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The art of making tiny holes

Posted By Graphene Council, Tuesday, August 4, 2020
Nobody can shoot a pistol bullet through a banana in such a way that the skin is perforated but the banana remains intact. However, on the level of individual atomic layers, such a feat has now been achieved - a nano-structuring method has been developed at TU Wien (Vienna), with which certain layers of material can be perforated extremely precisely and others left 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 2D material systems, for example to anchor certain metals on them, which can then serve as catalysts. The new method has now been published in the journal ACS Nano.

New materials from ultra-thin layers

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 was first produced, which consists of only a single layer of carbon atoms, 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 Dr. Janine Schwestka from the Institute of Applied Physics at TU WIen and first author of the current publication. "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 nanometres - for example, in order 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 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.

Two kinds of energy.

When an ion beam is used to treat a surface, it is usually the force of the impact of the ions that affects the material. At TU Wien, however, relatively slow ions are used, which are multiply charged. "Two different forms of energy must be distinguished here," explains Prof. Richard Wilhelm. "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 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: most of the potential energy has already been released.

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

"This provides us now with a wonderful new method for manipulating surfaces in a targeted manner," says Richard 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, one could create "masks" 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.

"We are very pleased that our excellent collaborations via the TU Doctoral College TU-D were able to contribute significantly to these results," says Janine Schwestka, who was a member of the TU-D for more than three years. "In addition, it distinguishes Vienna as a location for science and research that we were able to establish contacts with the University of Vienna through short distances in order to deepen our joint expertise and complement each other methodically".

Tags:  2D materials  Graphene  Janine Schwestka  Richard Wilhelm  TU WIen 

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Graphene on TEM grids for quality and integration studies

Posted By Graphene Council, Wednesday, July 29, 2020
Graphene suspended over grids for transmission electron microscopy (TEM) can have numerous interesting applications in studies of material properties for technological applications. Two recent publications from TU Wien demonstrate the use of graphene on TEM grids for studies of material quality and integration with indium oxide, another technologically relevant material.

The quality of growth of other materials on graphene is of fundamental importance for device applications. The crystallinity and orientation of indium oxide grown on graphene affects the quality of displays and sensors produced from such a heterostructure. In a study published in Advanced Functional Materials, researchers have shown that arrangement of indium oxide crystals on graphene depends on the pressure on which the crystals form. That can have a major impact on the application properties of the combined materials.

Crucial to the success of this study was the availability of free-standing graphene, on which indium oxide is grown. Having graphene which is suspended in vacuum provides a clean picture of the crystal structure, without any background from a substrate. This is achieved by using commercially available graphene suspended over a metallized mesh – a TEM grid. Transmission electron microscopy across such graphene has the best possible resolution, down to the atomic level.

In a second study, the team of scientists showed that graphene on TEM grids can be used to gauge the quality of the graphene itself. Although grown at a high quality on metal substrates with chemical vapour deposition, transfer of graphene to any useful substrate or a TEM grid requires first coating it with a transfer polymer. Numerous polymer removal processes are used at the industrial scale today, nevertheless it has been shown that some residue persists regardless of the cleaning process. The amount of residue directly impacts graphene film performance.

The study, published in the Journal of Chemical Physics, reveals that ion beam spectroscopy can be used as a tool to map with high resolution the local cleanliness of graphene. The results indicate that although some residue always remains, it clusters in small areas, leaving large clean areas that can be used for devices.

These novel studies highlight the usefulness of graphene on TEM grids as a tool in material science and technology research.

Tags:  chemical vapour deposition  Graphene  TU Wien 

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Graphene: It is all about the toppings

Posted By Graphene Council, Thursday, July 9, 2020
Graphene consists of a single layer of carbon atoms. Exceptional electronic, thermal, mechanical and optical properties have made graphene one of the most studied materials at the moment. For many applications in electronics and energy technology, however, graphene must be combined with other materials: Since graphene is so thin, its properties drastically change when other materials are brought into direct contact with it.

However, combining graphene with other materials at the molecular level is difficult: The way graphene interacts with other materials depends not only on which material you choose, but also on how these materials are brought into contact with the graphene. Rather than sticking a finished material layer to the graphene, the appropriate atoms are brought into contact with the graphene in such a way that they "grow" on the graphene in the desired crystal structure.

Until now the mechanisms of the "growth" of such other materials on graphene have often remained unclear. A new joint study by research teams from the TU Wien and the University of Vienna for the first time observes now how indium oxide grows on graphene. The combination of indium oxide with graphene is important, for example for displays and sensors. The results have now been presented in the scientific journal "Advanced Functional Materials".

Graphene pizza

"As with a pizza, graphene technology is not only dependent on the graphene pizza base but also on its toppings," explains Bernhard C. Bayer from the Institute of Materials Chemistry at the TU Wien, who led the study. "How these toppings are applied to the graphene is, however, crucial."

In most cases, atoms in the gaseous state are condensed on the graphene. In the case of indium oxide, these are indium and oxygen. "But there are many parameters such as background pressure, temperature or the speed at which these atoms are directed at the graphene that influence the result drastically," says Bernhard Bayer. "It is therefore important to develop a fundamental understanding of the chemical and physical processes that actually take place. But to do this, you have to watch the growth process as it proceeds. "

This is exactly what the research team has now succeeded in doing: for the first time, the individual steps of growing indium oxide on graphene were observed in the electron microscope at atomic resolution.

Randomly distributed or perfectly aligned

"What was particularly interesting for us was the observation that, depending on the background pressure, the indium oxide crystallites either arrange themselves randomly on the graphene's crystal lattice or snap perfectly on one another like Lego bricks. This difference in arrangement can have a major impact on the application properties of the combined materials," says Kenan Elibol, first author of the study. The new findings will be useful to make the integration of graphene with other materials more predictable and controllable with respect to future application requirements.

Tags:  Advanced Functional Materials  Bernhard C. Bayer  Graphene  indium oxide  Institute of Materials Chemistry  TU Wien  University of Vienna 

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