handbook of carbon, graphite, diamond and fullerenes

Diamond-Like Carbon 339
the near-surface of black, opaque materials such as graphite and DLC
since the laser light is able to penetrate the surface of these materials a few
tens of namometers. The Raman spectra are sensitive to disruption of
translational symmetry such as occurs in small crystals and, as such, is
particularly useful in the analysis of disorder and crystallite formation in
DLC materials.^ 7 '
2.3 Structure and Categories of DLC
A truly amorphous solid such as glass lacks any degree of crystallinity
and has no lattice long-range order. Unlike glass, DLC is composed of small
crystallites which have a local atomic configuration that can be either
tetrahedral (sp 3 ) or planar threefold (sp 2 ) but are small enough that electron
diffraction patterns indicate an amorphous material. DLC has no longrange
order but forms a random stable network, similar to that of vitreous
carbon (see Ch. 6, Sec. 3.1). As a result, DLC is generally considered to be
amorphous. It has also been clearly demonstrated that the physical and
chemical properties of the DLC films are closely related to the chemical
bonding and the miscrostructure.Wf 8 !
DLC can be divided into two closely related categories known as
amorphous DLC and hydrogenated DLC.
2.4 Amorphous DLC (a-C)
A sizeable proportion of the sites in amorphous DLC have the sp 3
coordination (diamond) in the form of individual or polycrystalline diamond
in small clusters (5 -10 nm), interspersed with amorphous regions. The sp 2
carbon sites are generally lacking as evidenced by the absence of n
plasmon absorption shown by EELSJ 4 ' The hydrogen content is less than
one atomic percent and a small amount of argon may remain trapped in the
lattice. 121 The exact overall structure is still uncertain. The material is
generally produced by sputtering from a solid carbon target (see Sec. 3.2)
and is usually referred to as "a-C".
2.5 Hydrogenated DLC (a-C:H or H-DLC)
Hydrogenated DLC is also considered amorphous but, unlike a-C, it
contains a variable and appreciable amount of hydrogen (up to fifty atomic