Kavli Affiliate: David A. Muller
| First 5 Authors: Desheng Ma, Desheng Ma, , ,
| Summary:
Tilt-corrected imaging methods in four-dimensional scanning transmission
electron microscopy (4D-STEM) have recently emerged as a new class of direct
ptychography methods that are especially useful at low dose. The operation of
tilt correction unfolds the contrast transfer functions (CTF) of the virtual
bright-field images and retains coherence by correcting defocus-induced spatial
shifts. By performing summation or subtraction of the tilt-corrected images,
the real or imaginary parts of the complex phase-contrast transfer functions
are recovered, producing a tilt-corrected bright field image (tcBF) or a
differential phase contrast image (tcDPC). However, the CTF can be strongly
damped by the introduction of higher-order aberrations than defocus. In this
paper, we show how aberration-corrected bright-field imaging (acBF), which
combines tcBF and tcDPC, enables continuously-nonzero contrast transfer within
the information limit, even in the presence of higher-order aberrations. At
Scherzer defocus in a spherically-aberration-limited system, the resultant
phase shift from the probe-forming lens acts as a phase plate, removing
oscillations from the acBF CTF. We demonstrate acBF on both simulated and
experimental data, showing it produces superior performance to tcBF or DPC
methods alone, and discuss its limitations.
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