The detailed structural characterizations of the ZnO:Eu nano-sponges are presented in Figure . The magnified HAADF-STEM images showed brighter contrast between the ZnO:Eu nano-crystals in both samples, suggesting an Eu-richer region. The 20Eu sample exhibited more of the Eu-rich inter-granular regions which were also wider at typically 5-10 Å thick, compared to the 5Eu sample. STEM-EDX mapping (Figure , 3G) further confirmed the increase in Eu concentration in these inter-crystalline regions. The STEM-EDX line profile (Figure ) gives the elemental concentration variation from the boundary towards the ZnO:Eu nano-crystal. It shows a discernible decrease in the Eu signal, accompanied by an increase in the Zn signal for the ZnO:Eu crystal, while the O concentration remains relatively stable. This provides evidence for a higher concentration of Eu at the grain boundaries of the ZnO:Eu nano-crystals. In the case of the 20Eu nano-sponge, the extracted spectrum from two distinct regions and subsequent calculation of the normalized composition yielded an approximate Eu concentration at the crystal boundaries of 13 at.% as compared to 3 at.% within the nano-crystals (Figure ). A similar comparison for the 5Eu sample yielded analogous results to those obtained for the 20Eu sample (Figure ) with the Eu concentration at the crystal boundaries and within ZnO:Eu crystal being approximately 6 at.% Eu and 2 at.% Eu, respectively. Given the identification of Eu within the ZnO crystals and the observed segregation of Eu-oxide to the crystal boundaries, the distribution of Eu atoms within ZnO nano-crystals was investigated with atomic-resolution HAADF-STEM images (Figure for 5Eu and Figure for 20Eu). Some bright-contrast atomic columns were observed indicating the presence of dopant Eu atoms within the ZnO structure. To get a deeper understanding of the influence of Eu doping on HAADF contrast, simulated HAADF-STEM images were made by leveraging the doped structure and the hexagonal ZnO structure along [001] zone axis. Certain brighter atomic columns in experimental data are marked by red circles in Figure doping in the ZnO structure lacks long-range periodicity, and instead exhibit a more random distribution. Moreover, it was found that the insertion of Eu atoms or clusters did not introduce significant lattice distortion into ZnO lattice which is line with the XRD data. The Eu cluster-like structure is labelled by a square mark in both the experimental and simulation data (Figure and 3D). To ensure the representativeness of these findings, additional data were collected which are presented in the Supplementary Materials (Figure ).