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62 Graphene – Synthesis, Characterization, Properties and Applications graphite is only ≈5Å (that is, less than 2 layers in thickness) and, hence, one must differentiate between the surface and the bulk even for films as thin as 5 layers. Earlier attempts to isolate graphene concentrated on chemical exfoliation. To this end, bulk graphite was first intercalated (to stage I) so that graphene planes became separated by layers of intervening atoms or molecules. This usually resulted in new 3D materials. However, in certain cases, large molecules could be inserted between atomic planes, providing greater separation such that the resulting compounds could be considered as isolated graphene layers embedded in a 3D matrix. Furthermore, one can often get rid of intercalating molecules in a chemical reaction to obtain a sludge consisting of restacked and scrolled graphene sheets. Because of its uncontrollable character, graphitic sludge has so far attracted only limited interest. There have also been a small number of attempts to grow graphene. The same approach as generally used for growth of carbon nanotubes so far allowed graphite films only thicker than ≈100 layers. On the other hand, single- and few-layer graphene have been grown epitaxially by chemical vapour deposition of hydrocarbons on metal substrates and by thermal decomposition of SiC. Such films were studied by surface science techniques, and their quality and continuity remained unknown. Only lately, few-layer graphene obtained on SiC was characterized with respect to its electronic properties, revealing high-mobility charge carriers. An illustration of a few layers of graphene is given in Figure 3. Epitaxial growth of graphene offers probably the only viable route towards electronic applications and, with so much at stake, a rapid progress in this direction is expected. The approach that seems promising but has not been attempted yet is the use of the previously demonstrated epitaxy on catalytic surfaces (such as Ni or Pt) followed by the deposition of an insulating support on top of graphene and chemical removal of the primary metallic substrate. Fig. 3. A few layers of graphene seen by scanning electron microscopy [Taken from www. sineurop-nanotech.com/en/graphene.html Courtsey SINEUROP Nanotech GmbH, Stuttgart, Germany] 1.3 How does one tell if it is graphene? Counting the number of graphene layers 1.3.1 Transmission electron microscopy (TEM ) Transmission electron microscopy is a two-dimensional microscopy technique. It is difficult to probe the third dimension by this technique. The only way to measure the thickness of graphene layers is by performing cross-sectional TEM which can be accomplished by mounting the sample such that the electrons from the gun run parallel to the surface of the graphite layer.PDF Image | GRAPHENE SYNTHESIS CHARACTERIZATION PROPERTIES
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