In-situ observation of the growth and decomposition processes is significantly important for guiding the fabrication of up-conversion nanocrystals (UCNCs) with high performance. However, the high crystallization energy and rapid nucleation rate of the corresponding crystals make real-time observation still a huge challenge. Herein, the in-situ nucleation-growth-degradation processes of the UCNCs are investigated by employing tailored metastable intermediates, which possess a slowing-down nucleation rate under electron beam irradiation. The non-classical nucleation processes of the UCNCs, containing the coalescence of clusters and the subsequent crystallization, are demonstrated. Moreover, the Ostwald ripening and oriented attachment processes, which determine the particle size and morphology, are unambiguously recorded. Furthermore, the degrading process of the UCNCs is observed to be triggered by surface defects. Our work provides an insight into the real-time evolution dynamics of the UCNCs, which further sheds light on the fabrication of nano-sized up-conversion phosphors.

Real-time stress sensing based on mechanoluminescence materials has been widely studied for structural health monitoring of bridges, buildings, high-pressure vessels, and other infrastructure surfaces. However, this approach is difficult to detect the stress information of closed mechanical structures. Here, we propose a delayed stress memory strategy to record the stress information of closed mechanical structure by the flexible film composed with CaAl₂O₄:Eu²⁺,Sm³⁺ phosphor. After the force is applied, the optical information on the film can be read out by the near-infrared laser after a period of time without real-time monitoring, and the stress distribution information of bearings and gears in the engine can be obtained. Furthermore, the regulation of trap depth from 0.662 to 1.042 eV allows the captured carriers to remain in the traps for a long time without being released as long persistent luminescence, which is beneficial to the delayed stress memory. Therefore, this work promotes the application prospect of mechanoluminescence materials in stress sensing, and provides a new idea to record the stress information of closed mechanical structures.