The highly charged ion penetrates all layers, but only creates a big hole in the top layer. The graphene layer below remains intact. Image: TU Wien.
The highly charged ion penetrates all layers, but only creates a big hole in the top layer. The graphene layer below remains intact. Image: TU Wien.

Nobody can shoot a bullet through a banana in such a way that the skin is perforated but the banana remains intact. But on the level of individual atomic layers, such a feat has now been achieved.

Researchers at the Vienna University of Technology (TU Wien) in Austria have developed a nano-structuring method that allows certain layers of a material to be perforated extremely precisely while others are left completely untouched, even though the projectile penetrates all layers. This is made possible through the use of highly charged ions to selectively process the surfaces of novel 2D material systems. The researchers report their new method in a paper in ACS Nano.

Materials that are composed of several ultra-thin layers are regarded as an exciting new field of materials research. Ever since the high-performance material graphene, which consists of a single layer of carbon atoms, was first produced, many new thin-film materials have been developed, often with promising new properties.

"We investigated a combination of graphene and molybdenum disulfide. The two layers of material are brought into contact and then adhere to each other by weak van der Waals forces," says Janine Schwestka from the Institute of Applied Physics at TU Wien and first author of the paper. "Graphene is a very good conductor, molybdenum disulphide is a semiconductor, and the combination could be interesting for the production of new types of data storage devices."

For certain applications, however, the geometry of the material needs to be specifically processed on a scale of nanometers, in order to change the chemical or optical properties of the surface. "There are different methods for this," explains Schwestka. "You may modify the surfaces with an electron beam or with a conventional ion beam. With a two-layer system, however, there is always the problem that the beam affects both layers at the same time, even if only one of them is supposed to be modified."

When an ion beam is used to treat a surface, it is usually the force of the ions’ impact that affects the material. But Schwestka and her colleagues used relatively slow ions, which are multiply charged.

"Two different forms of energy must be distinguished here," explains Richard Wilhelm, also from the Institute of Applied Physics at TU Wien. "On the one hand, there is the kinetic energy, which depends on the speed at which the ions impact on the surface. On the other hand, there is the potential energy, which is determined by the electric charge of the ions. With conventional ion beams, the kinetic energy plays the decisive role, but for us the potential energy is particularly important."

There is an important difference between these two forms of energy. While the kinetic energy is released in both material layers 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 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 ion hits immediately afterwards, remains intact: most of the potential energy has already been released.

The same experiment can also be reversed, so that the highly charged ion hits the graphene layer first and then the molybdenum disulphide layer. In this case, both layers remain intact: the graphene provides the ion with the electrons required 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. As a consequence, the ion crosses the graphene layer without leaving a permanent trace, and can no longer cause much damage to the molybdenum disulphide layer.

"This provides us now with a wonderful new method for manipulating surfaces in a targeted manner," says Wilhelm. "We can add nano-pores to surfaces without damaging the substrate material underneath. This allows us to create geometric structures that were previously impossible."

In this way, ‘masks’ can be created from molybdenum disulfide perforated exactly as desired, on which certain metal atoms are then deposited. This opens up completely new possibilities for controlling the chemical, electronic and optical properties of the surface.

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.