Effect of Focal Depth of HAADF-STEM Imaging on the Solute Enriched Layers in Mg Alloys

Abstract

This study has found that the Z-contrast of aberration-corrected high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) images of solute enriched layers in Mg-TM-RE (TM:Zn, RE:Y, Gd) alloys is sensitive to the imaging conditions: the defocus and the focal depth. Consequently, the depth position of solute enriched layers in the projected direction shows strong effect on the Z-contrast around each layer. The blurring of the Z-contrast is preferentially observed in both sides region along (0001) Mg basal planes. The boundary position between HCP-and FCC-type stacking sequence at edge regions does not change by defocus. The permissible defocus amount is comparable to the focal depth. The aberration-corrected HAADF-STEM has short focal depth less than 10 nm, so that it is critical for interpreting image contrast that the thin foil thickness and the depth position of the nano-size structure such as the solute enriched layers. In turn, it can be presumed, by the blurring of the Z-contrast, that the depth position of the solute enriched layers in the projected direction. Thus, the focal depth of aberration-corrected HAADF-STEM is quite sensitive to the depth position of solute enriched layers in the thin foil of specimens and the defocus of the electron probe, so that the imaging condition of aberration-corrected HAADF-STEM is critical in order to interpret correctly the Z-contrast of the images.

Atomic-Scale Observation of Titanium-Ion Shifts in Barium Titanate Nanoparticles: Implications for Ferroelectric Applications

“…This conclusion sounds reasonable, considering the focal depth (∼6.6 nm) of the ADF-STEM image acquired with the present experimental setting. The focal depth was calculated by the equation focal length = 1.77­(λ/α 2 ), where λ is the wavelength of electron and α is the probe-forming aperture semiangle …”

“…The focal depth was calculated by the equation focal length = 1.77(λ/α 2 ), where λ is the wavelength of electron and α is the probe-forming aperture semiangle. 23 Figure 4a shows the distribution of ferroelectric shifts near the top surface obtained from the STEM image in Figure 2a. Approximately half (41.5%) of the region exhibited a shift along [100] ± 22.5°(light-blue arrows) and shifts along [101] ± 22.5°(green arrows) in the other half (42.6%) of the region.…”

“…Because the titanium-ion shift should be closely related to the ferroelectricity of the NP, the shift will be called the ferroelectric shift, hereafter. The STEM images were obtained by focusing an electron probe on the top surface and on the middle of the NP, which allowed us to examine the structures near the surface and inside the NP, independently. , …”

“…The STEM images were obtained by focusing an electron probe on the top surface and on the middle of the NP, which allowed us to examine the structures near the surface and inside the NP, independently. 22,23 ■ METHODS BT NPs were fabricated by a hydrothermal method using a supercritical continuous-flow reaction system. Aqueous 0.05 M TiO 2 sols and 0.06 M Ba(OH) 2 solutions were used as the starting materials, and the reaction temperature, time, and pressure were set at 400 °C, 2 s, and 30 MPa, respectively.…”

Probing Crystal Dislocations in a Micrometer-Thick GaN Film by Modern High-Voltage Electron Microscopy

“…Let us consider a STEM probe with the convergence semiangle α and depth of focus Δ z . Assuming the smallest converged STEM probe (approximately 1 nm at 1 MV in the present experimental setup), a depth of focus as short as 110 nm is derived using the following equation where λ denotes the wavelength of incident electrons. , However, same dislocations were observed irrespective of the beam incidence direction to the wedge-shaped specimen with the thickness less than 2.5 μm, which is 23 times longer than the short depth of focus estimated above (Figure ). This result suggests that Δ z defined by the eq no longer serves as an indicator of the usable thickness.…”