image: Figure 1 | Theoretical investigation of self-recoverable ML emission in Al2O3:Cr3+. a Crystal structure of Al2O3:Cr3+. b Calculated formation energies of CrAl 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 CrAl0(4A2) defect under stress-free conditions, and the corresponding CrO6 octahedral structure (blue sphere: Cr; red sphere: O). Arrow diagram showing three electrons of Cr occupying the t2g energy level. d Electronic DOS for the ionized-state CrAl1+ defect under stressed conditions, and the corresponding CrO6 octahedral structure (blue sphere: Cr; red sphere: O). Arrow diagram showing two electrons of Cr occupying the t2g energy level. e Schematic diagram of the transitions between different states during electron excitation, relaxation, and recombination in Al2O3:Cr3+ in the ligand field. f Electronic DOS for the excited-state CrAl0(2E) defect under stress-free conditions, and the corresponding CrO6 octahedral structure (blue sphere: Cr; red sphere: O). Arrow diagram showing three electrons of Cr occupying the t2g 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.
Credit: Dengfeng Peng et al.
ML materials, which emit light in response to mechanical stimulation, have attracted increasing attention for applications in stress sensing, structural health monitoring, and intelligent photonic systems. Despite significant progress, most existing materials still face several critical challenges. Many reported systems rely on complex multi-component hosts or rare-earth doping, leading to high material and processing costs. In addition, their emission often depends on pre-irradiation or continuous external excitation to fill traps, which limits their practicality in real-world, energy-free environments. More importantly, the luminescence in many systems is not intrinsically self-recoverable, meaning the emission intensity gradually decays after repeated mechanical loading, reducing reliability and long term usability.
To address these limitations, the researchers developed a new class of ML material based on chromium-doped aluminum oxide. Aluminum oxide is widely known for its low cost, excellent chemical stability, and ease of large-scale production. By introducing chromium ions into the host lattice, the team successfully created a system that exhibits strong near-infrared emission under mechanical stimulation. More importantly, by combining first-principles calculations with systematic experimental investigations, they identified a fundamentally different luminescence mechanism. Under applied stress, the chromium-related centers undergo charge ionization, releasing carriers into the conduction band. When the stress is removed, these carriers are recaptured, leading to radiative recombination and light emission. This reversible ionization and recapture process enables intrinsic self-recoverable ML without the need for any external energy input.
Beyond the fundamental mechanism, the researchers further optimized the material performance through multiple strategies. By tuning the chromium doping concentration, annealing temperature, holding time, and synthesis atmosphere, they were able to regulate defect density and carrier concentration, which are key factors governing luminescence efficiency. High-temperature annealing was found to significantly increase the concentration of defects and free carriers, resulting in enhanced emission intensity. In addition, the introduction of aluminum oxide and gallium oxide heterostructures further improved carrier transport and recombination efficiency at the interface, leading to even stronger luminescence. As a result, the optimized material demonstrates high brightness, excellent stability, and remarkable durability, maintaining consistent emission over thousands of loading cycles without noticeable degradation.
The practical potential of this material was demonstrated through several application scenarios. By integrating the phosphor with cellulose fibers, the team fabricated flexible luminescent paper that can visualize mechanical stimuli in real time, enabling applications such as stress mapping, handwriting display, and multi-level anti-counterfeiting. Furthermore, using an in situ thermal oxidation strategy, the material can be directly formed on chromium–aluminum alloy surfaces as a dense and uniform luminescent layer. This coating is capable of converting mechanical deformation into visible signals without external power, making it highly suitable for real-time monitoring of structural stress in engineering components. The material also exhibits excellent environmental stability, maintaining its performance after long-term exposure to ambient conditions, which is essential for practical deployment.
These results highlight a new pathway for designing high-performance ML materials using simple oxide systems. By leveraging defect engineering and carrier dynamics control, the study demonstrates that complex compositions are not a prerequisite for achieving advanced functionality. The combination of low cost, scalability, and robust performance makes this material particularly promising for applications in structural health monitoring, smart sensing, security technologies, and next generation photonic devices.
“Our work shows that a simple and widely available oxide like aluminum oxide can be transformed into a high-performance ML material through control of defects” the researchers said. “This not only deepens our understanding of how mechanical energy can be converted into light, but also opens up a practical route toward durable, energy-free sensing systems for real world applications.”
Journal
Light: Science & Applications
Article Title
Self-Recoverable Mechanoluminescence in Simple Oxides: Al₂O₃:Cr