Biomedical Engineering Reference
In-Depth Information
by these two G and D modes of graphite, even when the carbons do not have particular
graphitic ordering [30].
The G mode of graphite at 1581 cm
−1
has
E
2g
symmetry, and it involves the in-plane bond-
stretching motion of pairs of carbon sp
2
atoms. This mode does not require the presence of
six fold rings, and so it can occur at all sp
2
sites, not only in rings. It is always in the range
between 1500 and 1630 cm
−1
. The D peak around 1355 cm
−1
is a breathing mode of
A
1g
sym-
metry. This mode is forbidden in perfect graphite and only becomes active in the presence
of disorder. And its peak intensity is connected to the presence of six fold aromatic rings
[30].
Figure 2.7 shows the various factors that can shift the G and D peaks in either direction
and alter their relative intensity.
Ferrari and Robertson [30] found that the Raman spectra of all disordered carbons can
be classified within a three-stage model of increasing disorder. The Raman spectrum is
considered to depend on
1. Clustering of the sp
2
phase
2. Bond disorder
3. Presence of sp
2
rings or chains
4. The sp
3
/sp
2
ratio
The above factors act as competing forces on the shape of the Raman spectra, as shown in
Figure 2.7. Ferrari and Robertson [30] defined an amorphitization trajectory ranging from
graphite to ta-C consisting of three stages:
1. Graphite
→
nanocrystalline graphite
2. Nanocrystalline graphite
→
a-C
3. a-C
→
ta-C
Stage 1 corresponds to the progressive reduction in grain size of ordered graphite layers,
while keeping aromatic rings. As the grain size decreases, phonon confinement causes the
sp
3
Bond disorder
Chains
Clustering
Clustering
D
G
1000
1100
1200
1300
1400
1500
1600
1700
1800
Raman shift (cm
-1
)
FIGURE 2.7
Schematic of the factors affecting position and heights of Raman G and D peaks of noncrystalline carbons.
(Reprinted with permission from Robertson,
J., Mater. Sci. Eng. R
, 37, 129, 2002.)
