Although just one atom thick, graphene posses outstanding mechanical, electronic, optical, thermal and chemical properties, described below. Graphene is a one atom thick sheet made of carbon atoms, arranged in a honeycomb (hexagonal) lattice.
- Its height was measured to be just 0.33nm, almost one million times thinner than a human hair!
- Graphene is the ultimate 2-dimensional carbon molecule.
- Graphite, the well known 3-dimensional carbon allotrope found in our pencils, is nothing more than a stack of several graphene planes.
Graphene shares its structure with two other materials which are exciting today's scientists: carbon nanotubes and fullerenes (also called bucky-balls), seen as a 1-dimension and 0-dimension rolled pieces of graphene, respectively.
One of the hottest areas of graphene research focuses on the intrinsic electronic properties; how electrons flow through a sheet – only one atom thick – while under the influence of various external forces.
So why is graphene such an exciting material?
Firstly, graphene is great conductor; electrons are able to flow through graphene more easily than through even copper. The electrons travel through the graphene sheet as if they carry no mass, as fast as just one hundredth that of the speed of light.
Secondly, the way electrons behave in graphene make it very useful to study some fundamental physical properties. Graphene’s near perfect crystal lattice mean it is a very clean system in which to experiment. By restricting the electrons to only two dimensions, they exhibit some interesting properties such as the 'anomalous quantum Hall effect' and 'Klein tunnelling'.
Graphene is a perfect thermal conductor.
Its thermal conductivity was measured recently at room temperature and it is much higher than the value observed in all the other carbon structures as carbon nanotubes, graphite and diamond (> 5000 W/m/K).
The ballistic thermal conductance of graphene is isotropic, i.e. same in all directions. Similarly to all the other physical properties of this material, its 2 dimensional structure make it particularly special. Graphite, the 3 D version of graphene, shows a thermal conductivity about 5 times smaller (1000 Wm-1K-1). The phenomenon is governed by the presence of elastic waves propagating in the graphene lattice, called phonons.
The study of thermal conductivity in graphene may have important implications in graphene-based electronic devices. As devices continue to shrink and circuit density increases, high thermal conductivity, which is essential for dissipating heat efficiently to keep electronics cool, plays an increasingly larger role in device reliability.
To calculate the strength of graphene, scientists used a technique called Atomic Force Microscopy. By pressing graphene that was lying on top of circular wells, they measured just how far you can push graphene with a small tip without breaking it.
It was found that graphene is harder than diamond and about 300 times harder than steel. To put this into context, it will take the weight of an elephant balanced on a needle-point in order to break this one atom thick fabric! The tensile strength of graphene exceeds 1 TPa.
Even though graphene is so robust, it is also very stretchable. You can stretch graphene up to 20% of its initial length. It is expected that graphene’s mechanical properties will find applications into making a new generation of super strong composite materials and along combined with its optical properties, making flexible displays.
Graphene, despite being the thinnest material ever made, is still visible to the naked eye. Due to its unique electronic properties, it absorbs a high 2.3% of light that passes through it, which is enough that you can see it in air (if you could manage to hold it up!).
To help enhance the visibility of graphene flakes we deposit them on to silicon wafers which have a thin surface layer of silicon dioxide. Light shining on to these three-layer structures will be partially transmitted and partially reflected at each interface.
This leads to complex optical interference effects such that, depending on the thickness of the silicon-dioxide layer (which we can control to a high degree of accuracy), some colours are enhanced and some are suppressed. This technique takes advantage of the same physics which causes the "rainbow effect" that you see when you have a thin layer of oil floating on water. In this case, the different colours correspond to longer/shorter optical path lengths that the light has had to travel through the oil film.
Similar to the surface of graphite, graphene can adsorb and desorb various atoms and molecules (for example, NO2, NH3, K, and OH).
Weakly attached adsorbates often act as donors or acceptors and lead to changes in the carrier concentration, so graphene remains highly conductive. This can be exploited for applications as sensors for chemicals.
Other than weakly attached adsobates, graphene can be functionalized by several chemical groups (for instances OH-, F-) forming graphene oxide and fluorinated graphene. It has also been revealed that single-layer graphene is much more reactive than 2, 3 or higher numbers or layers.
Also, the edge of graphene has been shown to be more reactive than the surface. Unless exposed to reasonably harsh reaction conditions, graphene is a fairly inert material, and does not react readily despite every atom being exposed and vulnerable to it's surroundings.