For the core/shells, the decay is monoexponential and independent of the monitored wavelength. We determine a lifetime of 5.3 ± 0.1 ms. This is similar to the longest values reported, indicating that the contribution of non-radiative decay, e.g. via interaction with surface or other defects, is limited. In the literature, long lifetimes of 4-5 ms only appear for low doping percentages (1%). Higher doping percentages are typically associated with shorter lifetimes. Here we report a core doped with 9% Eu, which features a lifetime of 5.3 ms after shelling and thus passivating of the surface defects. Some authors attribute the faster decay to monoclinic crystals phase and the slower decay to the tetragonal phase. Given that we find no evidence for a monoclinic phase, we assign the fast decay for the ZrO2 cores to Eu ions that are at the nanocrystal surface. Based on the changes in the emission spectrum and the increase in lifetime, we conclude that the shelling improves the incorporation of the Eu ions in the crystal. Indeed, Eu 3+ ions act as a probe for their local environment. Finally, we measured the emission spectra of the core/shell particles at low temperatures (Figure ). When excited at 300 nm, they feature the 4f-4f transitions of Eu 3+ , and a broadband contribution ranging from 350 nm to 600 nm, related to defect emission in ZrO2. This indicates that the Eu ions remain located in the core during the shelling, since the presence of Eu ions in the shell would suppress the ZrO2 defect emission (either by preventing the defect to form or by energy transfer to Eu). Therefore, the overall emission spectrum of the core/shells contains emission from Eu 3+ in the core and defect emission from the shells. This also explains the drastic increase in the lifetime of the core/shell particles, since the Eu 3+ ions are now in a perfect environment far from the outside. In summary, we reported the use of crystalline zirconium or hafnium oxide to shell nanocrystals. This method was applied to improve the local environment of europium dopants in zirconia nanocrystals and to suppress non-radiative de-excitation pathways. Our results are an invitation to explore intricate nanocrystal architectures with oxide hosts materials.