In contrast to bulk materials, nanoscale crystal growth is critically influenced by size-and shape-dependent properties. However, it is challenging to decipher how stoichiometry, in the realm of mixed-valence elements, can act to control physical properties, especially when complex bonding is implicated by short and long-range ordering of structural defects. Here, solution-grown iron-oxide nanocrystals (NCs) of the pilot wüstite system are found to convert into iron-deficient rock-salt and ferro-spinel sub-domains, but attain a surprising tetragonally distorted local structure. Cationic vacancies within chemically uniform NCs are portrayed as the parameter to tweak the underlying properties. These lattice imperfections are shown to produce local exchange-anisotropy fields that reinforce the nanoparticles' magnetization and overcome the influence of finite-size effects. The concept of atomic-scale defect control in subcritical size NCs, aspires to become a pathway to tailor-made properties with improved performance for hyperthermia heating over defect-free NCs.
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Results 1 Structural insights
Single-particle local structureFour nanoparticle samples were made available for this study, with specimen of spherical shape, entailing diameters of 8.1 ± 0.6 nm and 15.4 ± 1.3 nm and of cubic shape, obtaining edge-lengths of 12.3 ± 0.7 nm and 17.7 ± 1.6 nm ( Fig. S1). Henceforth, these are called S8, S15, C12 and C18, where 'S' stands for spherical and 'C' for cubic morphologies. Moreover, high-resolution transmission electron microscopy (HRTEM) ( Fig. 2a-d) suggests that the smaller S8 and C12 NPs entail a domain of a single-phase material, but the bimodal contrast in the larger S15 and C18 points that above a particle size of 12 nm two nanodomains are attained. The coexistence of dark and light contrast features in the S15 and C18 could be justified assuming that two chemical phases, of varying electron diffracting power, share the same nanoparticle volume. Crystallographic image processing by Fast Fourier transform (FFT) analysis of the relevant HRTEM images results in their corresponding (spot) electron diffraction patterns ( Fig. 2e-h). The unequivocal indexing of reflections in the S8 and C12 specimen may suggest that these adopt the magnetite type of phase ( Fig. 2e, f). On the other hand, a similar conclusion cannot be made after the evaluation of the FFT patterns of the apparently bimodal contrast S15 and C18 specimen as the cubic spinel and rock salt reflections (Fig. 2g, h) appear to be resolution limited. The behavior appears in line with the tendency of bulk wüstite for oxidative conversion, [28], [42] and infers a process analogous to its elimination from 23 nm core@shell FexO@Fe3-δO4 nanoparticles. [25] The long and short-range modulations of the selected (hkl) atomic planes could become more easily isolated with direct space images derived from the inverse FFT synthesis of chosen families of reflections. We find that the (400) (or (200) for the core@shell S15 and C18 NPs) family generates perfect atomic...