Chemistry Reference
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Figure 10.5
Scheme of the atom-probe set-up with a local counter-electrode and a
delay line detector.
(Reprinted from ref. 39, copyright
(2014), with permission from
Elsevier).
wide field of view and high data collection rates (up to 5
10
6
ions per mi-
nute), enabling much larger sample volumes to be studied. Equally im-
portant is the ability to use laser pulsing to remove ions rather than voltage
pulses (Figure 10.5). The use of green or especially UV-wavelength lasers now
enables the routine analysis of fragile, poorly conductive materials such as
thick oxide layers.
The combination of the flight time and the impact position of the ions
on the detector provides the necessary data to reconstruct a three-
dimensional map of the probed material. The accuracy of the reconstruction
algorithm in APT is not straightforward and directly relies on the under-
standing and knowledge of two physical mechanisms. The first is the ero-
sion of the typically conical sample by field evaporation and the second is
the trajectory of ions from the specimen surface onto the position-sensitive
detector. As discussed in the recent review by Vurpillot et al.,
40
standard
reconstruction algorithms have been mostly based on empirical description
of the field evaporation process and from experimental observations by FIM.
Examples of atom map reconstructions of materials of interest in hetero-
geneous catalysis are shown in Figure 10.6. APT reconstructions from the
samples exposed to oxidation were generated and focused on the oxide-
metal interfaces. In comparison with the aluminium samples, the extent of
oxidation is markedly less on Pt-based alloys. This is expected on these noble
metal surfaces, and demonstrates the applicability of APT methods for
studying relative behaviour trends in a range of metal systems, including
those relevant for catalysis.
.
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