Due to its stable, tear-resistant, and very thin atomic layers, graphene is considered to be the next-generation material. For instance, it is best suited for developing ultra-light electronics or mechanical components that are highly stable.
However, it is highly challenging to synthesize the wafer-thin carbon layers. Jürgen Kraus from the Technical University of Munich (TUM) has synthesized self-supporting graphene membranes, and has also systematically tested and optimized the graphene crystal growth. For his research, Jürgen Kraus was proffered with the Evonik Research Prize.
Graphene breaks all records. It is highly stable and the thinnest material ever known, and is also electrically conductive, ultralight, highly resilient, and tear-proof. Since it was discovered in 2004, the two-dimensional structures formed of carbon atoms have powered the creativity and innovational spirit. Authors of science fictions propose the material to be apt for developing cables for driving space elevators. Material scientists have been conducting experiments for developing transistors, displays, and electrodes made of graphene, which are considered to transform the futuristic electronics to be ultralight and highly stable, and with long service life. Scientists regard ultra-pure graphene to be highly valuable, as it enables liquids and gases to be confined in an ultra-dense way.
Currently, however, the basic requirements are still lacking. There are various manufacturing processes which are suitable for the mass production of graphene. However, this material is not free of defects. Graphene of the highest crystalline quality cannot be reproducibly manufactured in this manner.
Sebastian Günther, Professor for Physical Chemistry at the TUM.
At present, he and his colleagues have been successful in investigating, monitoring, and optimizing graphene crystal growth by means of chemical vapor deposition (or CVD). The outcomes of the study have been published in the Annalen der Physik, or Annals of Physics.
Theory and Caveats in Practice
Theoretically, graphene can be synthesized in a simple manner by using a heated glass vessel, a reactor filled with carbon-containing gas (e.g. methane), and copper as a catalyst. At a temperature of nearly 1000 °C, methane gets disintegrated on the copper surface and produces carbon and hydrogen. Eventually, the hydrogen leaves the copper surface, and the carbon atoms get collected on the surface of the copper film used for chemical precipitation from the gaseous state, a method known as chemical vapor deposition. In the process, the graphene atoms get cross-linked and form graphene “flakes,” that is, spot-like two-dimensional structures having a classic honeycombed structure. What remains is hydrogen, which is extracted by suction.
Yet, in practice, it is not so easy.
The biggest problem is that the two-dimensional crystal structure is often not entirely homogeneous, because growth begins simultaneously at multiple locations. At first glance, it appears that a continuous graphene film is appearing on the copper, but the hexagonal honeycombs are not all oriented in the same manner, and the structure is weakened at locations where they meet.
Flaws such as these can be eliminated by making sure that the copper surface is absolutely free from crystallization nuclei.
Jürgen Kraus used his experiments to show that the contaminants can be eliminated thoroughly by using oxygen gas, that is, by performing oxidation. Yet, to prevent unwanted after effects, care must be taken that the copper catalyst is exposed just to the minimum possible quantities of oxygen.
Crucial for Success: Gas Concentration and Temperature
In the latter part of his experiments, Kraus investigated the impact of a range of partial temperatures and pressures on the graphene formation at the time of chemical vapor deposition. When there is higher amount of hydrogen in the gas composition, there is no growth of graphene. By contrast, when the amount of hydrogen is very low, the layers become very thick. Ultra-pure graphene without any flaws can be formed in a crystal lattice only if all parameters are chosen in such a manner that the growth is “close enough” to the thermal equilibrium.
Quality Check in Italy
To authenticate the quality of the graphene flakes, the Munich-based scientists took their samples to Italy. They used a ring-shaped particle accelerator at the Research Centre Elettra Sincrotrone Trieste to structurally and chemically characterize the graphene layers by using a special microscope with a very high resolution due to the high-energy synchrotron radiation.
The results of the feasibility study were highly encouraging. The images have shown that reproducible results can be obtained by selecting the parameters during chemical vapor deposition.
Thus far, the best record for quality achieved by the TUM scientists is synthesizing graphene flakes with dimensions of 1 mm2 comprising 10 billion carbon atoms that are accurately aligned.
The advantage over other studies is not so much the ‘record size’ achieved, but lies in the fact that the flakes are formed with a predictable growth rate if the right CVD parameters are chosen, thereby allowing closed, highly crystalline graphene layers with a thickness of just one atom to be manufactured within just a few hours.
Mini Films for New Applications
Graphene opens the door for a broad array of innovative applications in fundamental research—for instance, the ultra-thin graphene films can be detached from the copper substrate and applied as covering films that are appropriate for trapping liquids inside a container. As slow electrons can easily pass through the films, the samples can be analyzed by means of electron spectroscopy and microscopy, although these methods are generally used in high or ultra-high vacuums.
In future, the scientists propose to use these films to analyze liquid-covered electrodes, living cells, as well as catalysts under high pressure through photoelectron spectroscopy. During the process, photons that can penetrate the film convey their energy to the electrons in the sample, making them to be set free and pass through the film to escape to outer side. Subsequently, the energy levels from the photons can be used to ascertain the chemical composition of the sample.
Jürgen Kraus was awarded the 2017 Research Prize from Evonik Industries AG for his research.
The DFG Priority Program “Graphene” (SPP 1459) funded the study.