CVD graphene is typically grown on a metal foil. Naturally the graphene grown forms on both sides of the substrate. Transferring the graphene requires the removal of one of these layers using a protective membrane and destroying the other. For this article, we will call the layer of graphene we want to keep the top layer and the layer we need to destroy the bottom layer.
Why can’t we use both sides of the graphene that grew on my foil?
This is the first question people getting into CVD graphene transfers typically ask as it feels like we are throwing away half of our expensive graphene. Although the graphene coats both sides of the metal foil during growth it is important to focus on only one of the sides of the foil. This is for two main reasons:
Commercial graphene manufacturers often use copper that is prepared for growth on only one side. This is through polishing and surface treatment. When the graphene grows on both sides of the foil it grows in a controlled manner on the treated surface. However, the backside forms in an uncontrolled manner. Leading to an obvious difference in the quality of the material.
The second reason is that graphene is a great chemical barrier. Although the layer is only one carbon atom thick it reduces the etch rate of the metal foil during the chemical processing. Hence, removing the bottom layer of graphene allows the etchant to directly contact the foil and ensure rapid removal.
Why protect the top layer of graphene
Graphene is often marketed as an ultra-tough strong man of a material, which is strictly only true on a nanoscale. When graphene is unsupported or subject to the forces of surface tension it can easily break apart.
The standard method of protection is to coat the graphene foil in a polymer to act as a barrier and support. PMMA (Poly(methyl methacrylate)) is normally selected as it is:
Chemically compatible – PMMA and its carrier solvent are chemically compatible with the graphene, the etchants used, and the labware used in the transfer process.
Easy to remove – PMMA is soluble in readily available laboratory reagents like acetone, anisole, and MIBK. This allows for the easy stripping of the PMMA once the transfer is complete.
Spin coated on the surface -Spin coating offers a reliable and scalable method of creating a thin layer of a polymer on a surface. Varying the concentration of polymer and spin speed/acceleration allows for the control of the polymer membrane thickness.
Floats in water – This property is critical for the wet transfer process to work as the etch process occurs when the graphene foil is floated on the etchant surface.
Once the top layer of the CVD graphene is protected the bottom layer needs to be removed.
Removing graphene using plasma
Plasma etching of graphene is a process used to selectively remove or modify specific areas of a graphene layer by exposing it to a reactive plasma. Plasma etching is a technique commonly employed in microfabrication and nanotechnology to pattern or shape materials with high precision.
The bottom layer of the graphene can be etched using this plasma etching process, leaving only the protected top layer of graphene on the foil.
For greater detail on the plasma etching process have a look at our plasma etching graphene knowledgebase article.
We have recently launched our manual graphene transfer system which is an affordable complete kit to allow for graphene transfers onto arbitrary substrates up to 4”.
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