Structural confinement toward suppressing concentration and thermal quenching for improving near-infrared luminescence of Fe³⁺

Abstract

Luminescence concentration quenching and thermal quenching are closely related to the energy transfer (ET) process between optically active ions. Herein, we utilize the structural confinement effect in Sr₉Ga(PO₄)₇ (SGP) to selectively control ET pathways so as to suppress luminescence concentration and thermal quenching. In the Fe³⁺-doped SGP compound, the relatively large Fe³⁺–Fe³⁺ distances can inhibit the ET between Fe³⁺ ions, leading to weak concentration quenching. The Sr₉Ga_0.8(PO₄)₇:0.2Fe³⁺ (SGP:0.2Fe³⁺) phosphor exhibits the highest near-infrared (NIR) luminescence intensity, and the emission intensity for 50% and 100% Fe³⁺-doped SGP phosphors is 73.46% and 18.25% of that for the optimal sample SGP:0.2Fe³⁺, respectively. Upon co-doping Yb³⁺ into SGP:0.2Fe³⁺, Fe³⁺–Yb³⁺ distances are much shorter than those of Fe³⁺–Fe³⁺, causing energy to quickly migrate from the quenching center Fe³⁺ to the thermally stable center Yb³⁺. The thermal stability of SGP:0.2Fe³⁺,0.07Yb³⁺ is greatly enhanced compared to SGP:0.2Fe³⁺. This study provides a strategy for enhancing NIR luminescence through utilizing structural confinement to control ET pathways toward suppressing concentration and thermal quenching. Finally, we demonstrate the potential applications of SGP:0.2Fe³⁺ and SGP:0.2Fe³⁺,0.07Yb³⁺ phosphors in night vision and optical thermometry fields.

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1. Introduction

Near-infrared (NIR) luminescent materials have aroused extensive research for their applications in the fields of night vision, plant growth, biomedical imaging, food analysis, etc. owing to the advantages of invisibility to human eyes, characteristic absorption by certain molecules and strong penetrating ability of NIR light.^1–6 Hitherto, the majority of research on NIR luminescent materials has been concentrated on Cr³⁺ activators with 3d³ electronic configuration, and plenty of excellent Cr³⁺-activated NIR phosphors have been developed, such as Sr₂ScSbO₆:Cr³⁺, LaMgGa₁₁O₁₉:Cr³⁺, Ca₃Sc₂Si₃O₁₂:Cr³⁺, ScBO₃:Cr³⁺, Ca₂LuZr₂Al₃O₁₂:Cr³⁺ and so on.^7–11 However, there exists the potential risk of Cr³⁺ ions in these materials being oxidized to Cr⁶⁺ ions, which may deteriorate the NIR luminescence efficiency severely.^12–14 In particular, the oxidation of chromium ions will greatly increase the toxicity of Cr³⁺-doped phosphors, potentially causing damage to any organisms and their structures, such as human cells and internal organs, thereby further inhibiting practical bio-applications especially in long-term in vivo therapies.^15,16 Therefore, it is of great importance to develop alternatives to Cr³⁺ emitters that exhibit NIR luminescence.

Trivalent transition metal Fe³⁺ ion, as an essential element in the human body, has been a promising environmentally friendly and chemically stable alternative activator for NIR emission. Fe³⁺ ions are characterized by their half-filled 3d⁵ electronic configuration, and both octahedral and tetrahedral sites are suitable for Fe³⁺ to exhibit NIR luminescence.¹⁷ Similar to Cr³⁺ ions, the emission wavelength of Fe³⁺ ions can be easily tuned because the bare 3d electrons are susceptible to the host environment.¹⁸ However, the d–d transition of Fe³⁺ ions is strictly restricted by the Laporte selection rule, especially in highly symmetric fields,^14,19 and moreover suffers from a strong electron–phonon coupling effect, leading to the poor luminescence property of Fe³⁺ ions. Several strategies have been proposed to enhance Fe³⁺ luminescence. For instance, a series of Sr_2−yCa_y(InSb)_1−zSn_2zO₆:Fe³⁺ phosphors with tunable emission from 885 to 1005 nm have been reported through crystallographic site engineering strategy, and thus an efficient double perovskite phosphor Ca₂InSbO₆:Fe³⁺ with ultra-high internal quantum efficiency (IQE) of 87% has been developed.¹⁴ Besides, Liu et al. reported the zero thermal quenching NIR luminescence up to 200 °C in KAl₁₁O₁₇:Fe³⁺ through multi-site energy transfer (ET).²⁰ In addition to the methods of crystallographic site engineering and multi-site ET, utilizing the structural confinement effect to suppress luminescence concentration and thermal quenching has also been proved to be an effective way to enhance luminescence in Cr³⁺-activated phosphors. As reported in Sr₉Ga(PO₄)₇:Cr³⁺ (SGP:Cr³⁺) and Sr₉Cr(PO₄)₇:Yb³⁺ phosphors,^21,22 the inhibited Cr³⁺–Cr³⁺ ET and induced Cr³⁺–Yb³⁺ ET are conducive to improving the NIR emission. The optimal Cr³⁺ dopant concentration in SGP is up to 80% with suppressed concentration quenching, and the emission intensity of Sr₉Cr(PO₄)₇:0.15Yb³⁺ at 375 K can be maintained at 100% of that at 80 K due to efficient Cr³⁺–Yb³⁺ ET. Considering that Fe³⁺ and Cr³⁺ have the same ionic valence and similar ionic radius, the SGP compound with structural confinement effect is an ideal host structure for Fe³⁺ occupation to achieve NIR emission. Furthermore, through guiding the ET from Fe³⁺ to the thermally stable center Yb³⁺, the luminescence thermal stability is expected to be improved.

In this work, based on SGP compound with unique structural characteristic, we investigate the effect of structural confinement on luminescence concentration and thermal quenching for enhancing NIR luminescence. In Fe³⁺-doped SGP phosphors, the distances between Fe³⁺ ions are relatively large, giving rise to suppressed Fe³⁺–Fe³⁺ ET and high dopant level of Fe³⁺ ions. Furthermore, when co-doping Yb³⁺ ions into Sr₉Ga_0.8(PO₄)₇:0.2Fe³⁺ (SGP:0.2Fe³⁺), the relatively short Fe³⁺–Yb³⁺ distances contribute to inducing the ET from the luminescence quenching center Fe³⁺ to the thermally stable center Yb³⁺. Compared with the SGP:0.2Fe³⁺ phosphor, the luminescence thermal stability of SGP:0.2Fe³⁺,0.07Yb³⁺ is significantly enhanced. Finally, the potential applications of SGP:0.2Fe³⁺ and SGP:0.2Fe³⁺,0.07Yb³⁺ phosphors in night vision and optical thermometry fields are explored.

2. Experimental section

2.1. Materials and synthesis procedures

Sr₉Ga_1−x(PO₄)₇:xFe³⁺ (SGP:xFe³⁺) and Sr_9−2yGa_0.8(PO₄)₇:0.2Fe³⁺,yYb³⁺,yNa⁺ (SGP:0.2Fe³⁺,yYb³⁺) phosphors were synthesized by the traditional high-temperature solid-state reaction. Upon doping Yb³⁺ ions, the equal molar weight of Na⁺ ions were added to balance the charge. According to the stoichiometric ratio, high purity (99.99%, Aladdin) raw materials of SrCO₃, Ga₂O₃, NH₄H₂PO₄, Fe₂O₃, Yb₂O₃ and Na₂CO₃ were weighed accurately. The compounds were ground for 30 min in an agate mortar to mix thoroughly. Afterwards, the mixtures were transferred into alumina crucibles and placed inside a box furnace to pre-sinter at 800 °C for 3 h at a heating rate of 2 °C min⁻¹. After cooling down to room temperature naturally, the mixtures were ground again and re-sintered at 1200 °C for 4 h under an air atmosphere inside a box furnace. Finally, the obtained samples were ground to fine powders for subsequent characterization.

2.2. Characterization

Powder X-ray diffraction (XRD) patterns were recorded on a diffractometer (SmartLab, Rigaku, Japan) with Cu Kα radiation (λ = 1.5406 Å) operating at 40 kV and 200 mA. The FullProf Suite package was used to refine the cell parameters.²³ A fluorescence spectrophotometer (Edinburgh Instruments, FLS-1000), equipped with a continuous 500 W xenon lamp and a microsecond flashlamp, was used to measure photoluminescence (PL) spectra, photoluminescence excitation (PLE) spectra and luminescence decay curves. The temperature-dependent PL spectra were recorded using the FLS-1000 instrument equipped with a variable-temperature liquid nitrogen optical cryostat Oxford OptistatDN2. An UV-visible-NIR spectrophotometer (Hitachi High-Tech Science Corporation, UH4150), with BaSO₄ as the standard, was used to measure diffuse reflection (DR) spectra. Scanning electron microscopy (SEM) images were recorded using a JEOL JSM-6510A microscope equipped with an energy dispersive X-ray analyzer. The IQE and external quantum efficiency (EQE) values were measured using an absolute PL quantum yield spectrometer (Quantaurus-QY Plus C13534-1, Hamamatsu Photonics).

2.3. LED device and thin film fabrication

The phosphor-converted light-emitting diode (pc-LED) devices were manufactured by coating phosphors on 365 nm UV chips (Sanan Optoelectronics). The phosphors and transparent silicone were mixed with a mass ratio of 1 : 2 and stirred for 15 min. After coating the glue on the chip, the pc-LED devices were solidified at 160 °C for 1 h. Then, an OHSP-350IRS handheld spectrometer was employed to measure the electroluminescence (EL) spectra of the fabricated pc-LED devices in a dark room under a driving voltage of 4.5 V and a current of 500 mA.

To prepare the thin film, the phosphor was dissolved in polydimethylsiloxane (PDMS) and a small amount of curing agent was added. The mixtures were stirred for 15 min to mix evenly. Then, a transparent mold with the characters “N I R” and a rectangular mold were placed on a polyethylene terephthalate (PET) sheet, and the mixtures were cast on the mold to form a smooth surface. After solidifying in an oven at 160 °C for 2 h, the thin film can be obtained by peeling it from the PET sheet.

3. Results and discussion

3.1. Crystal structure and XRD analysis

The β-Ca₃(PO₄)₂-type structural compound SGP belongs to the monoclinic crystal system with an I12/a1 space group.²⁴ Since SGP has similar crystal structure to Sr₉In(PO₄)₇, we used the crystallographic information of Sr₉In(PO₄)₇ (ICSD 59722) to clarify the structure of SGP. In the SGP structure, Sr²⁺ ions possess five different crystallographic positions marked as Sr1–Sr5 with the coordination number of eight or nine, while the Ga³⁺ ion is coordinated to six oxygen ions to form an octahedron.²⁴ Given the same ionic valence and the similar ionic radius of Fe³⁺ ions (r = 0.645 Å, CN = 6) and Ga³⁺ ions (r = 0.62 Å, CN = 6),²⁵ Fe³⁺ ions tend to occupy the sites of Ga³⁺ ions. The ionic radius of Sr²⁺ ions (r = 1.26 Å, CN = 8; r = 1.31 Å, CN = 9) is larger than that of Yb³⁺ ions (r = 0.985 Å, CN = 8; r = 1.042 Å, CN = 9),²⁵ meaning that Yb³⁺ ions are expected to substitute for Sr²⁺ ions. Fig. 1a shows the crystal structure viewed from the [001] direction, and it is apparent that [GaO₆] octahedra are well separated by Sr–O/P–O polyhedra and distributed in layers. Besides, as shown in Fig. 1b and c, [GaO₆] octahedra are positioned far away from each other (9.0173–18.3714 Å), while the distances between [GaO₆] and the nearest Sr1–Sr5 atoms are much shorter (3.7393–5.3357 Å). This special structural characteristic confers on the SGP structure the structural confinement effect, which has been discussed in SGP:Cr³⁺ and Sr₉Cr(PO₄)₇:Yb³⁺ phosphors.^21,22 In view of the structural confinement, the ET path can be controlled, and we speculate that the ET between Fe³⁺–Fe³⁺ ions in SGP can be restrained effectively while the Fe³⁺–Yb³⁺ ET can be strengthened. Therefore, it is expected to achieve suppressed luminescence concentration quenching in SGP:Fe³⁺ phosphor and suppressed luminescence thermal quenching in SGP:Fe³⁺,Yb³⁺ phosphor.

Fig. 1 (a) Crystal structure of SGP viewed from the [001] direction. (b) Distances between [GaO₆] octahedra. (c) Distances between Ga and the nearest Sr1–Sr5 atoms. (d) XRD patterns of SGP:xFe³⁺ (0 ≤ x ≤ 1). (e) Rietveld refinement of SGP:0.2Fe³⁺. (f) Rietveld refinement of Sr₉Fe(PO₄)₇.

Fig. 1d and S1† show the XRD patterns of SGP:xFe³⁺ (0 ≤ x ≤ 1), SGP:0.2Fe³⁺,yYb³⁺ (0.005 ≤ y ≤ 0.20) and SGP:0.1Yb³⁺ phosphors as well as the standard pattern of the SGP phase (PDF no. 53-0180). All the diffraction peaks can be well indexed to the standard pattern, demonstrating that all the samples are pure phases. Based on the crystallographic information of Sr₉In(PO₄)₇, the Rietveld refinement of SGP:0.2Fe³⁺ and Sr₉Fe(PO₄)₇ phosphors is conducted as presented in Fig. 1e and f. Table S1† gives the detailed crystallographic parameters, and the values with χ² = 2.54 and 3.01 ensure the reliability of the refinement. It can be seen that the cell parameters (a, b and c) and the unit cell volume V of Sr₉Fe(PO₄)₇ are a little larger than those of SGP:0.2Fe³⁺ due to the larger ionic radius of Fe³⁺ ions. Besides, Fig. S2a and b† present the SEM images and elemental mapping images of SGP:0.2Fe³⁺ and SGP:0.2Fe³⁺,0.07Yb³⁺ phosphors, indicating that the phosphors consist of several irregular particles with sizes of approximately 5–20 μm and all the elements (Sr, Ga, P, O, Fe, Yb, Na) are evenly distributed in the particles.

3.2. Suppressed luminescence concentration quenching in SGP:Fe³⁺

Fig. 2a displays the DR spectra of all the SGP:xFe³⁺ (0 ≤ x ≤ 1) phosphors. It can be seen that the absorption bands are mainly located in the UV region (270–400 nm), which can be attributed to the host absorption of SGP (300 nm) as well as the charge transfer (CT) transition between ligand O²⁻ ions and central metal Fe³⁺ ions (330 nm).²⁶ Compared with the Fe³⁺-free sample, in addition to the absorption band from O²⁻–Fe³⁺ CT transitions, there exist three other absorption bands peaking at about 425, 550 and 750 nm in Fe³⁺-doped phosphors originating from ⁶A₁(⁶S) → ⁴T₂(⁴D), ⁶A₁(⁶S) → ⁴T₂(⁴G) and ⁶A₁(⁶S) → ⁴T₁(⁴G) transitions of Fe³⁺ ions,²⁷ respectively. When enhancing the Fe³⁺ dopant concentration from 5% to 100%, the absorption intensity gradually increases. In particular, for samples with x = 0.8–1, the absorption intensity at around 400–650 nm significantly increases, but it has no effect on the luminescence performance. Fig. 2b and c exhibit the PLE and PL spectra of SGP:xFe³⁺ (0.05 ≤ x ≤ 1) phosphors. Monitoring at 915 nm, the excitation peaks in the range of 270–400 nm play a dominant role, while the peaks from spin- and parity-forbidden d–d transitions of Fe³⁺ ions are extremely weak. This indicates that the SGP:Fe³⁺ phosphor can be excited by 365 nm UV LED chips effectively. Besides, as shown in Fig. 2c, under the excitation of 330 nm, a broad NIR emission band ranging from 750 to 1200 nm centered at 915 nm with full-width at half-maximum (FWHM) value of 155 nm can be observed, which originates from the ⁴T₁(⁴G) → ⁶A₁(⁶S) transition of Fe³⁺ ions in an octahedral crystal field,¹⁶ while the emission intensity increases when enhancing the Fe³⁺ concentration from 5% to 20%, and then decreases. As shown in Fig. S3,† the emission intensity of 50% and 100% Fe³⁺-doped samples can be maintained at 73.46% and 18.25% that of the 20% Fe³⁺-doped phosphor, respectively. In general, the optimal Fe³⁺-dopant concentration in previously reported phosphors is relatively small, such as 0.3%, 1% and 2% in ZnGa₂O₄, LiAl₅O₈ and NaScSi₂O₆,^16,18,28 respectively. The suppressed luminescence concentration quenching in Fe³⁺-doped SGP phosphors is closely related to the unique structural characteristic of SGP. On introducing Fe³⁺ ions into the SGP host, [FeO₆] octahedra are positioned far away from each other, and the octahedra distributed in different layers are well separated by the Sr–O/P–O polyhedra. ET has a close association with the distance between luminescence centers,²⁹ and this special structural characteristic of the SGP can effectively inhibit the ET between Fe³⁺ ions. Therefore, it is expected to achieve high Fe³⁺ dopant concentration so as to increase the absorption intensity and thus enhancing the luminescence efficiency. The IQE/EQE values of the optimal sample SGP:0.2Fe³⁺ are 6.6%/5.3% (Fig. S4a†). A similar phenomenon has been observed in Cr³⁺-doped SGP phosphors, and the optimal Cr³⁺ doping concentration is up to 80%.²¹ All these phenomena are related to the structural confinement effect of the SGP compound. Both Cr³⁺- and Fe³⁺-doped SGP phosphors can achieve NIR luminescence and suppressed concentration quenching due to the presence of suitable octahedral sites and the structural confinement effect. However, introducing Cr³⁺ and Fe³⁺ ions into SGP has different effects on the luminescence performance. On the one hand, the excitation peak of SGP:Cr³⁺ matches well with the 450 nm blue LED chip while that of SGP:Fe³⁺ matches well with the 365 nm UV LED chip. On the other hand, the luminescence efficiency of the Cr³⁺-doped SGP is higher than that of the Fe³⁺-doped SGP, and thus the luminescence efficiency of the SGP:Fe³⁺ phosphor should be further improved in future works.

Fig. 2 (a) DR spectra of SGP:xFe³⁺ (0 ≤ x ≤ 1). (b) PLE and (c) PL spectra of SGP:xFe³⁺ (0.05 ≤ x ≤ 1). (d) Comparison of the PLE spectrum at 77 K and the DR spectrum of SGP:0.2Fe³⁺. (e) T–S energy-level diagram of the 3d⁵ electronic configuration in the octahedral field as well as the PLE and PL spectra of SGP:0.2Fe³⁺.

Generally, the lattice vibration can be greatly minimized at extremely low temperature. The PLE spectrum of SGP:0.2Fe³⁺ was recorded at 77 K to assign the energy levels accurately. As presented in Fig. 2d, an additional excitation peak at 383 nm appears when the PLE spectrum is recorded at 77 K, and this peak originates from the ⁶A₁(⁶S) → ⁴E(⁴D) transition of Fe³⁺ ions.²⁶ Fig. S5† shows the magnified PLE spectrum recorded at 77 K and the DR spectrum ranging from 450 to 800 nm of SGP:0.2Fe³⁺. It can be found that the absorption peaks match well with the excitation peaks. Similar to Cr³⁺ ions with the 3d³ electronic configuration, as can be seen from the Tanabe–Sugano (T–S) energy-level diagram in Fig. 2e,³⁰ the emission wavelength of Fe³⁺ is strongly influenced by the crystal field strength. The values of the octahedral crystal field parameter Dq, the Racah parameters B, C and the crystal field strength Dq/B of the SGP:Fe³⁺ phosphor can be estimated based on the following equations:^27,31

E(⁶A₁(⁶S) → ⁴T₂(⁴D)) = 13B + 5C (1)

E(⁶A₁(⁶S) → ⁴E₂(⁴D)) = 17B + 5C (2)

E(⁶A₁(⁶S) → ⁴T₁(⁴P)) = 18B + 7C (3)

E(⁶A₁(⁶S) → ⁴T₁(⁴G)) = 10D_q + 10B + 6C − (26B²/10D_q) (4)

E(⁶A₁(⁶S) → ⁴T₂(⁴G)) = 10D_q + 18B + 6C − (26B²/10D_q) (5)

Derived from the excitation peak positions of the PLE spectrum recorded at 77 K, the energy values corresponding to transitions ⁶A₁(⁶S) → ⁴E(⁴D), ⁶A₁(⁶S) → ⁴T₂(⁴D) and ⁶A₁(⁶S) → ⁴T₂(⁴G) can be determined as 26 109.66, 23 529.41 and 17 699.12 cm⁻¹ (383, 425 and 565 nm), respectively. According to eqn ((1), (2) and (5)), the values of the octahedral crystal field parameter Dq, and the Racah parameters B and C are calculated to be 1111.06, 645.06 and 3028.72 cm⁻¹, respectively. Thus, the crystal field strength Dq/B of the SGP:Fe³⁺ phosphor is determined to be 1.72. As indicated in Fig. 2e, the d–d transition from excited state ⁴T₁(⁴G) to ground state ⁶A₁(⁶S) gives rise to broadband NIR luminescence, and the energy difference between different excitation peaks matches well with that between different energy levels in the T–S diagram (Fig. 2e), further indicating the rationality for the attribution of excitation peaks. Besides, Fig. S6a† presents luminescence decay curves of SGP:xFe³⁺ (0.05 ≤ x ≤ 1) phosphors excited by 330 nm and monitored at 915 nm. As shown in Fig. S6b,† the lifetime values decrease from 636.99 to 171.59 μs when enhancing the Fe³⁺ concentration from 5% to 100% owing to the increased possibility of non-radiative ET.³² The lifetime values of Fe³⁺ ions in SGP are in line with those values of typical Fe³⁺-doped phosphors, such as NaScSi₂O₆:Fe³⁺.²⁸

To meet the requirements of practical applications, phosphors with excellent luminescence thermal stability are significantly important. Fig. S7a and b† show the temperature-dependent PL spectra at 80–500 K of the SGP:0.2Fe³⁺ phosphor as well as the dependence of PL intensity and PL peak wavelength on temperature. It can be seen that as the temperature increases from 80 to 500 K, the PL peak wavelength gradually shifts from 934 to 885 nm, accompanied by a decrease in the emission intensity. The blue shift of peak wavelength can be ascribed to the reduced crystal field strength. With increasing temperature, the lattice vibration intensifies, thus leading to lattice expansion.³³ Since the crystal field parameter Dq is inversely proportional to the 5th power of the metal–ligand distance,³⁴ lattice expansion will reduce the crystal field strength. As shown in the T–S diagram presented in Fig. 2e, the weaker the crystal field strength, the smaller the emission peak wavelength. On the other hand, the reduced emission intensity is ascribed to the luminescence thermal quenching related to non-radiative processes. Fig. S8† displays the configurational coordinate diagram to illustrate the thermal quenching behavior of the SGP:Fe³⁺ phosphor. The thermal activation energy is an important parameter that characterizes the luminescence thermal stability, which is defined as the energy difference between the lowest point of the ⁴T₁(⁴G) state and the intersection of ⁴T₁(⁴G) and ⁶A₁(⁶S) states. According to eqn (S1),†¹⁰ the plot of ln(I₀/I_T − 1) versus 1/kT for SGP:0.2Fe³⁺ is shown in Fig. S7c.† It is obvious that the data can be fitted to two lines with different slopes. The larger energy ΔE₂ (0.182 eV) in the high-temperature region may be related to the thermal ionization process, while the smaller energy ΔE₁ (0.093 eV) in the low-temperature region is the thermal activation energy for non-radiative relaxation.³⁵ The small value of ΔE₁ corresponds to the poor thermal stability of SGP:0.2Fe³⁺, and the T₅₀ value (temperature at which the luminescence intensity is half of its original intensity) of this phosphor is only 260 K (Fig. S7b†). Besides, the Huang–Rhys parameter S can be used to imply the strength of electron–phonon coupling. Based on the relationship between the FWHM value and temperature given in eqn (S2),†³⁶ the plot of FWHM versus T for SGP:0.2Fe³⁺ is depicted in Fig. S7d.† The FWHM values increase with rising temperature, and the fitted value of S is 2.594, demonstrating a strong electron–phonon coupling effect. Therefore, it is of great significance to take measures to enhance the luminescence thermal stability of the SGP:0.2Fe³⁺ phosphor.

3.3. Suppressed luminescence thermal quenching in SGP:0.2Fe³⁺, Yb³⁺

Efficient energy transfer from Fe³⁺ to Yb³⁺. Introducing rare earth ions Yb³⁺ as activators into Cr³⁺-doped phosphors has been proved to be an effective strategy to improve the luminescence property. Inspired by the Cr³⁺–Yb³⁺ ET method, it is expected to enhance the luminescence performance through Fe³⁺–Yb³⁺ ET by doping Yb³⁺ ions into the SGP:0.2Fe³⁺ phosphor. As displayed in Fig. 3a, there exists a large spectral overlap between the emission spectrum of SGP:0.2Fe³⁺ and the absorption spectrum of SGP:0.2Fe³⁺,0.15Yb³⁺ in the wavelength range of 860–1050 nm, indicating the probability of ET from Fe³⁺ to Yb³⁺ ions. The absorption band peaking at 965 nm of the Yb³⁺-doped sample originates from the ²F_7/2 → ²F_5/2 transition of Yb³⁺ ions.³⁷ Besides, the PL and PLE spectra can further indicate the occurrence of the ET process. As shown in Fig. 3b, under the excitation of 330 nm, all the Yb³⁺-doped phosphors display an extra emission band peaking at 978 nm with the wavelength range of 950–1100 nm, which is attributed to the ²F_5/2 → ²F_7/2 transition of Yb³⁺ ions.³⁷ Upon increasing the Yb³⁺ concentration, Fe³⁺ luminescence quenches rapidly, while Yb³⁺ luminescence significantly increases until y = 0.07, and then decreases due to the concentration quenching effect. The integrated PL intensity and peak intensity values at 978 nm of the optimal sample SGP:0.2Fe³⁺,0.07Yb³⁺ are 1.78 and 4.25 times higher than those of the Yb³⁺-free sample, and the IQE/EQE values of SGP:0.2Fe³⁺,0.07Yb³⁺ are enhanced to 11.8%/9.4% (Fig. S4b†). Compared with the Yb³⁺-free sample, the luminescence efficiency is significantly improved. In addition, Fig. S9† presents the PLE spectra of SGP:0.2Fe³⁺,yYb³⁺ (0 ≤ y ≤ 0.20) monitored at 915 and 978 nm. The PLE spectra monitoring at the Yb³⁺ emission wavelength presents the same excitation bands as those when monitoring at the Fe³⁺ emission wavelength, demonstrating the existence of ET from Fe³⁺ to Yb³⁺ ions. As depicted in Fig. S10a,† the excitation band of the Yb³⁺ single-doped SGP phosphor lies in the range of 270–350 nm peaking at around 300 nm, which does not match the 365 nm chip. In particular, through Fe³⁺ → Yb³⁺ ET, the emission intensity of the SGP:0.2Fe³⁺,Yb³⁺ phosphor can be significantly improved under 365 nm excitation (Fig. S10b†). Besides, as shown in Fig. 3d, with increasing Yb³⁺ dopant concentration, the PLE intensity at 330 nm reduces monotonously when being monitored at 915 nm while the PLE intensity is first enhanced up until y = 0.07 when being monitored at 978 nm, providing further evidence for the ET from Fe³⁺ to Yb³⁺.

Fig. 3 (a) PL spectrum of SGP:0.2Fe³⁺ and DR spectrum of SGP:0.2Fe³⁺,0.15Yb³⁺. (b) PL spectra of SGP:0.2Fe³⁺,yYb³⁺ (0 ≤ y ≤ 0.20). (c) Dependence of the integrated PL intensity and peak intensity at 978 nm on Yb³⁺ concentration y. (d) Relationship between the PLE intensity at 330 nm and Yb³⁺ concentration y monitored at 915 and 978 nm, respectively. (e) Luminescence decay curves of SGP:0.2Fe³⁺,yYb³⁺ excited by 330 nm and monitored at 915 nm. (f) Dependence of the ET efficiency on Yb³⁺ concentration y.

To better understand the mechanism of ET between Fe³⁺ and Yb³⁺ ions, the ET efficiency of SGP:0.2Fe³⁺,yYb³⁺ (0 < y ≤ 0.20) is calculated on the basis of luminescence decay curves. Fig. 3e shows the luminescence decay curves of SGP:0.2Fe³⁺,yYb³⁺ (0 ≤ y ≤ 0.20) excited by 330 nm and monitored at 915 nm. It is obvious that the lifetime values are attenuated with rising Yb³⁺ concentration, the lifetimes for the samples with y = 0, 0.01, 0.07, 0.10, 0.15 and 0.20 are fitted to be 599.79, 494.40, 368.25, 205.48, 179.23 and 112.76 μs, respectively. According to the calculation method for ET efficiency, given in eqn (S3),†³⁸ the ET efficiency of SGP:0.2Fe³⁺,0.2Yb³⁺ is up to 81.2% (Fig. 3f), indicating the efficient ET from Fe³⁺ to Yb³⁺ ions. The efficient ET from Fe³⁺ to Yb³⁺ can be attributed to the structural confinement effect of the SGP structure. ET is closely related to the distance between luminescence centers. As displayed in Fig. 1b and c, upon introducing Fe³⁺ and Yb³⁺ ions into the SGP host, Fe³⁺ substituting for Ga³⁺ and Yb³⁺ replacing Sr²⁺ will form a tight Fe³⁺–Yb³⁺ pair with small separation, which is conducive to the efficient Fe³⁺–Yb³⁺ ET. The efficient ET from Cr³⁺ to Yb³⁺ caused by the structural confinement effect has been verified in Sr₉Cr(PO₄)₇:Yb³⁺ with the same structure.²² Therefore, it is significant to choose host compounds with structural confinement to control the ET path so as to improve the luminescence performance.

Enhanced luminescence thermal stability in SGP:0.2Fe³⁺,Yb³⁺. Fig. 4a presents temperature-dependent PL spectra at 80–500 K of the SGP:0.2Fe³⁺,0.07Yb³⁺ phosphor, and the PL spectrum recorded at 77 K is displayed in Fig. 4b. Besides, as shown in Fig. S11,† the PLE spectra of SGP:0.2Fe³⁺,0.07Yb³⁺ monitored at 915 and 978 nm while being recorded at 77 K present the same shapes, demonstrating the existence of Fe³⁺–Yb³⁺ ET. Because of the efficient ET from Fe³⁺ to Yb³⁺, it can be found that the emission band is composed of several sharp-line emission peaks ranging from 950 to 1100 nm, which originate from the ²F_5/2 → ²F_7/2 transition of Yb³⁺ ions.³⁷ Apparently, the sharp-line emission peaks become more separated at low temperature due to the suppressed vibrational coupling effect. The energy-level diagram of Yb³⁺ in SGP:0.2Fe³⁺ is constructed roughly as shown in Fig. 4c. There are only two multiplet manifolds in the energy-level diagram of Yb³⁺, with the ground state ²F_7/2 splitting into three Stark levels and the excited state ²F_5/2 splitting into four.³⁹ These seven Stark levels are labeled 1 to 7 as the increments of energy. The rich Stark energy levels contribute to the luminescence with multiple phonon side bands. The emission at 971 nm can be determined as the zero-phonon line, corresponding to the transition between the lowest energy level of ²F_5/2 and ²F_7/2 (5 → 1).⁴⁰ Thus, the luminescence at around 964, 992, 1002, 1011, 1022 and 1068 nm can be attributed to the transition from different ²F_5/2 sub-levels to ²F_7/2 sub-levels as denoted in Fig. 4c.

Fig. 4 (a) Temperature-dependent PL spectra at 80–500 K of SGP:0.2Fe³⁺,0.07Yb³⁺. (b) PL spectrum of SGP:0.2Fe³⁺,0.07Yb³⁺ recorded at 77 K. (c) Energy-level diagram of Yb³⁺. (d) Relationship between the integrated emission intensity and the temperature of SGP:0.2Fe³⁺ and SGP:0.2Fe³⁺,0.07Yb³⁺.

In addition, as reported in many Cr³⁺ and Yb³⁺ co-doped phosphors, such as LiScP₂O₇:Cr³⁺,Yb³⁺, Ca₂LuZr₂Al₃O₁₂:Cr³⁺,Yb³⁺, Gd₃Sc_1.5Al_0.5Ga₃O₁₂:Cr³⁺,Yb³⁺, etc., thermal quenching can be effectively suppressed compared to the Yb³⁺-free sample.^11,41,42 Analogously, as shown in Fig. 4d, co-doping Yb³⁺ to the SGP:0.2Fe³⁺ phosphor can significantly enhance the luminescence thermal stability. It is obvious that the T₅₀ values are greatly raised from 260 K for SGP:0.2Fe³⁺ to 365 K for SGP:0.2Fe³⁺,0.07Yb³⁺. The improved thermal stability is ascribed to the efficient ET from Fe³⁺ to Yb³⁺ ions. Similar to transition metal Cr³⁺ ions, the d–d transition of Fe³⁺ ions with the 3d⁵ configuration is strongly coupled to the host lattice due to the bare 3d electron, and thus the non-radiative transition is strong, causing the poor luminescence thermal stability of the SGP:0.2Fe³⁺ phosphor. In contrast, the f–f transition of Yb³⁺ ions is hardly influenced by the host lattice because its 4f orbitals are shielded by the outer 5s and 5p orbitals, making the Yb³⁺ ion a thermally stable luminescence center. In the SGP compound with structural confinement, the Fe³⁺–Fe³⁺ distances are relatively large and the Fe³⁺–Yb³⁺ distances are extremely short, which contribute to the inhibited Fe³⁺–Fe³⁺ ET and the induced Fe³⁺–Yb³⁺ ET. Therefore, more energy is migrated from the luminescence quenching center Fe³⁺ to the thermally stable center Yb³⁺ so as to suppress the luminescence thermal quenching of the SGP:0.2Fe³⁺,0.07Yb³⁺ phosphor.

3.4. Potential applications in night vision and optical thermometry fields

Night vision. To demonstrate the potential applications of SGP:0.2Fe³⁺ and SGP:0.2Fe³⁺,0.07Yb³⁺ phosphors, two NIR pc-LED devices were fabricated by coating the abovementioned NIR phosphors on 365 nm UV chips. Fig. 5a and b present the EL spectra of the fabricated NIR pc-LED devices as well as photographs of the packaged lamps with the light off and on. The spectrum of the pc-LED device manufactured with SGP:0.2Fe³⁺ exhibits a broad NIR emission band over the range of 750–1100 nm, which is consistent with the PL spectrum shown in Fig. 2c. The spectral dip at about 910 nm in Fig. 5a originates from the instrumental error in this wavelength region. Then, we explored the applications of this NIR pc-LED device in the night vision field. As shown in Fig. 5c, when irradiated by NIR light from the fabricated NIR pc-LED device, the pattern on the box and the powder in the bottle are clearly visible in the dark with the aid of a NIR camera. In particular, the different colors in the pattern can be distinguished and displayed with shadows of different degrees, demonstrating promising application in distinguishing color. Besides, as shown on the left in Fig. 5d, a thin film with three characters “N I R” was made by mixing the SGP:0.2Fe³⁺ phosphor with PDMS. Under the irradiation of 365 nm light, the characters “N I R” can be observed in the dark (right-hand image in Fig. 5d). These results indicate that NIR pc-LED devices prepared from the SGP:0.2Fe³⁺ phosphor and 365 nm chip have application prospects in the field of night vision.

Fig. 5 EL spectra of fabricated NIR pc-LED devices with (a) SGP:0.2Fe³⁺ and (b) SGP:0.2Fe³⁺,0.07Yb³⁺ coated on 365 nm UV chips as well as photographs of the packaged lamps with the light off and on. (c) Photographs of the box and bottle captured under natural light using a visible camera (left) and under NIR light using a NIR camera (right). (d) Photographs of thin film made using the SGP:0.2Fe³⁺ phosphor and glue captured under natural light using a visible camera (left) and in the dark using a NIR camera under the irradiation of 365 nm light (right).

Optical thermometry. In addition, we investigated potential applications of the SGP:0.2Fe³⁺,0.07Yb³⁺ phosphor in the field of optical thermometry. Optical thermometry based on fluorescence intensity ratio (FIR) technology has great advantages of high detection precision and resolution, wide detection temperature range as well as tunable size, making it a promising candidate for non-contact thermometry.⁴³ As illustrated in Fig. 4a, the SGP:0.2Fe³⁺,0.07Yb³⁺ phosphor exhibits several distinct and separated emission peaks due to the rich energy-level structure of Yb³⁺ ions. With increasing temperature from 80 to 500 K, the positions of these emission peaks are almost unchanged while the emission intensity of different peaks display different variation trends, providing the possibility that this phosphor could be used in FIR-based optical thermometry. The FIR of anti-Stokes vibronic emission at 964 nm and zero-phonon line emission at 971 nm can be fitted well according to the following equation:⁴³where B and ΔE are constants, T is the temperature, and k represents the Boltzmann constant (8.617 × 10⁻⁵ eV K⁻¹). As shown in Fig. 6a, the experimental data can be well fitted in the temperature range of 170 to 380 K. The fitted function is ln(I₉₆₄/I₉₇₁) = −327.48/T + 0.5829, and thus the constants ΔE/k and B are 327.48 K and 1.79, respectively. Besides, the absolute sensitivity (S_a) and the relative sensitivity (S_r) are two parameters used to evaluate the applicability of optical thermometry quantitatively, both of which can be calculated according to the following equations:^44,45

Fig. 6 (a) Plot of I₉₆₄/I₉₇₁versus 1000/T. (b) The absolute sensitivity S_a and the relative sensitivity S_r values in the temperature range of 170–380 K.

According to the data fitted using eqn (6), the values of S_a and S_r are calculated and presented in Fig. 6b. It can be seen that S_a and S_r decrease when increasing the temperature from 170 to 380 K, and the maximum of S_r is up to 1.13% K⁻¹ at 170 K, indicating that the SGP:0.2Fe³⁺,0.07Yb³⁺ phosphor has the potential to be utilized as a low-temperature sensor.

4. Conclusions

In this work, a series of SGP:Fe³⁺ and SGP:0.2Fe³⁺,Yb³⁺ NIR phosphors were synthesized via the traditional high-temperature solid-state reaction. The effect of structural confinement on luminescence concentration and thermal quenching is discussed in detail. Upon doping Fe³⁺ ions into SGP, a broad NIR emission band peaking at 915 nm can be observed. The relatively large Fe³⁺–Fe³⁺ distances lead to weak luminescence concentration quenching. The optimal Fe³⁺ dopant concentration is up to 20%, and the emission intensity of Sr₉Fe(PO₄)₇ is maintained at 18.25% that of SGP:0.2Fe³⁺. Furthermore, by introducing Yb³⁺ ions into SGP:0.2Fe³⁺, the tight Fe³⁺–Yb³⁺ pair is conducive to strengthening the ET from the luminescence quenching center Fe³⁺ to the thermally stable center Yb³⁺, thus suppressing the luminescence thermal quenching. Finally, we fabricated two NIR pc-LED devices by combining SGP:0.2Fe³⁺ and SGP:0.2Fe³⁺,0.07Yb³⁺ on 365 nm UV chips, and demonstrated that the two phosphors are promising candidates for applications in the night vision field. Besides, the application of SGP:0.2Fe³⁺,0.07Yb³⁺ in the field of optical thermometry has been explored, indicating the potential application of the SGP:0.2Fe³⁺,0.07Yb³⁺ phosphor being utilized in low-temperature sensors.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (no. 12274023 and 51972020). We express our special thanks to Prof. Zhiguo Xia from South China University of Technology for the IQE/EQE measurements.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3qi01649h

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