Light emission is usually associated with current flow, a chemical reaction or heating an object to high temperatures. But it can also occur when a material is broken, compressed or rubbed.

This phenomenon, known as mechanoluminescence, has long been observed when certain crystals, including sugar, are crushed or when adhesive tape is peeled off. In everyday conditions, however, the emitted light is usually too faint to be easily detected.

Turning such a laboratory effect into a useful sensor is not straightforward.

An international team led by Justyna Barzowska, PhD, and Sebastian Mahlik, PhD, from the University of Gdańsk studied aluminium nitride doped with manganese ions, designated AlN+. Scientists from the Institute of Physics of the Polish Academy of Sciences and Adam Mickiewicz University in Poznań, as well as partners from Belgium, China and the United States, also took part in the project.

Aluminium nitride itself is a hard, chemically stable semiconductor with a large band gap, used in electronics, among other applications. The manganese ions introduced into its lattice act as luminescent centres – sites responsible for light emission.

Small defects in the crystal, where the regular arrangement of atoms is disrupted, are also important in the sensor’s construction.

When the material is exposed to light, charge carriers, including electrons, can become trapped in these defects, temporarily storing some of the energy. Heating or deforming the crystal can release the charges. They transfer energy to manganese ions, producing faint flashes of orange light.

Measurements support this description of the mechanism, although not all its details are yet known.

One potential application is visualising acoustic waves. Sound is a mechanical wave that causes particles to vibrate as it propagates through a medium.

When ultrasound reaches the material, it causes its grains to undergo repeated compression and stretching. These deformations can release trapped charges and induce luminescence, making the area exposed to a focused ultrasound beam visible.

Impact and friction produce a similar effect. In some experiments, the material was first irradiated with ultraviolet radiation to store energy, which was later partially used during emission.

A paper presenting the research results was published in the journal Science Advances.

The material exhibited several types of luminescence: it glowed when excited by light, retained its glow after the source was turned off, and responded to heating, friction, impact and ultrasound.

The researchers also embedded its particles in a flexible polymer. The resulting film could visualise traces made by writing, bending and stretching. A focused ultrasound beam, meanwhile, left a luminous pattern corresponding to the area affected by the wave.

The ability to obtain a spatial image of stress is more important than the luminosity itself. In the future, a phosphor coating could indicate overload points in a structural element, record stress on flexible electronics or help map an ultrasonic field.

Researchers also mention remote sensors, anti-counterfeiting protection and biomedical applications. However, these are only potential avenues for further research.

Before practical use, the sensitivity and durability of the composite, the dependence of brightness on stimulus strength and the repeatability of charging cycles must be determined. Medical use would also require separate safety and biocompatibility testing. (PAP)

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