There are several methods for exfoliating graphene, each with unique characteristics and applications. These methods include liquid-phase exfoliation, controlled sublimation of SiC, chemical vapor deposition (CVD), and mechanical exfoliation.
Liquid-phase exfoliation involves the use of a solvent with appropriate surface tension to stabilize graphene flakes produced from bulk graphite. This process typically uses non-aqueous solvents like n-Methyl-2-pyrrolidone (NMP) or aqueous solutions with added surfactants. Energy for exfoliation is initially provided by ultrasonic horn sonication, but high shear forces are increasingly used. The yield is generally low, necessitating the use of centrifugation to isolate monolayer and few-layer graphene flakes.
Controlled sublimation of SiC is a method used primarily in the electronics industry for producing epitaxial graphene. This process involves the thermal decomposition of a SiC substrate in an ultrahigh vacuum, using either e-beam or resistive heating. After silicon desorption, excess carbon on the surface rearranges to form a hexagonal lattice. However, this method is costly and requires significant amounts of silicon for large-scale production.
Chemical vapor deposition (CVD) is a versatile method that uses growth substrates and a hydrocarbon gas source. It can be achieved through carbon diffusion and segregation in high carbon solubility metals like nickel, or by surface adsorption in low carbon solubility metals like copper. CVD is particularly promising for producing large areas of monolayer graphene with high quality and is relatively inexpensive.
Mechanical exfoliation, famously demonstrated by Geim and Novoselov, involves peeling graphene layers from graphite using adhesive tape. This method is primarily used for fundamental studies and research due to its limited scalability and inability to control the number of exfoliated layers.
Each of these methods has its advantages and disadvantages, and the choice of method depends on the specific requirements of the application, such as the need for large-area, high-quality graphene with minimal defects.
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