Graphene consists of a two-dimensional array of carbon atoms. It resembles a honeycomb structure as the atoms are arranged in a hexagonal lattice. It is an infinitely large aromatic molecule. Graphene is a single layer of the carbon graphite structure.
Graphene has existed since the early 1940s. Monolayer graphene sheets were first synthesized in 2004.
Andre Geim and Konstantin Novoselov were the people behind this and they received the Nobel prize in physics in 2010 for their discovery. The ‘Scotch tape method’ was really simple.
Mechanical exfoliation of graphite helped in isolating graphene monolayers. One layer is pulled away from the bulk at a time by using adhesive tape.
Other methods for producing graphene include sonication and centrifugation of graphite in a liquid for creating a graphene dispersion, Tang-Lau method (synthesis from sugar), and chemical vapor deposition (CVD) which involves reduction of silicon carbide and epitaxial growth.
Various researches are being done on graphene because of its highly impressive properties. The scientific interest in the material keeps on growing every passing day.
Graphene is mostly handled in a dispersion, on a substrate, or even as a powder of graphene oxide. Graphene has several outstanding properties. This includes electronic, optical, thermal and chemical properties. The special properties of graphene is attributed to the fact that it is only one atomic layer thick.
With a planar density of 0.77 mg/m2, graphene is an extremely light material. It has the toughest and hardest crystal structure. Its tensile strength is 125 GPa and its elastic modulus is 1.1 TPa. Graphene’s breaking strength is 42 N/m. It has way better mechanical strength when compared to steel.
With a transparency rate of 97.7 percent, Graphene is highly transparent to visible light.
Graphene possesses very high electron mobility (2 × 105 cm2/Vs). It is the most highly conductive substance at room temperature. It has a conductivity of 106 S/m and a sheet resistance of 31 Ω/sq.
There is a small overlap between the valence and conduction bands of graphene. As a result, it is regarded as a semimetal and as a zero-bandgap semiconductor. Even at absolute zero temperature, the presence of a particular concentration of electrons in the conduction band and of holes in the valence band, is the primary reason for graphene being called a semi-metallic material and is the reason for its high electrical conductivity.
A single layer of graphene displays 10,000 times higher electrical conductivity than a few layers of graphene.
The thermal conductivity of graphene is 5300 W/mK. It is ten times the thermal conductivity of copper. The thermal conductivity of graphene drops to around 500–600 W/mK, when carried in an amorphous material.
Tuning of properties
Surface functionalization helps in modifying the properties of graphene. It involves inclusion of oxygen or other chemical functional groups to the monolayer. Functionalization of graphene facilitates the tuning of properties which includes the electrical conductivity, thermal conductivity and the ability of the monolayers to be processed in solution and prevention of agglomeration. Graphene oxide is one of the most common chemically modified types of graphene.
Graphene inks, layers and powders
As a result of its two-dimensional nature, graphene can be presented either as layers on a substrate, as a dispersion (or ink) and also in the form of a graphene oxide powder.
Graphene ink, if applied to a surface produces an electrically conductive film, which has a sheet resistance of 15 W/sq.
After transferring it from a removable polymer substrate, the monolayer graphene can be added to another desired substrate. Monolayer graphene is fabricated through chemical vapor deposition.
Graphene on PET films are also obtainable. Top quality monolayer graphene is grown through CVD on a 188 μm-thick PET substrate which has dimensions of up to 600 mm × 500 mm. Following this, it can be transferred onto a new substrate as well.
Graphene can be dispatched in the form of graphene oxide powder. The powder will be available in an aqueous dispersion. It will have a mean particle size of 285 μm, a concentration of 4.0 mg/ml and a pH value of 2.20—2.50.
Chemical vapor deposition (CVD) is used for producing high-quality graphene. It is one of the most common methods of producing top-quality graphene. Here, gaseous reactants are put in a reaction chamber. As a result of the reaction, a film of the desired material is formed on the surface of the substrate.
The two steps involved in the formation of graphene through CVD are namely the creation of carbon and the formation of the graphene structure.
To begin with, carbon atoms are produced via pyrolysis. Thermal decomposition in an inert atmosphere is called pyrolysis. Inorder to avoid the formation of carbon clusters or soot, the process is carried out on the substrate surface. A metal catalyst is usually used as the substrate.
The second step involves reaction between the carbon atoms and the catalyst substrate resulting in the formation of graphene.
Applications of graphene
Graphene is used for manufacturing different devices. Many of them are already available in the market.
As a result of its high transparency and high electrical conductivity, graphene functions as a transparent conductor. It can be a less environmentally damaging replacement for indium tin oxide. Because of its flexibility, graphene is also used in flexible displays.
Meanwhile, chemical and thermal properties of graphene are used by strain sensors, gas sensors, biosensors and flow sensors. It is even used as a cooling device for integrated circuits in smartphones.
Polymer-based graphene composites and graphene fibers are made taking advantage of the mechanical properties of graphene. It is also used for improving mechanical properties of rubber.