An Atom Probe is a scientific instrument with the full name “atom probe field ion microscope”. The atom probe allows individual atoms to be seen and also identified.
How Atom Probes Work
An atom probe is a point projection microscope that uses field-ion-microscopy (FIM) and time-of-flight mass spectrometry to resolve the chemical identity and position of individual atoms. The sample material is required to be a sharply pointed specimen, from the tip of which surface atoms are ionised by a high voltage at cryogenically cooled temperatures. This process is known as field evaporation. These ions are positively charged and are projected from the specimen. As in field ion microscopy, the ions then hit a screen where they form a highly magnified image of their original position in the sample.
Figure 1. Basic components of an atom probe
A small (0.5-5nm) hole in the screen allows ions from a specific region in the sample to pass through to a detector. For a given charge, lighter elements will hit the detector fast than heavier elements. By pulsing the charge it is possible to work out the time-of-flight for each ion and therefore identify the individual elements in the sample. The numbers of atoms of various elements hitting the detector are recorded and a mass spectrum is extrapolated from that data.
3D Atom Probes
A 3D atom probe operates in much the same way as a basic atom probe but the screen doubles as the detector and has no hole. In this case the screen is a position sensitive detector and records both the time data for when the ion hits the screen as well as the location of impact. This location corresponds to the original location of the ion in the sample. As ions are projected from the sample, ions in the layer below follow. This allows a fully three dimensional picture of the sample material composition to be built up.
What Atom Probes Are Used For
We understand that adding alloying elements to metals for example can produce a new material with enhanced properties such as increases in strength, toughness, creep resistance and oxidation resistance. Predicting these properties can be an inexact science due to the number of permutations possible when the alloying elements combine with the base material. An experimental understanding of the material microstructure is therefore desirable. The atom probe can be used to exactly determine the location and distribution of all elements within the alloy microstructure.
Further to this, problems with a material may be diagnosed. A new alloy may be inexplicably brittle and the atom probe can be used to pinpoint problem areas such as precipitation of brittle phases along a grain boundary.