2.1.2. Synthesis
2.1.2.1. Exfoliation. Graphene was first isolated in 2004 at the
University of Manchester by Novoselov and Geim using scotchtape
exfoliation of highly oriented pyrolytic graphite (HOPG)
(Novoselov et al., 2004). This method, also called micromechanical
exfoliation or peel-off method, relies on isolation of individual
p-stacked layers in graphite by repeated peeling using a scotch
tape. Graphene obtained using this method is usually defect-free
and of high quality, however, the sample size is very small (few
microns) and suffers from poor yield. This method has been limited
to lab-scale fundamental studies and not for commercial synthesis
of graphene. To overcome these limitations, liquid-phase exfoliation
of graphite in solvents like N-methyl-pyrrolidone
(Hernandez et al., 2008) and surfactant (sodium dodecyl benzene
sulfonate) solution has been demonstrated (Lotya et al., 2009).
This technique, though facile, has its own limitations such as the
resulting graphene film dimensions and the incomplete removal
of solvents which tend to have high boiling points. Due to its low
cost and low number of processing steps involved, this technique
provides a potential route for large-scale synthesis of graphenebased
sensors.