Methods to Produce Graphene Flake and Film

Definition:  Graphene can be defined as an allotrope of carbon that consists of a single layer of carbon atoms arranged in a hexagonal lattice and with carbon-carbon sp2 hybridized bonds.  With each carbon atom covalently joined to 3 other carbon atoms in a 2-dimensional array, there is no opportunity for other chemistry unless the carbons become dissociated with one or more of these bonds, or at the edges of the sheets where unsaturated bonds exist.  The resultant structure is highly integrated, very physically strong, capable of extreme transmission of electrons and phonons, and nearly inert.  (By contrast, another allotrope of carbon, diamond, is sp3 hybridized, attached to 4 other carbons, creating a 3-dimensional crystalline structure.)

General Methods to Produce Graphene Flake:

• Top-Down Methods are those which begin with graphene structures already in place, such as with graphite.
• Bottom-Up Methods require the dissolution of carbon bonds, and all other bonds. The environment in which this occurs will determine the nature of the resultant structures: highly organized 2-dimensional sheets of pure carbon as in the CVD process; smaller sheets of few-layer flakes; non-graphene structures of various description.

Examples of Methods and Resultant Structures:

• Exfoliation of graphite by means of physical delamination with adhesive tape is the method first used to isolate graphene from graphite. This produces very small flakes of few- or single-layer graphene. 
• Chemical exfoliation of graphite can produce few-to-many-layered graphene, often in the form of Nanoplatelets.
• Flash Joule Heating will shock the sample, isolate the carbons and vaporize everything else, allowing the carbons to form into flakes of graphene.
• Chemical Vapor Deposition often uses methane as a precursor and copper film as a catalyst, which are brought together in an autoclave at 1000°C. This energetic environment causes the C-H bonds to dissolve and the carbons atoms to self-assemble on the copper film as single, double, few or many layered graphene, depending on the amount of time exposure.
• Epitaxial Growth where the precursor is silicon carbide, which is subjected to extreme heat to sublimate the silicon and allow the carbon to reassemble.
• Plasma energy has recently been used to disassociate the atoms in methane and through careful manipulation of the chamber environment cause the reassembly of the carbon into graphene and hydrogen into H₂ 

As would be expected, these methods each produce slightly different carbon allotropes, each with a different capability and use-case.  Some methods are more flexible with regard to the introduction of functionalization (the Plasma Method is particularly inclined toward this), while others tend to form the cleaner, purer and more regular grids of 2-dimensional graphene sheets (CVD is indicated here).  Other methods offer the possibility of marrying the characteristics of graphene with those of the substrate (Epitaxial Growth). 

Further variation can be explored with dopants, metallic occlusions, oxides and other non-carbon additions to the lattice.  As mentioned above, the Plasma Method is particularly flexible because it allow for non-carbon elements to be introduced during the re-formation process.  The physical format of the carbon itself is also subject to manipulation, which creates interesting options to be discussed in a later blog.

As always, ACS Material is ready to answer any questions you have about this topic, or any other, by simply contacting us through the website or email (contact@acsmaterial.com).