# Dual-Rare-Earth-Doped Piezoelectric Semiconductor Gallate Solids With Spectral Tuning for Energy Conversion and Photonic Applications

Fanlei Qin, Yujie Chen, Zhangyi Tian, Ange Zhu, Xianghua Zhang, Wasfa

Zulfiqar, Denghao Li, Yang Zhang, Gongxun Bai

► To cite this version:

Fanlei Qin, Yujie Chen, Zhangyi Tian, Ange Zhu, Xianghua Zhang, et al. Dual-Rare-Earth-Doped Piezoelectric Semiconductor Gallate Solids With Spectral Tuning for Energy Conversion and Photonic Applications. Rare Metals, 2025, pp.e70032. 10.1002/rar2.70032. hal-05494483

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ORIGINAL ARTICLE

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# Dual-Rare-Earth-Doped Piezoelectric Semiconductor Gallate Solids With Spectral Tuning for Energy Conversion and Photonic Applications

Fanlei Qin¹ | Yujie Chen¹ | Zhangyi Tian¹ | Ange Zhu¹ | Xianghua Zhang² | Wasfa Zulfiqar³ | Denghao Li¹ | Yang Zhang⁴ | Gongxun Bai¹

¹College of Optical and Electronic Technology, China Jiliang University, Hangzhou, China | ²ISCR—Institut des Sciences Chimiques de Rennes, UMR, Université de Rennes, Rennes, France | ³Department of Physics, University of Agriculture Faisalabad, Faisalabad, Pakistan | ⁴Institute of Modern Optics & Tianjin Key Laboratory of Micro-Scale Optical Information Science and Technology, Nankai University, Tianjin, China

**Correspondence:** Denghao Li (dhli@cjlu.edu.cn) | Yang Zhang (yangzhang@nankai.edu.cn) | Gongxun Bai (baigx@cjlu.edu.cn; baigongxun@126.com)

**Received:** 7 July 2025 | **Revised:** 23 September 2025 | **Accepted:** 25 September 2025

**Keywords:** mechanoluminescence | photonic applications | piezoelectric semiconductors | spectrum control

## ABSTRACT

Mechanoluminescent materials have gained significant attention for their unique capability to convert mechanical stimuli into light without external optical or electrical excitation, holding immense promise for applications in safety monitoring, biomedicine, optical anti-counterfeiting, and intelligent sensing. However, current mechanoluminescent materials are hindered by limitations such as a single functional mode, a restricted emission color gamut, and poor stability. To address these challenges, we report a layered piezoelectric semiconductor, (Sr, Ca)Ga₄O₇ host doped with dual rare-earth activators Tb³⁺ and Eu³⁺, which enables unprecedented multimodal luminescence responsiveness to mechanical force, UV light, X-rays, and temperature within a single crystalline framework. By precisely tuning the Tb³⁺/Eu³⁺ doping ratio, the material achieves continuously adjustable photoluminescence, mechanoluminescence, and X-ray-excited photoluminescence spectra across the green-to-red color gamut. Notably, the material demonstrates an ultralow mechanoluminescence activation threshold (≤ 10 kPa) and exhibits millisecond-level response kinetics. A series of potential applications, including digital information encryption, high-precision handwriting identification, modern motion analysis, and imaging detection, are designed based on its multimodal luminescence. This work establishes a paradigm for designing multifunctional luminescent materials through piezoelectric-semiconductor-rare-earth synergy.

## 1 | Introduction

Mechanoluminescent (ML) materials refer to materials that can spontaneously emit light when subjected to external mechanical stress or stimuli, such as stretching, compression, bending, vibration, or friction [1-5]. Unlike conventional luminescent materials that require optical or electrical excitation, ML materials operate through a self-powered light-emission mechanism, offering advantages such as portability and real-time responsiveness [6]. Their ability to convert mechanical force into light

enables a wide range of potential applications, including structural health monitoring, photonic anti-counterfeiting, flexible wearable devices, and intelligent sensing [7-10]. However, with the rapid advancement of technology, traditional ML materials remain limited in functionality, particularly under complex or dynamic environments, where single-function mechanical light-emitting materials are insufficient to meet the increasing demand for multifunctional and adaptive materials. Recent efforts have, therefore, focused on integrating ML with complementary optical modalities, aiming to create multifunctional ML platforms

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

© 2025 The Author(s). *Rare Metals* published by John Wiley & Sons Australia, Ltd on behalf of Youke Publishing Co., Ltd.
capable of robust performance, intelligent feedback, and envi-
ronmental adaptability [11–13]. Such advances not only broaden
the scope of applications of ML materials but also open significant
avenues for future applications in intelligent sensing, energy
management, and environmental monitoring [14–17].

In recent years, multifunctional mechanical light-emitting ma-
terials have made significant progress in multiple fields [18–20].
Peng et al. developed a multifunctional optical sensing platform
based on Sr₂MgSi₂O₇:Eu²⁺, Dy³⁺, which enables highly sensi-
tive detection of temperature, pressure, and stress [21]. The
material demonstrates excellent sensing performance in pho-
toluminescence, persistent luminescence, and ML modes,
showing strong potential for practical applications as a multi-
functional ML material. Xu's team realized five distinct emis-
sion modes in perovskite materials for optical anti-
counterfeiting, enabling applications in optical security, digital
encryption, and personalized signatures [22]. By leveraging
these multimodal light-emitting properties, they developed
multiple anti-counterfeiting strategies, demonstrating broad
prospects in information encoding and encryption, and further
promoting the development of multifunctional ML materials.
Zhang et al. introduced a novel artificial optical synapse that
simulates neural function by combining ML and photo-
stimulated luminescent materials [23]. The system converts
mechanical force and optical signal inputs into light signals,
thereby simulating the short-term and long-term plasticity of
neural synapses and exhibiting learning and memory functions.
These studies illustrate the significant potential of multifunc-
tional ML materials, yet several critical challenges remain un-
resolved. Current systems often lack broadband spectral
tunability, require relatively high stress thresholds for activa-
tion, and suffer from limited operational stability. Achieving
simultaneous multimodal responsiveness, temporal stability,
wide emission bandwidths, and mechanical flexibility continues
to be a formidable task [24–26]. Expanding the functional range,
improving stability, and extending the color gamut are key
strategies to advance ML materials toward versatile, adaptive,
and durable applications [27–30].

Here, we introduce a layered piezoelectric semiconductor (Sr, Ca)
Ga₄O₇ doped with Tb³⁺/Eu³⁺ as a versatile ML platform. Owing
to its wide bandgap (E_g = 4.46 eV), robust piezoelectric response,
and high thermal stability, the material readily accommodates
rare-earth dopants to achieve multimodal luminescence [31]. By
doping with Tb³⁺ and Eu³⁺ rare-earth ions, our design achieves
four emission modes (ML, PL, long-persistent luminescence, and
XEOL) and continuous color tuning from green to red simply by
adjusting the Tb³⁺/Eu³⁺ ratio. The system exhibits efficient force-
to-light conversion, with millisecond-scale response under kPa-
level stress, and maintains long-term temporal stability across
all emission pathways. Leveraging its multimodal output and
tunable color gamut, we demonstrate applications in dynamic
information encryption, high-precision handwriting authentica-
tion, and motion analysis, underscoring its potential for smart
photonic technologies. Overall, this gallate piezoelectric semi-
conductor provides a robust and adaptable platform for next-
generation applications spanning intelligent systems, security,
and health care.

2 | Results and Discussion

## 2.1 | Structure and Characterization

The microstructure of a material profoundly influences its
physical properties, so an in-depth study of its structural
features is key to enhancing the material's functional perfor-
mance. As shown in Figure 1A, X-ray diffraction (XRD)
pattern of the original sample after synthesis gradually
evolves from CaGa₄O₇ (ICSD #10351) to SrGa₄O₇ (PDF #71-
1580), and as the Sr doping amount increases, the diffraction
peaks shift slightly, indicating that Sr²⁺ (1.18 Å, CN 6) suc-
cessfully substitutes for the smaller Ca²⁺ (1.00 Å, CN 6) in
the host lattice, leading to lattice expansion. Figure 1B shows
the XRD patterns of the (Sr₀.₅Ca₀.₅)Ga₄O₇ (SCGO) matrix
doped with different ratios of Tb³⁺ and Eu³⁺. The incorpo-
ration of Tb³⁺ and Eu³⁺ does not induce the formation of
secondary crystalline phases, confirming that the rare-earth
ions are successfully accommodated within the SCGO lattice
while preserving the integrity of its original crystal structure.
Figure 1C presents the UV-visible diffuse reflectance spectra
of the SCGO matrix and the Tb³⁺, Eu³⁺ co-doped samples.
The doping of Tb³⁺ and Eu³⁺ lowers the reflectance of the
material across the ultraviolet-visible region, thereby
enhancing light absorption. This effect is attributed to the 4f-
4f electronic transitions of Eu³⁺ and Tb³⁺ ions [32]. Subse-
quent calculations of the diffuse reflectance data reveal that
the bandgap of the SCGO sample is 4.46 eV, as shown in
Figure 1D. Wide-bandgap materials are ideal host matrices
because they suppress intrinsic luminescence and facilitate
efficient emission from the characteristic transitions of rare-
earth ions [33].

The SCGO matrix is a piezoelectric semiconductor with a
noncentrosymmetric polar hexagonal space group. Figure 1E
compares the structures of CaGa₄O₇, SCGO, and SrGa₄O₇. All
are based on GaO₄ tetrahedral units, but different substitutions
of Ca²⁺ and Sr²⁺ result in distinct structural frameworks [22].
CaGa₄O₇ forms a compact structure due to the smaller Ca²⁺
ions, whereas the larger Sr²⁺ ions in SrGa₄O₇ lead to a more
expanded, less dense structure. Partial substitution in (Sr, Ca)
Ga₄O₇ improves structural stability. Figure 1F shows the scan-
ning electron microscope (SEM) image of the SCGO:Tb³⁺, Eu³⁺
sample, which reveals a characteristic layered tunnel structure
that provides high defect tolerance and a uniform element dis-
tribution. Energy-dispersive X-ray spectroscopy (EDS) confirms
that Tb³⁺ and Eu³⁺ have been successfully incorporated into the
matrix (Supporting Information S1: Figure S1). Figure 1G
compares the full-range X-ray photoelectron spectroscopy (XPS)
spectra of pristine and doped samples, further confirming the
successful doping of Tb³⁺ and Eu³⁺. In Figure 1H, characteristic
Tb 3d₅/₂ and Tb 3d₃/₂ peaks are located near 1245 and 1235 eV,
respectively, whereas the Eu 3d₅/₂ peak is observed around
1150 eV, confirming successful dopant incorporation. The Sr
3d₃/₂ and Sr 3d₅/₂ peaks shift 0.2 eV toward lower binding en-
ergies upon doping. Core-level spectra of O 1s, Ca 2p, and Ga 2p
also show leftward shifts of 0.1–0.3 eV, indicating a decrease in
chemical binding energy due to Fermi-level movement (Sup-
porting Information S1: Figure S2) [25].
<figure><img src="image_2.png" /></figure>

**FIGURE 1** | Atomic structure and morphological characteristics of the samples. (A) XRD patterns of phosphors with different $Sr^{2+}$ and $Ca^{2+}$ ratios. (B) XRD patterns of phosphors with different $Tb^{3+}$ and $Eu^{3+}$ doping ratios. (C) Comparison of diffuse reflectance spectra of co-doped and undoped samples. (D) Bandgap of undoped samples. (E) Crystal structure and ionic substitution diagram of SCGO. (F) SEM and EDS morphology and element distribution of $Tb^{3+}$- and $Eu^{3+}$-doped samples. (G) Comparison of XPS spectra of co-doped and undoped samples. (H) XPS spectra of $Tb^{3+}$ and $Eu^{3+}$ states in doped samples.

## 2.2 | Down-Conversion Luminescence

Photoluminescence (PL) characterization is a vital approach for evaluating the luminescent performance of materials. Figure 2A presents the emission spectra of SCGO doped with different rare-earth ions, with corresponding photographs shown in the inset. The $Tb^{3+}$-doped sample exhibits characteristic emission peaks under 260 nm UV excitation, corresponding to the 4f–4f transitions of $Tb^{3+}$: $^5D_4 \rightarrow {}^7F_J (J=6, 5, 4, 3)$ at 486, 549, 590, and 627 nm, respectively, along with an additional peak at 452 nm, which is assigned to the $^5D_3 \rightarrow {}^7F_5$ transition. Under 390 nm UV excitation, the $Eu^{3+}$-doped sample displays four emission peaks at 589, 617, 655, and 702 nm, corresponding to the $^5D_0 \rightarrow {}^7F_J (J=1, 2, 3, 4)$ transitions of $Eu^{3+}$. The co-doped sample exhibits all emission features observed in the singly doped samples. As illustrated in Figure 2B, the wide bandgap of SCGO ($E_g = 4.46$ eV) allows

effective modulation of luminescence through rare-earth ion doping. The luminescence intensity is highly sensitive to dopant concentration. With increasing dopant concentration, both $Eu^{3+}$ and $Tb^{3+}$ exhibit the typical trend of concentration-dependent luminescence enhancement followed by concentration quenching. Optimal emission intensity is achieved at 3% $Eu^{3+}$ and 2% $Tb^{3+}$, beyond which quenching occurs (Supporting Information S1: Figure S3) [34].

The unique crystal structure of SCGO favors defect-related luminescent behavior, including long-persistent luminescence (LPL), as shown in Figure 2C. After UV excitation and subsequent removal of the excitation source, the SCGO:$Tb^{3+}$, $Eu^{3+}$ sample shows only the characteristic green emission of $Tb^{3+}$, lasting for over 6 min. The inset presents a comparison between PL and LPL spectra, along with photographs illustrating the
<figure><img src="image_3.png" /><figcaption>**FIGURE 2** | Spectral analysis and regulation of phosphors under UV excitation. (A) PL spectra of singly doped and co-doped samples. (B) Comparison of luminescence intensity for samples with different doping ratios of Tb³⁺ and Eu³⁺. (C) After 1 min of UV exposure at 254 nm at room temperature, afterglow spectral analysis and the corresponding images. (D) Spectral regulation of phosphors after doping with different concentrations of Tb³⁺ and Eu³⁺. (E) Spectral regulation of SCGO:Tb³⁺, Eu³⁺ phosphors at different excitation wavelengths. (F) Diagram of the luminescent energy-level transition mechanism.</figcaption></figure>

persistent luminescence. The PL color of this material can be effectively tuned by adjusting the Tb³⁺, Eu³⁺ doping ratio and excitation wavelength. As shown in Figure 2D,E, the emission chromaticity can be modulated from green to red. The Eu³⁺-only sample displays dominant red emission, whereas increasing Tb³⁺ content gradually shifts the emission toward green. Additionally, the excitation wavelength significantly influences the emission color. For the SCGO:Tb³⁺, Eu³⁺ sample, changing the excitation wavelength from 260 to 390 nm induces a shift from green to red (Supporting Information S1: Figure S4). This dual-mode tunability highlights the material's potential for multicolor luminescence, anti-counterfeiting, and tunable light sources. The phosphor also exhibits excellent environmental and thermal stability. After storage under ambient conditions for 1 year, SCGO:Tb³⁺, Eu³⁺ retains its emission intensity with negligible loss. Thermal testing reveals that SCGO:Tb³⁺ phosphor retains 82.3% of its emission intensity at 423 K relative to that at 303 K. This indicates strong resistance to thermal quenching and suitability for solid-state lighting applications

under high-temperature conditions (Supporting Information S1: Figure S5). As shown in Figure 2F, the PL mechanism involves excitation of electrons from the ground state to higher energy levels under UV irradiation. After partial nonradiative relaxation and energy transfer to activator levels, radiative transitions back to the ground state release energy in the form of light.

## 2.3 | ML Spectral Modulation

To investigate the ML properties of the material, a dedicated experimental setup was designed and constructed for the precise measurement of ML intensity (Supporting Information S1: Figure S6). Figure 3A shows the collected ML spectra of singly and co-doped samples. The Tb³⁺-doped sample exhibits green emission under mechanical stimulation, whereas the Eu³⁺-doped sample shows red emission. The co-doped sample displays all characteristic peaks observed in the singly doped
<figure><img src="image_4.png" /></figure>

**FIGURE 3** | ML performance analysis and spectral regulation. (A) ML spectra of singly doped and co-doped samples. (B) ML images of samples with different ion doping ratios. (C) Spectra of singly doped samples under different mechanical forces. (D) Force-optical sensitivity analysis of Tb<sup>3+</sup>- and Eu<sup>3+</sup>-doped samples under different mechanical forces. (E) Single response speed of the samples under mechanical stimulation. (F) ML response rate of the samples under repeated mechanical stimulation. (G) ML intensity of SCGO:Tb<sup>3+</sup>, Eu<sup>3+</sup> after 20 cycles under 15 N friction stress. (H) ML cycling performance of SCGO:Tb<sup>3+</sup>, Eu<sup>3+</sup> after 10 cycles of charge and discharge under 15 N friction stress. (I) Time stability test.

counterparts. Multicolor-tunable ML is achieved in this matrix
by simply adjusting the dopant concentration, as demonstrated
in Figure 3B. For the Tb<sup>3+</sup>-only sample, the emission peak at
545 nm is attributed to the <sup>5</sup>D<sub>4</sub> → <sup>7</sup>F<sub>5</sub> transition. With increasing
Eu<sup>3+</sup> content, the characteristic <sup>5</sup>D<sub>0</sub> → <sup>7</sup>F<sub>2</sub> transition becomes
more prominent, resulting in a shift in emission color from
green to red (Supporting Information S1: Figure S7). The inset
shows photographs of the recorded ML emission. The influence
of Sr-to-Ca ratios on ML intensity is also investigated, and the
ML intensity reaches its maximum when the Sr:Ca ratio is 1:1
(Supporting Information S1: Figure S8). Additionally, the
dopant concentration of rare-earth ions plays a critical role in
modulating ML behavior. Both Tb<sup>3+</sup> and Eu<sup>3+</sup> achieve their

highest ML intensity at a doping concentration of 2%, beyond
which the intensity decreases due to concentration quenching
(Supporting Information S1: Figure S9).

Figure 3C presents the effect of different mechanical forces on ML intensity. The corresponding signal-to-noise ratio values are also presented for each force condition (Supporting Information S1: Figure S10). A marked increase in emission intensity is observed with increasing force. A strong linear relationship between the applied force and emission intensity is established, as shown in Figure 3D, with linear-fit slopes of 23.69 and 22.72 for Tb<sup>3+</sup> and Eu<sup>3+</sup>, respectively. These results indicate that both ions exhibit comparable sensitivity to mechanical force, consistent with
findings reported in the literature [17]. Figure 3E displays the ML intensity as a function of time under a single mechanical excitation, revealing a millisecond-level response time and indicating rapid mechano-optical responsiveness [35]. The response speed of this piezoelectric semiconductor material is significantly superior to that of other ML materials (Supporting Information S1: Table S1). The intensity-time curve for 10 consecutive excitation cycles is shown in Figure 3F. Although the emission intensity exhibits minor fluctuations, the overall trend remains consistent with the single-excitation response, demonstrating excellent mechanical-optical stability. Stability assessment is crucial for evaluating the long-term reliability of ML materials. As shown in Figure 3G, during 20 consecutive cycles, a gradual decrease in ML intensity is observed with repeated friction. However, the intensity is fully restored to its initial value after UV irradiation, as shown in Figure 3H. As shown in Figure 3I, time-dependent stability tests reveal that 96.7% of the initial ML intensity is retained after 30 days. Notably, a sample stored for 1 year still maintains 92.6% of its original ML intensity, confirming the exceptional long-term stability of the material (Supporting Information S1: Figure S11).

## 2.4 | **XEOL Spectral Modulation**

X-ray-excited optical luminescence (XEOL) is a valuable tool for probing the luminescence response of materials in various radiation environments, particularly revealing electronic transitions and energy transfer under high-energy excitation. Figure 4A–D demonstrates the XEOL properties of SCGO:Tb³⁺/Eu³⁺. Under X-ray excitation, the co-doped sample exhibits all characteristic emission peaks observed in the singly doped samples, as shown in Figure 4A, with representative real-time photographs provided in the inset. Figure 4B presents the XEOL spectra of the sample under varying X-ray tube voltages, where increasing voltage leads to a significant enhancement in XEOL intensity. Radiation stability was evaluated under continuous X-ray irradiation at 50 kV and 200 µA, and, as illustrated in Figure 4C, negligible intensity loss was observed after 50 min, confirming excellent durability under X-ray excitation. Furthermore, the emission spectrum can be tuned by adjusting the Tb³⁺/Eu³⁺ ratio, as illustrated in Figure 4D.

Figure 4E–G highlights the spectral tunability, persistence, and stability of SCGO:Tb³⁺/Eu³⁺ under X-ray excitation. As depicted in Figure 4E, the CIE chromaticity diagram reveals that the tunable emission color under X-ray excitation spans from the green to red region, thereby extending the excitation band into the X-ray regime. Such spectral flexibility offers a promising route toward multifunctional luminescent materials, particularly for multidimensional optical anti-counterfeiting. Figure 4F shows that SCGO:Tb³⁺ additionally exhibits X-ray-excited persistent luminescence, maintaining noticeable emission intensity even 6 min after the removal of the excitation source. This long-persistent XEOL makes it highly suitable for applications in optical tagging, anti-counterfeiting, radiation detection, and imaging [36–39]. Long-term stability tests, illustrated in Figure 4G, indicate that 91.7% of the XEOL intensity is retained after 1 year, further verifying the material’s exceptional durability.

Understanding the luminescence mechanisms is critical for designing and optimizing multifunctional luminescent systems. Figure 4H illustrates the proposed mechanisms of XEOL, ML, and LPL in this material. Upon X-ray excitation, electrons of Tb³⁺ and Eu³⁺ are excited from the ground state to higher energy levels (Process ①). A portion of these electrons becomes trapped in defect or trap states (Process ②), whereas others directly transition to the excited states of emission centers, where they remain briefly before recombining with holes in the ground state, emitting photons corresponding to PL and XEOL emissions. The trapped electrons may later be released by thermal or mechanical stimuli (Process ③). Owing to the unique layered and tunnel-like structure of the material, which facilitates efficient electron tunneling and migration, electrons can more readily escape from traps and recombine with emission centers either through tunneling (Process ④) or via conduction band-mediated pathways (Process ⑤). The combined contributions of Processes ④ and ⑤ give rise to the observed ML and LPL emissions [22].

## 2.5 | **Photonic Applications**

Owing to its multimodal luminescence characteristics, this material demonstrates substantial potential for applications across a range of fields, including digital encryption, forensic handwriting analysis, modern sports analytics, and X-ray imaging and detection. As illustrated in Figure 5Ai, a dynamic digital-encryption device is fabricated by integrating SCGO:Eu³⁺ and pristine SCGO. Under ambient light, the device displays the dummy information “8888,” whereas under UV excitation, the encrypted message “2025” becomes visible. Figure 5Aii demonstrates spatial encryption using a combination of SCGO:Eu³⁺, SCGO, and SCGO:Tb³⁺. Upon UV excitation, the device exhibits the effective information “1717” in a composite red-green emission. After removing the excitation source following 3 min of irradiation, the encrypted message “1717” remains visible in the dark due to the LPL of SCGO:Tb³⁺, demonstrating encryption feasibility in the time dimension.

To explore the potential of this material in high-precision handwriting identification, composite films were fabricated by integrating SCGO:Tb³⁺, Eu³⁺, and PDMS. These films enabled visualization of stress distributions in handwritten signatures using ML imaging. As shown in Figure 5B, signatures from three persons were recorded. Grayscale analysis yielded two-dimensional and three-dimensional pressure distribution maps, along with precise stroke trajectories. These results confirm the material’s ability to achieve high-resolution handwriting anti-counterfeiting based on stress visualization. Furthermore, analyzing pressure distributions and key points in each stroke—such as initiation, turning, and termination—enables extraction of user-specific writing patterns, thereby enabling precise information recognition.

In the context of fencing, where scoring depends on striking designated target areas such as the chest, a wearable device is developed by incorporating SCGO:Tb³⁺, Eu³⁺, and PDMS. This device is applied to valid scoring regions, as shown in
<figure><img src="image_5.png" /><figcaption>**FIGURE 4** | XEOL performance analysis and spectral regulation. (A) XEOL spectra of singly doped and co-doped samples. (B) XEOL spectral analysis of samples under different X-ray excitation voltages. (C) Stability of radiation-induced luminescence within 50 min. (D) Spectral analysis of XEOL spectral regulation based on different ion doping concentrations. (E) CIE diagram of XEOL spectral regulation based on different ion doping concentrations. (F) Afterglow spectral analysis and the corresponding images after 1 min of X-ray exposure. (G) XEOL time stability test. (H) Multimodal emission mechanism diagram.</figcaption></figure>

Figure 5Ci. Upon impact, the material emits a bright ML signal that can be captured by a high-resolution camera. Different impact actions—such as thrust marks or scratches—produce distinct ML trajectories, as illustrated in Figure 5Cii. As shown in Figure 5Ciii, grayscale and trajectory analyses enable detailed tracking of motion dynamics and subtle variations between techniques. This facilitates accurate identification of technical details during fencing. Continuous monitoring of such data can help optimize training strategies and enhance athletic performance.

X-ray luminescence imaging technology integrates the high-penetration capability of X-rays with the high-sensitivity luminescence performance of phosphors, enabling broad applications across multiple fields. As shown in Figure 5D, imaging experiments were conducted using samples including a peanut, a capsule containing a spring and screw, and electronic components. During the imaging process, X-ray voltage and current were set to 40 kV and 200 μA, respectively. Figure 5Di presents real-time X-ray imaging of the spring and screw inside the capsule, clearly revealing the internal structures. Figure 5Dii–iv displays both real-time and delayed X-ray imaging results, in which the internal features of the peanut and electronic devices

remain visible even 1 minute after the X-ray source is turned off. These results demonstrate a novel approach to delayed luminescence-based detection and promote further development of high-penetration, high-signal-to-noise imaging, and anti-counterfeiting applications.

### 3 | Conclusions

In this study, a piezoelectric semiconductor SCGO with a tunnel-like layered structure was successfully synthesized via a high-temperature solid-state method. By doping with dual rare-earth ions Tb³⁺ and Eu³⁺, a multifunctional ML material was obtained, combining a wide bandgap, excellent piezoelectric properties, and facile luminescence tunability by lanthanide activators. The material responds to various stimuli (ultraviolet light, mechanical force, X-rays, and heat) and exhibits corresponding multimodal emissions (PL, ML, LPL, and XEOL). By adjusting the ratio of doped ions, continuous color tuning from green to red can be achieved within a single matrix. The material demonstrates excellent ML-to-light conversion efficiency, achieving millisecond-level light response under kPa-level
<figure><img src="image_6.png" /><figcaption>**FIGURE 5** | Exploration of potential applications based on multimodal emission. (A) Potential application in digital information encryption based on PL. (B) Potential application in high-precision handwriting identification based on stress visualization. (C) Potential application in modern sports analysis. (D) Potential application based on X-ray imaging detection.</figcaption></figure>

mechanical stress. It also exhibits high stability and repeatability, making it a promising ML candidate. Based on its multimodal emission properties, various potential applications have been designed in fields such as motion analysis, information encryption, photonic anti-counterfeiting, and imaging detection, significantly enhancing its application potential in both time and space dimensions. This material provides promising insights and strategies for the development of multifunctional ML materials and promotes their advancement in various fields.

## Author Contributions

**Fanlei Qin:** writing – original draft, validation, supervision, methodology, investigation, formal analysis. **Yujie Chen:** supervision, writing – review and editing. **Zhangyi Tian:** writing – review and editing. **Ange Zhu:** writing – review and editing. **Xianghua Zhang:** writing – review and editing. **Wasfa Zulfiqar:** writing – review and editing. **Denghao Li:** writing – review and editing. **Yang Zhang:** writing – review and editing. **Gongxun Bai:** writing – review and editing, resources, methodology, funding acquisition.

## Acknowledgments

This work was financially supported by Zhejiang Provincial Natural Science Foundation of China (Grant No. LR24F050002) and the National Natural Science Foundation of China (Grant No. 62175225).

## Conflicts of Interest

The authors declare no conflicts of interest.

## Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.

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**Supporting Information**

Additional supporting information can be found online in the Supporting Information section.

**Supporting Information S1:** rar270032-sup-0001-suppl-data.docx.