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Zhou et al. Microstructures 2023;3:2023043 https://dx.doi.org/10.20517/microstructures.2023.38 Page 9 of 23

Figure 5. APT specimen preparation from a bulk nanoporous gold (NPG) with frozen heavy water (D O ice) in a plasma FIB. (A) NPG
2
and D O ice before annular milling with the 200- and 75-μm milling patterns. (B) D O ice/NPG pillar during ion milling. (C) Ice/NPG tip
2 2
[32]
after milling. (D) High-resolution view of the final APT tip. Reproduced with the permission of Ref. Copyright 2023, Science
Advances.
the metallic microstructure . Among these traps, dispersoids, such as second-phase precipitates, are of
[66]
great interest as they enhance material toughness while reducing the diffusible hydrogen content in the
matrix .
[67]

APT exhibits exceptional sensitivity to light elements, including hydrogen, making it a promising technique
for hydrogen mapping in materials. However, the high diffusivity of hydrogen in metals and alloys,
regardless of the presence of traps, poses challenges, as hydrogen-charged metallic specimens are prone to
desorption and signal loss. Recent advancements in cryo-transfer have enabled the study of frozen-in-place
hydrogen atoms in metals and alloys using APT [40,46,68-73] . To ensure high-quality APT hydrogen analyses, a
key experimental requirement is introducing deuterium ( H or D) into APT tip specimens. Deuterium is a
2
less-abundant isotope of hydrogen with a similar chemical reactivity but an additional neutron, resulting in
higher atomic mass (and slightly lower diffusivity) than protium ( H), the most abundant hydrogen isotope.
1
Charging deuterium into APT specimens allows the distinction of experimentally introduced signals from
the hydrogen background noise arising from the residual gas in the APT chamber . While some studies
[15]
have shown that using hydrogen charging can also provide qualitative measurements of hydrogen
trapping , deuterium charging has been demonstrated to reduce ambiguity in observations .
[74]
[15]
Furthermore, deuterium charging must be combined with voltage-pulsed APT, as laser-pulsed APT can
produce a significant amount of H peak at 2 m/z, leading to confusion when assigning 2 m/z to deuterium
+
1
2
( H ) . A comprehensive discussion on using APT for high-quality hydrogen mapping can be found in a
2
+ [75]
recent review . The following section will present several example findings made possible by cryo-APT.
[15]