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Page 4 of 23 Zhou et al. Microstructures 2023;3:2023043 https://dx.doi.org/10.20517/microstructures.2023.38

Apart from providing excellent chemical resolution, the APT analysis also offers valuable spatial
information. This is achieved by using a position-sensitive ion detector that enables the back projection of
[1,2]
the spatial origins of ions based on the nominal ion trajectory path in the equipment [Figure 2] . As a
result, a 3-D reconstructed atom map is produced, providing a detailed visualization of the atomic
arrangement within the material under investigation.

To achieve the ultra-high vacuum required for ion evaporation and detection, the modern Local Electrode
Atom Probe instrument (LEAP, developed by CAMECA) employs a three-chamber system . The first
[13]
chamber, known as the “load lock”, is responsible for pumping down from ambient pressure to around
10 Pa. Once this step is completed, the APT specimen can be transferred to the second chamber, called the
-5
-7
“buffer” chamber, which maintains a constant pressure of approximately 10 Pa. Finally, the APT specimen
-9
is moved to the "analysis chamber", where the pressure is maintained below 10 Pa. This chamber is
equipped with various components, including the specimen cooling system, the ion detector for ToF
measurements, and the pulsing system.

In 2017, the groundbreaking development of cryogenic electron microscopy (cryo-EM) was honored with
the Nobel Prize in Chemistry . This prestigious award recognized the seminal progress made in cryogenic
[24]
specimen handling, enabling electron microscopy specimens to be rapidly cooled to extremely low
temperatures and maintained in a "frozen" state throughout the transfer process. The key to this
advancement lies in the ability to quickly quench the specimens, leading to the formation of amorphous ice
from the water within them. This amorphous ice prevents ice crystallization, which could otherwise damage
delicate structures during slow cooling processes . The innovation in cryo-specimen transfer has opened
[25]
up exciting possibilities for preserving biological specimens in their natural states before observation in
[26]
electron microscopes . This breakthrough also served as inspiration for the development of a cryo-
specimen handling workflow dedicated to APT by Gerstl and Wepf . This adaptation of cryogenic
[27]
techniques created opportunities to apply APT to soft matter and specimens containing diffusive solutes,
which require low temperatures to immobilize them. Following their pioneering work at ETH Zurich, other
APT laboratories have invested substantial efforts in enhancing and innovating cryo-workflows and
applications, leading to the emergence of a new microanalysis technique called cryogenic APT (cryo-
APT) [28-32] . This cutting-edge approach represents a promising frontier in the field of microanalysis.

CRYO-APT
When it comes to cryo-APT analysis, the critical aspect lies in maintaining the low temperature of the
specimen between its preparation and the actual APT analysis. This involves two key processes: the "cryo-
transfer" (cryogenic transfer) to keep the specimen at a low temperature and the "cryo-specimen fabrication"
to create the specific APT tip geometry. It is important to mention that the temperatures used for cryo-
specimen fabrication and cryo-transfer can vary depending on the purpose, the type of coolant used, and
the efficiency of thermal conduction from the coolant. Generally, temperatures ranging from 120 to 140 K
are required for forming vitrified ice, which helps prevent damage during ice crystallization in biological
specimens . On the other hand, temperatures around 150 K are necessary to inhibit the desorption of
[26]
diffusive hydrogen solute from a metallic specimen [33,34] .

The cryo-transfer process involves plunge-freezing or direct cooling in liquid nitrogen, which allows the
specimen to reach temperatures as low as 77 K. Using liquid helium enables reaching even lower
temperatures of around 4 K. However, in an indirect cooling process, the lowest temperature that the
specimen can achieve is somewhat compromised due to limitations in thermal conduction efficiency at
various interfaces of the physical components. As a rule of thumb, utilizing copper bands for cooling with