Force-induced charge carrier storage: a new route for stress recording

Stress sensing is the basis of human-machine interface, biomedical engineering, and mechanical structure detection systems. Stress sensing based on mechanoluminescence (ML) shows significant advantages of distributed detection and remote response to mechanical stimuli and is thus expected to be a key technology of next-generation tactile sensors and stress recorders. However, the instantaneous photon emission in ML materials generally requires real-time recording with a photodetector, thus limiting their application fields to real-time stress sensing. In this paper, we report a force-induced charge carrier storage (FICS) effect in deep-trap ML materials, which enables storage of the applied mechanical energy in deep traps and then release of the stored energy as photon emission under thermal stimulation. The FICS effect was confirmed in five ML materials with piezoelectric structures, efficient emission centres and deep trap distributions, and its mechanism was investigated through detailed spectroscopic characterizations. Furthermore, we demonstrated three applications of the FICS effect in electronic signature recording, falling point monitoring and vehicle collision recording, which exhibited outstanding advantages of distributed recording, long-term storage, and no need for a continuous power supply. The FICS effect reported in this paper provides not only a breakthrough for ML materials in the field of stress recording but also a new idea for developing mechanical energy storage and conversion systems. Lung disease: Mechanoluminescence: stress-sensitive phosphors hold deep memories A material that records mechanical impacts and provides optical readouts at later dates shows promise for anti-counterfeiting devices and structural damage analysis. Phosphors, such as rare earth-doped silicates, can emit light in response to physical stress because they store charge carriers in easy-to-access energy states. Rong-Jun Xie from China's Xiamen University and colleagues now report development of phosphors that release charges to less accessible 'deep' energy states after being stimulated mechanically. These carriers are retained in the deep states and then released on-demand as photon emissions following thermal treatments. The team demonstrated several applications for the new phosphors including sensors that can record signature traces and composite films that attach to vehicles to monitor for potential collisions.