Atomic and Crystal Structures of Diamond

Diamond has two basic crystal stmctures, one with a cubic symmetry (more common and stable) and the other with a hexagonal symmetry (rare but well established, found in nature as the mineral lonsdaleite). The close-packed layers, 111 for cubic and 100 for hexagonal, are identical. The cubic structure can be visualized as stacking of puckered planes of six-membered saturated carbon rmgs man EO EO sequence along (111) direction, referred to as 3C diamond (Fig. 1). All ofthe rings exhibit the chair 

The cubic structure is the dominant crystal structure in both natural and synthetic diamond since the staggered conformation is more stable than the eclipsed due to the slightly lower energy (0.1-0.2 eV per carbon atom). Diamond polytypes and carbyne phases form only during the homogeneous nucleation and growth of diamond powder, 

In Fig. 3a, cubic and octahedral faces are evident, and in Fig. 3b the twinned crystals with pseudo-fivefold symmetry can be clearly seen. This twinned fivefold symmetry is prevalent in CVD diamond thin films and apparently never develops on homoepitaxially grown crystals.Balllike diamond crystals are grown at high supersaturationsf (Fig. 3c). 

In Fig. 4, it ran be seen that, with the a value increasing from 1 to 3, the crystal shape changes from cube to cubo-octahedron and then to octahedron (upper) and the crystals grown with a 1.5 exhibit rough 111) facets whereas the crystals grown witli a 1.5 show very smooth 111 fecets 

Faceted diamond dendrites may form in jElame synthesis (Fig. 5a). Twinning, or stacking feult, occurs frequently on the 111 planes. A 

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By making an updated and systematic review of diamond CVD processes, the objective of this book is to familiarize the reader with the scientific and engineering aspects of diamond CVD, and to provide experienced researchers, scientists, and engineers in academic and industry community with the latest developments in this growing field. The scope of the present book encompasses the development and applications of diamond CVD, starting with a brief description of atomic and crystal structures of diamond and a review of the various processing techniques used in diamond CVD. It is followed by an extensive discussion of fundamental phenomena, principles and processes involved in diamond CVD, with emphasis on the... 

The element carbon occurs in nature in two so-called allotropic forms, different crystal structures with the same chemical formula. In Fig. 3.13 the crystal structure of diamond and graphite have been represented. In diamond the C atoms are closely packed and each C atom is linked with four other C atoms. Thus a tight network of atoms is formed which, together with the binding strength, is responsible for the extreme hardness of diamond. Graphite has a layered structure and the space between the layers is relatively large. 

FIGURE 1.7. The crystal structure of diamond. Stereoview showing several unit cells. Each carbon atom is tetrahedrally surroimded by four others. The arrangement of carbon atoms found in cyclohexane and in tetramethylmethane (shown with black bonds) can also be picked out from this rigid structure,... 

FIGURE 8.11 Crystal structures of diamond-type monophosphides (a) AlP, (b) BN, (c) ZnSiP and (d) diamond (or Si). All atoms are tetrahedrally coordinated. 

Diamond has the highest atom density of any material with a molar density of 0.293 g-atom/cm. As a result, diamond is the stiffest, hardest and least compressible of all substances. In comparison, the molar density of graphite is 0.188 g-atom/cm. The atomic and crystal structure data of diamond are summarized in Table 11.1 Also included in the table are the data for hexagonal diamond (see Sec. 2.5). 

Fig. 1. Crystal structures of (a) cubic diamond and (b) lonsdaleite. A, B, and C indicate the stacking sequence of sheets of atoms.

Figure 29.1 Crystal structures of ZnS. (a) Zinc blende, consisting of two, interpenetrating, cep lattices of Zn and S atoms displaced with respect to each other so that the atoms of each achieve 4-coordination (Zn-S = 235 pm) by occupying tetrahedral sites of the other lattice. The face-centred cube, characteristic of the cep lattice, can be seen — in this case composed of S atoms, but an extended diagram would reveal the same arrangement of Zn atoms. Note that if all the atoms of this structure were C, the structure would be that of diamond (p. 275). (b) Wurtzite. As with zinc blende, tetrahedral coordination of both Zn and S is achieved (Zn-S = 236 pm) but this time the interpenetrating lattices are hexagonal, rather than cubic, close-packed.

FIGURE 14.30 Structure of diamond. Each carbon atom is sp hybridized and forms tetrahedral rr-bonds to four neighbors. This pattern is repeated throughout the crystal and accounts for diamond s great hardness. 

A number of chemical elements, mainly oxygen and carbon but also others, such as tin, phosphorus, and sulfur, occur naturally in more than one form. The various forms differ from one another in their physical properties and also, less frequently, in some of their chemical properties. The characteristic of some elements to exist in two or more modifications is known as allotropy, and the different modifications of each element are known as its allotropes. The phenomenon of allotropy is generally attributed to dissimilarities in the way the component atoms bond to each other in each allotrope either variation in the number of atoms bonded to form a molecule, as in the allotropes oxygen and ozone, or to differences in the crystal structure of solids such as graphite and diamond, the allotropes of carbon. 

Two later sections (1.6.5 and 1.6.6) look at the crystalline structures of covalently bonded species. First, extended covalent arrays are investigated, such as the structure of diamond—one of the forms of elemental carbon—where each atom forms strong covalent bonds to the surrounding atoms, forming an infinite three-dimensional network of localized bonds throughout the crystal. Second, we look at molecular crystals, which are formed from small, individual, covalently-bonded molecules. These molecules are held together in the crystal by weak forces known collectively as van der Waals forces. These forces arise due to interactions between dipole moments in the molecules. Molecules that possess a permanent dipole can interact with one another (dipole-dipole interaction) and with ions (charge-dipole interaction). Molecules that do not possess a dipole also interact with each other because transient dipoles arise due to the movement of electrons, and these in turn induce dipoles in adjacent molecules. The net result is a weak attractive force known as the London dispersion force, which falls off very quickly with distance. 

Figure 7 shows the crystal structures of graphite, ordinary (cubic) diamond, and hexagonal diamond. The layers of carbon atoms lie in flat sheets in graphite, but in diamond the sheets are more wrinkled and lie closer together. Taken separately, the sheets are similar, but they may be stacked in various lateral positions and still have bonding between them. 

In a similar way Kekule s theory of the benzene structure has been very completely established by the whole development of aromatic chemistry. The direct physical verification of the presumably planar arrangement was in this case more delayed. The crystal structure of graphite was examined almost as soon as that of diamond, but the early results were inconclusive. The structure is not determined by the symmetry alone, and the later detailed investigation by Bernal (1924) showed that the carbon atoms in the hexagonal net must be coplanar to within at least 0-38 A. Later work by Ott (1928) narrowed this limit still further. Although it is generally assumed that the atoms are coplanar, the exactness with which this can be established depends on... 

The three-dimensional network structure of diamond can be considered as constructed from the linkage of nodes (C atoms) with rods (C-C bonds) in a tetrahedral pattern. From the viewpoint of crystal engineering, in a diamondoid network the node can be any group with tetrahedral connectivity, and the linking rods (or linker) can be all kinds of bonding interactions (ionic, covalent, coordination, hydrogen bond, and weak interactions) or molecular fragment. 

Germanium — (Ge, atomic number 32) is a lustrous, hard, silver-white metalloid (m.p. 938 °C), chemically similar to tin. Ge is a low-band-gap - semiconductor that, in its pure state, is crystalline (with the same crystal structure as diamond), brittle, and retains its luster in air at room temperature. Anodic dissolution of the material occurs at potentials more positive than ca. -0.2 V vs SCE. Peaks in the voltammograms of germanium in acidic electrolyte are ascribed to a back-and-forth change between hydrogenated and hydroxy-lated surfaces [i]. Studies are often conducted at p-doped and n-doped Ge electrodes [ii] or at Ge alloys (e.g., GeSe) where photoelectrochemical properties have been of considerable interest [iii]. 

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