Reversible contrast in focus series of annular bright field images of a crystalline LiMn2O4 nanowire

A through-focus series of annular bright field (ABF) images were observed simultaneously with high-angle annular dark field (HAADF) images of very thin lithium manganese oxide (LiMn2O4), a typical cathode material used in lithium ion batteries, using a spherical aberration corrected electron microscope with a 50 pm resolution (R005). The ABF images showed dark dips at the positions of Li and Mn–O atomic columns, which reversed to bright peaks when the defocus sign was changed, as commonly observed in phase contrast images. The optimal defocus for the ABF images was about 2 nm of over-focus, while that for the HAADF images was 2 nm of under-focus for an incident probe with a convergent semi-angle of 30 mrad. These experimental results are interpreted based on a weak-phase-object approximation.