Sheet structures graphene

FIGURE 4.2 Representation of different carbon types on cobalt, (a) Atomic carbon/ surface carbide in a threefold hollow site, (b) CHX species located in threefold hollow sites, (c) Subsurface carbon lying in octahedral positions below the first layer of cobalt, (d) Cobalt carbide (Co2C) with an orthorhombic structure, (e) Polymeric carbon on a cobalt surface, (f) A sheet of graphene lying on a cobalt surface. The darker spheres represent carbon atoms in all the figures. 

The structure of CNTs can be understood as sheets of graphene (i.e. monolayers of sp2 hybridized carbon, see Chapter 2) rolled-up into concentric cylinders. This results in the saturation of part of the dangling bonds of graphene and thus in a decrease of potential energy, which counterbalances strain energy induced by curvature and thus stabilizes the CNTs. Further stabilization can be achieved by saturating the dangling bonds at the tips of the tubes so that in most cases CNTs are terminated by fullerene caps. Consequently, the smallest stable fullerene, i.e. C60, which is - 0.7 nm in diameter, thus determines the diameter of the smallest CNT. The fullerene caps can be opened by chemical and heat treatment, as described in Section 1.5. 

The synthesis of composite structures from Ni/Ni3P/Co2P207 components has recently been reported [94]. The electrical, optical and thermal properties of BN nanotubes are reported to be drastically different to those of carbon nanotubes [88]. The properties of Pnanotubes (Chapter 4.1) might prove to be even more interesting. While the flat sheets of graphene can be bent into carbon nanotubes, the puckered sheets which are present in black P would seem less likely candidates for similar treatment. 

Since 1976, studies on the thermal treatment of hydrocarbons (CVD) allowed researchers to isolate and observe long fibers and filaments (hollow fibers), carbon of several micrometers in length, but with very small diameter. However, this discovery made jointly by French and Japanese researchers [26] had a limited impact because the crystalline structure of these objects was inaccessible. In the early 90s, another Japanese team from NEC Labs proposed the helical structure of carbon nanocylinders (Fig. 5. If), which was quickly confirmed by several teams around the world [1]. In 1993, tubes with almost no thickness (formed from a single sheet of graphene) were synthesized, isolated and analyzed [27]. These were named single-wall carbon nanotubes (SWNT) because of their nanometer diameter. 

The ripples in graphene are intrinsic structures, and differait curvatures can be envisioned. With the outlined strong correlation betweai the graphene sheet structure and the resulting physical and chani-cal properties, such properties of graphene can be controlled via manipulation of gnphene ripples. 

The structure of a SWNT can be conceptualized by wrapping planar sheet of graphene into a seamless cylinder as shown in fig. 6. The way, the graphene sheet is wrapped is represented by a pair of indices (n, m). The integers, n and m, denote the number of unit vectors along two directions in the honeycomb crystal lattice of graphene. If m = 0, the nanotubes are called zigzag nanotubes, and if n = m, the nanotubes are called armchair nanotubes. Otherwise, they are called chiral nanotubes. Diameter of ideal CNT can be calculated from its unit vector (n, m) as follows ... 

The possible reactions during the process of restoring graphene sheet structure upon thermal treatment. (Adapted from ref. 14.)... 

Carbon nanotubes (CNTs) discovered by lijima (1991), are seamless cylinders made of rolled up hexagonal network of carbon atoms. Single-wall nanotubes (SWNTs) consist of a single cylindrical layer of carbon atoms. The details of the structure of multi-wall carbon nanotubes (MWNTs) are still being resolved, but can be envisioned as a tubular structure consisting of multiple walls with an inter-layer separation of 0.34 nm. The diameter for inner most tube is on the scale of nanometer. In one model, the MWNTs are considered as a single sheet of graphene that is rolled up like a scroll to form multiple walls [33]. 

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