The atomic structure of nanomaterials is often studied using transmission electron microscopy. In addition to image formation, the energetic electrons impinging on the sample may also cause damage. In a good conductor such as graphene, the damage is limited to the knock-on process caused by elastic electron-nucleus scattering. This process is determined by the kinetic energy an atom needs to be sputtered, i.e. its displacement threshold energy Ed. This is typically assumed to have a fixed value for all electron impacts on equivalent atoms within a crystal. Here we show using density functional tight-binding simulations that the displacement threshold energy is affected by thermal perturbations of atoms from their equilibrium positions. This effect can be accounted for in the estimation of the displacement cross section by replacing the constant threshold energy value with a distribution. Our refined model better describes previous precision measurements of graphene knock-on damage, and should be considered also for other low-dimensional materials.

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Abstract. By hydrothermal decomposition of the Fe-EDTA complex, at 230°C, after 38 h of high -pressure treatment time, magnetite octahedrons with micrometric dimensions (15-45 μm) were obtained. Unusual and notable for these crystallites is the very small hysteresis surface, indicating a superparamagnetic-like behavior at micrometer scale, which means that the agglomeration of these crystallites is avoided after the removal of the magnetic field. This behavior can be correlated with a low level of imperfections in magnetite octahedrons, involving a low level of internal stresses, which leads to coercivity decreasing. The large dispersion in the crystallite axes is consistent with low remanence. We assumed that these attributes are related to the hydrothermal crystal growth process. To elucidate some issues regarding the magnetite crystallites development during the hydrothermal process, a careful observation of occurring intermediate phases by optical and SEM microscopy has been performed. Also the purity of the ferrous carbonate/magnetite mixture was analyzed by FTIR and Mössbauer spectroscopy

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This is an Accepted Manuscript for the Microscopy and Microanalysis 2020 Proceedings. This version may be subject to change during the production process.