Investigating Nano-structured Catalysts at the Atomic scale by Field Ion Microscopy and Atom Probe Tomography - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 255

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Figure 10.4 (a) Schematic diagram of a field ion microscope (b) Ionization of a

polarized gas atom at the vicinity of the tip apex.

10.2.2.1 Principles
A schematic diagram of a FIM is presented on Figure 10.4a. The specimen
tip is positively biased, cooled to cryogenic temperatures and a low amount
of image gas—typically helium or neon at 10 5 mbar—is introduced into
the system. A highly magnified image of the surface of the tip apex is
formed on the screen placed in front of the sample. The field ion micro-
scope is elegant in its simplicity of design and operation. As the voltage on
the specimen is increased, the image gas atoms close to the apex of the
specimen are polarized and attracted to the specimen by the inhomo-
geneous high electric field. When the voltage on the specimens is increased
to generate a suciently high field, typically 30-50 V nm 1 , the image gas
atoms which come close to the surface are field ionized. The resulting ions
are projected towards a screen positioned about 50 mm in front of the
specimen. The field ion image is the result of field ionization of gas atoms
over individual atoms on the surface of the entire apex region of the spe-
cimen. Accordingly, a field ion image such as shown in Figure 10.1c de-
pends on ionization probability of a gas above a curved and charged
conducting surface. By further increasing the field on the specimen, the
surface atoms themselves can be field ionized and evaporated from the
specimen as charged species. This process is termed field evaporation and
is useful to clean the tip sample and also to permit the interior of the
specimen to be examined. The phenomenon is fundamental to the devel-
opment of APT. Microchannel plate image intensifiers are positioned in
front of the phosphor screen to increase the brightness on the screen by a
factor of approximately 10 000.

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10.2.2.2 Applications in Catalysis
The aim of the present chapter is to demonstrate the importance of pro-
viding real-time information on surface reactions by imaging and chemically