In high-angle annular dark-field imaging scanning transmission electron microscopy, electrons scattered to large angles give predominantly incoherent mass–thickness and atomic-number contrast. For comparable thickness, gold is much brighter than nickel–phosphorus because its atomic number is much larger. carbon in the protective cap scatters weakly, whereas its tungsten component scatters strongly. The cap is not a homogeneous low-atomic-number reference.
The gold layer follows a curved, locally distorted interface. The dark central opening in the overview is consistent with a crack, void or locally missing/thinned foil: little material along the beam path gives little high-angle scattering. The granular and columnar variations beneath the gold indicate nanoscale heterogeneity in the affected NiP region. Differences in thickness, density, porosity and composition can all contribute; diffraction/channeling effects are reduced in high-angle annular dark-field imaging but not eliminated in every geometry.
A black region is therefore evidence of low projected scattering mass, not by itself the identity of a chemical phase. In particular, a single high-angle annular dark-field imaging image does not distinguish a pre-existing void from material lost during specimen preparation.
The thin scanning transmission electron microscopy foil restricts scattering to a much smaller projected specimen volume than bulk SEM at . It reveals the nanoscale shape and continuity of the gold/NiP interface, the narrow low-density opening and the fine heterogeneous region underneath the gold, which the larger SEM interaction volume and surface-sensitive collection obscure.
The enlarged view resolves structure on the tens-of-nanometres scale: local interface roughness, small bright and dark domains and the relationship of the gold layer to the damaged or porous-looking NiP region. This helps distinguish an interface-associated defect from merely a change between nominal alloy layers. It still does not identify a crystalline phase or quantify composition without diffraction or spectroscopic analysis. Since the SEM surface and focused ion beam milling lamella need not cut precisely the same volume, correlations should allow for section position.
The conspicuous approximately vertical streaks are focused ion beam milling curtaining: different sputtering rates beneath the rough, compositionally varying cap and interfaces leave stripes of different foil thickness. The uneven lower foil edge, contrast gradient and locally thinned regions are also consistent with nonuniform milling; redeposition can add material to other regions.
A dark gap or pull-out may have been enlarged or created during milling, so its original dimensions cannot be inferred securely from this one image. ion implantation and an amorphized surface layer are additional possible focused ion beam milling effects, but they are not independently demonstrated by these intensity variations. A lower-energy final polish, improved protective cap and examination of another lamella could test the preparation contribution.
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