Graphite

The ideal crystal structure of graphite (see Fig. 2) consists of layers in which the carbon atoms are arranged in an open honeycomb network containing two atoms per unit cell in each layer, labeled A and B. The stacking of the graphene layers is arranged, such that the A and A' atoms on consecutive layers are on top of one another, but the B atoms in one plane are over the unoccupied centers of the adjacent layers, and similarly for the B' atoms on the other plane [17]. This gives rise to two distinct planes, which are labeled by A and B. These distinct planes are stacked in the 'ABAB' Bernal stacking arrangement shown in Fig. 2, with a very small in-plane nearest-neighbor distance aC-c of 1.421 A, an in-plane lattice constant a0 of 2.462 A, a c-axis lattice constant c0 of 6.708 A, and an interplanar distance of c0/2 = 3.354 A. This crystal structure is consistent with the D|h(P63/mmc) space group and has four carbon atoms per unit cell, as shown in Fig. 2.

Fig. 2. (a) The crystal structure of hexagonal single crystal graphite, in which the two distinct planes of carbon hexagons called A and B planes are stacked in an ABAB... sequence with P63/mmc symmetry. The notation for the A and B planes is not to be confused with the two distinct atoms A and B on a single graphene plane (note a rhombohedral phase of graphite with ABCABC... stacking also exists [17]). (b) An STM image showing the trigonal network of highly oriented pyrolytic graphite (HOPG) in which only one site of the carbon hexagonal network appears, as for example, the B site, denoted by black balls in (a)

Fig. 2. (a) The crystal structure of hexagonal single crystal graphite, in which the two distinct planes of carbon hexagons called A and B planes are stacked in an ABAB... sequence with P63/mmc symmetry. The notation for the A and B planes is not to be confused with the two distinct atoms A and B on a single graphene plane (note a rhombohedral phase of graphite with ABCABC... stacking also exists [17]). (b) An STM image showing the trigonal network of highly oriented pyrolytic graphite (HOPG) in which only one site of the carbon hexagonal network appears, as for example, the B site, denoted by black balls in (a)

Since the in-plane C-C bond is very strong and the nearest-neighbor spacing between carbon atoms in graphite is very small, the in-plane lattice constant is quite stable against external perturbations. The nearest neighbor spacing between carbon nanotubes is essentially the same as the interplanar spacing in graphite (~3.4 A). One consequence of the small value of aC-C in graphite is that impurity species are unlikely to enter the covalently bonded in-plane lattice sites substitutionally (except for boron), but rather occupy some interstitial position between the graphene layer planes which are bonded by a weak van der Waals force. These arguments also apply to carbon nano-tubes and explain why the substitutional doping of individual single wall carbon nanotubes with species other than boron is difficult. The weak inter-planar bonding of graphite allows entire planes of dopant atoms or molecules to be intercalated between the carbon layers to form intercalation compounds. Also carbon nanotubes can adsorb dopant species on their external and internal surfaces and in interstitial sites between adjacent nanotubes, as is discussed in Sect. 6.

The graphene layers often do not stack perfectly and do not form the perfect graphite crystal structure with perfect Bernal 'ABAB' layer stacking. Instead, stacking faults are often formed (meaning departures from the ABAB stacking order). These stacking faults give rise to a small increase in the interlayer distance from the value 3.354 A in 3D graphite until a value of about 3.440 A is reached, at which interplanar distance, the stacking of the individual carbon layers become uncorrelated with essentially no site bond ing between the carbon aatoms in the two layers. The resulting structure of these uncorrelated 2D graphene layers is called turbostratic graphite [1,18]. Because of the different diameters of adjacent cylinders of carbon atoms in a multiwall carbon nanotube [15,16], the structural arrangement of the adjacent carbon honeycomb cylinders is essentially uncorrelated with no site correlation between carbon atoms on adjacent nanotubes. The stacking arrangement of the nanotubes is therefore similar in behavior to the graphene sheets in turbostratic graphite. Thus, perfect nanotube cylinders at a large spatial separation from one another should be able to slide past one another easily.

Of significance to the properties expected for carbon nanotubes is the fact that the electronic structure of turbostratic graphite, a zero gap semiconductor, is qualitatively different from that of ideal graphite, a semimetal with a small band overlap (0.04 eV). The electronic structure of a 2D graphene sheet [15] is discussed elsewhere in this volume [14], where it is shown that the valence and conduction bands of a graphene sheet are degenerate by symmetry at the special point K at the 2D Brillouin zone corner where the Fermi level in reciprocal space is located [19]. Metallic carbon nanotubes have an allowed wavevector at the K-point and therefore are effectively zero gap semiconductors like a 2D graphene sheet. However, semiconducting nanotubes do not have an allowed wavevector at the K point (because of quantum confinement conditions in the circumferential direction) [14,15], thus resulting in an electronic band gap and semiconducting behavior, very different from that of a graphene sheet.

Several sources of crystalline graphite are available, but differ somewhat in their overall characteristics. Some discussion of this topic could be helpful to readers since experimentalists frequently use these types of graphite samples in making comparisons between the structure and properties of carbon nanotubes and sp2 graphite.

Natural single-crystal graphite flakes are usually small in size (typically much less than 0.1 mm in thickness), and contain defects in the form of twinning planes and screw dislocations, and also contain chemical impurities such as Fe and other transition metals, which make these graphite samples less desirable for certain scientific studies and applications.

A synthetic single-crystal graphite, called "kish" graphite, is commonly used in scientific investigations. Kish graphite crystals form on the surface of high carbon content iron melts and are harvested as crystals from such high temperature solutions [20]. The kish graphite flakes are often larger than the natural graphite flakes, which makes kish graphite the material of choice when large single-crystal flakes are needed for scientific studies. However, these flakes may contain impurities.

The most commonly used high-quality graphitic material today is Highly Oriented Pyrolytic Graphite (HOPG), which is prepared by the pyrolysis of hydrocarbons at temperatures of above 2000° C and the resulting pyrolytic carbon is subsequently heat treated to higher temperatures to improve its crystalline order [21,22]. When stress annealed above 3300° C, the HOPG exhibits electronic, transport, thermal, and mechanical properties close to those of single-crystal graphite, showing a very high degree of c-axis alignment. For the high temperature, stress-annealed HOPG, the crystalline order extends to about 1 ^m within the basal plane and to about 0.1 ^m along the c-direction. This material is commonly used because of its good physical properties, high chemical purity and relatively large sample sizes. Thin-film graphite materials, especially those based on Kapton and Novax (polyimide) precursors, are also prepared by a pyrolysis/heat treatment method, and are often used.

0 0

Post a comment