A post-reduction strategy to enhance near-infrared-II emission from Li₄SrCa(SiO₄)₂:Cr⁴⁺ phosphors†

Yixin Sun, Yining Wang, Minliang Deng, Xiaole Xing, Yiying Zhu and Mengmeng Shang*
Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials Ministry of Education, School of Material Science and Engineering, Shandong University, 17923 Jingshi Road, Jinan 250061, P. R. China. E-mail: mmshang@sdu.edu.cn

First published on 2nd August 2024

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1. Introduction

NIR (700–1700 nm) light finds numerous applications due to its robust resistance to interference from visible light, enabling broad identification of the absorption spectrum of specific functional groups, and deeper penetration into biological tissues.^1–9 Broadly, NIR light is categorized into two distinct windows: NIR-I (700–1000 nm) and NIR-II (1000–1700 nm), each with distinct applications. Compared with NIR-I, NIR-II is more suitable for information storage, biological imaging and NIR spectroscopy due to less light scattering and absorption, and not being affected by natural light.

Thereinto, lanthanide ions (Yb³⁺, Nd³⁺, Er³⁺, and Ho³⁺)^10–15 and transition metal ions (Ni²⁺ and Cr⁴⁺)^16–18 are the main doping ions that can allow NIR-II luminescence. However, the absorption and emission peaks of lanthanide ions with f–f transitions are narrow and inefficient, which is not suitable for NIR spectroscopy. The luminescence of Ni²⁺ ions is mostly located at 1100–1700 nm, but it is poorly matched with blue LED chips and often requires Cr³⁺ ions as sensitizers for energy transfer. Notably, the Cr⁴⁺ ion matches well with the blue LED chip, and its emission band is located at 1000–1700 nm (the emission peak usually exceeds 1200 nm), which seems to be the most suitable NIR-II region luminescence center.

The Cr⁴⁺ ions with 3d² electronic configuration usually occupy the tetrahedral positions, but Cr³⁺ ions are inclined to occupy octahedron sites. Consequently, meticulous selection of the host material is imperative to obtain Cr⁴⁺-activated NIR phosphors. Recently, Cr⁴⁺-activated NIR phosphors have been widely studied, such as Li₂ZnGeO₄:Cr⁴⁺, Li₂CaGeO₄:Cr⁴⁺ and Ca₂Ga₂SiO₇:Cr⁴⁺.^19–21 Typically, Li₂ZnGeO₄ has only tetrahedral sites, and is devoid of octahedral sites, yielding an emission peak at 1218 nm in the NIR-II region when Cr⁴⁺ is doped into the Li₂ZnGeO₄ lattice.¹⁹ Xia's research group achieved an ultra-long wavelength NIR emission with a luminescence center at 1330 nm in CaYGaO₄:Cr⁴⁺, exhibiting a FWHM of 233 nm.²² Although [YO₆] and [CaO₆] octahedrons are present in the CaYGaO₄ crystal structure, their ionic radii significantly differ from those of Cr³⁺/Cr⁴⁺, leading to the absence of [CrO₆] octahedrons and Cr ions occupying the [GaO₄] site in the +4 valence state. Therefore, the choice of matrix is crucial to achieve effective luminescence of Cr⁴⁺.

Li₄SrCa(SiO₄)₂ has been developed as an excellent luminescence material host for Eu²⁺ and Ce³⁺ doping,^23–25 but there is no report on the doping of Cr⁴⁺ ions. In the Li₄SrCa(SiO₄)₂ structure, the Li⁺ and Si⁴⁺ ions form a tetrahedron, Sr²⁺ ions form a polyhedron with a coordination number of 10, and Ca²⁺ ions form an octahedron. Owing to the significant disparity in charge and size between Ca²⁺ and Cr³⁺, [CrO₆] is difficult to form in the Li₄SrCa(SiO₄)₂ structure. The presence of [SiO₄] enables the formation of [CrO₄], where Cr is present in the +4 valence state. Based on the above considerations, the Li₄SrCa(SiO₄)₂ compound is chosen as the matrix compound, and a series of Li₄SrCa(SiO₄)₂:Cr⁴⁺ phosphors have been successfully prepared. At a 465 nm excitation wavelength, the Li₄SrCa(SiO₄)₂:Cr⁴⁺ emission peak is located at 1215 nm, and the FWHM is more than 230 nm, covering the ultra-wide range of 950–1600 nm. An effective NIR-II luminescence is realized. Additionally, a novel post-reduction strategy is proposed to enhance NIR-II luminescence of Cr⁴⁺. Through the complete substitution between Ca²⁺ and Sr²⁺, the universality of the post-reduction strategy is verified, and the corresponding applicable conditions are concluded, which is of guiding significance. Thanks to the ultra-long wavelength emission of Li₄SrCa(SiO₄)₂:Cr⁴⁺, this material has been verified to have great application potential in optical temperature measurement and spectral analysis.

2. Experimental section

2.1. Materials and synthesis

Powder samples with compositions of Li₄SrCa(SiO₄)₂:xCr⁴⁺ (x = 0.5–11%) and Li₄Sr_1+zCa_1−z(SiO₄)₂:3%Cr⁴⁺ (z = −1 to 1) were synthesized by a high-temperature solid state reaction. The raw materials Li₂CO₃ (98%, Sinopharm Chemical Reagent Co.), SrCO₃ (99%, Sinopharm Chemical Reagent Co), CaCO₃ (99.99%, Sinopharm Chemical Reagent Co), SiO₂ (99%, Sinopharm Chemical Reagent Co.), and Cr₂O₃ (99.95%, Aladdin) were weighed according to the stoichiometric amounts and then ground in an agate mortar for 30 min. Subsequently, the powder mixture was transferred into an alumina crucible and sintered at 920 °C for 4 h in air. Finally, the as-prepared phosphors were cooled down to RT and ground into fine powders for subsequent testing. The preparation steps of H-Li₄SrCa(SiO₄)₂:yCr⁴⁺ (y = 0.5–11%) and H-Li₄Sr_1+zCa_1−z(SiO₄)₂:3%Cr⁴⁺ (z = −1 to 1) except for secondary sintering are the same as above. The second sintering was carried out at 920 °C for 4 h in 10%H₂/90%N₂.

2.2. Characterization

The composition and phase purity of products were studied by powder X-ray diffraction (XRD) measurements using a D8 focus diffractometer (Bruker) with Cu Kα radiation (λ = 0.15405 nm). The general structure analysis system program was used to conduct the structure refinements. The morphology and elemental composition were obtained using a field emission scanning electron microscope (FESEM, S4800, Hitachi) equipped with an energy-dispersive spectrometer (EDS). The DR spectra were recorded using an UV-vis-NIR spectrophotometer equipped with an integrating sphere (Cary 5000). The PL, PLE spectra, decay curves, TRS, and temperature-dependent PL spectra (77–425 K) were recorded using an Edinburgh fluorescence spectrometer (FLS1000) equipped with a continuous 450 W xenon lamp as the steady-state excitation source, a pulsed high energy xenon flash lamp (μF2) as the transient excitation source, and a heating stage (TAP-02) as the temperature-controlled stage. The quantum efficiency for NIR emission of the as-prepared phosphors was recorded using a Vis-NIR absolute quantum efficiency test system (Hamamatsu C9920-02) equipped with a 3.3-inch integrating sphere. Raman spectra were recorded using a Raman spectrometer (Renishaw in Via, UK) equipped with a solid-state laser (532 nm). X-Ray photoelectron spectra (XPS) were recorded on a Thermo Scientific K-Alpha with Al Ka as the X-ray source.

3. Results and discussion

3.1. Phase identification and photoluminescence properties of Li₄SrCa(SiO₄)₂:Cr⁴⁺ prepared in an air atmosphere

Fig. 1a illustrates the crystal structure of Li₄SrCa(SiO₄)₂ (labeled as LSCS), which originates from the aluminum carbonitride structure and is classified under the Pbcm space group.^26,27 Li and Si atoms each bond with four oxygen atoms to create [LiO₄] and [SiO₄] tetrahedrons. While Li atoms are linked together to form a strong framework, Si atoms are isolated within the crystal lattice. The coordination of Ca atoms with six oxygen atoms results in the formation of a [CaO₆] octahedron, whereas the Sr atoms form an irregular [SrO₁₀] polyhedron. Within LSCS, the octahedral site consists of [CaO₆] octahedra, resulting in significant charge and ion radius disparities between Ca²⁺ (CN = 6, r = 1.0 Å) and Cr³⁺ (CN = 6, r = 0.615 Å). Hence, it is difficult to substitute the [CrO₆] octahedrons (in the form of Cr³⁺) for [CaO₆] in the LSCS lattice. Therefore, Cr ions tend to occupy the [SiO₄] tetrahedrons with the +4 valence state (Cr⁴⁺, CN = 4, r = 0.41 Å; Si⁴⁺, CN = 4, r = 0.26 Å) in a weakly oxidizing environment.²⁰

Fig. 1b presents the X-ray diffraction (XRD) patterns of LSCS:xCr⁴⁺ (x = 0, 1%, and 5%) samples prepared in an air atmosphere. All diffraction peaks match well with standard Li₄SrCa(SiO₄)₂ (PDF # 83-0763) data. This confirms the formation of the pure LSCS phase. The XRD Rietveld refinements of LSCS:0.1%Cr⁴⁺ and LSCS:3%Cr⁴⁺ are carried out, and the refinement results are drawn in Fig. 1c and d. The residual factor values are provided in Table S1 (ESI†). The favorable fitting data not only demonstrate the reliability of the refined structure model but also confirm the high purity of the phase. The lattice parameters are calculated to be V = 693.13 ų for LSCS:0.1%Cr⁴⁺ and V = 693.22 ų for LSCS:3%Cr⁴⁺. The increased unit cell volume further indicates that Cr⁴⁺ should occupy the site of Si⁴⁺ and no existence of Cr³⁺. This is because there are no octahedral sites suitable for Cr³⁺ ions to occupy in the LSCS lattice. Even if Cr³⁺ enters the only [CaO₆] octahedron, it would cause a reduction in the unit cell volume.

Energy-dispersive spectrometry (EDS) mapping images of LSCS:3%Cr⁴⁺ are provided in Fig. S1 (ESI†). The images reveal uniform distribution of Sr, Ca, Si, O and Cr elements throughout the sample, whereas the presence of the Li element remains undetectable due to its low energy.

Fig. 2a is the photoluminescence excitation (PLE) and photoluminescence (PL) spectra of the LSCS:3%Cr⁴⁺ sample prepared in an air atmosphere. Under excitation wavelengths of 465 and 645 nm, the sample emits NIR-II broadband emission from 950 to 1600 nm, with a peak at approximately 1215 nm, which is the characteristic of the ³T₂ (³F) → ³A₂ spin-allowed transition of Cr⁴⁺ ions. Additionally, a continuous PLE spectrum consisting of two bands peaking at 465 nm and 645 nm is observed when monitoring the emission at 1215 nm. These bands correspond to the ³A₂ → ³T₁ (³P) and ³A₂ → ³T₁ (³F) energy level transitions of Cr⁴⁺ ions, respectively.

Fig. 2b illustrates the PL spectra of LSCS:xCr⁴⁺ (x = 0.5–9%) samples. The FWHMs of the PL spectra for all samples are around 235 nm. When excited at a wavelength of 465 nm, both the emission peaks (1215 nm) and spectral shapes remain unchanged with increasing doping amounts. The luminescence intensity of LSCS:xCr⁴⁺ samples initially rises with increasing doping concentration, peaking at x = 3% before decreasing due to concentration quenching, as illustrated in Fig. 2c.

Based on the crystal field theory, the Cr⁴⁺ ion possesses a 3d² electronic configuration,²⁸ and its Tanabe–Sugano (T–S) energy level diagram resembles that of 3d⁸ in an octahedral environment. Therefore, the energy level splitting of LSCS:xCr⁴⁺ is delineated by the T–S diagram of the 3d⁸ configuration within the octahedral field, as depicted in Fig. 2d. The crystal field intensity is calculated from the ratio of the crystal field's splitting energy (D_q) to the Racah parameter (B), as shown in the following equation:^19,29,30

10Dq = E(ZPL)	(1)

	(2)

In the formula, x is as follows:

	(3)

Herein, the zero phonon line (ZPL) is approximately 1000 nm, corresponding to the starting wavelength of the higher energy band in the PL spectra.¹⁹ The transition energies E(³A₂ → ³T₁ (³P)) and E(³A₂ → ³T₁ (³F)) are derived from the PLE spectra. The calculated D_q/B from the excitation spectra is ∼1.41, which is to the left of the intersection of the ³T₂ and ¹E₁ levels, as depicted in Fig. 2d. Therefore, the ³T₂ level functions as the lowest excited state, leading to the broadband emission from the ³T₂ (³F) → ³A₂ transition.

Fig. S2 (ESI†) depicts the luminescence decay curves measured for the LSCS:xCr⁴⁺ (x = 1–11%) samples. These curves can be analyzed by using the single exponential decay model, as follows:³¹

It = I0 + Ae(−t/τ)	(4)

where I₀ and I_t denote the PL intensity at the beginning and at time t, while A stands for constant parameters, and τ represents the decay time in the single-exponential fitting. The fitted lifetimes are shown in Fig. S2 (ESI†), and the lifetimes of LSCS:Cr⁴⁺ fall within the microsecond range. With an increase in the Cr⁴⁺ concentration, the lifetime of the LSCS:xCr⁴⁺ decreases from 9.14 to 8.29 μs. And the good single-exponential property suggests the existence of only one luminescent center in the LSCS:xCr⁴⁺ phosphor. To obtain insight into the nature of ion–ion interaction in the lattice, the critical distance R_c = 22.26 Å is calculated (eqn (S1), (ESI†)), indicating that the non-radiative energy transfer mechanism is multipolar interaction. Fig. S3 (ESI†) depicts the linear fitting of log(I/x) to log(x), yielding a slope of −1.10 and θ as 3.30 (eqn (S2), (ESI†)). This indicates that the energy transfer between neighboring ions serves as the main concentration quenching mechanism of LSCS:xCr⁴⁺, since θ is close to 3.

Fig. 2e shows the PL 3D-mapping spectra of the LSCS:3%Cr⁴⁺ sample. Intuitively, in the PL 3D-mapping spectra, there is only one broadband emission at 1215 nm. And the same decay rate of the time-resolved emission (TRS) spectra (Fig. 2f) further indicates that there is only a single Cr⁴⁺ ion luminescence center in LSCS:3%Cr⁴⁺. It is worth mentioning that increasing the Cr⁴⁺ doping concentration enhances the NIR-II luminescence intensity to some extent, but the luminescence intensity of LSCS:3%Cr⁴⁺ is still low. Moreover, the internal quantum efficiency (IQE) generally diminishes with the red shift of λ_max, presenting a significant challenge for NIR phosphors. Therefore, the IQE of the LSCS:3%Cr⁴⁺ sample is poor, only 2.2% (Fig. S4 (ESI†)).

3.2. Post-reduction strategy to improve the PL properties

The LSCS:Cr⁴⁺ samples exhibit weak luminescence, severely limiting their utility in spectral analysis applications. Previous studies indicate that due to the same ionic radius of Cr⁶⁺ (CN = 4, r = 0.26 Å) and Si⁴⁺, a significant fraction of Cr ions will inevitably be oxidized to the +6 valence state during air sintering, thereby resulting in an adverse effect on Cr⁴⁺ luminescence.³²

The diffuse reflectance (DR) spectra test is conducted to confirm the existence of Cr⁶⁺. Fig. S5a (ESI†) illustrates the DR spectra of LSCS:xCr⁴⁺ (x = 0, 0.5% and 3%) samples. Besides the absorption peak of Cr⁴⁺ in the 420–1000 nm range, additional absorption peaks at 280 nm and 375 nm are also observed, which are consistent with the absorption peak of Cr⁶⁺ reported in the literature.^20,33 The Cr⁶⁺ has ground state of (3p⁶ 3d⁰) which transforms into Cr⁵⁺ (3p⁶ 3d¹) by promoting an electron from 2p⁶ of O²⁻ to 3d⁰ of Cr⁶⁺ through charge transfer [Cr⁶⁺ O²⁻ (3d⁰ 2p⁶) → Cr⁵⁺ O⁻ (3d¹ 2p⁵)].^34–37 Therefore, the transitions at 280 nm and 375 nm in DR spectra are resulting from the splitting of this charge transfer state in the crystal field. At the same time, the optical band gap (E_g) calculated according to the Tauc and Kubelka–Munk function formulas (eqn (S3) and (S4), (ESI†)) is shown in Fig. S5b (ESI†). As the Cr ion concentration increases, E_g decreases from 5.7 eV to 4.5 eV, indicating the successful doping of Cr. Subsequently, the PLE spectrum of the extended scan range is then re-examined and shows an additional excitation peak at 375 nm (Fig. S5c (ESI†)), consistent with the results of DR spectra. Moreover, Fig. S6 (ESI†) presents X-ray photoelectron spectroscopy (XPS) test results of the LSCS:3%Cr⁴⁺ sample, detecting signal peaks for Li 1s, Si 2p, Sr 3d, Ca 2p, O 1s, and Cr 2p. In the high-resolution Cr 2p XPS spectrum, four signal peaks at 574.2 eV, 585.7 eV, 578.9 eV and 590.4 eV are observed, corresponding to the 2p_3/2 and 2p_1/2 states of Cr⁴⁺ and Cr⁶⁺, respectively.^33,38–40 Therefore, all these results prove the coexistence of Cr⁴⁺ and Cr⁶⁺ in the sample.

During the initial air sintering process, Cr ions are allowed to be incorporated into the lattice in a hexavalent (+6) state. If a reducing atmosphere is directly adopted during the initial sintering process, it is easy to reduce Cr⁶⁺ ions to Cr³⁺ and even to Cr²⁺ with lower valence states, and thus it is difficult to obtain Cr⁴⁺-activated NIR-II phosphors. So, a post-reduction strategy is used to optimize the luminescence performance of LSCS:Cr⁴⁺. That is, after initial air sintering, a H₂ reduction atmosphere is introduced during the secondary sintering process to reduce the Cr⁶⁺ ions. Fig. 3a illustrates that the luminous intensity of the sample obtained through the post-reduction strategy is significantly improved, approximately 8 times higher than that of the sample sintered in air, whereas the luminous intensity of the sample directly sintering in a reducing (H₂) atmosphere during the initial sintering process is the lowest. This is because Cr⁶⁺ ions are easily reduced to Cr³⁺ or even lower valence Cr²⁺ when sintered only in a reducing (H₂) atmosphere. However, Cr³⁺ ions lack appropriate lattice sites for occupation, thereby hindering the incorporation of Cr³⁺ ions into the lattice. The post-reduction strategy overcomes this issue by initially sintering in an air atmosphere. The Cr⁶⁺ ions are allowed to enter the radius-matched Si⁴⁺ site. This process is often accompanied by the creation of cation vacancies or interstitial oxygen defects. Subsequently, secondary sintering in a reducing H₂ atmosphere reduces the Cr⁶⁺ ions at the Si⁴⁺ sites, resulting in efficient Cr⁴⁺ luminescence. Simultaneously, the reduction of cation vacancies and interstitial oxygen defects reduces non-radiative transitions, thereby enhancing the luminescence intensity. In addition, the emission spectra sintered under different atmospheres have no obvious changes except the intensity, indicating that the properties of the luminescent center remain unchanged, displaying Cr⁴⁺ luminescence.

The PL spectra of post-reduction-Li₄SrCa(SiO₄)₂ (labeled as LSCSH):yCr⁴⁺ (y = 1–9%) samples are shown in Fig. S7 (ESI†). The emission peaks are still located at 1215 nm, and the luminescence intensity is improved. Fig. S8 (ESI†) presents the XRD patterns of LSCSH:yCr⁴⁺ (y = 0, 1%, and 5%) samples. All diffraction peaks match well with standard Li₄SrCa(SiO₄)₂ (PDF # 83-0763) data. This suggests that the LSCSH:Cr⁴⁺ prepared using the post-reduction strategy maintains a pure phase. Fig. S9a and b (ESI†) show the scanning electron microscopy (SEM) images of LSCS:3%Cr⁴⁺ and LSCSH:3%Cr⁴⁺, respectively. The results show that after the post-reduction strategy, the surface and particle size of the samples remain basically unchanged, indicating that secondary sintering has little effect on the morphology. Furthermore, to eliminate the influence of secondary sintering on luminescence enhancement, three kinds of luminescent materials are prepared at the same sintering temperature, time and sintering times, and only the sintering atmosphere (air + air, air + H₂, air + N₂) is changed. The emission spectra are shown in Fig. S10 (ESI†). It can be seen that the samples obtained through the post-reduction strategy still exhibit the highest luminescence intensity, followed by those sintered under air + N₂ conditions, and the weakest under air + air conditions. Therefore, the enhancement in luminescence intensity achieved by the post-reduction strategy is not due to an increase in crystallinity.

As discussed earlier, the DR spectra (blue curve in Fig. 3b) of the LSCS:0.5%Cr⁴⁺ sample reveal distinct absorption peaks attributed to Cr⁶⁺ at 280 nm and 375 nm.^20,33,41 After applying the post-reduction strategy, there is a notable increase in the absorption peaks (red curve in Fig. 3b) that correspond to the Cr⁴⁺ ion. And the absorption peaks of Cr⁶⁺ are weakened. This suggests that the post-reduction strategy is highly effective at converting Cr⁶⁺ to Cr⁴⁺. Meanwhile, the optical band gap E_g calculated according to the Kubelka–Munk function formula^42–44 (eqn (S3) and (S4), (ESI†)) is shown in Fig. S11 (ESI†). After reduction, E_g increases from 4.95 (air) to 5.1 eV (air + H₂). This alteration is attributed to the post-reduction strategy, which reduces Cr⁶⁺ to Cr⁴⁺, prompting electron rearrangement within the host material and inducing distortion in the local coordination environment. These factors collectively contribute to the widening of the band gap.^45,46 After the post-reduction strategy, the excitation peak of the sample exhibits alterations consistent with the DR spectra. The excitation peak attributed to Cr⁶⁺ at 375 nm vanishes, while the intensity and broadness of the excitation peaks (at 465 nm and 645 nm) corresponding to Cr⁴⁺ notably increases (Fig. 3c). This occurs because a significant portion of Cr⁶⁺ ions occupying the [SiO₄] tetrahedra is reduced to Cr⁴⁺, with a larger ionic radius through the post-reduction strategy. This reduction induces increased crystal field splitting and lattice distortion, thereby broadening the absorption peaks at 465 nm and 645 nm. Simultaneously, the increase in Cr⁴⁺ ion concentration enhances the energy transfer and electron–phonon coupling effects between Cr⁴⁺ ions, further contributing to the broadening of the absorption peaks. In addition, the excitation peak of Cr⁶⁺ also vanishes in the normalized wavelength-dependent PLE spectra (Fig. S12, (ESI†)).

At the same time, the decay curves (Fig. 3d) can still be fitted by a single exponential function, and the lifetime increases from 8.52 μs (air) to 38.31 μs (air + H₂). And the reasons for the increased fluorescence lifetime may include the following aspects: firstly, the post-reduction strategy reduces Cr⁶⁺ to Cr⁴⁺, which reduces the non-radiative transition and causes increased fluorescence lifetime. Secondly, an increase in Cr⁴⁺ concentration improves luminous efficiency and fluorescence lifetime. Finally, changes in the local environment of the central Cr⁴⁺ ion promote energy level splitting and increase the radiative transition probability, thereby further extending the fluorescence lifetime. In general, the increase in fluorescence lifetime is the result of a combination of factors. The post-reduction strategy can effectively extend the fluorescence lifetime of LSCSH:Cr⁴⁺ by reducing non-radiative transitions, optimizing the doping concentration and local environment.

Through the post-reduction strategy, the IQE exhibits a notable enhancement, from 2.2% to 27% (Fig. S13, (ESI†)). Additionally, the radius matching principle is exceptionally important in doped luminescent materials, even playing a decisive role, according to the distortion index D_r:⁴⁷

	(5)

where D_r represents the difference in the radius between the substituted ion and the doped ion. R_m and R_d represent the radii of substituted ions and doped ions, respectively. According to the D_r, the value of Cr⁴⁺ (CN = 4, r = 0.41 Å) occupying Si⁴⁺ (CN = 4, r = 0.26 Å) is calculated to be 57.69% implying that doping is extremely difficult. Consequently, current research on Cr⁴⁺ occupying Si⁴⁺ sites for NIR emitting materials is limited, with only a few early studies available.^48–50 As shown in Table S2 (ESI†), the luminescence properties of Cr⁴⁺-doped Si-based NIR luminescent materials are compared. The post-reduction strategy has achieved a significant leap in the IQE of Cr⁴⁺ at Si⁴⁺ sites. However, the radius mismatch still exists, and the efficiency improvements still need further research and enhancement in the future.

Fig. 3e and Fig. S14 (ESI†) illustrate the XRD Rietveld refinement results of LSCSH:yCr⁴⁺ (y = 0.1% and 3%). After post-reduction, LSCSH:yCr⁴⁺ still maintains the physical phase of Li₄SrCa(SiO₄)₂. The related results are presented in Table S3 (ESI†). Notably, the calculated unit cell volumes are recorded as V = 693.99 ų (y = 0.1%) and V = 694.02 ų (y = 3%), both of which are larger than the unit cell volume before post-reduction (air, V = 693.13 ų, LSCS:0.1%Cr⁴⁺ and V = 693.22 ų, LSCS:3%Cr⁴⁺). This means that the post-reduction strategy reduces Cr⁶⁺ with a smaller ionic radius to Cr⁴⁺ with a larger ionic radius, which is consistent with the spectral analysis results.

To analyze the effect of the post-reduction strategy on the local environment of Cr ions, the Raman spectra of typical samples LSCS:3%Cr⁴⁺ and LSCSH:3%Cr⁴⁺ are shown in Fig. 3f. Six distinct bands at 383, 430, 812, 844, 863, and 897 cm⁻¹ are observed in the Raman spectra. The Raman peak at 383 cm⁻¹ is attributed to the vibrational bending mode of the Si–O–Si group, while the intense Raman peaks in the range of 810–865 cm⁻¹ correspond to the stretching mode of the Si–O bonds. Additionally, the Raman peaks in the 865–1000 cm⁻¹ range are associated with the stretching mode of the Ca–Si–O and Sr–Si–O groups.^23,27 Compared with LSCS:3%Cr⁴⁺, the Raman spectra of LSCSH:3%Cr⁴⁺ reveal a noticeable decrease in intensity and symmetry of the vibrational bending mode of the Si–O–Si group and the stretching mode of the Si–O bond. This phenomenon supports that the post-reduction strategy facilitates the reduction of Cr⁶⁺ ions located in [SiO₄] sites to Cr⁴⁺ ions with larger ionic radii, resulting in an increase in lattice distortion. This distortion not only reduces lattice symmetry but also alters the local environment (such as charge rearrangement). Additionally, variations in crystal field strength and symmetry impact the transition probability between the excited state and ground state of the doped ions, thereby affecting the luminescence intensity. In summary, the post-reduction strategy significantly enhances the luminescence intensity, lifetime and quantum efficiency of LSCSH:Cr⁴⁺ luminescent materials.

3.3. Universal verification of the post-reduction strategy

Generally speaking, the reduction strategy is commonly used to prepare Cr³⁺-activated phosphors to avoid oxidation of Cr³⁺ to Cr⁴⁺. Throughout the reported articles on Cr⁴⁺-activated NIR-II phosphors, the reduction strategy for improving Cr⁴⁺ has rarely been reported, especially the post-reduction strategy. This is because the reducing atmosphere is more likely to reduce Cr⁴⁺ to Cr³⁺, making it impossible to obtain Cr⁴⁺-activated phosphors. Therefore, the proposal of a new strategy should give its applicable conditions and explore whether it is universal. Based on this, a series of H-Li₄Sr_1+zCa_1−z(SiO₄)₂:3%Cr⁴⁺ (z = −1 to 1) samples are prepared using the post-reduction strategy and their corresponding XRD patterns are presented in Fig. S15 (ESI†). When z values fall within the range of −0.4 and 0.5, the diffraction peaks are attributable to the pure Li₄SrCa(SiO₄)₂ (PDF#83-0763) phase. When z > 0.5, additional diffraction peaks emerge, corresponding to the Li₂SrSiO₄ phase (the same structure as Li₂EuSiO₄, PDF#89-0133). When the sites originally occupied by Sr²⁺ ions are instead filled with Ca²⁺ ions in H-Li₄Sr_1+zCa_1−z(SiO₄)₂:3%Cr⁴⁺ (−1 < z < −0.4), the diffraction peaks of Li₂CaSiO₄ (PDF#72-1729) appear. The samples with z = 1 and z = −1 are completely pure phases of Li₂SrSiO₄ and Li₂CaSiO₄. And the crystal structure of Li₂SrSiO₄ (z = 1, hexagonal structure) and Li₂CaSiO₄ (z = −1, tetragonal structure) are completely different from that of Li₄SrCa(SiO₄)₂ (orthorhombic structure), as Li₂SrSiO₄ is composed of [LiO₄], [SrO₈] and [SiO₄] polyhedrons, while Li₂CaSiO₄ is composed of [LiO₄], [CaO₈] and [SiO₄]. Thus, their luminescent properties need to be discussed separately.

After the post-reduction strategy, the luminescence intensities of H-Li₄Sr_1+zCa_1−z(SiO₄)₂:3%Cr⁴⁺ (−1 < z < −0.4) samples improved, and the PL spectra are illustrated in Fig. S16 (ESI†). The effect of the post-reduction strategy (Fig. 4a) on improving the luminescence intensity of Cr⁴⁺ is very significant. In addition, the PLE and PL spectra of H-Li₂SrSiO₄:3%Cr⁴⁺ and H-Li₂CaSiO₄:Cr⁴⁺ samples are shown in Fig. 4b. When excited by light at 465 nm, the emission peak of the H-Li₂CaSiO₄:3%Cr⁴⁺ sample is situated at 1150 nm, spanning the NIR-II region from 900 to 1600 nm. In contrast, the emission wavelength of H-Li₂SrSiO₄:3%Cr⁴⁺ undergoes a red shift to 1260 nm because the d orbitals of Cr⁴⁺ ions experience different energy level splits under varying crystal fields, which influences the absorption and emission. Although the crystal structures of Li₂SrSiO₄ and Li₂CaSiO₄ samples differ from that of Li₄SrCa(SiO₄)₂, the luminescence intensities are also notably enhanced after the post-reduction strategy. As depicted in Fig. S17 (ESI†), the PL intensities increase 34 times (Li₂SrSiO₄:3%Cr⁴⁺) and 15 times (Li₂CaSiO₄:3%Cr⁴⁺) of those obtained under the air-sintering conditions. Hence, the post-reduction strategy proves to be universally effective in enhancing Cr⁴⁺ luminescence. In summary, the applicable conditions for obtaining efficient Cr⁴⁺-activated NIR phosphors by the post-reduction strategy are as follows: (i) the matrix should lack suitable sites for Cr³⁺ occupation to prevent facile reduction of Cr ions to trivalent (+3) states. (ii) In the [AO₄] tetrahedron occupied by Cr⁴⁺ (CN = 4, r = 0.41 Å), the radius of the cation A^x+ should be smaller than that of Cr⁴⁺. The larger radius difference between A^x+ and Cr⁴⁺, the better the post-reduction strategy, such as Si⁴⁺ (CN = 4, r = 0.26 Å), Ge⁴⁺ (CN = 4, r = 0.39 Å) and Al³⁺ (CN = 4, r = 0.39 Å).

3.4. High-temperature thermal stability

Fig. S18a (ESI†) and Fig. 5a are the PL spectra of LSCS:3%Cr⁴⁺ and LSCSH:3%Cr⁴⁺ excited with 465 nm light at different temperatures. As the temperature increases from 100 to 425 K, the PL intensity gradually diminishes, although the spectral shape remains mostly unchanged. Fig. 5b displays the PLE and PL spectra of LSCSH:3%Cr⁴⁺ at 100 K, and the symmetric PL spectra consistent with room temperature once again validate the single luminescent center of the sample. Furthermore, the fluorescence lifetimes of Cr⁴⁺ (Fig. 5c) exhibit a notable decrease as the temperature rises from 100 to 423 K, attributable to the heightened probability of non-radiative transitions at elevated temperatures. And the single exponential decay curve also shows that LSCSH:3%Cr⁴⁺ has only one Cr⁴⁺ luminescence center occupying the [SiO₄] site. Concurrently, with rising temperature, there is a decrease in the luminescence intensity of LSCS:3%Cr⁴⁺ (Fig. S18b (ESI†)) and LSCSH:3%Cr⁴⁺ (Fig. 5d) samples. At 375 K, the emission intensity of these samples is about 64% (LSCSH:3%Cr⁴⁺) and 46% (LSCS:3%Cr⁴⁺) of that at RT. Therefore, the post-reduction strategy significantly enhances the thermal stability of LSCSH:3%Cr⁴⁺.

Evidently, the LSCSH:3%Cr⁴⁺ sample, characterized by ultra-long wavelength emission, demonstrates a significant Stokes shift that renders it highly sensitive to temperature variations, thereby indicating poor thermal stability.⁵¹ The activation energy (ΔE) can further evaluate thermal stability and can be calculated by using the Arrhenius formula^52–55 (eqn (S5) (ESI†)). The calculated ΔE = 0.19 eV for LSCSH:3%Cr⁴⁺ is shown in Fig. 5d. Electron–phonon coupling induces cross-relaxation, causing electrons to return from the excited state to the ground state via nonradiative transitions rather than radiative transitions. The energy barrier ΔE necessary for these nonradiative transitions is illustrated in Fig. S19 (ESI†). The extremely small ΔE = 0.19 eV of LSCSH:3%Cr⁴⁺ indicates that electrons readily surmount the energy barrier, leading to more significant dissipation of energy in the form of heat through non-radiative transitions.

In luminescence temperature measurement, emission intensity, lifetime and emission spectrum shape are the three most commonly used features. For NIR-II luminescent materials, the environmental background luminescence has minimal impact, providing distinct advantages in optical temperature measurement. The spectral shift (Δλ) and decay time (τ) are chosen as indicators for optical temperature measurement in LSCSH:3%Cr⁴⁺ luminescent materials, relying on its temperature-dependent luminescence properties. The absolute sensitivity (S_A) and relative sensitivity (S_R) are used to assess the optical temperature sensor's responsiveness to temperature changes. The fitting formula is as follows:^22,56–59

	(6)

	(7)

where Λ denotes the optical thermometry indicators, specifically Δλ and τ in this work. Fig. 5e describes the variation of Δλ and τ with temperature, and the data are fitted by polynomial and single exponential function, respectively. By Δλ fitting, the maximum values of S_A and S_R obtained are 0.28 nm K⁻¹ (425 K) and 1.71% K⁻¹ (150 K), as shown in Fig. S20 (ESI†). And Fig. 5f shows that the maximum values of S_A and S_R are estimated to be 0.29 μs K⁻¹ (425 K) and 1.69% K⁻¹ (425 K) by fitting the τ. Additionally, Table S4 (ESI†) provides a summary of optical temperature measurements of previously reported materials based on Δλ and τ. Therefore, LSCSH:3%Cr⁴⁺ materials with NIR-II luminescence have potential in optical temperature measurement.

3.5. NIR spectroscopy analysis application

The non-destructive testing characteristics of spectral analysis technology have great advantages in the field of food safety.^60–63 In contrast to the NIR-I region, the NIR-II region exhibits heightened sensitivity in detecting absorption peaks attributed to organic functional groups. Here, the LSCSH:3%Cr⁴⁺ phosphor covers the NIR-II region range from 950 nm to 1600 nm, which can satisfy the above purpose. To ascertain the potential application value of the LSCSH:3%Cr⁴⁺, ethanol, water and peanut oil are selected to measure the NIR transmission spectra of these foods, as shown in Fig. 6. Obviously, since each substance has its own unique absorption of NIR light, a specific absorption spectrum can be observed. The transmission spectrum of ethanol exhibits absorption peaks near 1200 and 1400 nm. The former is associated with the second overtone of C–H stretching vibration, while the latter corresponds to the first overtone of O–H and the overlap of several C–H stretching vibrations. Water mainly presents three absorption bands near the wavelengths of 1150, 1200 and 1350 nm, corresponding to the characteristic overtone of O–H stretching. The absorptions observed in peanut oil at near 1215 and 1380 nm are attributed to various C–H overtone combinations. These indicate that the prepared LSCSH:3%Cr⁴⁺ phosphor with ultra-broadband NIR-II emission can effectively distinguish the different components of foodstuffs.

4. Conclusions

In summary, a series of LSCS:Cr⁴⁺ phosphors with ultra-broadband NIR-II emission are successfully synthesized. Under excitation with 465 nm blue light, LSCS:3%Cr⁴⁺ emits NIR light centered at 1215 nm with a FWHM of 233 nm, spanning a wide spectrum from 950 to 1600 nm. A novel post-reduction strategy is proposed to achieve a leap-forward improvement in the luminescence intensity, lifetime and quantum efficiency, so that the luminescence intensity of LSCSH:3%Cr⁴⁺ is 8 times higher than that of LSCS:3%Cr⁴⁺, accompanied by an increase in quantum efficiency from 2.2% to 27%. In addition, through the mutual substitution between Ca²⁺ and Sr²⁺, a series of Li₄Sr_1+zCa_1−z(SiO₄)₂:3%Cr⁴⁺ (z = −1 to 1) samples are prepared, which confirmed the universality of the post-reduction strategy, and the applicable conditions of the post-reduction strategy are summarized. It provides valuable guidance for the practical application of the post-reduction strategy. Due to the ultra-broad NIR-II region emission of LSCSH:Cr⁴⁺, this material has broad application prospects in optical temperature measurement and spectral analysis. Therefore, this work takes a solid step towards the development of the next generation of efficient and stable NIR-II region luminescent materials.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (NSFC No. 12374376), the Natural Science Foundation of Shandong Province (ZR2021ZD10), the Guangdong Basic and Applied Basic Research Foundation (2024A1515010408), and the Project of the Qilu Young Scholar Program of Shandong University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc02781g

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