Quantitative analysis of atomic displacements in iron/iron oxide core-shell nanoparticles and their effect on high-resolution scanning transmission electron microscopy imaging

The main purpose of this project is to investigate the impact of atomic level displacement fields on the oxide shell of iron/iron oxide shell NPs (Fe@Fe3O4). This effect could play an important role for the reactivity of these NPs because their atomic structure is influenced by strain, which is essential for their applications in areas such as in biomedicines, environmental remediation, data storage or catalysis. To systematically study this impact, finite element (FEM) simulations (COM- SOL Multiphysics® programme) were utilized to obtain a realistic 3D displace- ment field which was then applied to 3D atomistic oxide shell models. Quantitative TEM/STEM simulations (QSTEM) were used to simulate the impact of such strain on image formation. The results reveal that the strain has a significant effect on high-resolution scan- ning transmission electron microscopy (HRSTEM) imaging. The image intensities of Fe(II) atomic columns in the strained model show a reversal in the intensity dis- tribution in comparison to the unstrained case. This is expressed by a decrease of column by column intensity variation from the core/shell interface towards the outer edge of the domain in the presence of strain whereas for the unstrained/relaxed case the column by column intensity from the core/shell interface towards the outer edge of the domain is increasing. The results also reveal that diffusion in the NPs de- pends on their size. For example, the displacement field mapping results reveal a relatively higher lattice strain in small (<20 nm) NPs which is distributed across the oxide lattice, inducing lattice diffusion, while for large NPs ( >34 nm) strain is concentrated around the grain boundaries (GBs), which could be related to enhance GB diffusion.