Scientists at the Vienna University of Technology (TU Wien) in Austria have succeeded in a new feat: They have developed a method that allows them to perforate certain layers of material precisely while leaving others completely untouched. Is it just art? No, this could be interesting for the production of data storage devices.
It’s actually pretty obvious: You can’t shoot a bullet through a banana, perforate the skin and leave the banana undamaged. But at the level of ultra-thin atomic layers this is now possible, as recently proven by scientists at the Vienna University of Technology (TU Wien). They developed a nano-structuring method with which they can precisely perforate one layer of material while leaving the other layer completely untouched, even though the “projectile” penetrates all layers.
This is made possible with the help of highly charged ions. They can be used to selectively process the surfaces of novel, ultra-thin 2D material systems. In this way, certain metals, which then serve as catalysts, can be anchored on them. “We investigated a combination of graphene and molybdenum disulphide,” says Dr. Janine Schwestka from the Institute of Applied Physics at the Vienna University of Technology and lead author of the current publication, describing the new feat. “The two layers of material are brought into contact and then adhere to each other by weak van der Waals forces.
To understand this, you have to know that materials that are composed of several ultra-thin layers are regarded as a great area of hope in materials research. Ever since the high-performance material graphene – which consists of only a single layer of carbon atoms – was first produced, new thin-film materials have been developed again and again. And these often have promising new properties.
What’s more, for certain applications, the geometry of the material on a scale of nanometers needs to be specifically processed, for example, to change the chemical properties by adding additional types of atoms or to control the optical properties of the surface. “There are different methods for this,” explains Janine Schwestka. “You can change the surfaces with an electron beam or with a conventional ion beam. With a two-layer system, however, you always have the problem that the beam changes both layers at the same time, even if you actually only want to process one of them.
Two kinds of energy
When a surface is treated with an ion beam, the force of the impact of the ions normally changes the material. At the Vienna University of Technology, however, relatively slow ions were used, but were electrically charged several times. “You have to differentiate between two different forms of energy here,” explains Prof. Richard Wilhelm, who was awarded the FWF’s START Prize in 2019 for building the world’s first ultrafast ion source. “On the one hand, there is kinetic energy, which depends on the speed at which the ions strike the surface. On the other hand, there is potential energy, which is determined by the electric charge of the ions. With conventional methods, the kinetic energy was decisive, but for us the potential energy is particularly important.
The essential difference is that while the kinetic energy is released in both layers of material when penetrating the layer system, the potential energy can be distributed very unevenly among the layers. “The molybdenum disulfide reacts very strongly to the highly charged ions,” says Richard Wilhelm. “A single ion arriving at this layer can remove dozens or hundreds of atoms from the layer. What remains is a hole, which can be seen very clearly under an electron microscope.” The graphene layer, on the other hand, which the projectile hits immediately afterwards, remains intact, since most of the potential energy has already been released.
New data storage devices conceivable
Schwestka also describes possible areas of application. “Graphene is a very good conductor, molybdenum disulfide is a semiconductor, and the combination could be interesting for the manufacture of new types of data storage devices, for example.”
Incidentally, the same experiment can also be reversed, so that the highly charged ion first hits the graphene and then the molybdenum disulfide layer. In this case, both layers remain intact: The graphene provides the ion with the electrons it needs to neutralize it electrically in a tiny fraction of a second. The mobility of the electrons in the graphene is so high that the point of impact also “cools down” immediately. The ion traverses the graphene layer without leaving a permanent trace. Afterwards, it can no longer cause much damage in the molybdenum disulphide layer.
“This provides us with a wonderful new method of manipulating surfaces in a targeted manner,” says Richard Wilhelm. “We can add nano-pores to the surface without damaging the substrate material underneath. This way we can create geometric structures that were previously impossible.” In this way, one could create “masks” from molybdenum disulfide perforated exactly as desired, on which certain metal atoms are then deposited exactly in the holes. This opens up completely new possibilities for controlling the chemical, electronic and optical properties of the surface.
The new method was recently published in the technical journal “ACS Nano.”