Self-recoverable mechanoluminescence in simple oxides: Al(2)O(3):Cr.

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Learn more: PMC Disclaimer | PMC Copyright Notice Light Sci Appl . 2026 Apr 15;15:200. doi: 10.1038/s41377-026-02274-w Search in PMC Search in PubMed View in NLM Catalog Add to search Self-recoverable mechanoluminescence in simple oxides: Al 2 O 3 :Cr Ziyi Fang Ziyi Fang 1 College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060 China Find articles by Ziyi Fang 1, # , Xiaofeng Pan Xiaofeng Pan 2 Key Laboratory of Quantum Materials and Devices of Ministry of Education, School of Physics, Southeast University, Nanjing, 21189 China Find articles by Xiaofeng Pan 2, # , Qi’an Zhang Qi’an Zhang 1 College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060 China Find articles by Qi’an Zhang 1, # , Mingzhi Wu Mingzhi Wu 1 College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060 China Find articles by Mingzhi Wu 1 , Yang Liu Yang Liu 1 College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060 China Find articles by Yang Liu 1 , Qidong Ma Qidong Ma 1 College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060 China Find articles by Qidong Ma 1 , Biyun Ren Biyun Ren 1 College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060 China Find articles by Biyun Ren 1 , Yanze Wang Yanze Wang 1 College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060 China Find articles by Yanze Wang 1 , Shengqiang Liu Shengqiang Liu 1 College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060 China Find articles by Shengqiang Liu 1 , Maryam Zulfiqar Maryam Zulfiqar 1 College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060 China Find articles by Maryam Zulfiqar 1 , Ming-Gang Ju Ming-Gang Ju 2 Key Laboratory of Quantum Materials and Devices of Ministry of Education, School of Physics, Southeast University, Nanjing, 21189 China Find articles by Ming-Gang Ju 2, ✉ , Jiulin Gan Jiulin Gan 3 State Key Laboratory of Luminescent Material and Devices, and Guangdong Provincial Key Laboratory of Fibre Laser Materials and Applied Techniques, Guangdong Engineering Technology Research and Development Center of Special Optical Fiber Materials and Devices, South China University of Technology, Guangzhou, 510641 China Find articles by Jiulin Gan 3, ✉ , Leipeng Li Leipeng Li 4 College of Physics Science and Technology, Hebei University, Baoding, China Find articles by Leipeng Li 4 , Feng Wang Feng Wang 5 Department of Materials Science and Engineering, City University of Hong Kong, Hong Kong SAR, China 6 Hong Kong Institute for Clean Energy, City University of Hong Kong, Hong Kong SAR, China Find articles by Feng Wang 5, 6, ✉ , Dengfeng Peng Dengfeng Peng 1 College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060 China 7 Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province,Shenzhen University, Shenzhen, Guangdong, 518060 China 8 China State Key Laboratory of Radio Frequency Heterogeneous Integration, Shenzhen University, Shenzhen, Guangdong, 518060 China Find articles by Dengfeng Peng 1, 7, 8, ✉ Author information Article notes Copyright and License information 1 College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060 China 2 Key Laboratory of Quantum Materials and Devices of Ministry of Education, School of Physics, Southeast University, Nanjing, 21189 China 3 State Key Laboratory of Luminescent Material and Devices, and Guangdong Provincial Key Laboratory of Fibre Laser Materials and Applied Techniques, Guangdong Engineering Technology Research and Development Center of Special Optical Fiber Materials and Devices, South China University of Technology, Guangzhou, 510641 China 4 College of Physics Science and Technology, Hebei University, Baoding, China 5 Department of Materials Science and Engineering, City University of Hong Kong, Hong Kong SAR, China 6 Hong Kong Institute for Clean Energy, City University of Hong Kong, Hong Kong SAR, China 7 Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province,Shenzhen University, Shenzhen, Guangdong, 518060 China 8 China State Key Laboratory of Radio Frequency Heterogeneous Integration, Shenzhen University, Shenzhen, Guangdong, 518060 China ✉ Corresponding author. # Contributed equally. Received 2025 Dec 17; Revised 2026 Mar 6; Accepted 2026 Mar 9; Collection date 2026. © The Author(s) 2026 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/ . PMC Copyright notice PMCID: PMC13083877  PMID: 41986338 Abstract Materials exhibiting mechanoluminescence (ML) that directly convert mechanical stimuli into light hold significant potential for real-time stress sensing and intelligent photonic systems. However, most high-performance ML systems rely on complex multicomponent compounds that often suffer from limited intensity, stability, and scalability, largely due to poorly understood mechanisms. Herein, we report a simple Al 2 O 3 :Cr 3+ oxide that exhibits unprecedented ML intensity, enabled by a well-defined mechanical-to-optical energy conversion process. The self-recoverable ML arises from stress-induced ionization of electrons from luminescence centers, followed by their recapture upon stress release. By precisely tuning the doping levels, annealing conditions, and heterojunction interfaces, Al 2 O 3 :Cr 3+ phosphors achieved intense, reproducible, and thermally stable near-infrared emission. Notably, high-temperature annealing dramatically enhanced the ML intensity, with thermodynamic and kinetic analyses revealing increases in the carrier and defect concentrations by several orders of magnitude, accounting for the exceptional brightness. By leveraging the chemical robustness, abundance, and low cost of alumina, we demonstrated the flexible ML paper for stress visualization and multi-level anti-counterfeiting, as well as in-situ grown Al 2 O 3 :Cr 3+ luminescent layers on Cr–Al alloys for passive, real-time stress monitoring. This study establishes Al 2 O 3 as a durable and scalable oxide platform for next-generation self-recoverable ML materials, bridging fundamental research and practical sensing technologies. Subject terms: Optical materials and structures, Optical physics A low-cost Al 2 O 3 :Cr 3+ platform exhibits self-recoverable mechanoluminescence (ML) via reversible Cr charge transitions. Defect engineering and Al 2 O 3 /Ga 2 O 3 heterojunctions produce strong ML for real-time stress visualization. Introduction Mechanoluminescence (ML) refers to the phenomenon in which a material emits light in response to mechanical stimuli such as fracture 1 , friction 2 , compression 3 , tension 4 , or ultrasound 5 , 6 . Unlike conventional electroluminescence, photoluminescence (PL) 7 , and chemiluminescence, ML enables the direct conversion of mechanical energy into light without requiring any external energy input source 8 , 9 . Owing to their real-time optical response, ML materials hold significant promise for applications in stress sensing and visualization 10 – 12 , multi-level anti-counterfeiting 13 , 14 , and biomedical technologies 15 – 17 . Current research on ML has been primarily focused on materials that emit in the ultraviolet (UV) to visible spectral range, such as SrAl 2 O 4 :Eu 18 , ZnS:Mn/Cu 19 – 23 , CaZnOS:Mn 24 , SrZnOS:Mn 1 , BaSi 2 O 2 N 2 :Eu 25 , Sr 2 P 2 O 7 :Pr 26 , MgF 2 :Mn 27 , CaF 2 :Tb 4 , SrF 2 :Pr 3 , and AlN 28 . However, visible light is susceptible to environmental scattering and background interference, which limits its effectiveness in complex environments. By contrast, near-infrared (NIR) ML materials offer greater penetration depth and reduced visible-light interference, making them well suited for specialized applications such as long-range detection, internal stress monitoring of structures, and tissue bioimaging. In parallel with these application demands, increasing attention has been paid to self-recoverable ML systems, in which luminescence can be repeatedly triggered under cyclic mechanical stimulation without external recharging. Several representative systems exhibiting cyclic ML responses have been reported in recent years, such as Sr 3 (BO 3 ) 2 :Pr 8 and SrZnOSe:Nd 15 . Nevertheless, NIR ML systems remain limited in number and are frequently non–self-recovering, relying on UV pre-excitation to generate trap-controlled emission (e.g., LiNbO 3 :Nd 29 ) or on irreversible fractoluminescence 30 . These mechanisms suffer from delayed response and lack of self-recovery, significantly restricting their utility in dynamic or real-time monitoring applications. Therefore, the development of novel NIR ML materials featuring low cost, self-recoverability, and high emission efficiency is essential for advancing their practical applications. Despite the growing interest in ML, its underlying mechanisms remain incompletely understood. Identifying specific defect states and probing ultrafast carrier dynamics under mechanical stress pose significant experimental challenges, causing most ML advancements to rely on empirical trial-and-error approaches. Consequently, the diversity of ML systems remains limited, and their development trails behind that of more established luminescent technologies. Current theoretical efforts primarily emphasize the calculation of defect formation energies and post-doping electronic structures; however, the pivotal question of how external stress modulates charge-carrier behavior and emission processes has been inadequately explored. Owing to its unique [Ar]3d 3 electronic configuration, Cr 3+ exhibits exceptional luminescent properties in the NIR region. Its emission spectrum includes sharp R-line emission ( 2 E → 4 A 2 , ~700 nm) and broad-band emission ( 4 T 2 → 4 A 2 , ~750–1200 nm), with the emission wavelength tunable via the modulation of the local crystal field 31 . This electronic configuration renders Cr 3+ particularly effective for NIR emission because its luminescence primarily arises from d–d transitions, which exhibit relatively high transition probabilities in the NIR range, thereby enabling efficient light emission. Compared with trivalent rare-earth ions, Cr 3+ ions possess the advantages of strong light absorption, broad absorption range, and continuously tunable emission peaks 32 . In recent years, Cr 3+ has garnered significant attention in the design of ML materials (e.g., LiGa 5 O 8 33 , ZnGa 2 O 4 34 , Ga 2 O 3 13 , 35 , MgGa 2 O 4 36 , SrGa 12 O 19 34 , Gd 3 Ga 5 O 12 37 , MgO 38 , 39 , and LaAlO 3 40 ). Nonetheless, despite their promising performance, these materials are constrained by high production costs and complex crystal structures, which substantially limit their large-scale application and further commercialization. By contrast, Al 2 O 3 offers a structurally simple, earth-abundant, and industrially mature oxide host, yet its ML has remained largely unexplored compared with complex gallate- or garnet-based systems. In this study, we present Al 2 O 3 :Cr 3+ as a simple and durable oxide platform that delivers unprecedented ML intensity and practical usability. Its self-recoverable emission mechanism was clarified via theoretical calculations and experiments; ML self-recovery relies on reversible Cr Al 0 /Cr Al 1+ charge transitions, whereas NIR emission originates from radiative transitions between the 2 E to 4 A 2 states of Cr 3+ . We synthesized Al 2 O 3 :Cr 3+ phosphors using low-cost, high-temperature solid-state methods, optimizing the Cr 3+ concentration and annealing conditions. High-temperature annealing significantly enhances carrier and defect concentrations by orders of magnitude, yielding intense, stable, and thermally robust ML. Further optimization was performed using Al 2 O 3 /Ga 2 O 3 :Cr 3+ heterojunctions to enable synergistic defect and ML modulation. Leveraging the robustness, abundance, and scalability of alumina, we extended the material to two applications: (1) embedding Al 2 O 3 /Ga 2 O 3 :Cr 3+ powders into cellulose pulp for flexible NIR ML paper for invisible handwriting, stress mapping, anti-counterfeiting; and (2) performing in situ thermal oxidation of Cr–Al alloys to generate rigid Al 2 O 3 :Cr 3+ layers for real-time, excitation-free engineering stress visualization. Results Al 2 O 3 exhibited a direct bandgap of 8.61 eV, consistent with the experimentally measured value of 8.70 eV and demonstrated its intrinsic insulating characteristics (Fig. S1 ) 41 . With Cr doping, the atoms tended to substitute Al, forming Cr Al defects with CrO 6 octahedra (Fig. S2a ), as illustrated in Fig. 1a . To investigate the stress response mechanism underlying the ML process, we systematically calculated the formation energies of all intrinsic and extrinsic point defects in Al 2 O 3 :Cr 3+ under both stressed and stress-free conditions (Figs. S2 , S3 ). The phase stability of Al 2 O 3 :Cr 3+ was determined by calculating the total energies of competing phases within the Al–O–Cr chemical space (see Supplementary Information ). Considering the experimental synthesis conditions, the formation energies of all defects were calculated under O-rich equilibrium growth conditions. Notably, only minimal changes were observed in the formation energies (Fig. S2b, c ) and transition levels (Fig. S2d, e ) of most defects under stress. Interestingly, the Cr Al defect—which serves as the dominant active center in the Al 2 O 3 :Cr 3+ system—exhibited subtle changes under stress. To determine the stable form of defects under specific synthesis conditions, the evolution of the self-consistent Fermi level (E F ) with growth conditions is shown in Fig. S2f . Remarkably, the Cr Al defect was preferentially stabilized in a neutral state (Cr Al 0 ) under stress-free conditions, whereas applied stress induced a valence transition to the +1 charged state (Cr Al 1+ ), demonstrating a stress-driven ionization mechanism (Fig. 1b ). Compared with pristine Al 2 O 3 , the neutral state of Cr Al 0 created relatively shallow spin-up and spin-down defect states adjacent to the valence band maximum (VBM) and conduction band minimum (CBM), respectively (Fig. 1c ). Under applied strain, Cr Al 1+ produced distinct spin-polarized impurity states within the bandgap (Fig. 1d ). In addition, Cr Al 1+ exhibited a Jahn–Teller distortion, in which the bond length is typically contracted compared with the ground-state Cr Al 0 configuration (insets in Fig. 1c, d ). Upon stress release, the ionized electrons undergo detrapping and recombination, thereby repopulating the active centers and restoring the system to Cr Al 0 , which accounts for the observed self-recoverable ML behavior. Fig. 1. Theoretical investigation of self-recoverable ML emission in Al 2 O 3 :Cr 3+ . Open in a new tab a Crystal structure of Al 2 O 3 :Cr 3+ . b Calculated formation energies of Cr Al defects under stress-free and stressed conditions. Self-consistent Fermi levels are indicated by vertical dashed lines. Local atomic structure: Cr atoms (blue spheres) and charge density distribution (yellow isosurfaces). c Electronic densities of states (DOS) for the ground-state Cr Al 0 ( 4 A 2 ) defect under stress-free conditions, and the corresponding CrO 6 octahedral structure (blue sphere: Cr; red sphere: O). Arrow diagram showing three electrons of Cr occupying the t 2g energy level. d Electronic DOS for the ionized-state Cr Al 1+ defect under stressed conditions, and the corresponding CrO 6 octahedral structure (blue sphere: Cr; red sphere: O). Arrow diagram showing two electrons of Cr occupying the t 2g energy level. e Schematic diagram of the transitions between different states during electron excitation, relaxation, and recombination in Al 2 O 3 :Cr 3+ in the ligand field. f Electronic DOS for the excited-state Cr Al 0 ( 2 E) defect under stress-free conditions, and the corresponding CrO 6 octahedral structure (blue sphere: Cr; red sphere: O). Arrow diagram showing three electrons of Cr occupying the t 2g energy level, one with opposite spin. g Upon strain excitation, bound excitons stored in activators are ionized and excited to CBM (process 1), followed by relaxation (process 2) and radiative recombination between electrons in the excited states and holes in the ground state (process 3). The driving forces (denoted as D) in this process include band bending, enhanced dipole moment, and enhanced electron–phonon coupling To illustrate the luminescence process involving different electronic states, Fig. 1e depicts a schematic of the transitions between different states during electron excitation, relaxation, and recombination in Al 2 O 3 :Cr 3+ . In particular, the 3d 3 configuration of Cr 3+ induced by the ligand field was split into e g and t 2g energy levels 42 , 43 , corresponding to the formation of ground-state Cr Al 0 ( 4 A 2 ) (inset in Fig. 1c ) and low-energy excited-state Cr Al 0 ( 2 E) (inset in Fig. 1f ). Structural analysis showed that, compared with the ground state, the 2 E excited state exhibited obvious Jahn–Teller distortion, characterized by compressed equatorial bond lengths. The density of states (DOS) further indicated that the 2 E excited state exhibited stronger occupied states near the Fermi level, reflecting stronger interactions within the CrO 6 unit in the excited state and resulting in an overall higher energy (Fig. 1f ). Under applied stress, the ionized-state Cr Al 1+ remained stable. However, upon stress removal, Cr Al 1+ readily recaptured an electron, converting it into the neutral- and excited-state Cr Al 0 ( 2 E), which subsequently underwent radiative recombination to return to the ground state (Fig. 1e ). Therefore, the observed luminescence stemmed from electron transition between the excited and ground states ( 2 E → 4 A 2 ) 44 . The theoretical transition energies calculated using constrained density functional theory (cDFT) and the ΔSCF method were determined to be 1.8 eV after functional correction, showing an excellent agreement with the experimental result of 696 nm (1.79 eV) 44 . This work further systematically investigated the charge redistribution and band-structure modulation in Al 2 O 3 :Cr 3+ systems under external stimuli. Analysis of the average charge density difference (Δρ) and electrostatic potential profiles between the ground and excited states (Fig. S4 ) demonstrated stress-induced charge depletion (~ 0.2 e/Å) and potential change (~ 0.15 eV) around Cr 3+ sites, accompanied by enhanced dipole moments. This pronounced charge separation, comparable to that observed in transition-metal dichalcogenides 45 , establishes the fundamental driving force for electron migration. Combined non-adiabatic molecular dynamics (NAMD) and DFT calculations revealed strong electron–phonon coupling during ionization. Fourier transform analysis of state transitions at 300 K (Fig. S5 ) indicated that electron–phonon coupling was enhanced by more than 150% under strain 46 , which significantly reduced the electronic excitation barrier through non-adiabatic effects. Moreover, substantial restructuring of the band edge in Al 2 O 3 :Cr 3+ systems was triggered by a strain of 0–5% along the c-axis, as shown via rigorous band alignment correction 47 , 48 , resulting in the bending of VBM and CBM by 0.5 eV and 1 eV, respectively (Fig. S6 ). This strain-mediated hybridization of the band edge significantly enhances the coupling between shallow defect states and band edges 11 , creating additional carrier channels for radiative recombination. Upon stress application, the bound carriers stored in Cr Al activator sites were ionized, transitioning the defect charge state from the ground-state Cr Al 0 ( 4 A 2 ) to the ionized-state Cr Al 1+ . Driven by band bending, an enhanced dipole moment, and strengthened electron–phonon coupling, these carriers subsequently underwent relaxation. This process returned the defect charge state to the excited-state Cr Al 0 ( 2 E), ultimately leading to radiative recombination between electrons and holes at the luminescence center of Cr Al (Fig. 1g ). To investigate the effect of Cr 3+ doping on the crystal structure of Al 2 O 3 , Rietveld refinements were performed on Al 2 O 3 :1%Cr 3+ and undoped Al 2 O 3 to assess their phase purities (Fig. 2a, S7a ). The refinement results indicated that both samples exhibited a typical corundum phase structure (space group R-3c, No. 167), with good agreement between the experimental data and fitted curves. The refinement factors were R wp = 5.68% and R p = 4.33% for Al 2 O 3 , and R wp = 6.44% and R p = 4.38% for Al 2 O 3 :1%Cr 3+ . Compared with undoped Al 2 O 3 (a = b = 4.761 Å, c = 12.997 Å), Al 2 O 3 :1%Cr 3+ exhibited slightly increased lattice parameters (a = b = 4.763 Å, c = 13.004 Å), which can be attributed to the partial substitution of smaller Al 3+ ions (0.535 Å) by larger Cr 3+ ions (0.615 Å). Figure S7b shows a comparison of the X-ray diffraction (XRD) patterns of Al 2 O 3 :1%Cr 3+ and undoped Al 2 O 3 with the standard PDF#10-0173. All diffraction peaks matched well with the standard pattern. In addition, Fig. S8a, b show the XRD patterns of Al 2 O 3 :1%Cr 3+ samples annealed at different temperatures (1573–1923 K) and Al 2 O 3 :xCr 3+ samples with various doping concentrations (x = 0.01, 0.1, 0.5, 1, 2, and 4%) annealed at 1923 K, respectively. All diffraction peaks of Al 2 O 3 :Cr 3+ samples were consistent with the standard data for Al 2 O 3 (PDF#10-0173), indicating that Cr 3+ was successfully incorporated into the Al 2 O 3 lattice without altering the host phase structure. A schematic of the Al 2 O 3 crystal structure is shown in Fig. 2b , where oxygen ions form a hexagonal close-packed array and Al 3+ ions occupy partial octahedral interstitial sites, forming stable [AlO 6 ] coordination environments that provide ideal sites for Cr 3+ ion substitution. To further verify the crystal structure of Al 2 O 3 :1% Cr 3+ , high-resolution transmission electron microscopy (HRTEM) was conducted on samples prepared via focused ion beam milling. The HRTEM image clearly reveals the atomic arrangement of Al 2 O 3 (Fig. 2c ). The dark-blue region in Fig. 2c was subjected to fast Fourier transform (FFT), and the resulting diffraction pattern clearly exhibited the lattice features of the sample (Fig. 2d ). Fig. S9a shows an area scan of the light-blue region in Fig. 2c , revealing a pronounced increase in brightness in the central region, which is attributed to the incorporation of Cr 3+ ions. With a higher atomic number (Z = 24) than that of Al (Z = 13), Cr exhibits stronger electron scattering. As a result, the incorporation of Cr 3+ ions into the lattice led to enhanced local contrast in the HRTEM image 49 . This result provides additional evidence of the successful incorporation of Cr 3+ into the Al 2 O 3 lattice. Fig. S9b shows the line profile in Fig. 2d , from which the lattice spacing was measured to be 0.1428 nm. A selected area electron diffraction pattern of the sample is shown in Fig. 2e . Figure 2f shows the TEM image and the corresponding energy-dispersive X-ray spectroscopy (EDS) elemental mapping, confirming the presence of Al, O, and Cr in the sample. The elements were uniformly distributed throughout the sample, as observed in the mapping results. Additional scanning electron microscopy (SEM) images and EDS mappings (Fig. S10 ) further verified the uniform elemental distribution and reliability of the doping process at high Cr 3+ concentrations (4%). Fig. 2. Structure and composition of Al 2 O 3 : Cr 3+ . Open in a new tab a Rietveld refinement profile of Al 2 O 3 :1%Cr 3+ , confirming the corundum phase with high fitting accuracy. b Schematic illustration of the Al 2 O 3 crystal structure, where Al 3+ ions occupy octahedral interstices within a hexagonal close-packed oxygen framework. c HRTEM image showing the atomic lattice of Al 2 O 3 :1%Cr 3+ . d FFT pattern obtained from the dark-framed region in ( c ), revealing distinct lattice fringes and an interplanar spacing of 0.1428 nm. e Selected area electron diffraction pattern, confirming the crystalline nature of the sample. f Elemental mapping images of Al, O, and Cr from energy-dispersive X-ray spectroscopy, demonstrating a uniform elemental distribution in the Al 2 O 3 :1%Cr 3+ sample The effects of different Cr 3+ doping concentrations and annealing temperatures on the PL and ML properties of Al 2 O 3 :xCr 3+ were investigated. Fig. S11a shows the PL excitation (PLE) and PL spectra of Al 2 O 3 :xCr 3+ samples with various Cr 3+ concentrations (x = 0.01–4%) after annealing at 1923 K. When monitored at an emission wavelength of 694 nm, the PLE spectra exhibited two broad and intense absorption bands centered at 404 and 558 nm, corresponding to the spin-allowed transitions of Cr 3+ ions: 4 A 2 ( 4 F) → 4 T 1 ( 4 F) and 4 A 2 ( 4 F) → 4 T 2 ( 4 F). A sharp R-line emission at 694 nm was observed in the PL spectra, attributed to the spin-forbidden 2 E → 4 A 2 transition of Cr 3+ ions in a strong crystal field. This transition is spin-forbidden, and the resulting emission is narrow and exhibits high optical stability. The emission intensity was significantly influenced by the doping concentration, reaching a maximum at x = 0.5%. At lower concentrations (x < 0.5%), the emission intensity was weak due to an insufficient number of luminescent centers. At higher concentrations (x > 0.5%), the reduced distance between Cr 3+ ions facilitated energy transfer, promoting non-radiative transitions and resulting in a typical concentration quenching effect with significantly reduced emission intensity. Notably, the positions of excitation and emission peaks remained unchanged across all doping concentrations. Under excitation at 404 nm and monitoring at 694 nm, the fluorescence decay curves of the samples followed a single-exponential fitting function: I t = I 0 × e − t τ 1 where I t represents the fluorescence intensity at time t, I 0 is the initial intensity (at t = 0), and τ represents the fluorescence lifetime. Fig. S11b illustrates the influence of the doping concentration on the PL lifetime. As the Cr 3+ concentration increased, τ gradually decreased from 3.50 to 0.05 ms. This reduction is attributed to the shorter distances between Cr 3+ ions, which enhance non-radiative transitions 50 . Fig. S11c shows the PLE and PL spectra of Al 2 O 3 :1%Cr 3+ samples annealed at temperatures ranging from 1573 to 1923 K. Similarly, the annealing temperature did not affect the positions of excitation and emission peaks. However, the PL intensity increased with increasing annealing temperature, indicating that higher temperatures help optimize the local environment of Cr 3+ centers and improve radiative recombination efficiency. Fig. S11d shows the variation in the fluorescence lifetime with the annealing temperature. As the annealing temperature increased, τ gradually increased from 0.85 to 2.3 ms. Thermoluminescence (TL) measurements were performed to characterize the trap states in Al 2 O 3 :Cr 3+ (Fig. S12 ). A dominant TL peak centered at approximately 546 K indicates the presence of thermally stable traps. The corresponding trap depth, estimated using the improved peak position method, is about 1.41 eV. This indicates a much deeper trap depth compared with that of persistent luminescent phosphors of ~0.5–0.8 eV 33 . To evaluate ML performance, the phosphor was embedded in a polyethylene terephthalate (PET) matrix and tested using a custom-built setup. A representative demonstration of ML from the Al 2 O 3 :1%Cr 3+ –PET composite under mechanical scratching is provided in Video S1 . Meanwhile, infrared thermal imaging recorded during the scratching process confirms that no discernible surface temperature rise occurs during ML generation, excluding a thermal-origin contribution (Video S2 ). Figure 3a shows the ML spectra of samples with varying Cr 3+ doping concentrations under a constant applied force of 30 N. The peak positions and spectral shapes closely matched those of the PL spectra. The maximum ML intensity was observed at a doping concentration of 1% Cr 3+ , whereas higher concentrations led to luminescence quenching 51 . Notably, no detectable ML signal was observed for undoped Al 2 O 3 under identical testing conditions, further demonstrating that the ML behavior is induced by Cr 3+ doping rather than the host lattice itself. Figure 3b shows the ML spectra and corresponding intensities of samples annealed at different temperatures, all tested under a constant applied force of 30 N. With increasing annealing temperature, the ML intensity increased significantly, indicating that high-temperature annealing effectively enhances ML performance. Figure S13a, b summarizes the correlations among the integrated ML intensity, doping concentration, and annealing temperature, further confirming the trends discussed above. Fig. 3. ML properties of Al 2 O 3 :Cr 3+ and enhancement strategies. Open in a new tab a ML spectra of Al 2 O 3 :xCr 3+ samples (x = 0.01, 0.1, 0.5, 1, 2, and 4%) under an applied force of 30 N. b ML spectra of Al 2 O 3 :1%Cr 3+ samples annealed at different temperatures (1573–1923 K) under an applied force of 30 N. c ML spectra of Al 2 O 3 :1%Cr 3+ under varying applied loads ranging from 5 to 35 N. d Reported ML spectra of Cr 3+ -doped material systems and comparison of their integrated ML intensities. e ML repeatability of Al 2 O 3 :Cr 3+ under 7000 loading cycles at 10 N without any pre-irradiation. f ML spectra of Al 2 O 3 /Ga 2 O 3 :Cr 3+ heterojunctions with varying Al 2 O 3 to Ga 2 O 3 ratios, indicating compositional tuning effects on emission characteristics. g Integrated ML intensity of Al 2 O 3 /Ga 2 O 3 :Cr 3+ heterojunctions with varying Al 2 O 3 to Ga 2 O 3 ratios The strategies for enhancing the ML performance of Al 2 O 3 :Cr 3+ were systematically investigated. Fig. S13c shows the effect of varying holding times (1–6 h) on the ML intensity, and Fig. S13d shows the integrated ML intensity of Al 2 O 3 :1%Cr 3+ at different holding durations. The results indicated that the ML intensity initially increased and then decreased with prolonged holding time, reaching a maximum at 3 h. A further increase to 6 h led to a significant decline in intensity, suggesting the existence of an optimal thermal treatment window. Fig. S13e demonstrates the influence of different annealing atmospheres on ML performance for all samples annealed at 1923 K. Compared to single-atmosphere annealing, the sample treated by a stepwise annealing process—first in air for 2 h followed by an N 2 /H 2 mixed atmosphere for another 2 h—exhibited the highest ML emission intensity. This enhancement may be attributed to a synergistic optimization mechanism during stepwise atmospheric annealing: the initial air treatment stabilizes the Cr 3+ luminescent centers, while the subsequent N 2 /H 2 treatment introduces an appropriate concentration of oxygen vacancies (V O ), collectively optimizing the carrier trapping and release processes to boost the ML intensity. To satisfy the practical application requirements, the mechanical response characteristics of Al 2 O 3 :Cr 3+ were evaluated. Figure 3c illustrates the effects of varying external stress (5–35 N) on the ML intensity and its integrated intensity. The ML intensity increased progressively with increasing applied load, whereas the emission peak position remained unchanged, indicating an excellent mechanoresponsive behavior. Moreover, the integrated intensity exhibited an almost linear increase with the applied load (Fig. S13f ). Figure S14 further shows that the NIR ML intensity increases nearly linearly under higher applied loads from 1000 to 4000 N, demonstrating stable luminescence behavior even under large mechanical stress. Variable-temperature ML measurements were carried out from 233 to 373 K under identical mechanical loading conditions to evaluate the thermal robustness of Al 2 O 3 :1%Cr 3+ . Representative ML spectra at different temperatures are shown in Fig. S15a , while the corresponding temperature dependence of the integrated ML intensity is summarized in Fig. S15b . With increasing temperature, the ML intensity exhibits a gradual decrease. This behavior is commonly observed in ML materials and is generally associated with enhanced non-radiative relaxation at higher temperatures. Fig. S16 presents the XRD patterns and ML responses of Al 2 O 3 :1%Cr 3+ samples after storage in air for one year and after water immersion for one month. No noticeable structural change or degradation of NIR ML intensity is observed, indicating that the material maintains its structural integrity and ML response under these environmental conditions. Fig. S17 shows that the Al 2 O 3 :1%Cr 3+ exhibits millisecond-scale decay behavior, with an extracted ML lifetime of about 2.1 ms, comparable in timescale to the corresponding PL lifetime. Notably, among reported Cr 3+ -doped materials, Al 2 O 3 :Cr 3+ exhibited the highest ML performance, as shown in Fig. 3d . The inset shows a magnified view of the dotted line region in the spectrum. The bar chart displays the ML integrated intensity for each material. Based on the above findings regarding the annealing-temperature dependence, the annealing temperature for the other Cr 3+ -doped materials was increased to 1923 K for comparison. Because ZnGa 2 O 4 and SrGa 12 O 19 melt at this temperature, these two materials were annealed at 1823 K instead. After high-temperature annealing, a significant enhancement in ML intensity was observed. All samples were synthesized via high-temperature solid-state reactions. The synthesis conditions and corresponding ML intensities are summarized in Table 1 , and the XRD patterns and PL spectra are shown in Figs. S18 and S19 , respectively. To quantitatively evaluate the self-recoverable nature of the ML response, cyclic loading tests were performed on the Al 2 O 3 :Cr 3+ /PET composite with an applied force of 10 N and a rotation speed of 500 r min⁻¹. As shown in Fig. 3e , although a moderate decrease in intensity occurs during the initial cycles, the ML signal remains stable and repeatedly triggerable over long-term cycling. Even after 7000 loading cycles, approximately 54% of the initial ML intensity is retained. Table 1. Reported Cr 3+ -doped material systems: synthesis conditions and ML intensity Host Concentration Temperature Time Peak Intensity (a.u.) Integrated Intensity (a.u.) Ref. Al 2 O 3 1% 1923 K 4 h 5808 (P = 694 nm) 220,728 This work Ga 2 O 3 1% 1923 K 4 h 1768 (P = 716 nm) 207,100 35 MgO 0.5% 1923 K 4 h 883 (P = 808 nm) 151,137 39 MgGa 2 O 4 0.5% 1623 K 6 h 206 (P = 712 nm) 14,064 36 LiGa 5 O 8 0.6% 1673 K 6 h 192 (P = 721 nm) 12,551 33 ZnGa 2 O 4 0.5% 1673 K 6 h 160 (P = 696 nm) 10,687 34 SrGa 12 O 19 0.5% 1733 K 6 h 115 (P = 702 nm) 12,228 34 LaAlO 3 0.6% 1773 K 10 h 48 (P = 737 nm) 1844 40 Gd 3 Ga 5 O 12 3% 1673 K 6 h 13 (P = 719 nm) 1366 37 Open in a new tab Previous studies have shown that the presence of anomalous bonding states at heterojunction interfaces can induce significant band offsets, lowering the energy barrier for electron transitions associated with ML and thereby enhancing ML performance 52 . Inspired by this, this work synthesized Al 2 O 3 /Ga 2 O 3 :Cr 3+ heterojunction materials by introducing Ga 2 O 3 and adjusting the phase ratio to optimize ML behavior. As shown in Fig. S20 , the XRD patterns of Al 2 O 3 /Ga 2 O 3 :Cr 3+ samples with different ratios confirmed the presence of both Ga 2 O 3 (PDF#76-0573) and Al 2 O 3 (PDF#10-0173) phases, indicating the successful formation of hybrid heterostructures. The PL properties of heterojunction samples were further investigated (Table S1 ). With increasing Ga 2 O 3 content, the excitation peaks (Ex I and Ex II) red-shifted from 402 nm and 558 nm to 424 nm and 589 nm, corresponding to the Cr 3+ transitions of 4 A 2 ( 4 F) → 4 T 1 ( 4 F) and 4 A 2 ( 4 F) → 4 T 2 ( 4 F), respectively. Furthermore, the emission peak Em II shifted from 694 nm to 702 nm, accompanied by the emergence of two additional peaks, Em I and Em III. Em I/Em II were attributed to the narrow-line 2 E → 4 A 2 transitions (R1 and R2 lines), while Em III corresponded to the broad 4 T 2 → 4 A 2 transition. In addition, the full width at half maximum of the PL spectrum markedly increased from 2 nm to 92 nm, which may have resulted from new energy-level structures or local electronic environment changes induced at the heterojunction interface. Figure 3f shows the ML properties of Al 2 O 3 /Ga 2 O 3 :Cr 3+ samples with varying heterojunction ratios. As the Ga 2 O 3 content increased, the ML spectra exhibited red-shifting and band-broadening behavior similar to that observed in the PL spectra, which was attributed to the weakened crystal field strength from the higher Ga 3+ content. Figure 3g shows the integrated ML intensity as a function of the Al 2 O 3 /Ga 2 O 3 ratio, with the highest value observed at a ratio of 1:0.4. This indicates that, at this composition, the interface region effectively facilitates charge carrier migration and recombination, leading to optimal ML enhancement. Table S2 summarizes the quantum yield (QY) performances of Al 2 O 3 :Cr 3+ and Al 2 O 3 /Ga 2 O 3 :Cr 3+ heterojunction materials under different Cr 3+ doping concentrations and annealing temperatures. The QY of Al 2 O 3 :Cr 3+ decreased markedly with increasing Cr 3+ concentration but improved significantly with higher annealing temperatures. In Al 2 O 3 /Ga 2 O 3 :Cr 3+ heterojunctions, the QY slightly decreased with increasing Ga 2 O 3 content; however, optimized Cr 3+ doping still delivered near-ideal efficiency for NIR emissions. Furthermore, this work performed DFT calculations to gain insights into the influence of temperature on ML performance. Our results revealed that the thermally activated exponential increase in the defect concentration (Fig. 4a ) underlay the luminescence enhancement observed at elevated annealing temperatures, aligning with experimental observations. The relationship between the carrier concentration and the self-consistent Fermi level (E F ) further demonstrated an increase by an order of magnitude as the temperature increased from 1600 K to 1950 K (Fig. 4b ). This exponential increase in carrier concentration with annealing temperature was consistently observed under various chemical synthesis conditions (Fig. 4c ). The resulting higher carrier density significantly enhanced the radiative recombination efficiency, leading to superior luminescence performance. Fig. 4. DFT explanation of the temperature-dependent ML properties of Al 2 O 3 :Cr 3+ . Open in a new tab a Equilibrium defect concentration at 300 K in Al 2 O 3 crystals grown at 1950 K as a function of the annealing temperature. b Carrier concentration at the initial annealing temperature of 1600 K and the optimal annealing temperature of 1950 K. c Calculated carrier concentration in Al 2 O 3 :Cr 3+ at room temperature, as a function of annealing temperature and chemical potential conditions The following section describes the application of this simple oxide system on both flexible and rigid substrates. For flexible substrates, a novel ML paper was designed and fabricated using Al 2 O 3 /Ga 2 O 3 :Cr 3+ heterojunction materials, leveraging their high emission brightness, excellent thermal stability, and environmental durability. Potential applications in anti-counterfeiting and stress visualization were explored. The ML paper was successfully fabricated by mixing Al 2 O 3 /Ga 2 O 3 :Cr 3+ powder with dehydrated paper pulp at a mass ratio of 4:1, followed by a conventional handmade papermaking process, yielding a material with outstanding mechano-optical responsiveness. Under visible light, the paper retained its natural white appearance; however, under UV illumination at 365 nm, it emitted distinct NIR fluorescence (Fig. S21a ). Notably, the embedded powder retained its excellent ML performance, emitting NIR light stably even after incorporation into the paper matrix. Videos S3 and S4 further demonstrate the luminescent behavior of the ML paper under binocular night-vision observation. Distinct ML signals were observed when scratching with a glass rod or cutting with scissors, highlighting the unique stress-triggered emission pattern. This emission pattern is difficult to replicate, enhancing the technical reliability of multi-level anti-counterfeiting and invisible data encryption, thereby offering robust support for applications such as secure labeling and personalized information storage. To evaluate the long-term environmental stability of the ML paper, samples were stored under ambient laboratory conditions for up to one year. After storage, the ML paper still exhibited excellent ML emission under mechanical stimulation, with no obvious degradation compared to its initial state (Fig. S21b ). Furthermore, the mechanical fatigue stability of the ML paper was examined through repeated loading–unloading tests. As shown in Fig. S21c , the ML intensity remained at a relatively stable level over 100 consecutive mechanical cycles, indicating good durability under repeated mechanical stimulation. In a darkroom environment, motion trajectories written on the paper were clearly visualized using binocular night-vision devices. Figure 5a shows a handwritten keyboard pattern on the ML paper, clearly capturing the motion of the applied forces. Additionally, Fig. S21d shows the handwritten emission trails of numbers 1 through 9 and letters a through z, further demonstrating the potential of this material for stress visualization applications. In addition, a composite polydimethylsiloxane (PDMS) film was fabricated by mixing Al 2 O 3 :Cr 3+ , ZnS:Cu + , and PDMS in a mass ratio of 1:1:2, which exhibited both visible and NIR PL and ML emissions (Fig. S22a, b ). Video S5 shows the stretching-induced luminescence of the PDMS film recorded using both infrared and visible cameras with a 600-nm high-pass filter. In addition, the ML response of the Al 2 O 3 :Cr 3+ /PDMS composite can also be triggered under non-contact ultrasonic excitation, as demonstrated in Video S6 . Fig. 5. Application exploration of Al 2 O 3 :Cr 3+ . Open in a new tab a Visualization of a handwritten keyboard pattern on the ML paper. b Schematic illustration of the formation of an Al 2 O 3 :Cr 3+ thin film on the Cr–Al alloy via thermal oxidation. c ML emission of the Cr–Al alloy after 1-year natural storage under ambient conditions. Surface morphology of the alloy ( d ) before thermal treatment and ( e ) after thermal oxidation. f PL of the Al 2 O 3 :Cr 3+ thin film under 365-nm UV excitation. g Flexibility of the Cr–Al alloy with the Al 2 O 3 :Cr 3+ thin film under bending deformation. h ML image of the Al 2 O 3 :Cr 3+ thin film under external force application, and ( i ) the corresponding grayscale intensity map. j Cross-sectional SEM image and EDS elemental mapping of the Cr–Al alloy and the surface of the Al 2 O 3 :Cr 3+ thin film In addition to its application on flexible substrates, this material system demonstrates promising potential for stress visualization on rigid substrates, which are widely used in practical engineering scenarios. Aluminum alloys are widely employed as structural materials in the aerospace, automotive, and other industries owing to their low density and high strength. However, in practical applications, structural components often experience complex and nonuniform stress distributions, which vary significantly with the working conditions. Although finite element simulations are typically employed during the design phase to predict stress distributions and optimize structures, discrepancies often exist between the simulated results and actual stress states in real-world scenarios. To address this issue, we proposed a novel stress-responsive optical monitoring strategy involving the in-situ construction of an Al 2 O 3 :Cr 3+ ML layer on Cr–Al alloy surfaces via thermal oxidation, enabling passive and visual monitoring of ML. A schematic illustration of the thermally induced formation of the Al 2 O 3 :Cr 3+ layer on Cr–Al alloys is shown in Fig. 5b . Upon heating the Cr–Al alloy in air, surface oxidation occurred, leading to the formation of a uniform Al 2 O 3 :Cr 3+ layer. The in situ-formed layer exhibited ML upon mechanical stimulation. After long-term natural storage under ambient conditions for up to one year, the in situ-formed Al 2 O 3 :Cr 3+ layer on the Cr–Al alloy still exhibited clear ML emission under mechanical stimulation, demonstrating good environmental stability (Fig. 5c ). The XRD pattern of the Cr–Al alloy is shown in Fig. S23a . The alloy surface exhibited a metallic luster before heating (Fig. 5d ), whereas it became rough and oxidized after heating (Fig. 5e ). Under 365-nm UV excitation, the oxide layer displayed distinct PL (Fig. 5f ) while maintaining sufficient flexibility (Fig. 5g ) to accommodate various complex deformations encountered in practical applications. Furthermore, mechanical fatigue stability was evaluated by subjecting the Cr–Al alloy to repeated mechanical loading–unloading cycles. As shown in Fig. S23b , the ML intensity remained relatively stable over 100 consecutive cycles, indicating good durability of the oxide layer under repeated mechanical stimuli. Fig. S24a shows the PL spectrum of the Al 2 O 3 :Cr 3+ layer under 402-nm excitation, showing a sharp emission peak at 694 nm. The fluorescence decay curves at 402-nm and 558-nm excitation, monitored at 694 nm, exhibited millisecond-scale lifetimes (Fig. S24b ). The temperature-dependent PL spectra (Fig. S24c, d ) showed a gradual intensity variation with increasing temperature, whereas the emission peak position remained nearly unchanged. Upon external mechanical loading, the layer emitted stable NIR ML (Fig. 5h and Video S7 ). The color-encoded grayscale image (Fig. 5i ) clearly reveals the correlation between the luminescence intensity and applied stress, demonstrating the excellent pressure sensitivity of the material. Cross-sectional SEM and EDS elemental mapping (Fig. 5j ) further confirmed the compact structure and uniform elemental distribution of the oxide layer. Al was distributed throughout the sample, originating from the alloy substrate and extending into the oxide layer. Oxygen was primarily enriched at the surface, reflecting the oxidation process, whereas chromium was uniformly distributed within the layer, indicating its successful incorporation as a luminescent center. Compared with conventional stress-sensing technologies, this Al 2 O 3 :Cr 3+ -based method does not require an external power supply or complex electronics and relies solely on stress-induced optical signals for real-time stress detection and visualization. The in-situ growth process ensures robust interfacial bonding between the oxide layer and the alloy substrate. The resulting dense and uniform layer exhibits excellent mechanical stability and flexibility, making it suitable for structural monitoring in complex environments. This method provides a novel optical solution for dynamic structural status detection and health monitoring and shows significant promise for high-end engineering applications, particularly in the aerospace, energy pipelines, and large-scale industries. This straightforward thermal treatment enables the functionalization of structural materials, thereby laying a solid foundation for the development of next-generation smart components with self-sensing capabilities. Discussion In summary, this work demonstrated that Al 2 O 3 :Cr 3+ (in the form of crystals, powders, and Al alloy-related products) is a robust and scalable oxide platform for self-recoverable NIR ML. DFT calculations confirmed that this material exhibited efficient self-recovery behavior combined with a high-brightness emission potential. Guided by these mechanistic insights, we optimized the material via controlled doping, high-temperature annealing, and Ga 2 O 3 heterostructure engineering, thereby significantly boosting ML efficiency and stability. Validated across both flexible and rigid devices, the material enables applications such as NIR ML stress-mapping and anti-counterfeiting paper, as well as real-time structural monitoring of layers formed on Cr–Al alloys. This work clarifies the mechanical-to-optical conversion mechanism, bridges fundamental photonic research with practical stress-sensing technologies, and provides a general strategy for designing oxide-based ML systems for intelligent photonic and sensing applications. Materials and methods Materials Al 2 O 3 (99.99%, Sinopharm Co., Ltd.), Ga 2 O 3 (99.99%, Aladdin), and Cr 2 O 3 (99.95%, Aladdin) were used as starting materials. Preparation method Al 2 O 3 : xCr 3+ samples (x = 0.01, 0.1, 0.5, 1, 2, and 4%) were prepared using the high-temperature solid-phase method. First, Al 2 O 3 and Cr 2 O 3 powders were accurately weighed according to the desired Cr 3+ doping ratio. The raw materials were wet-ground in an agate mortar with a small amount of anhydrous ethanol for 40 min. The resulting slurry was dried at 80 °C, placed in a corundum crucible, and compacted. The mixtures were annealed in a chamber furnace at different temperatures (1573, 1623, 1673, 1723, 1773, 1823, 1873, and 1923 K) for 4 h in air. After sintering, the samples were cooled naturally to room temperature and sieved through a 150-mesh sieve to obtain a homogeneous powder for subsequent testing and characterization. Al 2 O 3 /mGa 2 O 3 :1% Cr 3+ samples (m = 0.2, 0.4, 0.6, 0.8, and 1) were prepared using the high-temperature solid-phase method. Al 2 O 3 , Ga 2 O 3 , and Cr 2 O 3 powders were first weighed in stoichiometric ratios and subjected to the same mixing, wet milling, and drying procedures described above. The treated powders were then placed in a corundum crucible, compacted, and annealed in a box furnace at 1923 K for 4 h to complete the reaction in air. After sintering, the samples were naturally cooled to room temperature and sieved through a 150-mesh sieve to obtain a homogeneous powder for subsequent testing and characterization. Preparation method for ML test films For ML films, 0.3 g of the powder and 0.06 g of UV curing adhesive (LEAFTOP 9307) were added to a test tube along with 9 mL of anhydrous ethanol. The mixture was shaken well and placed in an ultrasonic cleaner for ultrasonic dispersion to ensure uniformity. The well-dispersed mixture was poured into a 3 × 3-cm square mold and left to stand until the ethanol completely evaporated. After film formation, the mold was removed, and the dried powder layer was sandwiched between the top and bottom layers of the Ethylene Vinyl Acetate (EVA)–PET plastic sealing film (Deli No. 3817). The assembly was then irradiated and cured using a UV lamp to ensure that the layers were firmly bonded. Finally, the sandwich film was placed in a hot-press laminator for pressing and laminating to enhance its mechanical strength and ensure uniform thickness, yielding a highly stable ML test film for subsequent mechanical testing. Material characterization XRD measurements were performed using a SmartLab multifunctional diffractometer (Rigaku, Japan) operated at 45 kV and 200 mA. ML measurements were performed using a custom-built testing system comprising a digital force gauge, uniaxial controller, stepper motor, fiber-optic spectrometer (Ocean Optics QE65Pro), glass substrate, and a computer. For testing, the ML film/paper was fixed on a glass plate, and a metal probe with a hemispherical tip of 0.1 mm was brought into contact with the film by adjusting the force gauge position to apply pressure. The opposite side of the glass plate was connected to an optical fiber coupled to a spectrometer for light collection. The uniaxial controller drove the stepper motor to move the slider and fiber assembly horizontally, enabling real-time acquisition of dynamic ML signals. For the cyclic repeatability test, the ML powder was first encapsulated in a PET film. Cyclic mechanical stimulation was applied using a motor-driven setup with a rotation speed of 500 r min⁻¹ under a constant normal force of 10 N. The emitted ML signal during repeated cycles was collected by an optical fiber and recorded using a spectrometer. For large-pressure ML measurements, the ML powder was embedded in an epoxy resin matrix with a mass ratio of powder to resin of 1:4. After curing, the composite sample was subjected to external mechanical loading. For the Cr–Al alloy sample, ML measurements were carried out by scraping the alloy surface under an applied force of 3 N. An optical fiber was covered with a glass test tube and positioned on the same side of the scraping region to collect the emitted ML signal, which was then recorded by a spectrometer. PL, fluorescence lifetime, and PLQY were characterized using an FLS1000 spectrofluorometer (Edinburgh Instruments Ltd., UK) equipped with a xenon lamp as the excitation source. Temperature-dependent PL measurements were performed using a Hitachi F-7100 spectrofluorometer (Japan) with continuous excitation from a 405-nm laser for 16 min. A 510-nm optical filter was used to eliminate residual excitation light. The sample temperature was regulated and recorded using a heating stage combined with an infrared thermal imager. Spectral data were collected using an Ocean Optics USB65 Pro spectrometer at an integration time of 100 ms and stored at 10-°C intervals. SEM images were obtained using a Thermo Scientific APREO S high-resolution field-emission scanning instrument, whereas TEM images were acquired using a Thermo Fisher Titan Cubed Themis G2 300 microscope equipped with a double spherical aberration correction. Computational methodology This study employed first-principles calculations based on density functional theory (DFT), as implemented in the Vienna Ab initio Simulation Package (VASP). The electron–ion interactions were described using the projector augmented-wave (PAW) pseudopotential method, with a plane-wave cutoff energy of 520 eV. Structural optimizations were performed using the PBEsol functional, and Brillouin zone integrations were carried out on a 3 × 3 × 3 k-point mesh. To accurately capture the localized nature of Cr 3 d orbitals, electronic structure calculations were conducted using the HSE06 hybrid functional. In these calculations, the screened Fock exchange mixing parameter α was set to 0.31, a value optimized to reproduce the experimental band gap of Al 2 O 3 (DFT 8.61 eV, Exp. 8.7 eV). This hybrid functional approach effectively mitigates self-interaction error and significantly improves the accuracy of defect energy level predictions. Detailed computational methodology are reported in Supplementary Information . Supplementary information Supplementary Information (27.3MB, docx) Video S1. PET (2MB, mp4) Video S2. Thermal imaging (3.4MB, mp4) Video S3. Paper Bending and Sliding (9.6MB, mp4) Video S4. Pepper Cutting (1.3MB, mp4) Video S5.PDMS (4.1MB, mp4) Video S6. Ultrasound (4.6MB, mp4) Video S7. Alloy (1.3MB, mp4) Acknowledgements This work was supported by the Natural Science Foundation of China (62275170, 22173019 and 62505188), the National Key Research and Development Program of China (2021YFA1200700), the Guangdong Provincial Science Fund for Distinguished Young Scholars (2022B1515020054), the Key-Area Research and Development Program of Guangdong Province (2024B0101080001), Shenzhen Fundamental Research Project (JCYJ20240813141624033), the Research Grants Council of Hong Kong (11211922), Scientific Research Foundation as Phase III construction of high level University 2035 plan (0000050101), as well as the Medical-Engineering Interdisciplinary Research Foundation of Shenzhen University (2023YG031). We thank the Big Data Computing Center of Southeast University for providing the facility support on the calculations. We thank the Electron Microscope Center of Shenzhen University. We gratefully acknowledge Prof. Yixi Zhuang and Yuantian Zheng for their assistance with ML decay and TL measurements. We also thank Shengbin Xu for conducting large-load tests and ML repeatability measurements, and Prof. Pengfei Shen and Hongli Liang for performing high-pressure photoluminescence spectroscopy measurements. Author contributions Z. Y. F., X. F. P., and Q. A. Z. contributed equally to this work. D. F. P. and M. G. J. conceived and supervised the project, while F. W. and J. L. G. provided additional strategic guidance. Z. Y. F. and Q. A. Z. synthesized the phosphors, conducted preliminary characterizations, and contributed to manuscript writing. X. F. P. performed the theoretical calculations, analyzed the computational data, and contributed to mechanism interpretation and manuscript writing. Z. Y. F., Q. A. Z., and M. Z. W. carried out the spectral measurements, processed the experimental data, and verified result reproducibility. Y. L., Q. D. M., B. Y. R., and Y. Z. W. assisted with sample preparation, experimental setup, and measurement calibration. S. Q. L. and M. Z. supported manuscript revision, language polishing, and figure optimization. L. P. L. provided essential equipment support for experimental testing. J. L. G., F. W., M. G. J., and D. F. P. served as corresponding authors and contributed to manuscript review, editing, and overall supervision. All authors discussed the results and approved the final version of the manuscript. Conflict of interest The authors declare no competing interests. Footnotes These authors contributed equally: Ziyi Fang, Xiaofeng Pan, Qi’an Zhang Contributor Information Ming-Gang Ju, Email: [email protected]. Jiulin Gan, Email: [email protected]. Feng Wang, Email: [email protected]. Dengfeng Peng, Email: [email protected]. Supplementary information The online version contains supplementary material available at 10.1038/s41377-026-02274-w. References 1. Wang, H. et al. Oscillatory mechanoluminescence of Mn 2+ -doped SrZnOS in dynamic response to rapid compression. Nat. Commun. 16 , 548 (2025). [ DOI ] [ PMC free article ] [ PubMed ] [ Google Scholar ] 2. Jeong, H. I. et al. Super elastic and negative triboelectric polymer matrix for high performance mechanoluminescent platforms. Nat. Commun. 16 , 854 (2025). [ DOI ] [ PMC free article ] [ PubMed ] [ Google Scholar ] 3. Cai, C. Y. et al. Multi-stimulated far-UVC luminescence for solar-blind imaging. Nat. Commun. 16 , 6224 (2025). [ DOI ] [ PMC free article ] [ PubMed ] [ Google Scholar ] 4. Wang, W. X. et al. Contact-separation-induced self-recoverable mechanoluminescence of CaF 2 :Tb 3+ /PDMS elastomer. Nat. Commun. 15 , 2014 (2024). [ DOI ] [ PMC free article ] [ PubMed ] [ Google Scholar ] 5. Chang, K. et al. Achieving ultrasound-excited emission with organic mechanoluminescent materials. Adv. Mater. 36 , 2407875 (2024). [ DOI ] [ PubMed ] [ Google Scholar ] 6. Runowski, M. et al. Sound, force and light induced emissions from Er 3+ -Mn 2+ doped ZnS/CaZnOS heterostructure for remote temperature monitoring via photo- and mechanoluminescence. Adv. Mater. 37 , e10117 (2025). [ DOI ] [ PMC free article ] [ PubMed ] [ Google Scholar ] 7. Tao, L. et al. High-quality perovskite quantum dots with excellent reproducibility and amplified spontaneous emission by optimization of cesium precursor. Light Adv. Manuf. 6 , 12 (2025). [ Google Scholar ] 8. Lv, X. L. et al. Self-powered mechanoluminescent elastomer for solar-blind ultraviolet emission. Light Sci. Appl. 15 , 61 (2026). [ DOI ] [ PMC free article ] [ PubMed ] [ Google Scholar ] 9. Xie, Z. Dual-channel mechano-phosphorescence: a combined locking effect with twisted molecular structures and robust interactions. Light Sci. Appl. 13 , 85 (2024). [ DOI ] [ PMC free article ] [ PubMed ] [ Google Scholar ] 10. Sun, X. L. et al. 3D printing of auxetic self-powered mechanoluminescent photonic skins for underwater communication and safety monitoring. Adv. Mater. 37 , 2502743 (2025). [ DOI ] [ PMC free article ] [ PubMed ] [ Google Scholar ] 11. Wang, X. D. et al. Dynamic pressure mapping of personalized handwriting by a flexible sensor matrix based on the mechanoluminescence process. Adv. Mater. 27 , 2324–2331 (2015). [ DOI ] [ PubMed ] [ Google Scholar ] 12. Guo, J. X. et al. In-sensor computing with visual-tactile perception enabled by mechano-optical artificial synapse. Adv. Mater. 37 , 2419405 (2025). [ DOI ] [ PubMed ] [ Google Scholar ] 13. Liu, S. Q. et al. Bright chromium-sensitized lanthanide NIR-II mechanoluminescence in a piezoelectric oxide. Adv. Mater. 37 , e06957 (2025). [ DOI ] [ PubMed ] [ Google Scholar ] 14. Zhang, P. et al. High defect tolerance breaking the design limitation of full-spectrum multimodal luminescence materials. Adv. Mater. 37 , 2411532 (2025). [ DOI ] [ PubMed ] [ Google Scholar ] 15. Wang, Y. Z. et al. Synthesis of SrZnOSe crystals with low phonon energy for enhancing near-infrared mechanoluminescence. Adv. Mater. 36 , 2406899 (2024). [ DOI ] [ PubMed ] [ Google Scholar ] 16. Liu, S. Q. et al. Chromium-activated phosphors: from theory to applications. Chem. Soc. Rev. 55 , 1954–1998 (2026). [ DOI ] [ PubMed ] [ Google Scholar ] 17. Dubey, V. et al. Mechanoluminescence behavior of rare-earth-activated phosphors. in Rare-Earth-Activated Phosphors (eds Dubey, V. et al.) (Amsterdam: Elsevier, 2022), 283-319. 18. Xu, C. N. et al. Direct view of stress distribution in solid by mechanoluminescence. Appl. Phys. Lett. 74 , 2414–2416 (1999). [ Google Scholar ] 19. Xu, C. N. et al. Artificial skin to sense mechanical stress by visible light emission. Appl. Phys. Lett. 74 , 1236–1238 (1999). [ Google Scholar ] 20. Hou, B. et al. An interactive mouthguard based on mechanoluminescence-powered optical fibre sensors for bite-controlled device operation. Nat. Electron. 5 , 682–693 (2022). [ Google Scholar ] 21. Jeong, H. I. et al. Defect localized mechanoluminescence model in copper doped zinc sulfide. ACS Nano 19 , 35027–35036 (2025). [ DOI ] [ PubMed ] [ Google Scholar ] 22. Jeong, H. I. et al. High-resolution mechanoluminescent haptic sensor via dual-functional chromatic filtration by a conjugated polymer shell. Adv. Mater. 37 , e08917 (2025). [ DOI ] [ PMC free article ] [ PubMed ] [ Google Scholar ] 23. Jeong, H. I. et al. Interfacial dipole moment engineering in self-recoverable mechanoluminescent platform. Mater. Today 81 , 4–11 (2024). [ Google Scholar ] 24. Zhang, J. C. et al. An intense elastico-mechanoluminescence material CaZnOS:Mn 2+ for sensing and imaging multiple mechanical stresses. Opt. Express 21 , 12976–12986 (2013). [ DOI ] [ PubMed ] [ Google Scholar ] 25. Botterman, J. et al. Mechanoluminescence in BaSi 2 O 2 N 2 :Eu. Acta Mater. 60 , 5494–5500 (2012). [ Google Scholar ] 26. Li, H. M. et al. Force-induced ultraviolet C luminescence of Pr 3+ -Doped Sr 2 P 2 O 7 for X-ray dosimetry. Adv. Mater. 36 , 2411804 (2024). [ DOI ] [ PubMed ] [ Google Scholar ] 27. Ning, J. J. et al. MgF 2 :Mn 2+ : novel material with mechanically-induced luminescence. Sci. Bull. 67 , 707–715 (2022). [ DOI ] [ PubMed ] [ Google Scholar ] 28. Zheng, T. et al. Mechanoluminescent aluminum nitride crystal for super-sensitive optical manometry, thermometry and force sensing. Adv. Mater. 38 , e11943 (2026). [ DOI ] [ PMC free article ] [ PubMed ] [ Google Scholar ] 29. Xiong, P. X. & Peng, M. Y. Near infrared mechanoluminescence from the Nd 3+ doped perovskite LiNbO 3 :Nd 3+ for stress sensors. J. Mater. Chem. C. 7 , 6301–6307 (2019). [ Google Scholar ] 30. Tiwari, R. et al. Fracto-mechanoluminescence induced by impulsive deformation of II–VI semiconductors. Luminescence 30 , 883–890 (2015). [ DOI ] [ PubMed ] [ Google Scholar ] 31. Ming, J. et al. High-brightness transition metal-sensitized lanthanide near-infrared luminescent nanoparticles. Nat. Photonics 18 , 1254–1262 (2024). [ Google Scholar ] 32. Liu, S. Q. et al. Intervalence charge transfer of Cr 3+ -Cr 3+ aggregation for NIR-II luminescence. Light Sci. Appl. 12 , 181 (2023). [ DOI ] [ PMC free article ] [ PubMed ] [ Google Scholar ] 33. Xiong, P. X. et al. Self-recoverable mechanically induced instant luminescence from Cr 3+ -doped LiGa 5 O 8 . Adv. Funct. Mater. 31 , 2010685 (2021). [ Google Scholar ] 34. Liu, S. Q. et al. Near-infrared mechanoluminescence of Cr 3+ doped gallate spinel and magnetoplumbite smart materials. Adv. Funct. Mater. 33 , 2209275 (2023). [ Google Scholar ] 35. Suo, H. et al. A broadband near-infrared nanoemitter powered by mechanical action. Matter 6 , 2935–2949 (2023). [ Google Scholar ] 36. Yu, X. et al. Intense NIR mechanoluminescence from Al 3+ -regulated MgGa 2 O 4 :Cr 3+ . Chem. Eng. J. 491 , 152155 (2024). [ Google Scholar ] 37. Bu, W. F. et al. Near-infrared mechanoluminescence of Gd 3 Ga 5 O 12 :Cr 3+ , La 3+ for biological stress imaging. Laser Photonics Rev. 19 , 2400893 (2025). [ Google Scholar ] 38. Wu, S. et al. Self-powered near-infrared mechanoluminescence through MgO/MgF 2 piezo-photonic heterojunctions. Nat. Commun. 16 , 8912 (2025). [ DOI ] [ PMC free article ] [ PubMed ] [ Google Scholar ] 39. Zhao, F. Y. et al. Self-recoverable broadband near-infrared mechanoluminescence in Cr 3+ -doped MgO. Sci. China Mater. 68 , 4440–4447 (2025). [ Google Scholar ] 40. Shao, P. S. et al. Self-recoverable NIR mechanoluminescence from Cr 3+ doped perovskite type aluminate. Adv. Powder Mater. 3 , 100165 (2024). [ Google Scholar ] 41. Mashiko, H. et al. Multi-petahertz electron interference in Cr:Al 2 O 3 solid-state material. Nat. Commun. 9 , 1468 (2018). [ DOI ] [ PMC free article ] [ PubMed ] [ Google Scholar ] 42. Dang, P. P. et al. Recent advances in chromium-doped near-infrared luminescent materials: fundamentals, optimization strategies, and applications. Adv. Opt. Mater. 11 , 2201739 (2023). [ Google Scholar ] 43. Zheng, G. J. et al. Rare-metal-free ultrabroadband near-infrared phosphors. Adv. Mater. 37 , 2415791 (2025). [ DOI ] [ PubMed ] [ Google Scholar ] 44. Shang, L. B. et al. Understanding near-infrared luminescence in Cr-doped La-gallogermanates through first-principles calculations and ligand-field theory. Phys. Rev. B 108 , 155136 (2023). [ Google Scholar ] 45. Liu, J. et al. An efficient descriptor for rapid determination of dipole moments and band alignments of 2D janus transition-metal dichalcogenides. Adv. Funct. Mater. 34 , 2401737 (2024). [ Google Scholar ] 46. Dodani, S. C. et al. Discovery of a regioselectivity switch in nitrating P450s guided by molecular dynamics simulations and Markov models. Nat. Chem. 8 , 419–425 (2016). [ DOI ] [ PMC free article ] [ PubMed ] [ Google Scholar ] 47. Zhang, Y. Y. et al. Origin of the type-II band offset between rutile and anatase titanium dioxide: classical and quantum-mechanical interactions between O ions. Phys. Rev. B 95 , 155308 (2017). [ Google Scholar ] 48. Zhang, Y. et al. Synthesis of mixed-dimensional 1D-graphene nanoribbon/2D-CuSe heterostructures with controllable band alignments. Nat. Commun. 16 , 5988 (2025). [ DOI ] [ PMC free article ] [ PubMed ] [ Google Scholar ] 49. Borshon, I. Z. et al. Predicting column heights and elemental composition in experimental transmission electron microscopy images of high-entropy oxides using deep learning. npj Comput. Mater. 10 , 275 (2024). [ Google Scholar ] 50. Huang, D. C. et al. A highly efficient and thermally stable broadband Cr 3+ -activated double borate phosphor for near-infrared light-emitting diodes. J. Mater. Chem. C. 9 , 164–172 (2021). [ Google Scholar ] 51. Zhang, Y. et al. Blue LED-pumped intense short-wave infrared luminescence based on Cr 3+ -Yb 3+ -co-doped phosphors. Light Sci. Appl. 11 , 136 (2022). [ DOI ] [ PMC free article ] [ PubMed ] [ Google Scholar ] 52. Peng, D. F. et al. A ZnS/CaZnOS heterojunction for efficient mechanical-to-optical energy conversion by conduction band offset. Adv. Mater. 32 , 1907747 (2020). [ DOI ] [ PubMed ] [ Google Scholar ] Associated Data This section collects any data citations, data availability statements, or supplementary materials included in this article. Supplementary Materials Supplementary Information (27.3MB, docx) Video S1. PET (2MB, mp4) Video S2. Thermal imaging (3.4MB, mp4) Video S3. Paper Bending and Sliding (9.6MB, mp4) Video S4. Pepper Cutting (1.3MB, mp4) Video S5.PDMS (4.1MB, mp4) Video S6. Ultrasound (4.6MB, mp4) Video S7. 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