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 254

rhodium and platinum), electropolishing in molten salt mixtures at 520 1C

gives the best results. However, if this procedure is used, rigorous cleaning
of the specimens must be carried out after electropolishing. The evolution of
sample preparation is usually followed by optical microscopy. In spite of
limited resolving power (

B

d n 9 r 4 n g | 8

200 nm) well above the typical size of tip apex,
it usually suces to qualitatively assess the sharpness of a specimen and
thus its suitability for imaging by APM. Electropolishing systems make use
of inexpensive basic materials comprising a AC- and DC power supplies,
beakers, melting pots, furnaces, counter electrode material, and an optical
microscope.

B

10.2.1.2 Focused Ion Beam Methods
For materials which are not suited for electropolishing, focused ion beam
(FIB) methods are the alternative now widely used across the world. The
general concept is to make use of accelerated ions so as to confer a desired
shape of the region of interest by erosion. To develop needle shaped
geometry, annular ion milling is performed with decreasing inner dia-
meters, as illustrated in Figure 10.3b. The level of control of FIB methods
allows a wide number of materials to be prepared as sharp tips. FIB methods
have made a significant contribution to widening the application of APM.
The first successful results of FIM specimen preparation by the FIB techni-
que was reported by A. R. Waugh et al. using a liquid metal ion source. 30
Later, Larson et al. applied the FIB technique to prepare FIM specimens
from multilayer thin films. 31,32 By fabricating pre-thinned blank specimens
using an annular ion beam, the authors succeeded in preparing FIM
tips from multi-layer thin films in the direction perpendicular to the film
layers. They also succeeded in analysing chemical compositional changes at
the interface of the multi-layers with atomic layer resolution. Today, the
development of so-called 'lift-out' procedures allows for the production of
hybrid samples. In this case, the sample is prepared by using a truncated
cone of a conducting metal as a base, with the material of interest being
fixed on the flat part of the post. Examples of this procedure are shown in
recent papers 33 and one is illustrated by Figure 10.3c. Well-chosen regions of
flat materials can now be routinely characterized by APT. Single catalytic
particles can also be fixed on the top of sharp posts for the same purpose
(Section 10.3.3.2).

.

10.2.2 Field Ion Microscopy
The Field Ion Microscope (FIM) is an evolution of the Field Electron Emis-
sion Microscope (FEM) developed earlier by the same inventor, Erwin W.
M¨ller. The FIM permitted single atoms to be imaged for the first time in
October 1955. 7 A historical account of this event has been reported by
A. Melmed. 34