Aberration-corrected ADF-STEM depth sectioning and prospects for reliable 3D imaging in S/TEM

Area of Science:

• Materials Science
• Electron Microscopy
• Nanotechnology

Background:

• Aberration-corrected scanning transmission electron microscopes (STEMs) offer high resolution for nanomaterial imaging.
• Short depth of focus in STEMs suggests potential for 3D reconstruction via through-focal series.
• Annular dark-field (ADF)-STEM depth sectioning is hindered by a contrast transfer function with a missing-cone problem, analogous to tilt-series tomography.

Purpose of the Study:

• To investigate the feasibility and limitations of 3D reconstruction using STEM depth sectioning.
• To analyze the missing-cone problem and image distortion in ADF-STEM depth sectioning.
• To compare the performance of different STEM/TEM imaging modes for 3D reconstruction.

Main Methods:

• Depth-sectioning experiments were conducted using a C(5)-corrected aberration-corrected STEM (100 keV, 33-mrad probe-forming angle).
• Analysis of the contrast transfer function and image distortion in both real and reciprocal space.
• Comparison of imaging modes including bright-field STEM, ADF-STEM, hollow-cone ADF-STEM, and coherent scanning confocal electron microscopy (SCEM).

Main Results:

• The study confirms a significant missing-cone problem in ADF-STEM depth sectioning.
• An elongation factor greater than 30 was observed for depth sectioning under optimal conditions, leading to severe 3D object distortion and artifacts.
• Bright-field STEM, ADF-STEM, hollow-cone ADF-STEM, and coherent SCEM all exhibit a missing cone of information.

Conclusions:

• The current ADF-STEM depth sectioning method is severely limited by image distortion and information loss due to the missing-cone problem.
• Only incoherent scanning confocal electron microscopy (SCEM) effectively overcomes the missing-cone limitation.
• Further development is needed to realize the potential of STEM-based 3D reconstruction of nanomaterials.