Zirconium and hafnium oxide nanocrystals (NCs) are appealing (host) materials due to their high thermal and chemical stability, and large band gap. Colloidal ZrO2 NCs are components for (in)organic composites while HfO2 NCs find applications in memory devices. Due to the high atomic number of hafnium and the high density of hafnia, HfO2 NCs are developed as CT contrast agents, scintillators, and as radiation therapy enhancers. Both ZrO2 and HfO2 NCs serve as hosts for optically active lanthanide ions, e.g, europium. Fluorides (e.g., NaYF4 or NaGdF4) are another class of nanocrystals that are widely used as host for lanthanides with application in both upconversion and downconversion. In the fluoride system, the syntheses are well developed, allowing the precise positioning of dopants inside the nanocrystal and the growth of undoped shells on the doped core. This procedure protects the lanthanides from surface effects and thus increases the quantum efficiency of both up-and downconversion processes. Additionally, layered structures offer controlled energy cas-cades. Higher quantum efficiencies coupled with long lifetimes enabled their use in, e.g., time-gated fluorescence imaging. The oxide hosts have found less widespread use, due to the synthetic challenge of producing colloidally stable oxide nanocrystals with a complex (e.g., core/shell) architecture. However, the oxide hosts are more chemically stable while the fluorides dissolve in highly dilute aqueous media. Surfactant-assisted, nonaqueous syntheses have allowed to synthesize ZrO2 and HfO2 NCs and gain control over their size. Even more, solid solutions of various compositions (ZrxHf1-xO2) were obtained as colloidal nanocrystals. Hf0.5Zr0.5O2 is an interesting ferroelectric material, making it promising for non-volatile memory devices. Here, we leverage the control of nonaqueous synthesis and report metal oxide core/shell nanocrystals. We epitaxially grow HfO2 onto ZrO2, and ZrO2 onto ZrO2:Eu 3+ . The beneficial effect of the shell on the optical properties of the europium dopant is established. Having access to these novel heterostructures (unavailable in bulk) opens up possibilities for their application in different areas, from microelectronics to scintillators. Zr and Hf are chemically very similar and both oxide nanocrystals can be synthesized at 340 °C from the metal choride and isopropoxide in tri-n-octylphosphine oxide (TOPO). Under these conditions, HfO2 forms nanorods with the monoclinic crystal structure, while ZrO2 forms spheres with the tetragonal crystal structure. We first focus on the ZrO2/HfO2 core/shell structure, since one can conveniently distinguish growth of HfO2 onto the spherical ZrO2 cores from separately nucleating HfO2 nanorods. ZrO2 and HfO2 form similar crystal structures (monoclinic, tetragonal and cubic) with negligible lattice mismatch, thus allowing for epitaxial growth. We follow a two-step approach towards the core/shell structure. In a first step, we synthesize and isolate ZrO2 cores from ZrCl4(THF)2 and Zr(O i Pr)4. i PrOH in TOPO. The cores are spherical, with an average diameter of 3.8 nm according to bright field transmission electron microscopy (BF TEM, Figure ). The cores have the tetragonal (P42/nmc) crystal structure (Figure ), and their surface is covered with a mixture of protonated TOPO, dioctylphosphinate and dioctyl pyrophosphonate (Figure ). In a second step, the purified cores are added to a new reaction mixture of HfCl4(THF)2 and Hf(O i Pr)4. i PrOH in TOPO, see Figure . After 2 hours at 340°C, the core/shell structures are isolated and purified. The nanocrystal surface is covered by the same ligands as mentioned before (see Figure for the 31 P NMR spectrum). Atomic resolution, high angle annular dark field (HAADF) scanning TEM (STEM) analysis shows that the resulting nanocrystals are no longer spherical, but more clearly faceted, exposing {01-1} and {10-1} surfaces (Figure ). Nevertheless, to compare to the core diameter, we analyze the BF TEM image (Figure ) by measuring the projected area of the faceted nanocrystals and calculate an apparent 'diameter', using the formula of a circle. The core/shell structures have an average 'diameter' of 4.9 nm, clearly indicating that the cores have grown to larger sizes. Crystal growth is further confirmed by the narrower reflections peaks in XRD (Figure ). The core/shell interface was further analyzed by atomic resolution HAADF STEM, see Figure (and Figures ).