At the end of May, an atom probe tomography (APT) laboratory started operating at the Institute of Thermomechanics of the Czech Academy of Sciences. It is an instrument that can examine a small piece of material atom by atom. The laboratory was funded by the OP JAK FerrMion project, in which the Institute of Physics of the Czech Academy of Sciences also plays a significant role.
How the atom probe works
Atom probe tomography is a powerful microscopy method. From a small piece of material, it creates a three-dimensional (3D) image of how the atoms are arranged, showing where each atom lies and which element it is.
First, a small sample is cut out of the studied material using a method called focused ion beam (FIB). The sample has the shape of a thin needle with a cone-like tip, and the point of this tip has a radius of less than 100 nm. The tip of the needle is then placed in a strong electric field (usually 1 to 10 kV), just below the level at which atoms start to come loose from the surface.
A short laser or voltage pulse pushes past this level and releases one or more atoms from the tip — or, more precisely, charged particles called ions. These ions reach a position-sensitive detector, which catches almost every one of them.
For each ion, the detector measures two values:
the time between the pulse and the impact, which is used to calculate the ratio of mass to charge and so identify which element the ion is;
the place where the ion hits the detector, which is used to work out the atom's original position in the studied sample.
The whole time, the sample is cooled to very low (cryogenic) temperatures, between −250 and −190 °C, to limit the movement of the atoms so their positions can be measured more accurately. With every pulse, the tip loses a few atoms, so the needle slowly becomes shorter. From the measured data, scientists then build a 3D model of the material (the needle), atom by atom.
One of the few instruments in the world
There are not many atom probes used for scientific purposes in the world. In Central and Eastern Europe, this is the first installation of its kind. Others are still being prepared.
"In a little over thirty years since it was invented, the atom probe has become an essential method for chemical mapping of materials at the nanometre scale," says scientist Petr Šittner from the Institute of Physics. "It has helped to design advanced nanostructured materials that solve challenges in modern metallurgy, energy storage and the production of semiconductor chips."
The OP JAK project Ferroic Multifunctionalities (FerrMion) brings together researchers from three academic institutes and three universities in Prague. Most of them are materials physicists and metallurgists who work with metallic and dielectric materials. They hope this unique method will help them understand the behaviour of the multiferroic materials they study.
Ferroic materials have unusual abilities. For example, they can change shape or react to a magnetic or electric field. This is caused by a rearrangement of their inner structure, called a martensitic transformation, which happens even while the material stays solid. How the material behaves often depends on tiny differences in chemical composition, at the nanometre scale. Only the atom probe can reliably measure differences this small.
Photo: Josef Landergott, External Relations Division, SSČ AV ČR
