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  • Subject area(s): Science
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  • Published on: 15th October 2019
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Discovery

While graphene had been discussed and considered for many decades before its isolation, it was assumed to be impossible to separate, and believed to be simply an ‘academic’ material [3]. Thin films were believed to be highly unstable, even at 10 or 12 atomic layers thick, and with the melting temperature of such films decreasing rapidly as the material was made thinner. [3] However, this was unexpectedly disproved in 2004 by researchers Andre Geim and Konstantin Novoselov [4], when they did isolate single layer graphene through mechanical exfoliation of pyrolytic graphite (repeated application and peeling off of sticky tape to graphite to remove single layers of the graphite sample). For this experiment, Geim and Novoselov won the 2010 Nobel Prize in Physics. [5]

Structure

Graphene is a single 2-dimensional layer of carbon atoms, hexagonally arranged and joined by sp2 atomic bonds. [2] The atoms are all 0.142nm apart [6]. The material is made up of pure carbon, with one electron per carbon atom ‘unbonded’ and free to roam about the surface of the graphene plane. The carbon lattice experiences small ‘ripples’, which provide a possible explanation for graphene’s ability to retain stability despite its single atom thick structure [7].

Properties

Graphene is most extraordinary for its mix of incredible physical properties. Graphene is a zero-gap semiconductor, and due to its unbonded electrons it has very high conductivity and low (10-8m-1) electrical resistivity at room temperature, values better than normal metals and semiconductors [2]. It is also very strong, with a Modulus of Elasticity of 1TPa and tensile strength of 130GPa, significantly stronger than steel [2]. Graphene is also transparent, and has a thermal conductivity of 5000W/mK.

Graphene production

The most significant impediment in graphene use is in its production. It remains difficult to synthesise or extract large quantities of graphene using current techniques, though this is a particular area of research in graphene science.

Some techniques for synthesising graphene are discussed below:

Mechanical exfoliation

This method, as initially used in the discovery of graphene, still provides the highest quality graphene with the fewest defects. [1] It is inexpensive and reasonably effective, however it is difficult to scale up to large quantities. This method is thus often still used for graphene research when small quantities of pure graphene are required.

Chemical exfoliation

The process of chemical exfoliation usually involves first increasing the interplanar distance in graphite, for example through intercalating (adding extra atoms between the graphene layers), to lower the bond energy, and then using this to easily separate the individual layers. [1].  Some specific forms of chemical exfoliation include liquid-phase exfoliation, which have little defects but low yield, extraction from graphene oxide, which has high yield but produces low quality graphene and furthermore utilises multiple expensive acids and takes a long time, making this uneconomical for industrial applications, and electrochemical exfoliation or supercritical fluid exfoliation, which show the most promise but but are still currently being developed. [8]

Chemical Vapour Deposition

This process is another promising method of producing larger quantities of graphene. It uses transition metals and hydrocarbons, as well as changing temperatures to create graphene. The gaseous hydrocarbons (such as CH4) are exposed to a single crystal of a transition metal (such as Nickel, Platinum or Iridium) at an elevated temperature (about 900-1000℃). The carbon and transition metal atoms form a solid solution at the elevated temperature, and the carbon precipitates out into a graphene film when this temperature is lowered. [9]

Applications

Graphene is proving to have many potential applications in diverse fields. Some possible applications include, but are in no way limited to: a replacement for the indium tin oxide currently used in touchscreens, where graphene would provide greater flexibility and strength than the currently used materials [6], use in transistors to replace silicon, due to graphene’s high electron mobility, and the limits of silicon due to quantum effects, [6] the use of graphene powder in electric batteries, due to graphene’s high surface area to volume ratio [3], and for use in solar cells or supercapacitors [6].

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