When charged particles are blasted through ultra-thin layers of material, sometimes spectacular micro-explosions occur, and sometimes the material remains nearly intact. The reasons for this have now been explained by researchers at TU Wien.
It looks a bit like a magic trick: some materials can be traversed by fast electrically charged ions without showing holes afterwards. What would be impossible at the macroscopic level is permitted at the level of individual particles. However, not all materials behave the same in such situations – in recent years different research groups have conducted experiments with very different results.
At TU Wien (Vienna, Austria) it has now been possible to find a detailed explanation of why some materials are perforated and others are not. This is interesting, for example, for the treatment of thin membranes, which are supposed to have tailor-made nano-pores in order to trap, retain or let through very specific atoms or molecules.
Ultra-thin materials: graphene and its peers
“Today, there is a whole range of ultrathin materials that consist of just one or a few atomic layers,” says Professor Christoph Lemell from the Institute for Theoretical Physics at TU Wien. “Probably the best known of these is graphene, a material made up of a single layer of carbon atoms. But research is also underway on other ultrathin materials in the world today, such as disulphide of molybdenum.”
In the research group of Professor Friedrich Aumayr at the Institute for Applied Physics of TU Wien, these materials are bombarded with very special projectiles, highly charged ions. They take atoms, usually noble gases such as xenon, and strip them of large numbers of electrons. This creates ions with 30 to 40 times the electrical charge. These ions are accelerated and then strike the thin layer of material with high energy.
“This results in completely different effects depending on the material,” says Anna Niggas, an experimental physicist at the Institute of Applied Physics. “Sometimes the projectile penetrates the layer of material without any noticeable change in the material. Sometimes the layer of material around the impact site is also completely destroyed, many atoms are dislodged, and a hole a few nanometers in diameter appears. form.”
The speed of electrons
These differences can be explained by the fact that it is not the momentum of the projectile that is mainly responsible for the holes, but its electrical charge. When a multiple positively charged ion hits the layer of material, it attracts more electrons and carries them with it. This leaves a positively charged region in the layer of material.
The effect this has depends on how fast electrons can move through this material. “Graphene has an extremely high electron mobility. Thus, this local positive charge can be balanced there in a short time. The electrons simply arrive from elsewhere”, explains Christoph Lemell.
In other materials such as molybdenum disulphide, however, things are different: there the electrons are slower, they cannot be delivered in time from outside to the impact site. This is how a mini-explosion occurs at the point of impact: the positively charged atoms, from which the projectile has taken their electrons, repel each other, fly away, which creates a pore of nanometric size.
“We have now been able to develop a model that allows us to estimate very well in which situations the holes are forming and in which they are not, and it depends on the mobility of the electrons in the material and the state of charge of 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 expelled from the material move relatively slowly: the high speed of the projectile does not matter to them; they are removed from the material by electrical repulsion only after the projectile has already passed through the layer of material. And in this process, not all the energy of electrical repulsion is transferred to the sputtered atoms – much of the energy is absorbed in the remaining material in the form of vibrations or heat.
Experiments and simulations were performed at TU Wien. The resulting deeper understanding of atomic surface processes can be used, for example, to specifically equip membranes with tailored “nanopores”. For example, one could construct a “molecular sieve” or retain certain atoms in a controlled manner. There is even thought to use such materials to filter CO2 from the air. “Thanks to our discoveries, we now have precise control over the manipulation of materials at the nanoscale. This provides a completely new tool for manipulating ultra-thin films in a precisely calculable way for the first time,” says Alexander Sagar Grossek.
Alexander Sagar Grossek et al, Model of Nanopore Formation in Two-Dimensional Materials by Highly Charged Ion Impact, Nano-letters (2022). DOI: 10.1021/acs.nanolett.2c03894
Provided by Vienna University of Technology
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