The Hidden Limits of Top-Down Approach: Cerium-Doped Garnet Nanophosphors as a Case Study

Despite the growing demand for cerium-doped yttrium aluminum garnet nanocrystals (YAG:Ce), their elaboration remains challenging. Although promising results have been obtained through the glycothermal route, the nanoparticles exhibit a low internal quantum yield compared to micron-sized YAG:Ce. Top-down approaches are less studied than bottom-up ones, and little information is available regarding the resulting optical properties. In this work, a top-down approach involving high-energy colloidal ball-milling is employed to achieve cerium-doped yttrium garnet nanocrystals, and the consequences of this process have been investigated through a comparative study of samples milled for different durations, using a multitechnique characterization approach. Structural characterization reveals the rapid (≤5 min) formation of a hydrated yttrium carbonate, as well as a progressive amorphization of the garnet lattice, starting at the surface and propagating to the core of the nanocrystals. Mechanically induced Ce3+ oxidation to Ce4+ is reported for the first time in garnet materials, to the best of our knowledge. The consequences on garnet optical properties are discussed in details. The study describes a top-down route to achieve cerium-doped garnet nanocrystals. However, it also demonstrates that the deterioration of optical properties with a decreasing crystal size is not dependent on the synthesis used to produce the nanoparticles.