Graphite, Diamond, and DLC

The differences between graphite, diamond, and DLC are summarized in Table 14.1.1  

Composition Pure carbon Essentially Up to 50 at.% carbon hydrogen ( 1 at.% hydrogen)  

Atom-bonding state sp2 only sp only sp2, sp , sp (variable ratio) 

Raman spectrum Sharp peak at 1580cm Sharp peak at 1332 cm Broad humps at 1330 1550 cm 

Electrical conductivity Conductor (ab direction) Insulator Insulator 

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Jellnek M, Kocourek T, Remsa J, Miksovsky J, Zemek J, Smetana K Jr, Dvof ikova B, Luxbacher Z (2010) Diamond/graphite content and biocompatibility of DLC films fabricated by PLD. Appl. Phys. A101 579-583. 

These allotropes are sometimes found in combination such as some diamond-like carbon (DLC) materials produced by low-pressure synthesis, which are actually mixtures of microcrystalline diamond and graphite (see Ch. 14). 

Pyrolytic graphite is different from another standpoint although produced in bulk form, its main use is in the form of coatings, deposited on substrates such as molded graphite, carbon fibers, or porous carbon-carbon structures. As such, it is part of a composite structure and is not as readily identifiable as other forms of carbon. It is similar in this respect to CVD diamond and diamond-like carbon (DLC) described in Chs. 13 and 14. 

A new form of carbon coating is now available which is neither diamond nor graphite and is known as diamond-like carbon (DLC).bl DLC can be considered as a metastable carbon produced as a thin coating with a broad range of structure and composition.l b[42]... 

In Section 2 we showed that the properties of amorphous carbon vary over a wide range. Graphite-like thin films are similar to thoroughly studied carbonaceous materials (glassy carbon and alike) in their electrode behavior. Redox reactions proceed in a quasi-reversible mode on these films [75], On the contrary, no oxidation or reduction current peaks were observed on diamondlike carbon electrodes in Ce3+/ 41, Fe(CN)63 4. and quinone/hydroquinone redox systems the measured current did not exceed the background current (see below, Section 6.5). We conventionally took the rather wide-gap DLC as a model material of the intercrystallite boundaries in the polycrystalline diamond. Note that the intercrystallite boundaries cannot consist of the conducting graphite-like carbon because undoped polycrystalline diamond films possess excellent dielectric characteristics. 

Figure 16 shows the Raman spectrum of a DLC film deposited by the IBAD technique. The Raman spectra for diamond like materials provide information on the sp bonding. The characteristic features of Raman spectra of diamond like materials consist of a graphite-like (G) peak and a disorder (D) peak in the regions 1500-1550 cm and 1330-1380 cm respectively. The relative intensities of the G and D peaks can be used to indicate qualitatively the concentration of graphite crystallites of... 

The theoretical model ing of the kinetic aspects of diamond nucleation processes is indeed scarce in published literature. Although attempts have been made to model the nucleation kinetics,as reviewed above, the approaches require an accurate estimation of the kinetic rate constants, necessitating fitting the kinetic model to experimental data, thereby making the model system- (or experiment-) dependent. In addition, the kinetics of surface diffusion of adatoms and the formation of intermediate carbonaceous phases were not considered in these studies. As indicated in Ref. 217, a kinetic model is expected to contribute to a better understanding of the role of SiC formation in the nucleation of diamond on a Si substrate. However, the kinetic scheme employed in these studies was, in fact, unable to distinguish between a Si and a SiC surface. To capture the possibil ity that an intermediate carbonaceous phase (such as DLC, carbide or graphite) may form prior to diamond nucleation, the kinetic model should be modified to includea time dependence of the density of nucleation sites determined by the kinetics of the formation of the intermediate phase. Further studies are therefore needed to construct a clear picture of the kinetics of diamond nucleation processes in CVD. 

Spectra from a-C and a-C,(Cu,Zr) films are characterized by the presence of two smeared peaks 1340 cm (D peak) and 1550 cm" (G peak). This kind of spectmm with two smeared D and G peaks is characteristic of the so-called diamond-like carbon (DLC) films [1,2,6]. The a-C film, obtained by CAVD compared with typical DLC films is characterized by a) G-peak shift from 1580 cm" position for crystalline graphite to lower values b) IJI relation is larger than 2. 

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