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Discovery of the Principle of Graphene Formation by Solid Phase Separation

The first two-dimensional crystals discovered by mankind
Graphene, also known as the first two-dimensional crystal discovered by mankind, is a thin-film carbon isotope with carbon atoms intertwined in the shape of a hexagonal honeycomb. It was discovered in 2004 and was the subject of a study that won the 2010 Nobel Prize in Physics. Graphene is a thin film one atom thick, that is, 0.34 nm in thickness, which is about 3 billionths of a meter. Although it is extremely thin, it still has high physical and chemical stability. Graphene has been in the spotlight in academia due to its many excellent properties. It is 100 times more conductive than copper and can move electrons more than 100 times faster than silicon.
It is 200 times stronger than steel in intensity, has a high thermal conductivity that is more than twice than diamond, which is known as having the best thermal conductivity. It also has excellent elasticity. Even if it is physically increased or bent by 20%, its various electrical properties do not change at all. Also, since graphene is composed of one layer of atoms, the transmittance of light is as high as 97.7%, which makes it highly transparent. These characteristics make graphene the next big thing that will revolutionize the semiconductor and display industries.

Development of easier and faster synthesis technology

A team of domestic researchers developed a technique that can easily synthesize graphene using a laser. A joint team, led by Prof. Sung-Yool Choi from the Dept. of Electrical Engineering at KAIST and Prof. Keon-Jae Lee from the Dept. of Materials Science and Engineering at KAIST said they discovered the solid phase separation phenomenon of single crystal silicon carbide (SiC) by irradiating ultrashort pulsed lasers for the first time in the world. They used this process to identify the principle of graphene generation. Conventionally, the chemical vapor deposition (CVD) method was mainly used in graphene synthesis.

The CVD method requires a considerable amount of time and a high temperature process in which a gaseous raw material is injected onto a substrate to form a thin film for a raw material on a substrate through chemical reactions in a high temperature environment. Unfortunately, there were a couple of disadvantages with
this method: the quality of graphene can deteriorate and the substrate can be damaged during the process due to the process’ complexity. Meanwhile, the laser heat treatment method developed by the team is enabling the synthesis of graphene in a short time in room temperature, thus significantly broadening the application of graphene in the future.

In this new method, the researchers irradiated the surface of the single crystal silicon carbide material with laser pulses for a very short period of time, which was only tens of nanoseconds (a nanosecond is one billionth of a second), so that the surface was instantaneously melted and re-solidified. Then, the phase separation was observed in which the silicon carbide surface was separated into an ultra-thin carbon layer and an underlying silicon (Si) layer. They found that the underlying silicon layer evaporated and the carbon layer turned into graphene when the laser pulses were irradiated again. The researchers also confirmed this by molecular dynamics. In particular, the interaction of hetero-structured elements such as silicon carbide with an ultrashort laser is a complex non-equilibrium phenomenon that has not been easy to identify until recently. Yet, the researchers took a picture of the carbon and silicon ultrathin layers separated instantaneously by laser pulses with a high-resolution electron microscope and found that the light reflectance of semiconductor materials such as silicon in a liquid state was different from the light reflectance in its solid state. Based on such findings, the team succeeded in identifying the phase separation phenomena of silicon carbide.

The laser heat treatment technique used in the study is widely used in the production processes of commercial displays such as Active-Matrix Organic Light-Emitting Diode (AM OLED). This method can be applied to heat-sensitive plastic substrates because the method only instantaneously heats the surface of the material. In addition, the range of applications is expected to be broadened in the field of flexible electronics in the future because this new method enables the synthesis of graphene selectively at a desired position on a substrate. Thanks to its significance, the study was published in a recent issue of Nature Communications, an internationally-renowned academic journal in the fields of natural and applied sciences. Prof. Choi said, "In the future, we will be able to develop new nanomaterials by identifying the interactions of various solid compounds with lasers and utilizing their phase separation phenomena.” Prof. Lee also added, "The results of this study is meaningful in that they will contribute to the wider applications of laser technology in 2D nanomaterials such as graphene.”

Prof. Choi, Sung Yool
2016 Annual Report

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