The model developed at TU Wien explains why tiny holes – only a few nanometers in size – are formed in some two-dimensional materials when they are bombarded with highly charged ions, but not in others. Image: TU Wien.
The model developed at TU Wien explains why tiny holes – only a few nanometers in size – are formed in some two-dimensional materials when they are bombarded with highly charged ions, but not in others. Image: TU Wien.

It sounds a bit like a magic trick, but some materials can be shot through with fast, electrically charged ions without exhibiting holes afterwards. What would be impossible at the macroscopic level is allowed at the level of individual particles. However, not all materials behave in the same way in such situations – in recent years, different research groups have conducted experiments that produced very different results.

Now, researchers at the Vienna University of Technology (TU Wien) in Austria have come up with a detailed explanation for why some materials are perforated by ion projectiles and others are not. This explanation could prove of use for applications such as the processing of thin membranes, which are supposed to have tailor-made nanopores for trapping, holding or letting through very specific atoms or molecules.

"Today, there is a whole range of ultrathin materials that consist of only one or a few atomic layers," says Christoph Lemell of the Institute of Theoretical Physics at TU Wien. "Probably the best known of these is graphene, a material made of a single layer of carbon atoms. But research is also being done on other ultrathin materials around the world today, such as molybdenum disulfide."

In Friedrich Aumayr's research group at the Institute of Applied Physics at TU Wien, such materials are bombarded with highly charged ions. The researchers take atoms, typically noble gases such as xenon, and strip them of a large number of electrons, creating ions with 30 to 40 times the electrical charge. They then accelerate these ions towards a thin layer of material, hitting it 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 material layer without any noticeable change in the material as a result. Sometimes the material layer around the impact site is also completely destroyed, numerous atoms are dislodged and a hole with a diameter of a few nanometers is formed."

These astonishing 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 electric charge. When an ion with multiple positive charges hits the material layer, it attracts a large number of electrons as it passes through, leaving behind a positively charged region in the material layer.

What effect this has depends on how fast electrons can move in this material. "Graphene has an extremely high electron mobility,” Lemell explains. “So this local positive charge can be balanced there in a short time. Electrons simply flow in from elsewhere."

In materials such as molybdenum disulfide, however, things are different: There, the electrons are slower and cannot be supplied to the impact site in time. This results in a mini-explosion at the impact site: the positively charged atoms, created by the projectile taking electrons with it, repel each other and fly away, leaving behind a nano-sized pore.

"We have now been able to develop a model that allows us to estimate very well in which situations holes are formed and in which they are not – and this depends on the electron mobility in the material and the charge state of the projectile," says Alexander Sagar Grossek of the Institute of Theoretical Physics, who is first author of a paper on this work in Nano Letters.

The model also explains why atoms knocked out of 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 material layer. And in this process, not all the energy of the electric repulsion is transferred to the sputtered atoms – a large part of the energy is absorbed in the remaining material in the form of vibrations or heat.

Both the experiments and the simulations were performed at TU Wien. The resulting deeper understanding of atomic surface processes could be used, for example, to specifically equip membranes with tailored ‘nanopores’. Such membranes could be used as a ‘molecular sieve’ or to hold certain atoms in a controlled manner. There are even thoughts of using them to filter carbon dioxide from the air.

"Through our findings, we now have precise control over the manipulation of materials at the nanoscale,” says Grossek. “This provides a whole new tool for manipulating ultrathin films in a precisely calculable way for the first time."

This story is adapted from material from TU Wien, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.