The components of bone assemble hierarchically to provide stiffness and toughness. However, the organization and relationship between bone's principal components-mineral and collagen-has not been clearly elucidated. Using three-dimensional electron tomography imaging and high-resolution two-dimensional electron microscopy, we demonstrate that bone mineral is hierarchically assembled beginning at the nanoscale: Needle-shaped mineral units merge laterally to form platelets, and these are further organized into stacks of roughly parallel platelets. These stacks coalesce into aggregates that exceed the lateral dimensions of the collagen fibrils and span adjacent fibrils as continuous, cross-fibrillar mineralization. On the basis of these observations, we present a structural model of hierarchy and continuity for the mineral phase, which contributes to the structural integrity of bone.

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Heterogeneous gas‐solid catalyst reactions occur on the atomic scale and there is increasing evidence single atoms and very small clusters can act as primary active sites in chemical reactions. Aberration corrected environmental scanning transmission electron microscope (AC ESTEM) has been developed for novel studies of nanoparticle catalysts with Angstrom (0.1 nm) resolution in gas. It extends high vacuum analyses with full analytical facilities and unrestricted high angle annular dark field (HAADF) imaging of single atoms, small clusters and nanoparticles in dynamic in‐situ experiments with controlled gas reaction environments at initial operating temperatures up to > 500 °C.

The structural, chemical, and magnetic properties of magnetite nanoparticles are compared. Aberration corrected scanning transmission electron microscopy reveals the prevalence of antiphase boundaries in nanoparticles that have significantly reduced magnetization, relative to the bulk. Atomistic magnetic modelling of nanoparticles with and without these defects reveals the origin of the reduced moment. Strong antiferromagnetic interactions across antiphase boundaries support multiple magnetic domains even in particles as small as 12–14 nm.