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Graphene is a material of great interest for many applications. Whilst graphene was first made by chemical vapor deposition (CVD) in 2004 by scientists in Manchester, there have since been a lot of other methods prophesized and utilized to create graphene in either a more efficient or commercially viable way.
While many established methods are used in both academia and industry, the use of a plasma (and/or plasma reactor) to produce graphene is one method that is gathering interest from both sides. This article looks at some of the common plasma-based methods that are used to produce graphene.
Graphene can be made in many ways. Those who work with graphene in an academic capacity will often use chemical vapor deposition (CVD) to create single layer graphene from the bottom up. However, from an industry perspective there are many more commercially viable methods which exfoliate graphite into graphene (with many layer variations possible – some of which are known at the time of creation, some of which aren’t) using heat, chemicals, mechanical abrasion or solar rays.
There has been another method which has been gathering strength in the academic world and has recently made it to a commercial production level, and that is the production of graphene using plasmas – of which there are a few different reactors (or reaction chambers) and methods to produce graphene using plasmas.
Dielectric Barrier Discharge (DBD) Plasma Reactors
This is the type of reactor that has recently been used by industry to produce graphene commercially and is the only one in this article to do so as an actual standalone graphene product – the others are plasma assisted methods that have more academic merit or are for deposition onto a specific surface.
Dielectric barrier discharge (DBD) involves the formation of products through a ‘silent discharge’ between two electrodes. The name of the discharge comes from the fact that it is inaudible, whereas most discharges produce a “snapping/crackling” sound.
DBD creates a plasma within the region located between two electrodes, where at least one of these electrodes are coated with a dielectric material. The plasma is created by passing an alternating current in the presence of carbon atoms and other working gases (such as helium, argon etc).
The discharges, and the production of the product, can occur by the formation/deposition on the dielectric material of one of the electrodes, or in the space between the two coated electrodes through a series of microdischarges, where the product is collected below.
Plasma Enhanced Chemical Vapor Deposition (PE-CVD)
Plasma Enhanced Chemical Vapor Deposition (PE-CVD) is a method that can be used to produce graphene commercially, but it is often for lower quantities and more specific applications – such as in the production of conformal thin films on a substrate of interest. It is a technique that is not limited to graphene, but it is becoming a common choice, especially among academics.
In PE-CVD, the deposition of carbon atoms is achieved by using carbon-based precursor materials with other working gases between two electrodes – a ground electrode and a radio frequency (RF) energized electrode.
Just like in the DBD method, the key to this process is that there is either an alternating current (AC) or an RF signal between the two electrodes. This creates a capacitive coupling between the electrodes that excites the gases present between the two electrodes (with a much higher degree than is possible with a direct current).
Once these gases are highly energized, they form a plasma and chemically react with each other and the product of the reaction is deposited onto the substrate – which is located on the ground electrode. The process uses a much lower temperature than many other deposition methods, including conventional CVD), and often only uses temperatures between 250-300 °C.
Thermal Plasma Reactors
The final plasma approach mentioned here, which is purely academic, is the use of high temperature plasma to create graphene. However, it is a process that can involve temperatures up to 1000 °C, which often results in a low graphene yield and a high formation of impurities. For that reason, it is the least used method out of those discussed here.
The thermal plasma source in these reaction chambers is often produced from a high current divergent anode-channel DC plasma torch. The process involves feeding hydrocarbon materials alongside the working gas into the plasma torch. This then causes heating and decomposition within the plasma jet (in the region of the arc discharge) of the hydrocarbons before condensation onto a substrate – where the graphene (and other impurities) form.
Sources:
“Plasma Assisted Synthesis of Graphene Nanosheets and Their Supercapacitor Applications”- Kim S. J. et al, Science of Advanced Materials, 2014, DOI: 10.1166/sam.2014.1722
“Investigation on a DBD Plasma Reactor”- Ghomi H., et al, IEEE Transactions On Plasma Science, 2011, DOI: 10.1109/TPS.2011.2160735
“Characteristics of Dielectric Barrier Discharge Reactor for Material Treatment”- Kostov K. G. et al, Brazilian Journal of Physics¸ 2009, DOI: 10.1590/S0103-97332009000300015
“Towards large-scale in free-standing graphene and N-graphene sheets”- Tatarova E. et al, Scientific Reports, 2017, DOI: 10.1038/s41598-017-10810-3
“Plasma Synthesis of Graphene from Mango Peel”- Carreon M. L. et al, ACS Omega¸ 2018, DOI: 10.1021/acsomega.7b01825
“Synthesis of graphene-like materials by pyrolysis of hydrocarbons in thermal plasma and their properties”- Amirov R. Kh. et al, Journal of Physics: Conference Series, 2015, DOI: 10.1088/1742-6596/653/1/012161
Plasma-Therm- http://www.plasma-therm.com/pecvd.html
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