Specialty Metals, Minerals and Materials: Graphene
Graphene, the single layer of graphite, was first isolated by Professors Konstantin Novoselec and Andrew Geim at Manchester University in 2004. They won the Nobel Prize for physics in 2010 for “ground-breaking experiments regarding the two-dimensional material graphene”.
There are dozens, if not hundreds of different types of graphene commercially available. It is the thinnest material ever made. At a thickness of around 0.35 nanometres per layer, three million sheets would make a stack only 1 millimetre high. It is flexible, can be stretched, is extremely strong, almost invisible and conducts heat, light and electricity.
There is a myriad of potential uses for graphene, however most are still at the research or prototype stage. The following is a partial list of potential uses:
- Electronics: transistors and optoelectronics
- Energy: supercapacitors and electrodes
- Environment: water filtration and decontamination
- Light: photodetectors and infrared light detection
- Medicine: measuring devices and drug delivery
- Miscellaneous: lubricants and coolant additives
Three of the major methods are briefly outlined below. There are several others being used or developed, but the subject quickly becomes very technical. New manufacturing methods are constantly being discovered and existing methods refined.
Chemical Vapour Deposition (“CVD”)
There are various methods of using CVD to manufacture graphene. One of the most popular methods uses copper as a substrate, heated to around 1,000 degrees centigrade in a furnace, at either atmospheric or lower pressures. Gas is introduced, such as a combination of hydrogen and methane, and graphene is deposited on the copper as either a single layer, or very few layers.
Graphene produced by this method is very pure, of uniform thickness and can be large in size. It also typically has lower defects, structural disorders and wrinkles than graphene produced by other methods. Its main use is in electronic and optoelectronic devices.
Epitaxial Graphene (“EG”) on Silicon Carbide
Epitaxy means the deposition of a crystalline layer on a crystalline substrate. In this case silicon carbide (SiC) is heated to about 1,100 degrees centigrade, forming a silicon substrate with a graphene layer. This is potentially the most promising method to produce graphene for use in semiconductors. It is compatible with current complementary metal oxide semiconductor (“CMOS”) technology, which would be a huge advantage for potential future use of graphene.
Exfoliation techniques use graphite, either synthetic or natural, as a start point. There are various methods used; for example, microwave assisted, gas assisted, sonic assisted, mechanical, and solvent exchange. The method used depends upon the feedstock and desired graphene properties.
Graphene produced in this way is suitable for a wide array of uses. It can be used in polymer composites, conductive coatings and inks, fuel cell batteries, catalysts and capacitors.
Because there are many variables in determining graphene’s suitability for various uses, an extensive suit of analyses is required to determine a particular graphene’s characteristics. These analyses include elemental analysis, thermo-gravimetric analysis, scanning electron microscopy, X-ray photoelectron spectroscopy, nuclear magnetic resonance spectroscopy, Raman spectroscopy, and electrical conductivity.
I shall only touch upon one technique, Raman spectroscopy, as that seems to be the favoured result reported by junior companies that are proposing graphene production from natural graphite. In essence the sample is bombarded with laser light and the receiving molecules receive the photons, change vibrational state and then return to their original state. The change in state can possibly show the suitability of the graphene for particular uses.
The issue with this method is that 99.999% of photons that are received by the molecules (graphene in this case) are just scatter. The results are meaningless. It is very difficult to acquire high quality Raman data. Many instruments are required to filter the data to acquire the 0.001% of potentially reliable data. Notch filters, tuneable filters, laser stop apertures, double/triple spectrometric systems, the list is long. It is difficult technology.
The Graphene Market
It is difficult to get accurate numbers on the graphene market. It is dominated by small, opaque companies with unproven technical and commercial skills. Consequently, information that is available can often be contradictory.
Prices vary depending upon the type of graphene. For example, a single graphene layer on copper foil will cost around $250 for a 5cmx5cm sheet. Graphene nanopowder flakes will cost $100 to $250 for 25 grams.
Production growth has accelerated rapidly, mostly driven by China, from around 100 tonnes in 2012 to 900 tonnes per year in 2015. The market is now in oversupply as demand is growing more slowly than expected.
Still, there are a variety of predictions as to how fast the market will grow. The optimists are saying it will be $100 million dollars or more by the end of the decade. However, others are far more conservative. In fact, some compare it with the much hyped carbon nanotubes (“3D graphene”) that were going to be everywhere but now only find limited use in niche markets.
There are still many challenges before graphene can become more widely used. It is still too expensive for many uses and quality is often questionable. It also suffers from some fundamental issues that may not be solved. For example, it is often touted as a faster replacement for current CMOS technology. However electric current cannot be switched on and off in a graphene conductor as it can with silicon based conductors.
In my opinion graphene will become commercialised for many applications, but it will take much longer than most think. Decades rather than years.
The future of graphene may be comparable with the history of aluminium. Aluminium was first identified in 1808 and first produced in 1825. Production methods improved during the 19th century but were very expensive. Napoleon III (Emperor of France from 1852 to 1870) used aluminium for his table ware. Lesser mortals used gold and silver.
By the end of the 19th century the electrolytic process was discovered and by the 1930’s there was enough electricity in the US to commence cost effect production. Thus about 100 years went by before aluminium was truly commercialised.
Let’s hope the commercialization of graphene is a little speedier.