Angstronauts: Diving Deep into Atomic Realm
By Shyam Katnagallu & Baptiste Gault (Max-Planck-Institut für Eisenforschung GmbH)
Stone age, bronze age, iron age: the tripartite divisions of historic time periods defined by materials. Materials have, are, and will continue to be the quintessential support for mankind’s development. From the mundane, redundant task of driving to work, to putting Starman on a Tesla Roadster, to cruising the solar system, modern day technology was only made possible by the discovery of new materials.
The smartphone, tablet, or laptop on which you are probably reading this article is more powerful than the computers used by National Aeronautics and Space Administration (NASA) to put a man on the moon in the 1960s. This is a direct result of the explosive advances made in materials science and technology.
The road to these achievements has been long and winding. Scientists had to rely heavily on trial and error until they started to exploit a panoply of characterization tools to understand and decipher the relationships between the material’s physical properties (e.g. optical, mechanical) and its constituents and their arrangements. An examination of a piece steel from a kitchen utensil, say a knife, under a light microscope reveals a rich and complex architecture of small crystals called grains, which form what is referred to as a microstructure. This microstructure combines with the local composition (i.e. which atom is sitting where within this structure) to dictate the steel’s properties and thus determine if it is suitable as a kitchen utensil or be rather in a nuclear power plant where the constraints associated with its operation will be vastly different over its lifetime. Microstructures can evolve or tailored by the application of high temperatures or through deformations of the structure, thereby modifying these small crystals and the distribution of their constituents.
At this stage, we are well within the realm of atomistics, and the study of these microstructural features requires specific tools. Electron microscopes have been instrumental in this aspect as they can deliver part of this crucial information despite often being limited to analysing surfaces. Even with the most powerful microscopes, whose spatial resolution is sufficient to image columns of atoms through a very thin specimen (only 10s of nanometres, i.e. billionths of a meter), the information is averaged throughout the specimen such that the knowledge of the position and elemental nature of each atom within a material remains elusive.
Seeing atoms has been a daunting endeavour ever since they were hypothesized to exist in 480 BCE by ancient Greek philosopher Democritus or in 600 BCE by Indian sage Acharya Kanada. The first atomic theory was developed by John Dalton in 1810, but the first direct images of individual atoms only came thanks to Erwin Wilhelm Müller’s 1951 invention called the field ion microscope (FIM): On October 11th, 1955, Müller, together with his PhD student Kanwar Bahadur, saw the individual atoms of tungsten.
FIM is a relatively simple microscope and can be easily built for a high school science project. The atomic resolution is achieved by ionizing gas atoms right above the surface of a very sharp needle, whose end radius is less than 100 nanometres. The sharp tip can conveniently generate colossal electric fields at their apex when subjecting the specimen to 1-10 kV of electrostatic potential. Such electric fields can rip apart the material constituting the specimen in a process called field evaporation that turns the surface atoms into ions. By collecting these ions with a particle detector, the impact position can be recorded along with the time it took for the ions to fly from the specimen towards the detector. The times-of-flight allow for discerning the elemental nature of each evaporated ion (i.e. a heavier ion takes a longer time to reach the detector than a lighter atom). A technique that combines time-of-flight mass spectrometry with a high-resolution projection microscope is called atom probe tomography (APT). Knowledge of the ion projection allows the conversion of the impact positions and the arrival sequence of the ions into a three-dimensional map that reveals the location of each element with near-atomic resolution.
FIM and APT enable materials scientists to do a range of analysis, to extract interesting features, their distributions, compositions and their structure, all pertinent in the quest to decipher the structure-property relationship of materials. The Department of Microstructure Physics and Alloy Design at the Max Planck Institut für Eisenforschung houses three atom probes called LEAPTM (local electrode atom probes). These instruments have been instrumental in the research efforts to understand fundamental material physics, e.g. how phases form or how atoms of a specific element segregate to different microstructural features, and in what quantity etc. This research enables us to tailor new, advanced materials with enhanced performance. Scientists of this group are the curious “Angstronauts” and APT is their spaceship. The new crusade will target the tiniest of atoms, hydrogen, within complex materials that make up the computer on which you read this, the chair on which you sit, the rails of the tram that you took to work, and the plane that took you to your last conference.