Intense and repeatable orange mechanoluminescence of Mn²⁺ activated CaGa₄O₇ for visualized mechanics sensing

Abstract

Because of the unique and attractive mechanics–optics conversion without additional energy consumption, mechanoluminescence (ML) has shown widespread prospects in visualized mechanics sensing. However, the current ML-based sensing technique mainly relies on short-wavelength materials, which limits its application range. In this work, a novel ML material, Mn²⁺-doped CaGa₄O₇ (CGOM), is synthesized, and the composite elastomer CGOM/polydimethylsiloxane (PDMS) exhibits an intense and repeatable orange ML with a relatively long wavelength range of 500–710 nm. The initial ML intensity of the CGOM/PDMS is about 2.8 times that of ZnS:Cu/PDMS under the same testing conditions, which makes its ML visible in ambient light. The ML mechanism conforms to the interfacial triboelectricity-induced electron bombardment model, based on which the repeatable ML is realized without a pre-charging process. In addition, the ML of the CGOM/PDMS shows good responsiveness to the applied friction and tensile parameters. Based on these features, the CGOM/PDMS is further applied to visualized mechanics sensing by simulating the organism and the competitive sports scenarios.

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1. Introduction

Mechanoluminescence (ML) materials are smart functional materials that can convert mechanical energy into light signals under mechanical stimuli, such as tension, friction, compression and fracture.¹ This direct force–light conversion process does not require additional energy input or external circuits. Besides, the ML materials show numerous advantages in terms of tunable emission color, wireless detection, and accurate responsiveness.^2–5 Therefore, ML materials show particular potential for visualized mechanics sensing applications, such as stress sensing,⁶ biomechanics monitoring,⁷ smart skin,^8,9 and human–computer interfaces.^10,11

To date, a large number of ML materials have been developed, including sulphides (ZnS:Cu,¹² ZnS:Mn¹³), oxysulfides (CaZnOS:Mn¹⁴), fluorides (MgF₂:Mn,¹⁵ CaF₂:Tb¹⁶), aluminates (SrAl₂O₄:Eu,Dy,¹⁷ CaAl₂O₄:Eu¹⁸), phosphates (Ca₆BaP₄O₁₇:Ce,¹⁹ Sr₂P₂O₇:Eu,Y,²⁰ Ca₉Bi(PO₄)₇:Ce²¹), gallates (LiGa₅O₈,²² CaGa₅O₆:Tb²³), etc. However, most of the ML materials at present exhibit visible ML with short emission wavelength (blue or green), which have a weak penetration ability for biological tissues, limiting its application in biomechanics sensing and health monitoring. Some reported near-infrared ML materials, such as CaZnOS:Nd (850–1000 nm),²⁴ LiNbO₃:Nd (875–950 nm),²⁵ SrGa₁₂O₁₉:Cr (700–900 nm),²⁶ LiGa₅O₈:Cr (700–800 nm),²⁷ and Ga₂O₃:Cr (600–1100 nm),²⁸ can be used for biomechanics monitoring, but cannot realize the visualization of mechanical information by the naked eye. Recently, Liu et al. developed Y₃Al₅O₁₂:Cr (650–850 nm),²⁹ which can be embedded in chicken feet to display joint stress conditions.³⁰ However, the low light intensity can only be captured in a completely dark environment. ZnS:Mn and CaZnOS:Mn are reported to exhibit bright orange ML with desirable cycling stability under compression.^14,31 However, their flexible composites with polydimethylsiloxane (PDMS) show weak ML intensity and non-repeatability under stretching stimulus due to the largely changed modes by mechanics, which reduces its suitability for biological applications. Therefore, the development of long-wavelength visible ML flexible composites with high brightness and repeatability for biomechanical sensing is urgently needed.

Compared with the f–f narrow band transition of rare earth ions, the d–d broadband transition of transition metal ions shows greater application advantages in the fields of luminescence displays and sensing.^26,32–35 As a typical transition metal ion luminescence center, Mn²⁺ ions can emit light in the range of 400–800 nm depending on their coordination environment.^36,37 More importantly, Mn²⁺ ions have the advantages of abundant sources, wide excitation bandwidths, and high emission intensities.^38,39 In addition, CaGa₄O₇ belongs to the monoclinic crystal system and has a centrosymmetric structure with space group C2/c (No. 15), which is widely used as a matrix for luminescent materials.^40–44 The research by Piotrowski et al. shows that high-brightness orange PL could be obtained by incorporating trace amounts of Mn ions into the CaGa₄O₇ matrix.⁴⁰ Hence, the combination of Mn²⁺ and CaGa₄O₇ inspires us to prepare ML materials with long wavelength emission.

In this work, Mn²⁺-doped CaGa₄O₇ (CGOM) powders were synthesized, which were further composited into a flexible PDMS matrix to investigate the ML performance. The use of PDMS can ensure the flexibility and the mechanics transfer ability which can be driven by biomechanics. It is found that the as-fabricated CGOM/PDMS composites exhibit intense orange ML in the wavelength range of 500–710 nm. For the initial stretch, the ML intensity of CGOM/PDMS even exceeds that of the commercial ZnS:Cu/PDMS under the same tensile conditions. The ML of the CGOM/PDMS composite elastomer also shows good mechanical responsiveness and repeatability under stretching and friction stimuli. The force–light conversion mechanism is explained based on the interfacial triboelectricity-induced electron bombardment model. By utilizing the tissue penetration and mechanics visualization abilities of the long-wavelength visible ML, this work further validates the concept of visualized biomechanics detection.

2. Experimental

2.1 Synthesis of ML powders

The Ca_(1−x)Ga₄O₇:xMn powders were synthesized via a high-temperature solid-state method. CaCO₃ (Aladdin, 99.99%), Ga₂O₃ (Aladdin, 99.99%), and MnCO₃ (Aladdin, 99.95%) were employed as the raw materials, which were weighted in accordance with the stoichiometric ratio. The mixture was initially ground in an agate mortar for 30 min, and was then transferred into a covered alumina crucible. Subsequently, the mixture was calcined in a tube furnace under a reducing atmosphere of 90% N₂ and 10% H₂ at 1473 K for 6 h. The heating and cooling rates were maintained at 5 K min⁻¹. After the temperature dropped to 773 K, the mixture was naturally cooled to room temperature. Finally, the obtained samples were ground into powders for further use.

For the synthesis of MgF₂:Mn, ZnS:Mn and CaZnOS:Mn powders, MgF₂ (Aladdin, 99.99%), MnCO₃ (Aladdin, 99.95%), ZnS (Bidepharm, 99.99%), MnCO₃ (Aladdin, 99.95%) and CaCO₃ (Aladdin, 99.99%) were weighed in accordance with the stoichiometric ratio. After being mixed evenly in an agate mortar, they were transferred into a tube furnace for sintering. The sintering atmosphere was nitrogen gas in all cases. The sintering temperature was 1573 K for ZnS:Mn, 1473 K for MgF₂:Mn, and 1373 K for CaZnOS:Mn, respectively. The heating and cooling rates were maintained at 5 K min⁻¹. After being kept at the preset temperature for 3 h, the samples were cooled to room temperature. The obtained samples were ground into powders for further use. It should be noted here that the employed ZnS:Cu ML powders were purchased from Shanghai Keyan Photoelectric Technology Co., Ltd.

2.2 Fabrication of the CCOM/PDMS composite elastomer

The composite elastomers were fabricated by weighing PDMS precursors, PDMS curing agents, and CGOM powders in a Petri dish (diameter: 50 mm) based on the mass ratios. Then, the mixture of these three components was uniformly stirred and cured at 70 °C for 30 min.

2.3 Characterization

The crystal structure of CaGa₄O₇:Mn was characterized using an X-ray diffractometer (XRD-6100, Shimadzu, Japan) with a Cu-Kα radiation source and a scanning speed of 5° min⁻¹. The morphology and elemental distribution of CGOM were recorded using a scanning electron microscope (SEM, S-340, Hitachi) and an energy dispersive spectrometer (EDS, S-340, Hitachi), respectively. X-ray photoelectron spectroscopy (XPS) was carried out using an X-ray photoelectron spectrometer (ESCALAB 250 Xi instrument). The friction experiments were carried out using a multi-function friction tester (AS-5430, Ningbo Aosheng) equipped with an electrostatic measurement probe (SK050, KEYENCE (Japan) Co., Ltd). The tensile experiments were conducted on a universal uniaxial tensile testing machine (WDT-5, Tianshui Hongshan Testing Machine Co., Ltd). The luminescence signals were collected via a home-made optical fiber to a fluorescence spectrometer (Omni-λ300i, Zolix Instruments Co., Ltd) equipped with a CCD camera (iVac-316, Edmund Optics Ltd) and a photomultiplier tube (R943-02, Hamamatsu). Optical photos were captured using a digital camera (Canon EOS 77D). The thermoluminescence (TL) curves were measured using a TL meter (FJ427A1, Beijing Nuclear Instrument Factory). The cathodoluminescence (CL) spectra of CGOM were tested using a modified Mp-Micro-S instrument adsorbed on the SEM. The heat-treatment temperature was controlled using the THMS 600 heating stage from Linkam (temperature stability of 0.1 K, and setpoint resolution of 0.1 K). All curves were plotted and fitted using Origin software.

3. Results and discussion

CGOM samples were synthesized by the solid-state method under a reducing atmosphere. As illustrated in Fig. 1a, CaGa₄O₇ belongs to the monoclinic crystal system with the space group of C2/c (No. 15), featuring a centrosymmetric structure, which can be dissected into a Ca–O₅ trigonal bipyramid and Ga–O₄ tetrahedra. In the crystal structure, all Ga–O₄ tetrahedra are mutually interconnected via a shared vertex, and the interconnected Ga–O₄ tetrahedra form extensive tunnels at the center, which are occupied by Ca²⁺. The effective ionic radii (EIR) of the 5-coordinated Ca²⁺ are approximately 94 pm, and that of 4-coordinated Ga³⁺ is 47 pm.⁴⁰ Given that the EIR of Mn²⁺ is 75 pm, Mn²⁺ would substitute for Ca²⁺ with a comparable radius. To explore the impact of doped Mn²⁺ on the crystal structure, XRD analysis and Rietveld refinement were executed (Tables S1 and S2, ESI†). The results in Fig. S1 (ESI†) manifested that the diffraction peaks of all samples could be indexed to the standard card of CaGa₄O₇ (PDF#71-1613). According to the XRD refinement result in Fig. 1b, the residual factors (R_wp = 9.27% and R_p = 6.86%) are small, affirming that the phase purity is unaltered after the doping of Mn²⁺. Moreover, we investigated the peak shifts by amplifying the diffraction peak at 19.5°, and calculated the values of cell volume. Due to the smaller ion radius of Mn²⁺ than Ca²⁺, the diffraction peak gradually shifts to a large angle with an increase in the Mn²⁺ doping concentration from 0.25% to 3%, as shown in Fig. S2a and b (ESI†). Meanwhile, Fig. S2c (ESI†) shows that the cell volume decreases linearly with the increase of the Mn²⁺ doping contents. These results in terms of the peak shifts and cell volume changes confirm that Mn²⁺ has been successfully doped into CaGa₄O₇. The XPS survey spectra and high-resolution XPS spectrum of Mn 2p in Fig. S3 (ESI†) show that the synthesized phosphor was composed of the four elements of Ca, Ga, O, and Mn, and the valence of Mn cations is +2. From the SEM image in Fig. S4 (ESI†), it could be discerned that the particles present an irregular shape with relatively smooth surfaces, and a broad size distribution within the range from 4 to 20 μm. Such smooth particles were potentially induced by a relatively rapid cooling rate after high-temperature sintering. EDS analysis was undertaken on a randomly selected particle, and the uniform distribution of all targeted elements (Ca, Ga, O, and Mn) within a single particle structure was validated. The above results demonstrate that a high quality CGOM sample has been successfully synthesized.

Fig. 1 (a) Crystal structure and (b) refined XRD spectrum of the CGOM powders. (c) Optical photograph (I), SEM image of the cross-section (II), and (d) ML spectra under pressing, rubbing, and stretching stimulus of the CGOM/PDMS composite elastomer. (e) ML spectra of the CGOM/PDMS composites with different ratios of CGOM: PDMS, and (f) with different Mn²⁺ doping concentrations under the stretching stimulus (stretching frequency: 3 Hz, strain: 60%; one of three experimental groups is employed to draw the spectra in (f)).

To facilitate ML investigations, the CGOM powder was incorporated into the PDMS matrix to form a composite elastomer, as depicted in Fig. 1c. It can be observed from the cross-sectional SEM image (Fig. 1c) that the CGOM particles are well dispersed in the PDMS matrix and the thickness of the elastomer is around 922.73 μm. When mechanical stimuli (tension, friction, and compression) are applied, the CGOM/PDMS composite elastomer exhibits intense orange ML, which can be readily observed by the naked eye. Compared to the ML and PL spectra in Fig. 1d and Fig. S5 (ESI†), it is discerned that all the emission peaks exhibit identical shape in the broad range from 500 to 710 nm. This result indicates that the ML and PL emissions undergo the same radiative transition process, which can be attributed to the electronic transition of Mn²⁺ between ⁴T₁ and ⁶A₁. To evaluate the crystal field strength and confirm the occupation of Mn²⁺ in CGOM, the fine PL excitation spectrum was tested as shown in Fig. S6 (ESI†). The broadband peak at ca. 250 nm originates from the charge transfer band and ⁶A_1g(⁶S) → ⁴E_g(⁴D) transition.⁴⁵ In addition, the excitation peaks at 289, 354/373, 413/428, and 509 nm, are attributed to ⁶A_1g(⁶S) → ⁴T_2g(⁴D), ⁶A_1g(⁶S) → ⁴A_1g/⁴E_g(⁴G), ⁶A_1g(⁶S) → ⁴T_2g(⁴G), and ⁶A_1g(⁶S) → ⁴T_1g(⁴G) transitions of Mn²⁺, respectively.⁴⁵ Utilizing the correction by Trees,^45,46 the obtained crystal field parameter 10Dq/B is 12.82, indicating a strong crystal field strength of Mn²⁺ in CGOM. Based on the literatures, tetrahedrally coordinated Mn²⁺ usually emits in the green light range, as witnessed in MgAl₂O₄,^47,48 while octahedrally coordinated Mn²⁺ ions tend to emit in the red light range above 600 nm.^49,50 Therefore, the orange luminescence of CaGa₄O₇:Mn²⁺ suggests that Mn²⁺ ions should occupy the 5-coordinated Ca²⁺ sites in the structure of CaGa₄O₇.

Additionally, the composite ratio between the CGOM powders and the PDMS matrix (powder-gel ratio) is the crucial factor to optimize the ML intensity. We established four gradients of the powder-gel ratio ranging from 0.25 : 1 to 1.50 : 1 to prepare the composite elastomers, and found that the ML intensity of the composite elastomer increases along with the increase in the powder proportion as shown in Fig. 1e. This is because the increasing ML powder in the composite elastomer provides more luminescence centers. Nonetheless, with the increase in the ML powder concentration, the elastic modulus of the composite elastomer increases, and the stress–strain curve deviates from the linear relationship, and the fracture point is also advanced (Fig. S7, ESI†). After comprehensive comparison, the optimal powder-gel ratio was determined to be 0.50 : 1, in which a relatively high ML intensity and enhanced flexibility can be sustained. Moreover, the effect of Mn²⁺ doping amounts on the ML intensity was investigated under the same tensile conditions (tensile frequency: 3 Hz, tensile strain: 60%), as shown in Fig. 1f. It is found that the ML intensity of the composite elastomer initially increases and then decreases with the increase in the content of Mn²⁺ (the inset in Fig. 1f clearly shows this changing trend), and the optimal doping content of Mn²⁺ is determined to be 1%.

After optimizing the ML of the CGOM/PDMS composite elastomer, the mechanical responsiveness is further characterized in rubbing and stretching modes. Fig. 2a presents a schematic illustration and photographs of the rubbing process between a glass rod and the CGOM/PDMS composite elastomer. Obviously, the ML of the composite elastomer in the rubbing state can be facilely captured by the naked eye and cameras even in the bright environment. As shown in Fig. 2b and c, the ML intensity is modulated by altering the friction force and frictional speed. The ML intensity of the CGOM/PDMS composite elastomer linearly enhances with the increase of the applied load and speed. Analogously, Fig. 2d exhibits the schematic diagram and stretching state photographs of the CGOM/PDMS composite elastomer. The ML of the stretched composite elastomer can be readily observable even in a bright environment. The two key attributes of stretching are the stretch frequency and strain. Fig. 2e and f reveal that the ML intensity of CGOM/PDMS linearly increases with the increase of the stretch frequency and strain (the stress escalates from 0.4 to 1.7 MPa as revealed in Fig. S7, ESI†). Based on the above results, the ML of the CGOM/PDMS composite elastomer exhibits good mechanical responsiveness under rubbing and stretching modes. Furthermore, in order to quantificationally expound the ML intensity of the CGOM/PDMS composite elastomer, a comparison in ML intensity was made among CGOM/PDMS, MgF₂:Mn/PDMS, CaZnOS:Mn/PDMS, and ZnS:Cu/PDMS under the identical tensile conditions. It is observed that the CGOM exhibits the highest ML intensity in the PDMS matrix at the initial stretch, as depicted in Fig. S8 (ESI†). It should be noted that we can only compare the initial ML intensity at present due to the largely decreased intensity after cyclic stimulus. In future, we may further enhance the cyclic stabilization properties of the ML materials by achieving the reversible and self-repairable interfacial interactions.

Fig. 2 (a) Photograph and schematic diagram of the CGOM/PDMS composite elastomer when rubbed. (b) ML spectra of the CGOM/PDMS when rubbed under various loads (friction rotation speed: 240 rpm), the inset shows the relationship of ML intensity versus the load. (c) ML spectra under diverse rubbing speeds (load: 64 kPa), the inset is the relationship of ML intensity versus the rotation speed. (d) Photograph and schematic diagram of the CGOM/PDMS composite elastomer under stretching. (e) ML spectra under different stretch frequencies (strain: 60%), the inset is the relationship of ML intensity versus the frequency. (f) ML spectra under different tensile strains (frequency: 5 Hz), the inset shows the relationship of ML intensity versus the strain.

In view of the high-brightness ML of the CGOM/PDMS composite elastomer, the mechanics-optics conversion mechanism is attractive and worth studying. As depicted in Fig. 3a, it is found that there are profuse trap structures within the CGOM samples at room temperature, and the carriers within the traps can be thoroughly eliminated after heat treatment at 523 K for 6 min. However, the ML intensity of the CGOM/PDMS composite elastomer is not significantly attenuated even after the heat treatment of 523 K for 6 min (Fig. 3b). In addition, the ML intensity of CGOM/PDMS gradually decreases and approaches a stable value under continuous stretching cycles, but the UV pre-irradiation process cannot restore the ML in Fig. 3c. These results substantiate that the ML of the CGOM/PDMS composite elastomer is not contributed by the traps, which is different from many of the reported trap-dependent ML materials.^51–53 To further investigate the ML physics, the luminescence behaviors of CGOM powder and its composites in diverse matrices were studied. As shown in Fig. 3d(I), weak emission was observed through directly grinding the CGOM powders, while CGOM/epoxy resin (ER) did not emit light under mechanical compression. Combined with the result of the centrosymmetric structure, it can be confirmed that the ML of CGOM powders is fractured luminescence, and should be non-piezo-dependent.^21,54 As shown in Fig. 3d(II), when the powder is stimulated by force, large particles crack or break into small particles. During this process, chemical bond breakage forms an interfacial electric field to generate ML.⁵⁵ Moreover, as depicted in Fig. 3f and e, it is found that the ML of the CGOM composite elastomer is selective to the matrix, i.e., the CGOM/PDMS and CGOM/silicone gel (SG) manifested the ML under mechanical stimuli (stretching, compression, friction) with different brightness, while the CGOM/polyurethane (PU) and CGOM/ER cannot emit light. This result indicates that the CGOM can produce ML only in flexible matrices. Under mechanical stimuli, there is a relative displacement between the CGOM and the flexible matrices owing to a large deformation. Accordingly, the interfacial friction caused by the relative displacement should be the critical role for the generation of ML.

Fig. 3 (a) TL spectra of CGOM, and (b) ML spectra of CGOM/PDMS before and after heat treatment at 523 K for 6 min, in which the sample was firstly irradiated by a 254 nm light for 6 min. (c) Influence of UV irradiation on the ML intensity of the CGOM composite elastomer under stretching cycles. (d) (I) Photographs of the CGOM powder and CGOM/ER; (II) schematic diagram of the ML mechanism of the CGOM powders. (e) ML photographs of CGOM in different matrices. (f) Triboelectric potential, and (g) the attenuation curves of various materials after rubbing against CGOM for 1 min. (h) Normalized CL and ML spectra of CGOM samples. (i) Schematic illustration of the interfacial triboelectrification induced ML mechanism of the CGOM/PDMS composite elastomer.

To verify the role of interfacial friction, the triboelectric potentials of ER, PU, SG, and PDMS were tested respectively after rubbing with the CGOM hard block for 60 s. As shown in Fig. 3f and g, the ER and PU lost electrons to generate positive potential, while the SG and PDMS gain electrons to generate negative potential with the triboelectric potential gradually decayed within the subsequent 100 s. The PDMS shows a higher negative potential than that of SG, which is consistent with the ML intensity. Fig. S9 (ESI†) shows the triboelectric potential attenuation curves of CGOM after rubbing with PDMS, SG, PU, and ER, respectively, which agree well with those in Fig. 3f and g. Combined with the results of Fig. 3e, the ML of the CGOM-based flexible composites should be related to the interfacial triboelectricity, and the electrons should transfer from CGOM to matrices to form an interfacial triboelectric field. Recently, our group proposed an interfacial triboelectricity-induced electron bombardment model,⁵¹ which can explain the ML phenomenon of many composite elastomers. ML materials of this mechanism should fulfill the following three conditions: (I) non-piezoelectricity and independent of traps; (II) there should be a triboelectric role between the ML powder and the matrix, and the electrons should transfer from the powder to the matrix; (III) the ML powder has a radiation pathway under the bombardment of high-energy (HE) electrons. Herein, the ML of the CGOM sample satisfies conditions (I) and (II). Fig. 3h further reveals that the CGOM powders show luminescence under additional HE bombardment, which is called CL. The CL has a similar spectral characteristic to ML, indicating that the radiation of ML may be excited by the high-energy electron bombardment, which fulfills condition (III). Therefore, the ML processes of the CGOM/PDMS composite elastomer can be explained by the interfacial triboelectricity-induced electron bombardment model. Fig. S10 (ESI†) illustrates the electroluminescence (EL) test of the CGOM powders. First, the CGOM powders were put in the middle of a PVC pipe. An electrostatic generator with an output voltage of 30 kV was employed to generate the electric field, and the positive and negative electrodes were fixed at both ends of the pipe. The CGOM powders were directly excited by the electric field. After powering on, the CGOM powders were observed to emit orange light. The observed EL should be produced by the process in which the electrons were accelerated to collide with the luminescent centers, which is similar to the proposed interfacial triboelectricity-induced electron bombardment model in this work. Therefore, the observed EL signal supports the proposed ML mechanisms to a certain extent.

The specific ML mechanisms are depicted in Fig. 3e. Firstly, the external mechanical stimulus causes interfacial friction between the CGOM powder and the PDMS polymer chain, resulting in charge rearrangement and the formation of a frictional electric field. During this process, the electrons are transferred from the CGOM powders to the PDMS polymer chains to form an interfacial triboelectric field. Under the action of the triboelectric field, the electrons on the PDMS polymer chain are accelerated to bombard the CGOM particles, leading to the excitation of electrons from the ground state to the excited state. Finally, the excited electrons generate orange ML through the radiative transition from ⁴T₁ to ⁶A₁ of Mn²⁺.

To further prove the role of interfacial friction between CGOM particles and PDMS polymer chains, a set of experiments were designed. As depicted in Fig. 4a, the CGOM/PDMS composite elastomer underwent cyclic stretchings in different directions. Specifically, the CGOM/PDMS was horizontally stretched for 5 cycles first, and then vertically stretched for another 5 cycles. Initially, the PDMS polymers and CGOM particles were in intimate contact in the CGOM/PDMS composite elastomer (Fig. 4a(I)). When the stretching force was applied in the horizontal direction, the PDMS polymer chains and CGOM particles exhibited a relative separation/displacement with the interfacial friction in the horizontal direction, generating an interfacial triboelectric field. Under the triboelectric field, the CGOM presented bright ML (Fig. 4a(II)). However, due to the hysteresis effect of the polymer chains,⁵⁶ the relative displacement in the horizontal direction between the PDMS polymer chains and the CGOM particles could not be fully restored immediately (Fig. 4a(III)). Thus, the interfacial friction action was significantly decreased during the continuous stretching cycles. Then, the stretching direction was shifted from the horizontal direction to the vertical direction. Since the majority of the hysteresis effect existed in the horizontal direction, a relatively intense ML could still be generated from the interfacial friction in the vertical direction (Fig. 4a(IV)). The elastic hysteresis in this case would also occur, causing a decrease in the interfacial friction action in the vertical direction during cyclic stretchings (Fig. 4a(V)). Simultaneously, when stretched vertically, the hysteresis of the elastomer in the horizontal direction could be recovered. Thus, when the stretching direction reverted back to the horizontal direction, the interfacial friction could be remarkably restored. As shown in Fig. 4b, the ML intensity of CGOM/PDMS presents virtually the same variation trend as the interfacial friction action, and the ML intensity is partially restored after each stretching direction is switched. These results demonstrate the role of the interfacial friction in ML generation, which supports the proposed ML mechanism in Fig. 3.

Fig. 4 (a) Schematic diagram of the interfacial interaction between CGOM particles and PDMS polymer chains under continuously stretching in different directions. (b) ML intensity variations of the CGOM/PDMS composite elastomer when continuously stretched in different directions (time interval: 60 s). (c) ML repeatability tests of the CGOM/PDMS under different time intervals of continuous stretching.

Given this triboelectricity-involved ML mechanism, the produced ML is supposed to be reproducible and self-recoverable without a pre-charging process. Herein, the ML intensity of CGOM/PDMS was measured under multiple stretchings with different time intervals. As shown in Fig. 4c, the ML intensity gradually decays and then stabilizes under stretching, and the ML can be repeated up to 100 times. At the initial stretches, the ML intensity drops sharply due to irreversible breaking of hydrogen bonds or van der Waals forces. Importantly, the stable ML intensity during the multiple stretching process increases with the extension of the time interval between the two adjacent stretches. Due to the elastic hysteresis of the CGOM/PDMS composite elastomer, the recovery of the elastomer should take a certain time. A longer time interval contributes to the higher degree of self-recovery, thereby generating a stronger ML during continuous stretching.

In view of the visible, repeatable and mechanics responsive features of the long wavelength ML of the CGOM/PDMS, it shows high application value in the non-contact and wireless mechanical sensing. As shown in Fig. 5a and b, we simulated the scenario of CGOM/PDMS for biomechanical detection. The ML of the CGOM/PDMS composite elastomer can effectively penetrate a 4 mm-thick piece of pig meat to be detected or seen by the naked eye. Fig. 5c shows the detected ML spectra after penetrating the tissue under different strains. With the increase of the applied strain from 20% to 100%, the penetrated ML signal gradually increases, which enables CGOM/PDMS composite elastomer to quantificationally monitor the biomechanics in organisms. Moreover, the CGOM/PDMS can be used to record personalized handwriting. As depicted in Fig. S11 (ESI†), the ML photos and intensity mapping images present both the writing habit and strength. Finally, we demonstrate the potential of CGOM/PDMS in competitive sports (such as archery, shot put and pole vaulting). In Fig. S12 (ESI†), some special equipment with a ML layer of CGOM/PDMS composite elastomer was fabricated, such as a target, landing area and pole. As shown in Fig. 5d, when the arrow or shot hit the target or landing area, a light signal from the ML layer was detected. The intensity of the luminous signal is positively correlated with the force exerted upon the target. In the case of pole vaulting, the larger the deformation of the pole got, the stronger the detected ML signal became. Furthermore, we can observe the stress distribution of the equipment when they are stimulated by external forces. Therefore, for these competitive sports, the ML of the CGOM/PDMS could be utilized to achieve the precise hit position information and stress distribution on the sports equipment, which provides guidance for athletes to improve their performance.

Fig. 5 (a) Schematic illustration of the biomechanics detection by CGOM/PDMS (created with https://BioRender.com). (b) Photograph of the biomechanics detection platform by penetrating the pork meat. (c) ML spectra after penetrating the pork meat under different strains, the inset is the ML photograph under 100% strain. (d) Stress visualization of the sports equipment for competitive sports by the ML of CGOM/PDMS composite elastomer (the diagrams were created with https://BioRender.com).

4. Conclusions

In summary, CaGa₄O₇:Mn²⁺ samples were synthesized, and their ML properties were investigated by fabricating PDMS-based composites. Under mechanical stimulation, the CGOM/PDMS composite elastomer exhibits bright orange ML in the wavelength range of 500–710 nm. The ML of the CGOM/PDMS shows good mechanical responsiveness under the stimulation of friction or tension. Through the study of TL, CL, triboelectric potential, and the stretching experiments, it is suggested that the ML of the CGOM/PDMS fulfills the interface triboelectricity-induced electron bombardment model, based on which the CGOM/PDMS exhibits a certain ML repeatability. The visible orange ML of the CGOM/PDMS was further confirmed to have good biological penetration which is promising for the visualized biomechanics sensing application. Moreover, the CGOM/PDMS composite elastomer is also suggested to be useful for the visualized mechanics sensing in competitive sports.

Author contributions

Y. He, S. Fang and Z. Wang conceived and designed the study. Y. He developed the synthesis method. F. Long and B. Tian assisted in the sample preparation and data analysis. J. Wang, J. Qin, and S. Feng helped to characterized the materials and ML performance. Y. He wrote the first draft of the manuscript. Z. Wang and S. Fang reviewed and edited the manuscript. Z. Wang secured funding for the project. All authors discussed the results and commented on the paper.

Data availability

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

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB 0470201), the Taishan Scholars Program, the Supporting Fund of Yantai for Leading Researcher, the Shandong Province Natural Science Foundation (ZR2023QB249), and the Key Program of the Natural Science Foundation of Gansu Province (25JRRA471).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc04960h

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