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The carbon-carbon (sp2 hybridised) bond length in graphene is approximately 0.142 nm with an interplanar spacing of 0.335 nm when sheets are stacked upon each other. The unit cell of monolayer graphene (also known as single-layer graphene (SLG)) has two atoms in it, whereas there are four atoms in bilayer’s unit cell. The carbon atoms in graphene are bonded with in-plane, 𝜎 bonds (3 bonds/atom), which are extremely strong and form the rigid backbone of the hexagonal lattice (Figure 2.2a). The out-of- plane partially filled 𝑝𝑧 orbital, 𝜋 bonds, perpendicular to lattice, are responsible for the electrical conduction and the interactions between graphene layers or substrates. The known stacking arrangements (Figure 2.2b,c and d) of bi- (two atomic layers) and few- (three to ten atomic layers) layer graphene are: hexagonal (AA stacking), Bernal (AB stacking) and rhombohedral (ABC stacking).17 Among these, the lowest energy and the most abundant form (80 %) in single crystal graphite is Bernal stacking.17, 18 Figure 2.2: a) Schematic representation of in-plane 𝜎 bonds and out-of-plane 𝜋 bonds.17 Different stacking arrangements of graphene: b) Hexagonal AA stacking c) Bernal AB stacking and d) Rhombohedral ABC stacking.17 e) Intrinsic ripples in graphene by Monte Carlo simulations. Red arrows are ~8 nm long.15 Despite showing long-range crystalline order, transmission electron microscopy (TEM) studies on suspended SLG sheets shown that they were not perfectly flat due to the previously discussed Peierls’ instability.16 They exhibit intrinsic static microscopic roughening or ‘ripples’ appear randomly on suspended SLG sheets with 8-10 nm wavelength and 0.7-1 nm amplitude with these waves also being predicted by Monte Carlo simulations.15 Scanning tunnelling microscopy (STM) studies revealed smaller intrinsic ripples (< 0.5 nm height) in SLG on SiO2 had little influence on its electronic properties19, whereas lower local conductance was observed for larger ripples (> 3 nm height).20 Apart from intrinsic ripples, graphene can have other disorders or defects such Chapter 2 – Graphene: Properties and Production 30PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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