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16 Introduction to Carbon Materials (a) (b) Figure 1.10: (a) AFM image of a graphene flake on a SiO2 substrate. (b) Transmission- electron-microscopic image of free-hanging graphene. Both pictures have been taken by the Manchester group. 1.3 Fabrication of Graphene We briefly review the two techniques used in fabricating graphene, mechan- ical exfoliation of graphene from bulk graphite and graphitisation of epitax- ially grown SiC crystals. The resulting graphene probes have similar, but not always equal, physical properties. It is therefore important to describe both systems separately and to clearly indicate on what type of graphene, exfoliated or epitaxial, a particular measurement is performed. A systematic comparison of both graphene types is still an open research field. 1.3.1 Exfoliated graphene The mechanical exfoliation technique, elaborated mainly by the Manchester group, consistsy of peeling a layered material [13]. In the case of graphite, it uses its primary graphical capacity, which we have already alluded to above: if one scratches a piece of graphite on a substrate, thin graphite stacks are exfoliated from the bulk and left behind on the substrate. Most of these thin stacks comprise several (tens or hundreds) of graphene sheets, but a few of them only consist of a single graphene layer. The fabrication of graphene is, thus, extremely simple, and we produce single layers of graphene whenever we write with a pencil of sufficiently high graphitic quality.7 In practice, one does not use a pencil to fabricate graphene layers, but one prepares very thin samples by peeling a small graphite crystallite in a piece 7Not all pencils are of the required quality. The graphite lead of most pencils contains normally a certain amount of clay impurities.PDF Image | Physical Properties of Graphene
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