₃O₉:Cr³⁺ phosphors with different Li⁺ content were prepared and studied. Figure 3 presents the XRD patterns of the obtained Na_3−yLi_yScSi₃O₉:2%Cr³⁺ (y = 0, 0.05, 0.3, 0.5, 0.6, and 0.9) phosphors. It can be noted that the phase composition of the Na_3−yLi_yScSi₃O₉:Cr³⁺ phosphor is consistent with that of Na₃ScSi₃O₉ when the content of Li⁺ is less than 0.5 (y ≤ 0.5). This indicates that the doped Li⁺ ions successfully occupy the sites of Na⁺ ions and no impurity phase can be formed. However, when y > 0.5, the NaScSi₂O₆ impurity phase starts to appear, and the main phase composition changes to the NaScSi₂O₆ compound when the y value reaches 0.9. Therefore, the content of Li⁺ in Na_3−yLi_yScSi₃O₉:Cr³⁺ phosphor should be less than 0.5 from the perspective of phase composition.

The excitation spectra of the Na_3−yLi_yScSi₃O₉:0.02Cr³⁺ (y = 0, 0.05, 0.25, and 0.5) phosphors are displayed in Figure 4a. Two excitation bands in the blue and red spectral regions are clearly observed, which can be ascribed to the Cr^{3+ 4}A₂ → ⁴T₁ and ⁴A₂ → ⁴T₂ electron transitions, respectively [37]. The spectral shapes of the excitation spectra remain almost unchanged with varying Li⁺ content, especially the blue excitation peak at around 489 nm. The photoluminescence emission spectra of the Na_3−yLi_yScSi₃O₉:2%Cr³⁺ phosphors are shown in Figure 4b. Under the excitation of 489 nm, a significant enhancement in the infrared luminescence intensity and a blue shift of the emission peak can be observed with the increasing doping concentration of Li⁺. Compared with the Na₃ScSi₃O₉:Cr³⁺ phosphor, the emission intensity of the Na_2.5Li_0.5ScSi₃O₉:Cr³⁺ phosphor is enhanced by ~4.2 times, and the peak emission shifts from 898 nm to 850 nm with an FWHM value of ~159 nm. The values of Dq/B are increased from 2.11 to 2.25 with the increase in the doping content of Li⁺, indicating that the surrounding crystal field environment experienced by Cr³⁺ ions shows an increasing trend.

The dependence of the integrated infrared luminescence intensity on the test temperature of the Na_2.5Li_0.5ScSi₃O₉:2%Cr³⁺ phosphor is given in Figure 4c,d. The spectral shape and maximum emission peak wavelengths remain unchanged with the increase in the test temperature. When the test temperature increases to 100 °C, the integral emission intensity can keep 58.5% of the initial emission intensity at room temperature. As a result, it can be seen that the substitution of Li⁺ ions for Na⁺ ions has a significant effect on the luminescence thermal stability of the prepared Na_3−yLi_yScSi₃O₉:Cr³⁺ phosphors.

3.4. Enhancing Infrared Photoluminescence Performance by Yb³⁺ Codoping

The infrared luminescence intensity and thermal stability can be improved by tuning the Na/Li stoichiometric ratio in the Na_3-yLi_yScSi₃O₉:Cr³⁺ phosphor. However, the FWHM value of the obtained phosphor is still smaller than 200 nm and the maximum emission peak is less than 900 nm, which will limit the practical application for infrared LEDs. Therefore, a series of Na_2.5Li_0.5ScSi₃O₉:2%Cr³⁺,zYb³⁺ phosphors were synthesized by codoping Yb³⁺. As presented in Figure 5a, all the XRD patterns of the Na_2.5Li_0.5ScSi₃O₉:2%Cr³⁺,zYb³⁺ phosphors agree well with the standard JCPDS card of the Na₃YSi₃O₉ crystal, which confirms that the synthesized phosphors are all pure phases and that the Cr³⁺ and Yb³⁺ codoping does not cause any impurities. Figure 5b exhibits the emission and excitation spectra of the Na_2.5Li_0.5ScSi₃O₉:2%Cr³⁺ and Na_2.5Li_0.5ScSi₃O₉:2%Cr³⁺,0.5%Yb³⁺ phosphors, respectively. Under the excitation of 489 nm, the characteristic emission band of Cr³⁺ ions ascribed to the ⁴T₂ → ⁴A₂ transition can be clearly monitored. For Na_2.5Li_0.5ScSi₃O₉:2%Cr³⁺,0.5%Yb³⁺, an additional emission band is observed in the 950–1150 nm spectral range, which originates from Yb^{3+ 2}F_5/2 → ²F_7/2 transition [30]. Moreover, the spectral width of the emission spectrum of Na_2.5Li_0.5ScSi₃O₉:2%Cr³⁺,0.5%Yb³⁺ phosphor is significantly broadened. Moreover, it is noted that when monitored at 970 nm, the excitation spectrum of the Na_2.5Li_0.5ScSi₃O₉:2%Cr³⁺,0.5%Yb³⁺ phosphor shows the same spectral profiles with that of the Na_2.5Li_0.5ScSi₃O₉:2%Cr³⁺ phosphor, which further proves the efficient Cr³⁺ → Yb³⁺ energy transfer in the Na_2.5Li_0.5ScSi₃O₉:2%Cr³⁺,0.5%Yb³⁺ phosphor.

The photoluminescence emission spectra of the Na_2.5Li_0.5ScSi₃O₉:2%Cr³⁺,zYb³⁺ phosphors upon 489 nm excitation are presented in Figure 5c. As the Yb³⁺ doping concentration increases, the luminescence intensity of Cr³⁺ gradually decreases. However, the infrared luminescence intensity of Yb³⁺ first increases monotonously and reaches the maximum when z = 0.07, then starts to decrease. Figure 5d gives the photoluminescence decay curves monitored at 850 nm with varying Yb³⁺ doping concentrations. The monoexponential equation can be used to fit the luminescence decay curves well [38]:

I(t) = I₀ + A exp(−t/τ)

(1)

where I(t) represents the emission intensity at time t, A is the fitting constant, τ represents the lifetime. As shown in Figure 5d, the lifetimes of the Na_2.5Li_0.5ScSi₃O₉:2%Cr³⁺,zYb³⁺ phosphors monitored at 850 nm are determined to be 36.9, 36.2, 28.9, 24.2, 22.6, 17.9, 15.4, and 13.8 μs, respectively. The Cr³⁺ → Yb³⁺ energy transfer efficiency (η) can be calculated by using the equation:

η = 1 − τ_z/τ₀

(2)

where η is the energy transfer efficiency, τ_z and τ₀ are the lifetimes of the Cr³⁺ ions in the presence and absence of Yb³⁺ ions, respectively. The lifetime and Cr³⁺ → Yb³⁺ energy transfer efficiency as a function of Yb³⁺ doping concentrations are shown in Figure 5e. The Cr³⁺ → Yb³⁺ energy transfer efficiency (η) can reach 62.7% when the doping concentration of Yb³⁺ is increased to 10%.

Figure 6a,b depict the dependence of infrared luminescence spectra of the Na_2.5Li_0.5ScSi₃O₉:2%Cr³⁺,0.5%Yb³⁺ phosphor on the test temperature upon blue light excitation. When the temperature increases from 25 to 200 °C, the infrared luminescence intensity decreases gradually, which is ascribed to the thermal quenching mechanism. When the temperature increases to 100 °C, the integral infrared luminescence intensity drops to 66.8% of room-temperature emission intensity, respectively (Figure 6c).

3.5. Luminescence Properties of the Fabricated Phosphor-Converted Infrared LEDs

The phosphor-converted infrared emitters were assembled through the combination of the Na_2.5Li_0.5ScSi₃O₉:2%Cr³⁺,0.5%Yb³⁺ phosphor with a 490 nm LED chip. When the driving current is turned on, visible emission can be observed from the inside 490 nm LED chip, which can be fully covered by using a 650 nm long-pass filter, as shown in the insets in Figure 7a. Figure 7a displays the infrared emission spectra of the fabricated infrared LED under various drive currents. As the drive currents increase over 20–320 mA, the fabricated infrared LED shows a broad emission band in a wavelength range of 700–1100 nm, and the infrared luminescence intensity increases gradually. Meanwhile, the output power and photoelectric conversion efficiency of the fabricated infrared LED under various drive currents are given in Figure 7b,c. When the drive current increases over 20–320 mA, the infrared output power increases monotonously from 0.6 to 5.2 mW, while the photoelectric conversion efficiency declines from 1.2% to 0.5%. The decrease in conversion efficiency can be ascribed to the efficiency decrease in the 490 nm LED chip. Furthermore, the infrared luminescence performance of the assembled LEDs can be further improved through the optimization of the fabrication process. Therefore, the prepared Na_2.5Li_0.5ScSi₃O₉:Cr³⁺,Yb³⁺ phosphor shows a promising application for phosphor-converted infrared LEDs.

4. Conclusions

In this work, we report that the infrared photoluminescence intensity, thermal stability, and spectral width can be significantly improved through Li⁺ substitution and Yb³⁺ codoping in Na₃ScSi₃O₉:Cr³⁺ phosphor. When the Li⁺ content is 0.5, the integrated infrared luminescence intensity of the prepared Na_2.5Li_0.5ScSi₃O₉:Cr³⁺ phosphor increases by 4.2 times in comparison with that of the Na₃ScSi₃O₉:Cr³⁺ phosphor. Meanwhile, thanks to the efficient Cr³⁺ → Yb³⁺ energy transfer, the optimized Na_2.5Li_0.5ScSi₃O₉:Cr³⁺,Yb³⁺ phosphor exhibits a broadened spectral width and good luminescence thermal stability, whose integral luminescence intensity at 100 and 125 °C can maintain 66.8% and 57.5% of room temperature emission intensity, respectively. An ultrabroadband infrared LED prototype device was assembled through the combination of the optimized Na_2.5Li_0.5ScSi₃O₉:Cr³⁺,Yb³⁺ phosphor and a 490 nm LED chip, which gives an infrared output power of 5.2 mW with photoelectric conversion efficiency of 0.5% at 320 mA drive current. Overall, this work provides new insights into the design and development of efficient, broadband, and thermally stable infrared-emitting phosphors for high-power infrared LEDs.