It’s like a magic trick: some objects can pass through an electric ion very quickly without showing holes afterwards. Impossibilities are left at the macroscopic level at the level of individual objects. However, not everything behaves like that in such situations – in recent years different research groups have done experiments with very different results, How to shoot projectiles through objects without damaging anything.
When the charged particles are blasted through the soft layers, sometimes a small explosion occurs, sometimes the object is perfectly fine. This has now been reported at TU Wien.
At TU Wien (Vienna, Austria) it is now possible to find a detailed explanation of why some things are broken (How to shoot projectiles through objects without damaging anything) and others are not. This is interesting, for example, for the treatment of sensitive skin, which should have nano-pores that are sewn to trap, retain or release from specific atoms or molecules.
Very important materials – graphene and its peers
“Today, there are all kinds of ultrathin materials with one or a few atomic layers,” says Prof. Christoph Lemell from the Institute for Theoretical Physics of TU Wien. “The best-known of these materials is graphene, a material composed of a single layer of carbon dioxide. But research is being done on other ultrafine materials in the world today, such as molybdenum disulfide.”
In the research group of Professor Friedrich Aumayr at the Institute for Applied Physics of TU Wien, these objects are filled with special projectiles – highly charged ions. They take atoms, usually noble gases like xenon, and strip them of a large number of electrons. This produces ions with 30 to 40 times the electricity. These ions are accelerated and hit a thin layer of material with high energy.
“This causes completely different effects depending on the material,” says Anna Niggas, a physicist at the Institute of Applied Physics. Sometimes, the layer of material around the impact area is completely destroyed, many atoms are displaced and holes of a few nanometers in diameter are formed.”
The speed of the electron
These amazing differences are explained by the fact that it is not the fire of the projectile that is the main source of space, but its electric charge. When an ion with more charge hits a layer of material, it attracts more electrons and carries it. This leaves a well-charged area in the material layer.
The effect this has depends on how fast electrons can pass through the material. “Graphene has a very high electron mobility. Therefore, the positive charge in this area can be adjusted in a short time. The electrons come from somewhere else,” explains Christoph Lemell.
In other things like molybdenum disulphide, however, things are different: there the electrons move slowly, they cannot be delivered in time from the outside and impact site. Therefore, a mini-explosion occurs at the impact site: the appropriate charged atoms, the projectile took their electrons, meet each other, they fly away – and this creates a nanometer-sized pore.
“Now, we have been able to create a model that allows us to accurately describe the conditions in which holes are formed and in which they are not – and this depends on the mobility of electrons in the material and the position of the charge. The projectile”, explains Alexander Sagar Grossek, first author of the publication in the journal Nano Letters.
The model also explains the surprising fact that the atoms ejected from the object move slowly: the speed of the projectile does not matter to them; They are removed from the material by electricity only when the projectile passes through a layer of material. In this process, not all the energy of the electric current is transferred to the atoms that flow – most of the energy is involved in the rest in the form of vibrations or heat.
Experiments and simulations were carried out at TU Wien. Deep understanding of atomic structure can be used, for example, to prepare membranes with artificial “nanopores“. For example, one can construct a “molecular dog” or hold certain atoms in a controlled manner. It is thought that such materials will be used to filter CO2 from the atmosphere. “Thanks to our research, we now have the ability to use materials at the nanoscale. This provides a completely new tool for manipulating thin films in a way that can be precisely calculated for the first time, ” says Alexander Sagar Grossek.
