From the contrast we can recognize the presence of a core/shell structure as the shell appears brighter with respect to the core, due to the higher atomic number of Hf compared to Zr. The power spectrum (FFT) analysis in the reciprocal space, shows that that the ZrO2 core has the P42/nmc tetragonal structure and that the HfO2 shell has a similar structure. Tetragonal HfO2 has the following lattice parameters at room temperature: a=b= 3.65 Å, c=5.33 Å, slightly higher than tetragonal ZrO2 with a=b= 3.59 Å, c=5.18 Å. This is in good agreement with the observed crystal arrangement in our core/shell nanoparticles. The ( ) and (10-1) reflections are 'elongated' or doubled, indicating the presence of HfO2 that grows epitaxially and partially relaxed onto the indicated surfaces of the ZrO2 core. The frequency filtered map displays clearly the core in green and the shell in red. While pure HfO2 nanocrystals would crystallize in the monoclinic P21/c structure, epitaxial growth thus stabilizes hafnia in its tetragonal structure. Final confirmation of the core/shell structure is provided by energy-dispersive X-ray spectroscopy (EDX) compositional mapping in Figure . It shows regions with zirconium in the center and hafnium in the shell. Second, we turn to the optically active ZrO2:Eu/ZrO2 core/shell system. We synthesize europium doped zirconia (10.0% nominal doping) from europium acetate, zirconium propoxide and benzyl alcohol. After synthesis, we functionalize the nanocrystal surface with dodecanoic acid ligands (Figure ). The ZrO2:Eu cores have an average diameter of 3.5 nm (Figure ), possess the tetragonal crystal structure (Figure ) and have an actual doping concentration of 9.0 % (determined by ICP-OES). The cores were subjected to the shelling procedure using ZrCl4(THF)2 and Zr(O i Pr)4. i PrOH. After 2 hours at 340°C, colloidally stable nanocrystals were isolated and purified (Figure for 31 P NMR and Figure for DLS). BF TEM imaging shows an increase in the average 'diameter' to 5.1 nm (Figure ) and again sharper reflections are observed in XRD, confirming growth of the tetragonal crystal (Figure ). HAADF STEM and powder spectrum analysis further confirm that both ZrO2:Eu cores and ZrO2:Eu/ZrO2 core/shells have the tetragonal structure (Figures ). The europium content decreases in the core/shells to 1.8% metal content. This is close to the expected 2.25% considering the four-fold volume increase of the nanocrystals upon shelling, and also taking into account the limitations in determining the nanocrystal volume given the faceted nature of the core/shell nanocrystals. The optical properties of the europium dopant change substantially when the cores are shelled, see Figure . The emission spectrum of Eu in the ZrO2:Eu cores is consistent with previous reports of Eu doped into colloidal ZrO2 nanocrystals or ZrO2 powders. Upon shelling, certain transitions disappear and the remaining emission peaks are more narrow. This indicates a change in the coordination around the europium ion. This is further demonstrated by the shape of the 5 D0-7 F0 transition around 580 nm, measured at 10K (Figure ). This J=0  J'=0 transition is not split by the crystal field, and thus spectrally different contributions for this transition can be assigned to different coordinating environments. For the core/shells, the emission band corresponding to this transition is narrower and more symmetric, indicating a highly uniform environment for europium. The shell has also a big impact on the lifetime of the excited state (Figure ). For the ZrO2:Eu cores, the luminescence decay depends on the monitored wavelength but all feature a biexponential decay. This variation in lifetime points again to different europium coordination environments. We fit the decay using the formula: I(t) = I + a e / + a e / At 606 nm, we determine a slow component of 2.7 ± 0.1 ms and a fast component of 1.2 ± 0.1 ms. The slow component contributes 86% to the total emission, calculated via: