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• In this work, different transition metal ions (Mn, Fe, and Cu) were doped into ZnO nanocrystals. • The current study presents a detailed multi-frequency (X- and Q-band) EPR investigation that clarifies the localization of the dopant ion, its site symmetry, and the formation of intrinsic-extrinsic defect complexes in ZnO:TM. • Incorporation of the TM ions was observed to modify the intrinsic defect structure of ZnO nanocrystals. • Notably, a deviation from the core-shell model was also observed. The appearance of intrinsic-extrinsic defect complexes that may contribute to a peculiar spin-exchange. [Display omitted] Mn, Fe, and Cu ions, when doped into ZnO nanocrystals, impart magnetic phenomena to their semiconducting property. Although notable in the dilute magnetic semiconductor community, transition metal (TM) ion-doped ZnO lacks investigations that inform researchers on the local lattice structure around the dopant ion, its spin-exchange phenomena, and the interaction between its intrinsic defects and the doped metal ion. The current study presents a detailed multi-frequency (X- and Q-band) EPR investigation that clarifies the localization of the dopant ion, its site symmetry, and the formation of intrinsic-extrinsic defect complexes in ZnO:TM. The incorporation of TM ion is observed to modify the intrinsic defect structure of ZnO nanocrystals. Particularly, a deviation from the core-shell model is observed for ZnO:TM, and the appearance of intrinsic-extrinsic defect complexes that may contribute to a peculiar spin-exchange phenomenon are noticed. Additionally, the localization as observed from the resonance lines of defect complexes comprising Cu 2 + is different from those of Mn 2 + and Fe 3 + , showing charge selective substitutions in the matrix. [ABSTRACT FROM AUTHOR]

This study investigated the influence of the ZnO morphology on the energy storage capacity of symmetric supercapacitor devices, focusing on the defect centers present in the materials due to morpho-structural differences. Thus, ZnOs with five different morphologies were synthesized with varying methods in which the nanoparticles have the form of flowers, bullets, pyramids, hexagons, and rods. All materials were thoroughly characterized by scanning/transmission electron microscopy, X-ray diffraction, and spectroscopic techniques like photoluminescence, UV–Vis, Raman, and electron paramagnetic resonance spectroscopy. Subsequently, the ZnO-based materials were assembled in symmetric supercapacitor devices to test their energy storage capabilities. The ZnO with nanorod and hexagonal morphologies showed the best specific capacity (180 and 142 F/g), energy density (25 and 19 Wh/kg), and power density (211 and 252 kW/kg) values. The importance of the defect centers in the materials was highlighted, where the ratio between the Zn and O vacancies, evidenced by EPR spectroscopy, plays a crucial role in the ZnO materials' energy storage capacity. The Raman results support the proposed model, which underlines the importance of oxygen vacancies. In contrast, BET measurements show that the specific surface area of the morphologically distinct ZnO materials does not change drastically, validating the importance of the defect centers in the materials. A so-called "bottle-neck" effect is proposed to describe the relation between the paramagnetic vacancies and the electric properties of the materials. [Display omitted] [ABSTRACT FROM AUTHOR]