The Diamond Family

Diamond and graphite are polymorphs of the same carbon elements. The two structures are inter-related in such a way that by stretching its (111) planes a diamond can transform into graphite.

In reverse, by puckering the basal planes, graphite will transform into diamond. Hence, we may call diamond a puckered graphite; and graphite, a stretched diamond. The above illustration indicates that the two polymorphs may convert entirely from one phase to the other, and vice versa. In addition, diamond and graphite may transform into each other only partially. In this case, the intermediate structure may contain both diamond's (sp3) and graphite's (sp2) bonds. In fact, there is no pure diamond structure as it would require that perfect alignment of the lattice infinitely in all directions, so all diamonds are inevitably contain a certain amount of graphitic bonds.

There are several ways that graphitic bonds can be incorporated in a diamond structure. The most common source of graphite bonds are located on the surface of a diamond crystal where at least one of the diamond bonds of the carbon atom is terminated to form an unpaired single electron, i.e., a dangling bond. Because a grain boundary is the source of graphitic bonds, the smaller the grain, the more graphitic bonds are possible. In fact, a nano-crystalline diamond with about 1000 atoms (10 atoms on each edge in average) will have about the same number of graphitic atoms located on the surface as diamond atoms located in the body, so this nano particle should be renamed graphitic diamond. Another way to introduce graphitic bonds is to drill holes through diamond (e.g. by making an optical sieve) or to induce defects in the crystal. In this case, the situation is reversed, instead of graphitic bonds surrounding diamond bonds, graphitic bonds are enclosed in between diamond bonds.

Another way to form graphitic diamond is to mix the two types of bonds in the same volume. This is the structure of amorphous diamond, a diamond-like carbon that contains no foreign elements (e.g. hydrogen free).

A third way to mix the two types of bonds is to introduce a slight diamond character into each of the atom in graphite. The net result is to curl up the basal planes, and depending on the distribution of the curled atoms, the structure may form either a ball or a tube, the former is known as bucky balls; the latter, carbon nano-tubes. These curled graphitic layers may also be viewed as hallow diamond balls or tubes as they may also be derived by eliminating the interior atoms from a solid diamond.

Keeping the above relationship between graphitic and diamond bonds in mind, DLC, bucky balls, nano-tubes, and other

Planar Diamond sp2 (Graphite)

Figure 3.9. Carbon family with the diamond taste. This diagram shows the connection of many carbon materials that exhibits various degrees of diamond characters. In the above diagram, the nano-cubicles represent a horde of hypothetical structures that may be formed by assembling carbon nanotubes in various forms like using steel beams for constructing houses. This molecular manipulation of carbon nano-tubes may be developed in the future.

structures that contain partial characteristics of sp3 bond may be considered as diamondoids (partial diamond) materials (Fig. 3.9). This terminology is relevant as the unique shapes and properties of bucky balls and nano-tubes are indeed derived from this partial diamond character. Because of this diamond connection, bucky balls and nano-tubes have phenomenal properties and they have been envisaged to be the enabling material for many exotic applications (e.g. hydrogen storage, field emission, drug carrier, molecule scale, nano-transistor, etc.).

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