Chemistry Reference
In-Depth Information
To unravel the local chemical composition of the imaged surfaces with
nanometric or even atomic spatial resolution, accurate determination of the
location of ionization is needed. An attractive feature of FIM is that it can be
quite easily combined with a time-of-flight mass spectrometer to construct a
local chemical probe able to measure the surface composition on some tens
of square nanometers while imaging the reaction on the entire surface of the
tip apex. The first generations of such atom probe devices were based on
the concept of drilling a small aperture, a 'probe hole',
20-22
in the middle of
the imaging screen so that ions passing through the hole could be mass
separated. Figure 10.2 illustrates this principle. Because of the equivalence
of ion trajectories in electrostatic fields, surface atoms which are ionized and
removed from the small part of the imaged region aligned over the probe
hole will themselves pass through the hole and enter the mass spec-
trometer.
23
The dark region on the micrograph is therefore the area which is
subject to chemical analysis. In this example (see Section 10.3.1.2), the
surface composition of a rhodium tip sample is investigated during the
ongoing water production in the presence of a reactive H
2
/O
2
gas mixture.
16
A significant breakthrough in the development of atom probes came with
the introduction of position sensitive ion detectors
24
(Section 10.2.3.1). The
latest generations of atom probes are such that several millions of ion im-
pacts per minute can be processed individually.
19
In this case, a FIM screen
is no longer strictly necessary. These atom probes are mainly used for ma-
terials studies because they routinely provide three-dimensional maps of
individual atoms constituting a sample, with close-to-atomic spatial
d
n
9
r
4
n
g
|
8
.
Figure 10.2
(a) Schematic principles for the first design of the atom probe. Gas Ions
hitting the FIM screen provide a micrograph of the tip apex. Field
desorbed or field evaporated ions passing through the probe aperture
are mass separated and identified by their flight time. (b) Scheme of the
probe-hole principle: the hole in the screen of the device shown in (a)
selects a small area of the tip sample, possibly during a catalytic
reaction in progress. (c) Field ion micrograph during the ongoing
H
2
þ
O
2
reaction. The micrograph was taken under experimental
conditions suitable to investigate the bistability of the reaction
(pO
2
¼
1.5
10
3
Pa, pH
2
¼
2.5
10
3
Pa, T
¼
500 K). The field ion micro-
graph suggests the simultaneous presence of nanoscale regions of
strongly different local composition on the surface.
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