Nonstoichiometric Strategy for Enhancing Luminescence Performance of Cr³⁺‐Activated Ca₃Ga₂Ge₃O₁₂ Broadband NIR Phosphors

ABSTRACT

Near‐infrared (NIR) emitting phosphors with broadband emission enable their promising application in various fields. In this work, a series of Ca₃Ga₂Ge_3‐x O_12‐δ:Cr³⁺ NIR phosphors were synthesized to investigate the influence of Ge⁴⁺ vacancies on the structural and luminescent properties. X‐ray diffraction analysis confirms that the main garnet phase is retained at low vacancy concentrations, whereas minor impurity phases appear at higher x values. X‐ray photoelectron spectroscopy analysis reveals the presence of both GaO₆ and GeO₆ octahedrons in this garnet. Under 458 nm excitation, all samples exhibit a broad NIR emission band centered at 752 nm. The integrated emission intensity increases with Ge⁴⁺ vacancy concentration, reaching a maximum at x = 0.4. The optimized phosphor shows exceptional thermal stability, retaining 91.4% of its room‐temperature intensity at 423 K. This study demonstrates that Ge⁴⁺ vacancy engineering is an effective strategy for enhancing the NIR luminescence of Cr³⁺‐doped garnet phosphors.

Broadband NIR phosphors Ca₃Ga₂Ge_3‐x O₁₂:Cr³⁺ are prepared, and Ge⁴⁺ vacancy is created into the lattice to improve the luminescence property of this phosphor. The optimized phosphor shows exceptional thermal stability, retaining 91.4% of its room‐temperature intensity at 423 K.

1. Introduction

Near‐infrared (NIR) spectrum, ranging between 700 and 1400 nm, has gained considerable attention owing to its strong penetration capability and fast response sensitivity [1, 2, 3, 4]. These unique features enable a wide range of applications, including nondestructive testing in food and agriculture, biomedical imaging, optical communication, and other fields [5, 6, 7]. However, conventional NIR light sources such as halogen lamps suffer from high power consumption, short lifetimes, and safety risks. In recent years, phosphor‐converted light‐emitting diodes (pc‐LEDs) have emerged as a novel type of NIR light source, attracting significant interest owing to their low cost, high luminous efficiency, and long operational lifespan [8, 9, 10]. Notably, the phosphor component plays a critical role in NIR technology, as it governs both the spectral matching with the commercial chip and the thermal stability of the light source [11, 12, 13].

Developing high‐performance NIR phosphors is essential for advancing frontiers in materials science and optoelectronic engineering. Numerous studies have explored novel NIR phosphors activated by transition metal ions (Cr³⁺ or Ni²⁺) [14, 15, 16, 17, 18] or rare‐earth ions (Eu²⁺, Tm³⁺, Nd³⁺, and Yb³⁺) [6, 19, 20, 21, 22, 23]. Among these, transition metal–doped phosphors have attracted particular attention as promising candidate materials owing to their abundant energy‐level transitions and excellent photostability. Specifically, Cr³⁺‐activated phosphors have garnered considerable interest because the 3d³ electronic configuration of Cr³⁺ enables broad emission in the range of 650–1100 nm, corresponding to the ⁴T₂ → ⁴A₂ transition under a weak crystal field environment [24, 25]. Conversely, in a strong crystal field, Cr³⁺ exhibits narrowband emission centered at approximately 700 nm, which originates from the spin‐forbidden ²E → ⁴A₂ transition [26]. In general, Cr³⁺ is highly sensitive to the crystal field strength, and its luminescence characteristics can be readily tuned by selecting an appropriate host lattice.

An appropriate host matrix can fulfill the requirements of various spectral applications. To date, numerous Cr³⁺‐activated inorganic phosphors have been reported, such as Sr₃Sc₄O₉:Cr³⁺ [27], BaMgAl₁₀O₁₇:Cr³⁺ [28], La₃Ga₅GeO₁₄:Cr³⁺ [29], and La₂MgTiO₆:Cr³⁺ [30]. Their photoluminescence (PL) and photoluminescence excitation (PLE) spectra exhibit distinct optical characteristics depending on the local coordination environment of Cr³⁺. The key targets for NIR phosphors are well‐matched emission spectra and excellent thermal stability. However, in most reported studies, the thermal stability still falls short of commercial application requirements. Therefore, it is urgent to identify an optimal combination of host matrix and Cr³⁺ to address this issue.

The garnet structure, with the general formula A₃B₂C₃O₁₂, serves as a paradigmatic host. In this structure, Cr³⁺ ions can occupy octahedral sites with a moderate crystal field strength, yielding emission bands centered in the range of 700–800 nm, which are ideal for a wide range of applications [31, 32]. Moreover, the rigid crystal framework of garnets provides excellent thermal stability, ensuring minimal thermal quenching even at elevated operating temperatures typical of high‐power LED applications [33]. Among various garnets, Ca₃Ga₂Ge₃O₁₂, which features a cubic garnet structure, offers suitable octahedral sites for Cr³⁺ substitution. However, the effect of Ge⁴⁺ vacancies on the structural and luminescent properties of Ca₃Ga₂Ge₃O₁₂:Cr³⁺ remains largely unexplored. Germanium acts as a network‐forming cation in the structure; its deficiency may distort the lattice and introduce antisite defects, thereby altering the local environment surrounding Cr³⁺.

In this work, we systematically investigate the influence of Ge⁴⁺ vacancies on the phase structure, morphology, and PL properties of Cr³⁺‐doped Ca₃Ga₂Ge₃O₁₂ (CGG) NIR phosphors. A series of Ca₃Ga_1.96Ge_3‐x O_12‐δ:0.04Cr³⁺ samples were synthesized via the conventional high‐temperature solid‐state method. The results demonstrate that the introduction of Ge⁴⁺ vacancies does not alter the main garnet phase. Importantly, under 458 nm excitation (⁴A₂ → ⁴T₁), the NIR emission centered at approximately 752 nm (⁴T₂ → ⁴A₂) increases with increasing Ge⁴⁺ vacancy concentration, reaching a maximum at x = 0.4. Further increasing the vacancy concentration leads to a decline in intensity, which can be attributed to the formation of impurity phases and structural degradation. Moreover, the optimized phosphor exhibits exceptional thermal stability, retaining 91.4% of its initial intensity at 423 K. These findings indicate that Ge⁴⁺ vacancy engineering is an efficient approach to enhance the NIR luminescence of Cr³⁺‐doped Ca₃Ga₂Ge₃O₁₂ phosphors.

2. Experimental Section

The samples Ca₃Ga₂Ge_3‐x O_12‐δ:Cr³⁺ were synthesized via a conventional high‐temperature solid‐state method. Starting materials of CaCO₃ (99.9%), Ga₂O₃ (99.99%), GeO₂ (99.99%), and Cr₂O₃ (99.9%) were weighed according to the nominal composition of Ca₃Ga_1.96Cr_0.04Ge_3‐x O_12‐δ and mixed in a mortar for at least 30 min to obtain a homogeneous mixture. The resulting mixture was then transferred to an alumina crucible and calcined at 1450°C for 5 h in a muffle furnace. After cooling to room temperature, the products were ground again to obtain the final phosphors for subsequent characterization.

X‐ray diffraction (XRD, Rigaku D/max‐22) was employed to determine the phase information of the prepared samples. The microstructure of the representative sample Ca₃Ga_1.96Cr_0.04Ge_2.6O_12‐δ was characterized using transmission electron microscopy (TEM, JEM2010). X‐ray photoelectron spectroscopy (XPS, Thermo Fisher, Nexsa) was applied to investigate the chemical states of the constituent elements. PLE and emission (PL) spectra were recorded using a fluorescence spectrophotometer (FLS 1000, Edinburgh). Temperature‐dependent luminescence properties were characterized with the aid of an additional heating attachment.

3. Results and Discussion

Figure 1a presents a schematic illustration of the defect engineering strategy in the garnet Ca₃Ga₂Ge₃O₁₂. As shown, Ca²⁺, Ga³⁺, and Ge⁴⁺ ions occupy the A, B, and C sites of the garnet structure with the general formula A₃B₂C₃O₁₂, respectively. Ca²⁺ and Ge⁴⁺ ions are coordinated by eight and four O²⁻ ions, forming CaO₈ dodecahedron and GeO₄ tetrahedron, respectively. Ga³⁺ ions are surrounded by six O²⁻ ions, existing as GaO₆ octahedron, which provide the coordination environment for the activator Cr³⁺. In this experiment, Ge⁴⁺ content was reduced to create Ge vacancies within the garnet lattice. XRD patterns of the samples Ca₃Ga₂Ge_3‐x O_12‐δ:Cr³⁺ are presented in Figure 1b to investigate the effect of Ge vacancy on the phase structure. When the reduction in Ge⁴⁺ content (x value) is below 0.2, pure phase of Ca₃Ga₂Ge₃O₁₂ is obtained. Further decreasing the Ge⁴⁺ content leads to progressive structural degradation and the appearance of an impurity phase Ca₂Ga₂GeO₇. Based on the calculation of the XRD pattern in the software Jade, mole ratio of the impurity phase to purity phase of Ca₃Ga_1.96Cr_0.04Ge_2.6O_12‐δ is 0.95:0.05. Rietveld refinement was performed on the Ca₃Ga₂Ge₃O₁₂:Cr³⁺ sample to obtain detailed structural information. The refinement result is presented in Figure 1c, and the corresponding structure parameters are summarized in Table 1. The garnet Ca₃Ga₂Ge₃O₁₂ crystallizes in the cubic space group Ia‐3d with high symmetry. The lattice parameters a, b, and c are equal, yielding a unit cell volume of 1835.728 ų.

FIGURE 1.

(a) Schematic illustration of the defect engineering strategy in the garnet Ca₃Ga₂Ge₃O₁₂. (b) XRD patterns of the samples Ca₃Ga₂Ge_3‐x O_12‐δ:Cr³⁺. (c) Rietveld refinement result.

TABLE 1.

Lattice parameters of Ca₃Ga₂Ge₃O₁₂:Cr³⁺.

Sample	Ca3Ga2Ge3O12:Cr3+
Space group	Ia–3d
a = b = c	12.244 Å
Volume	1835.728 Å3
Ge1‐O1	1.756 Å
Ga1‐O1	1.991 Å
Ca1‐O1	2.378 Å

TEM was performed on the representative Ca₃Ga₂Ge₃O₁₂:Cr³⁺ sample to further corroborate the structural information. TEM image in Figure 2a reflects the irregular shape of the particle. High‐resolution TEM images in Figure 2b,c exhibit clear lattice fringes, indicating the high crystallinity of the sample. The measured interplanar distance between adjacent fringes is 0.386 nm (Figure 2d), which corresponds to a characteristic crystallographic plane (2 2 0) of Ca₃Ga₂Ge₃O₁₂. Selected area electron diffraction (SAED) pattern presented in Figure 2e shows diffraction rings that can be assigned to the (4 2 0), (4 3 1), and (6 4 2) planes of Ca₃Ga₂Ge₃O₁₂. The elemental mapping images in Figure 2f,g demonstrate that Ca, Ga, Ge, O, and Cr are homogeneously distributed throughout the particle. Collectively, these information confirm the successful synthesis of the garnet phosphor Ca₃Ga₂Ge₃O₁₂:Cr³⁺.

FIGURE 2.

(a) TEM image; (b–c) HRTEM images of Ca₃Ga₂Ge₃O₁₂:Cr³⁺; (d) Fourier transform of corresponding HRTEM image; (e) SAED image of Ca₃Ga₂Ge₃O₁₂:Cr³⁺; (f–g) element mapping images of Ca₃Ga₂Ge₃O₁₂:Cr³⁺.

XPS analysis was further employed to verify the chemical states of the constituent elements. XPS survey spectra of the representative samples Ca₃Ga₂Ge₃O₁₂:Cr³⁺ and Ca₃Ga₂Ge_2.6O_12‐δ:Cr³⁺ are shown in Figure 3a,b, respectively, where all the characteristic signals of Ca, Ga, Ge, Cr, and O are clearly detected. High‐resolution Ga 3d XPS spectra of the two samples are presented in Figure 3c,d, exhibiting two prominent peaks at approximately 19.2 and 24.5 eV. These two peaks result from the Ga 3d_5/2 and Ga 3d_3/2, indicating that the Ga³⁺ ions occupy both the GaO₆ octahedron site and the GaO₄ tetrahedron site in the garnet structure, respectively [34, 35, 36]. The existence of GaO₄ tetrahedron illustrates that antisite defects exist (GaO₄ tetrahedron and GeO₆ octahedron) in this structure, thereby providing two distinct doping sites for the Cr³⁺ luminescent centers. As the Ge⁴⁺ concentration decreases, GaO₄ tetrahedron content decreased compared with the GaO₆ octahedron. This trend can be attributed to the reduced amount of Ge⁴⁺ available to form GeO₆ octahedron, which consequently limits the formation of GaO₄ tetrahedron. O 1s XPS spectra of Ca₃Ga₂Ge₃O₁₂:Cr³⁺ and Ca₃Ga₂Ge_2.6O_12‐δ:Cr³⁺ are shown in Figure 3e,f, both of which exhibit a single broad band. This asymmetric broad band can be deconvoluted into two components at around 529.5 and 531.8 eV, corresponding to the lattice oxygen (O_I) and adsorption oxygen (O_II), respectively [37, 38]. The area ratio of O_I to O_II decreases with decreasing Ge⁴⁺ content, indicating an increase in oxygen vacancies within the lattice. As the Ge⁴⁺ content in the Ca₃Ga₂Ge₃O₁₂ lattice decreases, the number of O²⁻ anions also decreases accordingly, resulting in a higher concentration of oxygen vacancies. These results directly confirm the introduction of Ge⁴⁺ vacancies into the structure.

FIGURE 3.

XPS survey spectra of (a) Ca₃Ga₂Ge₃O₁₂:Cr³⁺ and (b) Ca₃Ga₂Ge_2.6O_12‐δ:Cr³⁺; Ga 3d XPS spectra of (c) Ca₃Ga₂Ge₃O₁₂:Cr³⁺ and (d) Ca₃Ga₂Ge_2.6O_12‐δ:Cr³⁺; O 1s XPS spectra of (e) Ca₃Ga₂Ge₃O₁₂:Cr³⁺ and (f) Ca₃Ga₂Ge_2.6O_12‐δ:Cr³⁺.

PLE and PL spectra of Ca₃Ga₂Ge_3‐x O_12‐δ:Cr³⁺ were tested to investigate the influence of Ge⁴⁺ vacancy on the luminescence performance of the phosphors. Monitored at 750 nm, the phosphors exhibited two broad excitation bands in the ranges of 400–520 nm and 570–700 nm in Figure 4a, which can be assigned to the characteristic transitions ⁴A₂ → ⁴T₁ and ⁴A₂ → ⁴T₂ of Cr³⁺, respectively [39, 40]. Notably, the stronger excitation band is centered at approximately 460 nm, matching well with the emission of commercial blue LED chips. Under blue light excitation, the phosphors Ca₃Ga₂Ge_3‐x O_12‐δ:Cr³⁺ displayed a broad emission band ranging from 700 to 900 nm with a full width at half maximum (FWHM) of 111 nm (Figure 4b). The emission of Cr³⁺ activated impurity phase is much lower than that of Cr³⁺ activated the garnet Ca₃Ga₂Ge₃O₁₂ NIR phosphor, indicating that the impurity phase interfered the emission of Cr³⁺ in the phosphor. This broad emission of Ca₃Ga₂Ge_3‐x O_12‐δ:Cr³⁺ originates from the ⁴T₂ → ⁴A₂ transition of Cr³⁺, indicating that Cr³⁺ resides in a weak crystal field environment within Ca₃Ga₂Ge₃O₁₂ [41]. All these emission spectra are asymmetric and can be deconvoluted into two bands centered at 750 and 810 nm via Gauss fitting in Figure 4c. This spectral feature has been attributed to the occupation of Cr³⁺ ions at distinct crystallographic sites within the Ca₃Ga₂Ge₃O₁₂ lattice. In other words, Cr³⁺ ions occupy different octahedral sites, which corroborates the Ga 3d XPS results shown in Figure 3c,d. The intensity variations of these two fitted bands, presented in Figure 4d, exhibit a similar trend: Both increase with x until reaching a maximum at x = 0.4, followed by a decrease. As shown in Figure 4e, PL quantum yield (PLQY) of the optimized phosphor Ca₃Ga_1.96Cr_0.04Ge_2.6O_12‐δ was measured to be 54.5%. It is well known that the enhanced emission of Cr³⁺ may result from the altered crystal field strength within the lattice. To evaluate the crystal field of Cr³⁺ in this garnet, the values of Dq/B of Ca₃Ga₂Ge_3‐x O_12‐δ:Cr³⁺ were calculated via Equations ((1), (2), (3)) and summarized in Table 2 [42, 43].

$$D_q=\frac{E\left({{}^{4}A}_2\to {{}^{4}T}_2\right)}{10}$$ (1)

$$B=D_q\frac{x^2-10x}{15\left(x-8\right)}$$ (2)

$$x=\frac{E\left({{}^{4}A}_2\to {{}^{4}T}_1\right)-E\left({{}^{4}A}_2\to {{}^{4}T}_2\right)}{D_q}$$ (3)

FIGURE 4.

(a) PLE and (b) PL spectra of Ca₃Ga₂Ge_3‐x O_12‐δ:Cr³⁺ and Ca₂Ga₂GeO₇:Cr³⁺; (c) Gauss fitting of the emission spectrum; (d) emission intensity summarization; (e) PLQY value of Ca₃Ga₂Ge_2.6O_12‐δ:Cr³⁺; (f) Tanabe–Sugano diagram for Cr³⁺.

TABLE 2.

The calculated Dq/B values of Ca₃Ga₂Ge_3 − x O_{12 − δ}:Cr³⁺.

x values	0	0.1	0.2	0.3	0.4	0.5
Dq/B values	2.32	2.34	2.36	2.37	2.41	2.31

The calculated Dq/B values were approximately 2.35, confirming the weak crystal field environment for Cr³⁺. As the Ge⁴⁺ content in the lattice decreases, the Dq/B ratio initially increases, indicating an enhanced crystal field strength. However, upon further reduction of the Ge⁴⁺ content, the crystal field weakens. The Tanabe–Sugano diagram for Cr³⁺ in this lattice is presented in Figure 4f. Under blue or red light excitation, electrons are promoted from the ground state ⁴A₂ to the excited states ⁴T₁ and ⁴T₂, respectively. Following non‐radiative relaxation, these electrons return to the ⁴A₂ ground state from the ⁴T₂ excited state, generating NIR emission.

Temperature‐dependent luminescence properties were investigated to evaluate the effect of Ge⁴⁺ vacancies on the thermal stability of the phosphors. Figure 5a,b show the emission spectra of the phosphors Ca₃Ga₂Ge₃O₁₂:Cr³⁺ and Ca₃Ga₂Ge_2.6O_12‐δ:Cr³⁺ recorded at different temperatures. For both phosphors, the emission intensity gradually decreases with increasing temperature. With the introduction of Ge⁴⁺ vacancies, the emission intensity declines more slowly than that of the pristine Ca₃Ga₂Ge₃O₁₂:Cr³⁺, confirming the beneficial role of Ge⁴⁺ vacancies in improving thermal stability. At 423 K, the emission intensity of Ca₃Ga₂Ge_2.6O_12‐δ:Cr³⁺ still retains 91.4% of its room‐temperature value. Decreased rates of the fitted Peak I and Peak II are shown in Figure 5c. Different decreased ratios of these two peaks lead to the band shift of the emission spectra. As known, the thermal stability of the phosphors is influenced by the activation energy (ΔE), which can be evaluated via the Arrhenius equation (Equation 4) and the simplified Equation (5) [44]:

$$\mathit{IT}=I0{\left[1+\text{cexp}\left(-\frac{\mathrm{\Delta E}}{\mathrm{kT}}\right)\right]}^{-1}$$ (4)

$$\ln \left(\frac{I0}{\mathit{IT}}-1\right)=-\frac{\mathrm{\Delta E}}{\mathrm{kT}}+c^{\prime }$$ (5)

where I ₀, I _T , and c stand for the emission intensity at room temperature, emission intensity at T, and a constant, respectively. Based on the fitted lines in Figure 5d,e, ΔE of the phosphors Ca₃Ga₂Ge₃O₁₂:Cr³⁺ and Ca₃Ga₂Ge_2.6O_12‐δ:Cr³⁺ are calculated to be 0.166 and 0.212 eV, respectively. Generally, a larger activation energy is beneficial for the thermal stability of a phosphor. Besides, a large amount of defects in the lattice will form the defect level, which can capture the electrons released via the non‐radiative transition. Then, the captured energy can be transformed to the excited level to compensate the energy loss caused by the non‐radiative transition, resulting in the thermal stability improvement of the phosphors.

FIGURE 5.

Emission spectra of (a) Ca₃Ga₂Ge₃O₁₂:Cr³⁺ and (b) Ca₃Ga₂Ge_2.6O_12‐δ:Cr³⁺ recorded at different temperatures. (c) Emission intensity summarization. Relationship between ln(I ₀/I _T  − 1) and 1000/T of (d) Ca₃Ga₂Ge₃O₁₂:Cr³⁺ and (e) Ca₃Ga₂Ge_2.6O_12‐δ:Cr³⁺. (f) Peak position and FWHM of Ca₃Ga₂Ge_2.6O_12‐δ:Cr³⁺ as a function of temperature. (g) Temperature‐dependent decay curves of Ca₃Ga₂Ge_2.6O_12‐δ:Cr³⁺. (h) Representative fitting line. (i) Lifetimes of Ca₃Ga₂Ge_2.6O_12‐δ:Cr³⁺ as a function of temperature.

With increasing temperature, a red shift and broadening of the emission band are also observed. Figure 5f summarizes the peak position and FWHM of Ca₃Ga₂Ge_2.6O_12‐δ:Cr³⁺ as a function of temperature. As the temperature increases from 298 to 473 K, the emission band maximum shifts from 754 to 774 nm, and the FWHM broadens from 109 to 134 nm. With increasing temperature, lattice vibrations become more vigorous. Such intensified vibrations disturb the energy levels of Cr³⁺ ions, leading to a wider distribution of emitted photon energies and consequently resulting in the broadening of the emission spectrum. Figure 5g presents the temperature‐dependent decay curves of the representative Ca₃Ga₂Ge_2.6O_12‐δ:Cr³⁺ sample, all of which can be well fitted using the quadratic exponential equation (Equation 6) [45].

$$I=I0+A1\exp \left(\frac{-t}{\tau 1}\right)+A2\exp \left(\frac{-t}{\tau 2}\right)$$ (6)

in which τ and A are the exponential information and constant, respectively. A representative fitting line is presented in Figure 5h. The lifetime can be calculated via Equation (7) [46]:

$$\tau \mathrm{av}=\frac{A1{\tau }_1^2+A2{\tau }_2^2}{A1{\tau }_1+A2{\tau }_2}$$ (7)

The calculated lifetimes, presented in Figure 5i and Table 3, decrease from 73.69 to 50.04 μs as the temperature rises from 298 to 423 K. This decrease is attributed to enhanced non‐radiative transitions of Cr³⁺ at elevated temperatures. Taken together, these results confirm the advantages of introducing Ge⁴⁺ vacancies into the lattice.

TABLE 3.

The calculated Dq/B values of Ca₃Ga₂Ge_3 − x O_{12 − δ}:Cr³⁺.

Temperature/K	298	323	348	373	398	423	448	473
τ 1/μs	13.90	13.65	14.58	14.48	14.77	13.11	14.05	14.83
τ 2/μs	98.34	91.87	88.06	82.67	78.52	72.91	70.75	67.10
τ av /μs	73.69	71.43	67.26	63.62	60.15	55.90	53.77	50.04

4. Conclusion

In summary, we have systematically investigated the influence of Ge⁴⁺ vacancies on the structural and PL properties of Cr³⁺‐doped Ca₃Ga₂Ge₃O₁₂ NIR phosphors. The introduction of Ge⁴⁺ vacancies does not alter the primary garnet phase up to x = 0.2, but further increase leads to structural deterioration and the gradual appearance of impurity phases. XPS analysis confirms the coexistence of GaO₆ and GeO₆ units, indicating multiple Cr³⁺ occupation sites. The optimized decreased Ge⁴⁺ concentration was x = 0.4, and the integral NIR emission intensity was enhanced compared with the vacancy‐free sample. Importantly, the optimized phosphor exhibits superior thermal stability with 91.4% intensity retention at 423 K and a higher activation energy than the pristine sample. These findings highlight that cation vacancy engineering is a promising approach to boost the luminescence performance of Cr³⁺‐activated NIR phosphors for potential pc‐LED applications.

Author Contributions

Jianjun Zhao: investigation, data curation, software. Jing Zhang: investigation, writing – original draft, conceptualization, methodology, software, data curation, formal analysis. Weijie Li: investigation, data curation, software, methodology, writing – original draft. Xiaoming Shen: supervision, resources, project administration. Feiyu Wang: software, formal analysis. Yang Zheng: formal analysis, investigation, software. Zhixiang Zhang: data curation, investigation, formal analysis. Naicen Xu: project administration, formal analysis. Quan Liu: writing – review and editing, conceptualization, project administration, funding acquisition.

Funding

The authors have nothing to report.

Conflicts of Interests

The authors declare no conflicts of interest.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.