“The core message is that there is diversity in the defects in luminescent materials. This allows us to better steer the performance of luminescent materials… and to bring more functionality to those materials by controlling the different types of defects”Philippe Smet
Scientists from Ghent University in Belgium have produced a pressure-sensitive light-emitting material that can visualize the location where it was hit up to three days after it happened, allowing for the analysis of impacts and deformations, and their associated stresses. The material could find uses in investigating the reasons behind an impact to part of an aircraft or the blade of a wind turbine.
As reported in Light: Science & Applications [Petit et al. Light Sci. Appl (2019) DOI: 10.1038/s41377-019-0235-x], the team demonstrated how different types of defects can be present in luminescent materials, and their specific responses to stimuli such as heat, light and pressure. In many luminescent materials defects can be a problem, while for other materials such as in glow-in-the-dark paints, they are essential to the storage process. In this new mechanoluminescent material, however, which can emit light either when deformed or pressure applied to it, sensitivities to pressure or mechanical action and infrared radiation can be identified.
A range of materials have already been developed that can repeatedly emit light when pressure is applied due to the storage of energy in the crystal lattice of the luminescent material after exposure to ambient or UV light, although the mechanoluminescent emission has to be constantly monitored since the emission only happens when pressure is applied. In this study, memory was added to a mechanoluminescent material, and its imperfections or defects were used to visualize where the pressure had been applied.
The mechanoluminescent material was incorporated in polymer plates and exposed to UV light, during which defects in the material are populated with electrons originating from the luminescent centers. When pressure is applied to the luminescent material, these electrons are released again from the defects, or traps. Some of the released electrons return to the centers, emitting light. Some of the electrons are transferred to “deep traps” where the electrons are not easily released again – by scanning an infrared laser beam over the sample surface, the areas where electrons had been stored light up as soon as the infrared photons push them out of their traps. The electrons return to the luminescent center and the material again lights up. As the deep traps are extremely stable, the signal was still visible after three days.
As co-author Philippe Smet told Materials Today, “the core message is that there is diversity in the defects in luminescent materials. This allows us to better steer the performance of luminescent materials… and to bring more functionality to those materials by controlling the different types of defects”. The team would now like to improve the sensitivity of the mechanoluminescent material, optimize the write and read conditions, and also apply it in applications where optimal use of the memory function can be exploited.
“The mechanoluminescent (ML) material is applied as a coating onto a test piece. First, illumination with UV radiation leads to the trapping of charge carriers at defects in the ML material. When the material is strained, deformed or impacted, some bluish-green light is being emitted, proportional to the local stress. Up to three days later, the places where the stresses occurred can be made to emit bluish-green light again by sweeping infrared radiation (e.g. by a laser) over the surface. It is important to notice that the process is repeatable and that no damage to the ML material is required for the phenomenon to occur.”