No one can shoot a banana with an ordinary pistol so that its skin is perforated and the flesh remains intact. However, at the level of individual atomic layers, such a feat has now been achieved – the nanostructuring method was developed by scientists from the Technical University of Vienna (TU Wien). With it, certain layers of material can be perforated extremely accurately, while others remain completely intact, even if the projectile penetrates all layers. This is made possible with the help of highly charged ions. They can be used to selectively surface new systems of 2D materials, for example, to attach certain metals to them, which can then serve as catalysts. The new method was published in the ACS Nano magazine.
Materials that are composed of multiple ultra-thin layers are an exciting new area of materials research. Since the first high-performance graphene material, which is made up of only one layer of carbon atoms, was first manufactured, many new thin-film materials have been developed, often with promising new properties.
Scientists have investigated a combination of graphene and molybdenum disulfide. Two layers of material come into contact and then adhere to each other under the influence of weak van der Waals forces. Graphene is a very good conductor, molybdenum disulfide is a semiconductor, and this combination could be interesting for the production of new types of storage devices, scientists say.
However, for certain applications, the geometry of the material must be specially processed at the nanometer scale – for example, to change the chemical properties by adding additional types of atoms or to control the optical properties of the surface. Surfaces can be modified using an electron beam or conventional ion beam. However, in a two-layer system, there is always a problem – the ion beam acts on both layers simultaneously, even if in the problem it is worth changing only one layer.
When an ion beam is used for surface treatment, usually the force of the ion impact affects the material. However, the scientists’ method uses relatively slow ions that are multiply charged.
“There are two different forms of energy to be distinguished here. On the one hand, there is kinetic energy, which depends on the rate 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. In ion beams, kinetic energy plays a decisive role, but for scientists, it is the potential energy that is especially important.
There is an important difference between these two forms of energy: while kinetic energy is released in both layers of a material as it penetrates the layer system, potential energy can be distributed very unevenly between the layers.
Molybdenum disulfide reacts very strongly to highly charged ions. A single ion entering this layer can remove tens or hundreds of atoms from the layer. Only a hole remains, which is very clearly visible under an electron microscope. On the other hand, the graphene layer, into which the projectile hits immediately after impact, remains intact: most of the potential energy has already been released.
The same experiment can also be reversed. The strongly charged ion first hits the graphene and then onto the molybdenum disulfide layer. In this case, both layers remain intact: graphene provides the ion with the electrons needed to electrically neutralize it in a fraction of a second. The electron mobility in graphene is so high that the impact point also immediately “cools down”. The ion traverses the graphene layer without leaving a permanent trace. After that, it can no longer cause significant damage to the molybdenum disulfide layer.
This gives scientists a new method for targeted surface control. Now it is possible to add nanopores on the surface without damaging the substrate material underneath. This will help you create geometric structures that were previously impossible. You can create “masks” of molybdenum disulfide, perforated exactly as desired, on which certain metal atoms are then deposited. This opens up completely new possibilities for monitoring the chemical, electronic and optical properties of surfaces.