Chromium doped lithium aluminate converts mechanical stress into near infrared light without pre charging or afterglow, enabling precise sensing and imaging in security and biomedical applications.
(Nanowerk Spotlight) Press your thumb into it and it answers with a flash you’ll never see. The glow hides in the near-infrared, invisible to the eye but visible to a camera through fabric, plastic, or a layer of skin. That invisible light could track strain in an aircraft wing, mark a product with an unclonable code, or monitor a patient’s movement without a single wire or battery.
Mechanoluminescence—turning mechanical force into light—has been explored for decades, but practical versions stumble. They need pre-charging with bright light, they keep glowing after the event, or they emit in colors that scatter before they can reach a sensor. A material that avoids all of those problems and works in the near-infrared would open new options in security, engineering, and medicine.
The idea of light from motion is older than the term mechanoluminescence itself. Some crystals and phosphors have long been known to glow when rubbed, struck, or bent. But turning that novelty into technology has been slow. Most early systems worked by storing charge in “traps” created by defects in the crystal lattice. Light or another energy source would fill those traps with electrons, and mechanical stress would release them, causing recombination at luminescent centers and the release of photons.
The catch was obvious: the material had to be “charged” before it would respond, and it often kept glowing after the stimulus ended. For real-time sensing, especially in bright environments or inside the body, both were unacceptable.
That led to the search for self-powered mechanoluminescence—materials that could take mechanical input and produce light without any prior charging. A few candidates emerged, often using manganese in polymer or composite hosts, and some achieved good durability. But nearly all emitted in the visible spectrum, usually orange or red. In tissue or opaque materials, visible light is absorbed or scattered within millimeters. Near-infrared (NIR) light, typically between 700 and 900 nanometers, travels much farther and can be detected without affecting the sample. It is also invisible to human eyes, making it ideal for hidden markings and anti-counterfeiting.
Adjacent technologies helped define the goal. Piezoelectric sensors can generate voltage from motion, but require wiring and electronics. Triboelectric generators can harvest energy from contact, but again need circuits to produce readable signals. Persistent phosphors can emit for hours, but cannot produce a clean, instantaneous signal. The missing piece was a purely optical, NIR, self-powered mechanoluminescent material with no afterglow and a direct, linear response to force.
That is the niche that scientists set out to fill in this work. They worked with LiAl₅O₈, a cubic spinel oxide known for its chemical stability, and introduced small amounts of trivalent chromium. The doping was designed to do two things at once: slightly distort the lattice by replacing smaller aluminum ions with larger chromium ions, and introduce controlled defects that would interact with the chromium’s electronic structure.
Schematic of defect and local distortion induced self-powered mechanoluminescence mechanism. The photoluminescence and mechanoluminescence spectra of LiAl4.99O8: 0.01Cr3+ under different excitation conditions (Xe lamp for 254 nm, friction, impact, stress). (Image: Reprinted from DOI:10.1002/advs.202510848, CC BY)
X-ray diffraction confirmed the lattice stayed cubic but expanded slightly with doping. The chromium took up octahedral sites normally occupied by aluminum, where the mismatch in size created local distortions. These distortions were not random damage—they altered the local crystal field in ways that affect how electrons move. Electron paramagnetic resonance and other spectroscopic tools confirmed chromium’s oxidation state and the absence of unwanted higher-valence forms. Raman and nuclear magnetic resonance measurements showed that doping increased the proportion of distorted octahedral environments, a sign that the lattice had been tuned at the atomic scale.
The optical properties followed. In photoluminescence tests, the chromium produced a sharp emission at 711 nanometers, a spin-forbidden transition favored in strong crystal fields. The host lattice contributed a separate visible emission, but the NIR signal dominated when the material was stressed. Crucially, tests showed no persistent luminescence: once the force was removed, the signal vanished almost instantly.
The team then explored how mechanical stress produced light without traps. Density functional theory calculations showed that chromium’s d orbitals hybridized with nearby oxygen and aluminum orbitals to create mid-gap states—energy levels between the main conduction and valence bands. These states aligned with the chromium emission levels, making it possible for electrons to tunnel directly into them when the lattice was deformed. In other words, stress could push charges exactly where they needed to go to produce light, without storing them first.
Defects played a supporting role. Lithium and oxygen vacancies, and “anti-site” defects where lithium and aluminum trade places, all altered the local electronic landscape. The researchers varied lithium content to change the number of vacancies, finding that more lithium vacancies boosted mechanoluminescence. Gallium substitution for aluminum expanded the lattice and shifted the emission wavelength, confirming that both defect type and lattice geometry controlled performance.
A small but measurable piezoelectric effect also appeared in the doped samples. The undoped spinel is centrosymmetric and normally non-piezoelectric, but chromium broke inversion symmetry locally. The resulting piezoelectric fields helped separate charges under stress, improving the efficiency of electron transfer into the chromium sites.
Performance testing covered multiple forms of mechanical input: compression, friction, and impact. In each case, the emission intensity scaled linearly with the applied force, up to several thousand newtons, making the material a precise stress sensor. Over 1,500 loading cycles, the signal showed no degradation. Thermal tests showed the emission retained more than 70 percent of its original strength at 190 °C, pointing to stability in demanding environments.
Two demonstrations illustrated potential uses. In one, the phosphor was embedded in a polymer film patterned with a QR code. Under visible light, the code appeared blank. Under mechanical stress, a near-infrared camera revealed the code, invisible to the eye, showing its suitability for secure marking. In the other, the film was pressed against 10 millimeters of pork tissue and rubbed. The NIR signal passed through the tissue and was detected cleanly, outperforming visible-light mechanoluminescent materials and hinting at possibilities for wearable or implantable strain sensors.
The study’s main contribution is not just the material itself, but the mechanistic framework behind it: a synergy between lattice distortion and specific defects that creates mid-gap states tuned for direct, stress-driven electron transfer. This avoids the traps that cause afterglow, enabling clean, instantaneous NIR emission. The approach could be adapted to other hosts and dopants, opening a broader design space for autonomous optical sensors.
By linking atomic-scale structure to macroscopic performance, Xiao and colleagues move mechanoluminescence research toward predictive design rather than empirical trial and error. For applications in security, engineering, and medicine, a material that can turn stress directly into a stable, invisible, penetrating optical signal is not just a novelty—it is a new class of tool.
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