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Zhou et al. Microstructures 2023;3:2023043 https://dx.doi.org/10.20517/microstructures.2023.38 Page 5 of 23

liquid nitrogen can reach temperatures as low as 120 K. These experimental details are crucial to enable
successful cryo-APT analysis and open up new possibilities for research in this field.

Cryo-transfer
The success of cryo-transfer for cryo-APT tips depends on three critical factors: environmental moisture
that may come into contact with cryo-specimens, the cooling process, including potential heat absorption
from the surroundings to the cryo-specimens, and the evacuation process required to reach the UHV levels
necessary for the APT experiment [29,35] . Out of these factors, the presence of moisture in the working space is
particularly crucial in cryo-APT experiments. When a sharp tip is exposed to moisture, it can lead to frost
formation on the surface of the tip, which compromises the required sharpness for APT field
[36]
evaporation . Therefore, it is imperative to conduct cryo-transfer for APT specimens in an extremely dry
environment. This can be achieved by using a glovebox purged with dry, non-reactive gases such as
nitrogen and argon. Figure 3A provides an example image of a glovebox used for cryo-APT.

After preparing and treating the specimen inside a glovebox, the next step in cryo-APT involves transferring
the cooled specimen from the glovebox to either a FIB system for further tip-shaping or directly to the APT
measurement setup. To accomplish this, a specimen loading mechanism is utilized. As shown in the inset of
Figure 3A, this connection is achieved using a vacuum specimen carrier, often referred to as a "suitcase".
The suitcase is equipped with a specimen stage containing a cold finger that connects to a dewar (indicated
by the blue arrow). The cryo-and-vacuum suitcase requires a docking mechanism to connect it to the
glovebox (shown as red circles in Figure 3A). Before the cryo-specimen is transferred into the suitcase
through a loadlock (indicated by the green arrow in Figure 3A). The loadlock needs to be pre-evacuated to
-5
sufficiently low pressure, similar to that of the loadlock of the APT instrument, typically between 10 and
-6
10 Pa. This ensures a seamless transition of the specimen into the APT vacuum chambers. Additionally,
the loadlock of the glovebox must be connected to a dewar (blue arrow in Figure 3A) that can be filled with
[34]
coolant to cool the specimen docking stage before it comes into contact with the cryo-specimen . Once the
specimen reaches the desired low temperature and pressure, it is removed from the loadlock using a long
magnetic-coupling transfer arm (the long rod on the left side of the inset in Figure 3A).

During the process of removing the specimen, the transfer arm operating at room temperature can
inadvertently introduce heat, jeopardizing the cryogenic state of the specimen. To counteract this issue, it is
necessary to use a thermos-insulating washer, which can be made of polyetheretherketone (PEEK), as
shown by the red arrow in Figure 3B, which also shows the APT specimen carrier "puck" (the orange
component) for commercial LEAP instruments. In commercial LEAP systems, a carousel is employed to
facilitate the movement of specimens across various chambers, including the loadlock, buffer, and analysis
chambers. To handle cryo-specimens, the carousel also requires a thermo-insulating adaptation. Figure 3C
illustrates the need for a special carousel with a cryo-specimen docking slot made of PEEK to ensure proper
cryo-transfer (red arrow). In addition to the cryo-transfer workflow shown in Figure 3 at the University of
Sydney, Perea et al. developed a vacuum-cryo-transfer system at the Pacific Northwest National Laboratory
in the US, which allows APT tips to expose a range of reactive gases such as hydrogen, oxygen, and carbon
[28]
monoxide . This new instrumentation opens up the possibility of applying APT for catalytic research at
the nanoscale, relatable to the dimension of APT tips.

Using the specialized cryo-transfer components mentioned earlier, a typical workflow for pre-sharpened tip
specimens mounted on a puck unfolds as follows: Firstly, the specimen undergoes the desired treatment
process inside the glovebox, such as electrolytic hydrogen charging as demonstrated in [33,34] . Subsequently,
the specimen is directly plunge-frozen in liquid nitrogen and then loaded into the pre-cooled loadlock of