Multicolor mechanoluminescence for integrated dual-mode stress and temperature sensing

Abstract

The advancements in the new industrial era demand higher standards for integrated intelligent sensors. These multi-parameter coupling sensors require sensing materials that can swiftly respond to various physical stimuli, including stress, temperature, humidity, etc. Mechanoluminescence (ML), with its unique mechanical–optical response, serves as an effective medium for stress sensing. In ML materials, constructing multiple luminescent centers not only enables multicolor ML but also allows for temperature sensing through the differential thermal quenching properties of various luminescent centers. In this study, we present an energy transfer strategy that leverages high-energy blue-emitting ML from the host material, combined with Tb³⁺ and Mn²⁺, to achieve multicolor ML. The mechanical response is correlated with the integrated intensity of ML, while the temperature response is governed by the dynamic ML intensity ratio of I_Tb/I_Mn. This approach simplifies the design of multifunctional sensors and facilitates remote, dual-mode sensing. Moreover, we develop a highly secure encryption system based on the integration of multicolor PL and ML with an ML-triggering mechanism that is both convenient and reliable. This work presents an effective strategy for developing multicolor ML materials and advancing multimodal sensing.

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1. Introduction

The emission spectra of traditional ML materials are typically confined to a fixed wavelength range, limiting their applicability across diverse scenarios. This constraint is particularly pronounced in high-precision mechanical and temperature sensing applications, where at least two emission bands are essential for ratiometric and accurate measurements.^1–10 Consequently, the development of a single material capable of emitting across the full visible spectrum holds substantial scientific and practical significance. Such materials would enable real-time, precise responses to mechanical stress and external environmental conditions, providing innovative solutions across a wide range of fields.^11–16

At present, the construction method for multicolor ML is to use multiple materials or multiple luminescence centers to achieve preparation, such as SrAl₂O₄:Eu²⁺ (green),¹⁷ LiNbO₃:Pr³⁺ (red),¹⁸ NaCa₂GeO₄F:Mn²⁺ (yellow),¹⁹ Sr₃Sn₂O₇:Sm³⁺ (reddish-orange),²⁰ CaZr(PO₄)₂:Eu²⁺ (blue)²¹ and BaSi₂O₂N₂:Eu (bluish-green).²² However, this method demands substantial resources, in terms of both manpower and materials, highlighting the need for a simpler and more cost-effective alternative. In 2016, Jeong et al. physically mixed a red fluorescent dye with ZnS, where the red dye completely absorbed the green ML emission of ZnS, resulting in a color shift from green to red.²³ Similarly, in 2018, Du et al. achieved tunable ML colors ranging from green to red by mixing CaZnOS:Tb (green) and CaZnOS:Mn (red), regulating the color through adjustments in the mass ratio.²⁴ The multicolor ML was controlled through external energy transfer (ET) between materials. Furthermore, in 2021, Chen et al. successfully adjusted the ML color from green to red in SrZnSO:Tb,Eu by varying the Tb³⁺ ion doping concentration, facilitating internal ET.²⁵ Despite these advancements, the limited range of ML colors remains insufficient for current application requirements, and achieving full RGB (red–green–blue) color tuning is resource-consuming. Thus, the selection of appropriate materials is critical. The blue domain, being the most energy-demanding, is key for generating a wide color range. Most ML materials that emit green and red light can be efficiently excited by blue light, so introducing blue-light emitting centers into the matrix is an effective approach. This facilitates dynamic ML color adjustment across the blue, green, and red regions via energy transfer. Additionally, the inclusion of dual-emission centers significantly improves operational efficiency. For material selection, we chose Sr₈MgCe(PO₄)₇, a phosphate-based material. The presence of Ce³⁺ ions in the matrix provides effective high-energy blue light, and the rich cation coordination in the matrix is conducive to the introduction of multiple luminescence centers.²⁶

In this study, we successfully incorporated the rare-earth ion Tb³⁺ and the transition metal ion Mn²⁺ into the blue ML matrix Sr₈MgCe(PO₄)₇. PL and lifetime measurements confirmed that energy transfer facilitated tunable PL and ML emissions from blue to green, blue to red, and green to red. Furthermore, we observed a temperature-dependent intensity ratio (I_Tb/I_Mn) due to the energy-level differences between the rare-earth and transition metal ions. By calculating the ratio of S_r and S_a, we demonstrated the feasibility of ML-based temperature sensing without a light source. Finally, we validated the material's significant potential in mechanical displays and electronic signatures through a full-spectrum ML information encryption system.

2. Results and discussion

We utilize Ce³⁺ ions as the luminescent centers in the blue region of the host, while the rare-earth ion Tb³⁺ and the transition metal ion Mn²⁺ are selected as the luminescent centers in the red and green regions, respectively. Co-doping strategies of SMCP:xTb³⁺ (0 ≤ x ≤ 0.4), SMCP:yMn²⁺ (0.1 ≤ y ≤ 0.5), and SMCP:0.4Tb³⁺,yMn²⁺ (0 ≤ y ≤ 0.4) were developed. Compared with the standard card, no impurity phase was detected, indicating that the incorporation of Tb³⁺ and Mn²⁺ ions does not alter the phase structure (Fig. S1†). Energy spectrum and morphology analyses further confirmed that in the co-doped sample, SMCP:0.4Tb³⁺,0.04Mn²⁺, both Tb³⁺ and Mn²⁺ ions were uniformly incorporated into the lattice structure. It is worth noting that the morphology image shows that the particle size is irregular and relatively large, ranging from 10 to 60 μm (Fig. S2a and b†). This can be attributed to the fact that the high-temperature solid phase method is relatively rough and it is difficult to achieve absolutely uniform mixing of materials. After further refining the matrix material, the lower distribution coefficient validates the successful preparation of the single-phase sample with a stable phase structure (Fig. 1a). Crystallographic analysis revealed that SMCP belongs to the monoclinic crystal system I2/a (No. 15) and possesses a centrosymmetric structure (Fig. 1b). Based on the various coordination environments of Sr and Mg in the matrix SMCP, the introduction of Tb³⁺ and Mn²⁺ luminescence centers ensures the realization of the RGB primary colors.

Fig. 1 (a) XRD Rietveld refinement of SMCP; (b) the unit cell structure of SMCP; and (c) the coordination environments of the five Sr/Mg sites; Tb³⁺ and Mn²⁺ ions were expected to replace the Sr/Mg sites.

In the later stages of the ML sensing module, polydimethylsiloxane (PDMS) was selected as the organic medium. Fig. 2a depicts the preparation process, which involves uniform mixing and high-temperature cross-linking. Phase characterization indicated that the XRD patterns of SMCP:0.4Tb³⁺,0.04Mn²⁺, PDMS, and the composite device were consistent with standard reference cards, confirming the absence of phase transitions (Fig. 2b). Morphological analysis further demonstrated uniform cross-linking between the powder and PDMS, while cross-sectional energy spectrum analysis validated the successful incorporation of RGB tricolor centers (Fig. 2c). Fig. 2d and e illustrate that after high-temperature cross-linking, the device retains flexibility and can be stretched to 100% strain. These advantages establish a foundation for subsequent ML temperature sensing and full-spectrum ML information readout.

Fig. 2 (a) The preparation process of the film; (b) the XRD patterns of the SMCP:0.4Tb³⁺,0.4Mn²⁺ and SMCP:0.4Tb³⁺,0.4Mn²⁺@PDMS films; (c) SEM and EDS images of the SMCP:0.4Tb³⁺,0.4Mn²⁺@PDMS film; and (d and e) the stretchability and flexibility of the film.

In order to investigate its spectral modulation properties, in this study, we first tested the PL spectra and lifetime curves of different color gamuts. The spectrum of SMCP is shown in Fig. 3a. Under 312 nm excitation, the sample exhibits a broad emission in the matrix ranging from 325 to 450 nm, attributed to the ⁵D₁–⁴F₁ transition of Ce³⁺ ions. The excitation peaks at 275 nm and 294 nm correspond to the 4f → 5d broadband transitions of Ce³⁺ ions. For the SMCP:0.4Tb³⁺ sample, excitation at 312 nm produces two distinct emission spectra: one centered at 373 nm, corresponding to the Ce³⁺ ion emission peak, and another set of peaks centered at 488 nm, 542 nm, 583 nm, and 620 nm, which are characteristic emission bands of Tb³⁺ ions due to the ⁵D₄ → ⁷F_J (J = 2–5) transitions (Fig. 3b). Under 312 nm excitation, the SMCP:0.4Mn²⁺ sample shows an emission band centered at 373 nm from Ce³⁺ ions and an additional emission band at 611 nm, attributed to the ⁴T₁–⁶A₁ transition of Mn²⁺ ions (Fig. 3c). In Fig. 3d, the emission spectrum of the SMCP:0.4Tb³⁺,0.04Mn²⁺ sample comprehensively displays all characteristic transitions of Ce³⁺, Tb³⁺, and Mn²⁺ ions. Of particular note, the characteristic excitation spectra of both singly and doubly doped samples overlap with the matrix emission spectrum in the 325–450 nm range. Additionally, in the co-doped SMCP:0.4Tb³⁺,0.04Mn²⁺ sample, the emission band of Tb³⁺ ions at 488 nm shows slight overlap with the PLE spectrum of Mn²⁺ ions (Fig. S3†), providing strong evidence for effective energy transfer pathways of Ce³⁺–Tb³⁺, Ce³⁺–Mn²⁺ and Tb³⁺–Mn²⁺.

Fig. 3 (a–d) PL and PLE spectra of SMCP, SMCP:0.4Tb³⁺, SMCP:0.4Mn²⁺ and SMCP:0.4Tb³⁺,0.4Mn²⁺; (e) relationship between the ion doping concentration and the PL intensity in SMCP:xTb³⁺, SMCP:xMn²⁺ and SMCP:0.4Tb³⁺,xMn²⁺ systems; (f) energy transfer efficiency (ETE) from the host to Tb³⁺ in SMCP:xTb³⁺ and ETE from the host to Mn²⁺ in SMCP:yMn²⁺; ETE from Tb³⁺ to Mn²⁺ in SMCP:0.4Tb³⁺,yMn²⁺; and (g) ETE process of Ce³⁺–Tb³⁺, Ce³⁺–Mn²⁺ and Tb³⁺–Mn²⁺.

To further verify the energy transfer between the matrix and Tb³⁺ ions and Mn²⁺ ions, as well as between Tb³⁺ ions and Mn²⁺ ions, the PL spectra and lifetime curves of SMCP:xTb³⁺, SMCP:yMn²⁺ and SMCP:0.4Tb³⁺,xMn²⁺ were recorded, as shown in Fig. 3b and c, respectively. As shown in Fig. S5,† as the Tb³⁺ doping concentration increases, the ⁵D₁–⁴F₁ transition peak of the Ce³⁺ ions gradually decreases, while the intensity of the characteristic transitions of Tb³⁺ ions progressively increases. The same trend was observed in the Mn²⁺ series. With the increase of Mn²⁺ doping concentration, the ⁵D₁–⁴F₁ transition peak of the matrix gradually decreased, while the characteristic emission of Mn²⁺ gradually increased. Additionally, for the co-doped SMCP:0.4Tb³⁺,yMn²⁺ series, as the concentration of Mn²⁺ ions increases, the characteristic emission of Tb³⁺ decreases, while that of Mn²⁺ increases (Fig. S6†). In order to further verify the energy transfer of Ce³⁺–Tb³⁺, Ce³⁺–Mn²⁺ and Tb³⁺–Mn²⁺, the fluorescence lifetime was tested by monitoring the emission at 373 nm (Ce³⁺) and 542 nm (Tb³⁺) in the three systems, and the data were well-fitted using eqn (1):²⁷

where I represents the emission intensity at time t, A₁ is the constant, and τ₁ is the decay lifetime of the exponential component. With the increase of ion doping concentration, the fluorescence lifetimes of the SMCP:xTb³⁺ and SMCP:yMn²⁺ samples monitoring the characteristic transition of Ce³⁺ ions gradually decrease and the fluorescence lifetime of the co-doped SMCP:0.4Tb³⁺,yMn²⁺ sample monitoring the Tb³⁺ ions at 543 nm gradually decreases (Fig. 3f). This further confirms the existence of energy transfer between Ce³⁺ and either Tb³⁺ or Mn²⁺, as well as between Tb³⁺ and Mn²⁺ (Fig. S3–5†). Energy transfer efficiency (ETE) between Ce³⁺–Tb³⁺, Ce³⁺–Mn²⁺ and Tb³⁺–Mn²⁺ was calculated according to formula (2):²⁸where η_T presents the ETE, and τ_s and τ₀ are the decay lifetimes of the host and without the Tb³⁺ ions. In SMCP:Tb³⁺, the energy transfer efficiency between Ce³⁺ and Tb³⁺ showed a positive increase as the concentration of Tb³⁺ ions increases. The same is true between Ce³⁺ and Mn²⁺ and Tb³⁺ and Mn²⁺. Fig. 3g shows the possible ET processes between Ce³⁺and Tb³⁺, Ce³⁺ and Mn²⁺, and Tb³⁺ and Mn²⁺.

As mentioned above, most studies focusing on full-spectrum ML responses utilize the monochromatic introduction of various luminescent centers, which restricts dynamic adjustment. In addition, direct dynamic doping of the three primary colors of red, green and blue is difficult to control. Consequently, the introduction of two additional color domain luminescent centers in a matrix containing Ce³⁺ ions and the realization of full spectral ML via energy transfer represent a more convenient approach. Initially, we developed encryption devices incorporating SMCP, SMCP:0.4Tb³⁺, SMCP:0.4Mn²⁺ and SMCP:0.4Tb³⁺,0.04Mn²⁺. The congruence of the ML and PL spectra suggests the homogeneity of the luminescent centers, which emit blue, green, red, and orange ML, respectively (Fig. 4a–d). Effective energy transfer between Ce³⁺–Tb³⁺, Ce³⁺–Mn²⁺ and Tb³⁺–Mn²⁺ also causes changes in ML color with varying luminescent center concentrations. The ML spectra at different doping concentrations are presented in Fig. S6–8.† In the SMCP:xTb³⁺ and SMCP:xMn²⁺ systems, an increase in the doping concentration results in a decrease in the matrix ML emission intensity, while the characteristic ML emissions of Tb³⁺ and Mn²⁺ progressively intensify. In samples from the SMCP:0.4Tb³⁺,0.04Mn²⁺ system, an increase in the concentration of Mn²⁺ ions leads to a ML intensity reduction of Tb³⁺ ions, whereas the ML emission intensity of Mn²⁺ ions gradually increases. Fig. 4e illustrates the relative intensity variations of Ce³⁺/Tb³⁺, Ce³⁺/Mn²⁺, and Tb³⁺/Mn²⁺ across different systems; as the doping concentration increases, the relative intensity diminishes accordingly. Further visualization of dynamic PL and ML images reveals consistent colors, confirming the homogeneity of the luminescent centers and the feasibility of ML energy transfer (Fig. 4f–h). Notably, in SMCP:xTb³⁺ (x = 0.04–0.4), a dynamic transition from blue to green was observed for both PL and ML, while in dual-doped SMCP:0.4Tb³⁺,yMn²⁺ (y = 0–0.4), a striking dynamic transition from green to yellow to orange to red occurred. By importing the PL spectral data from Fig. S3a–S5a† and the ML spectral data from Fig. S6–S8† into the CIE color space for analysis (Tables S2 and S3†), the PL and ML colors are represented within triangular regions defined by the vertices of red (SMCP:Mn²⁺), green (SMCP:Tb³⁺), and blue (SMCP) (Fig. 4i and j). Obviously, according to the above evidence, we achieve a dynamic full-spectrum modulation through energy transfer between luminous centers, which provides a new strategy for the simple and efficient design of multicolor ML.

Fig. 4 (a–d) ML spectra of SMCP, SMCP:0.4Tb³⁺, SMCP:0.4Mn²⁺ and SMCP:0.4Tb³⁺,0.4Mn²⁺ under the tensile state; (e) relative spectral intensity of the ML series; (f–h) photographs of the series samples under 254 nm UV irradiation and stretching; and (i and j) CIE chromaticity coordinates of PL and ML corresponding to figures (f–h).

After achieving tunable full-spectrum ML, we further investigated the mechanical properties of the device. As illustrated in the mechanical testing schematic (Fig. 5a), ML data were collected under varying mechanical responses using a mechanical testing machine, integrated with a photomultiplier tube and an Ocean Optics detector. The ML spectra of SMCP:0.4Tb³⁺,0.04Mn²⁺ under different mechanical stimuli are presented in Fig. 5b. As the applied force increased, the total integrated area of the ML spectra expanded, with the transition intensities of Ce³⁺, Tb³⁺, and Mn²⁺ progressively increasing (Fig. 5c). Simultaneously, photon counts were recorded using the photomultiplier tube, capturing the variations in force and the photon count over time. The study found that during the force application process, the photon number intensity gradually increased, and the increasing stage can be linearly fitted as y = −74.29984 + 4334.43x (Fig. 5d and e). As is known to all, the device's self-recovery characteristic is especially crucial across various fields. In 10 consecutive stretching cycles at a consistent speed and force, we observed that the ML intensity gradually diminished as the number of cycles increased. This reduction in intensity can be attributed to microfractures formed within the elastomer, decreasing friction with the inorganic components. Fortunately, after 24 hours of rest, cross-linking occurred between the organic and inorganic parts, resulting in a recovery of 84% and 62% of the original strength, respectively, during the subsequent stretching process. This self-recovery feature allows ML materials to minimize both material waste and resource consumption.

Fig. 5 (a) Mechanical schematic diagram; (b) ML spectra of SMCP:0.4Tb³⁺,0.04Mn²⁺ under different forces; (c) the normalized ML intensity of the total and different luminous centers Ce³⁺, Tb³⁺ and Mn²⁺ under different forces; (d) the first stretch cycle under the mechanical testing machine: the relationship between force and time and the relationship between ML strength and time; (e) linear relationship between stress luminous intensity and force in the first cycle; and (f) recoverability test: 10 cycles under a 2N force until the ML intensity reaches the background light, and the ML intensity can be restored after being placed in a dark environment for 24 hours.

Temperature sensitivity is a key performance parameter for environmental temperature detection devices. Traditional upconversion-based temperature sensing methods often suffer from drawbacks such as complex operation and high costs. Therefore, exploring cost-effective ML-triggered temperature sensing becomes particularly important, offering the advantages of simplicity and direct visualization.^29,30 Due to their different external electron configurations and excited state energy levels, rare-earth ions and transition metal ions have different temperature response sensitivities, this property is also used to study their temperature sensing performance. Therefore, Tb³⁺ and Mn²⁺ ions, which exhibit the most distinct color differences, were selected as the temperature sensing centers. Temperature sensing based on the ML intensity ratio (IR) not only minimizes environmental interference but also eliminates the need for external light excitation. First, the ML spectra of SMCP:0.4Tb³⁺,0.04Mn²⁺ devices were recorded at varying temperatures (Fig. 6a). As the temperature increased, the intensity of the ML spectra gradually decreased. It is important to note that the decrease in intensity for the Tb³⁺ and Mn²⁺ luminescence centers followed different trends. Normalizing the Mn²⁺ ion intensity revealed an upward trend in the ML intensity of Tb³⁺ ions, resulting in a significant change in the intensity ratio (Fig. S10†). Based on this observation, we established an exponential relationship between the MLR of I_Tb/I_Mn and temperature. As shown in Fig. 6b, the fitted exponential function is given as MLR = 0.23 + 2.83 exp(−4806/T), with a fitting factor of 96%, which is sufficient to enable remote temperature sensing based on the MLR. The sensing performance of ML-based IR was evaluated using absolute sensitivity (S_a) and relative sensitivity (S_r), with the values of S_a and S_r estimated using eqn (3) and (4):²⁴

Fig. 6 (a) ML spectra of SMCP:0.4Tb³⁺,0.04Mn²⁺ at various temperatures from 25 to 200 °C; (b) ML ratio (MLR) I_Tb/I_Mn in SMCP:0.4Tb³⁺,0.04Mn²⁺ as a function of working temperature; the blue line shows the corresponding fitting curve (MLR = 0.23 + 2.83exp(−4806/T)); (c) relative sensitivity (S_r) and absolute sensitivity (S_a) of temperature sensing based on the MLR in SMCP:0.4Tb³⁺,0.04Mn²⁺; and (d) MLR of Ce/Tb, Ce/Mn and Tb/Mn of SMCP:0.4Tb³⁺,0.04Mn²⁺ samples after 4 cycles at 25 °C.

Here, the calculated results are shown in Fig. 6c. With the increase of temperature, S_a increases gradually, while S_r increases first and then decreases, reaching the maximum value at 363 K. In order to ensure the reliability of temperature sensing based on the force-luminescence intensity ratio, the ML spectra under different forces were tested at room temperature. Fig. 6d shows the ML intensity ratios of each pair of luminescent centers, and it was found that the ML intensity ratios of Ce³⁺/Tb³⁺, Ce³⁺/Mn²⁺ and Tb³⁺/Mn²⁺ all fluctuated within a certain range across different forces, which was attributed to their excellent chemical properties and thermal stability.

Research on temperature sensing and information encryption has traditionally focused on fluorescence and phosphorescence techniques, with relatively limited exploration of ML. Specifically, dynamic encryption based on dual-doped full-spectrum systems remains underdeveloped. ML, due to its self-powered nature and high sensitivity, is easily triggered, making it highly effective in temperature sensing and information encryption under harsh conditions. In this study, we demonstrate the information encryption advantages of ML-based devices. Fig. 7a illustrates the preparation process of the encryption device. First, devices with different color gamuts (SMCP: blue, SMCP:0.08Tb³⁺: cyan, SMCP:0.4Tb³⁺: green, SMCP:0.4Tb³⁺,0.1Mn²⁺: yellow-green, SMCP:0.4Tb³⁺,0.2Mn²⁺: yellow, SMCP:0.4Tb³⁺,0.4Mn²⁺: orange, and SMCP:0.4Mn²⁺: red) were cut into circular shapes. Then, these different colored discs with the PDMS organic module were combined and maintained at 80 °C for four hours to form the final encryption device. Fig. 7b presents the images of the device under stretching and under 365 nm and 254 nm UV light, where full-spectrum ML is evident in the stretched state. The force-stimulated readout feature of the encryption device suggests that authenticity can be verified by analyzing the intensity of the ML generated by a signature. Importantly, we propose a new strategy to improve the security of electronic signatures by combining ML materials with three different luminous colors, making individuals’ writing habits difficult to replicate (Fig. 7c(i)). Fig. 7c(ii) shows the stress distribution and the corresponding images of the letter “M”. The electronic signature device comprises three materials: #1 (SMCP:0.4Tb³⁺,0.4Mn²⁺), #2 (SMCP), and #3 (SMCP:0.4Tb³⁺), with the “M” divided into three regions, each exhibiting varying ML intensities based on the applied force. Fig. 7c(iii) illustrates the relative ML intensity using a pseudocolor map, from which the applied force in each region can be inferred, enabling the authentication of the information. The integration of #1, #2, and #3 stress sensors allows for the recording and differentiation of individual writing patterns, enabling precise authentication. This personalized, high-resolution, and full-spectrum ML-based anti-counterfeiting method, enhanced by new materials and intelligent technologies, is poised to become a key technology in future anti-counterfeiting efforts, offering significant improvements in security, environmental sustainability, and user accessibility.

Fig. 7 (a) Schematic diagram of the preparation of the mechanical display device; (b) device photos taken under different states (stretching, 365 nm UV, and 254 nm UV); (c) electronic signature: (i) schematic writing on the composite film; (ii) photographs of the process of recording handwritten traces on rectangular composite materials; and (iii) corresponding 3D distribution of relative ML intensity when writing “M”.

3. Conclusion

In conclusion, this study proposes a dual-doped full-spectrum stress and temperature sensor based on energy transfer. The device achieves dynamic tuning of PL and ML emission colors through energy transfer among Ce³⁺–Tb³⁺, Ce³⁺–Mn²⁺, and Tb³⁺–Mn²⁺ ions, covering a wide range from blue to green and red. Additionally, in the SMCP:0.4Tb³⁺,0.4Mn²⁺ material, the ML emission color adjusts with temperature variations. The constructed dual-modal dynamic sensor, responsive to both stress and temperature, enables remote visualization of stress and temperature distribution without external light excitation, with a maximum relative sensitivity (S_r) of 0.06% K⁻¹ and an absolute sensitivity (S_a) of 0.0235 K⁻¹. Finally, by leveraging an energy transfer strategy, this study successfully designs an encrypted electronic signature device using the full PL/ML spectrum, significantly enhancing information security.

4. Experimental section

Raw materials and sample synthesis

The phosphors SMCP:xTb³⁺ (0 ≤ x ≤ 0.4), SMCP:yMn²⁺ (0 ≤ y ≤ 0.5) and SMCP:xTb³⁺,yMn²⁺ (0 ≤ x ≤ 0.4, 0 ≤ y ≤ 0.4) were prepared via a solid state reaction at high temperature. The SrCO₃ (99%), Mg(OH)₂·4MgCO₃·5H₂O (99.99%), CeO₂ (99.99%), (NH₄)₂HPO₄ (99.99%), Tb₄O₇ (99.5%) and MnCO₃ (99.95%) powders were weighed in an appropriate stoichiometric ratio, mixed and ground in an agate mortar for 30 min with an appropriate amount of alcohol. Then the mixed powder was calcined at 1450 °C for 6 h under a 95% N₂ + 5% H₂ reductive atmosphere. Finally, the sample was taken out and ground again into powder for further measurement.

Preparation of PDMS-based ML composites

Construction of the composite device: polydimethylsiloxane (PDMS) acts as an elastic matrix to provide internal stress to the ML powder (Sylgard 184, Dow Corning). First, the powder was ground in an agate mortar for a few minutes to obtain more uniform particles. Second, 10 g of PDMS base resin and 1 g of curing agent were mixed in a disposable cup. A magnet was added and the mixture was stirred on a stimulating stirrer for half an hour. Finally, the stirred mixture was poured into a homemade glass grinding tool and solidified at 80 °C for 2 h to obtain a PDMS-based ML elastomer.

Measurements and characterization

The X-ray diffraction (XRD) data of the samples were used for phase characterization and crystal structure analysis using a Bruker D2 Advanced XRD. The samples were tested in the 2θ range from 10° to 80° at 40 kV and 40 mA. General structure analysis system (GSAS) software, which can offer Rietveld refinement analysis and a production structural model, was used. Software named “Diamond” was used to map the crystal structure of a sample. The morphology and element distribution of SMCP:0.4Tb³⁺,0.4Mn²⁺ were determined by scanning electron microscopy (SEM, TESCAN VEGA 3, China) and energy dispersive spectroscopy (SEM, TESCAN VEGA 3, China), respectively. The PL and PLE spectra were recorded using an FLS980 spectrofluorometer equipped with a Xe lamp (FLS980, Edinburgh, UK). ML spectra were recorded using a Marine optical spectrometer (FLAME-S-XP1-ES). The mechanical luminescence signal was detected using a multi-mode force luminescence detection system (QKLN-ML-2, China).

Data availability

The data that support the findings of this study are available in the ESI† of this article.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Natural Science Foundation of Shaanxi Province of China (2024JC-YBMS-042), the National Natural Science Funds of China (52303293), the Science and Technology Research Program of Chongqing Municipal Education Commission (KJZD-M202301302 and KJQN202301314), the National Natural Science Foundation of China (12374368), the Liaoning Revitalization Talents Program (XLYC2203011), the Science Foundation for Outstanding Youth of Liaoning Province (2024JH3/10200050), the Natural Science Foundation of Chongqing (Grant No. CSTB2023NSCQ-MSX0003) and the Fundamental Research Funds for the Central Universities (31920240125-06).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi03338h

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