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.

Perovskite materials have received extensive attention as optical gain media. However, it is tough to realize lasing using such materials on account of the unstable structure of MAPbI₃/CsPbI₃ nanocrystals (NCs), and the propensity for mixed-halogen perovskites to undergo phase decomposition into bromine-rich and iodide-rich regions under intense laser irradiation. To solve this issue, we fabricated CsPbX₃ (X = Br, I) NCs, which were embedded into a glassy matrix with high stability. In addition, by doping Br^- ions into the CsPbI₃ NCs to partially replace I^- ions, the optical efficiency and the optical gain properties were found to be significantly improved. Here, under 800 nm pulse laser excitation, red random lasing was realized with the assistance of such anion modulation. Moreover, we demonstrate that the perovskite NCs glasses (PNG) show strong water stability after immersion in water for one week, seeding a promise for application in high-definition (HD) displays and photonic devices.