Biomedical Engineering Reference
In-Depth Information
strain dependence of the electronic polarizability. It is the light scattering by the change
in polarizability due to lattice vibration. The difference in energy between the incident
photon and the Raman scattered photon is equal to the energy of a vibration of the scat-
tering molecule. A plot of intensity of scattered light versus energy difference (or shift) is
a Raman spectrum.
The Raman spectra of diamond, graphite, and some disordered carbons are compared in
Figure 2.6. Diamond has a single Raman active mode at 1332 cm
−1
, which is a zone-center
mode of
T
2g
symmetry. Single crystal graphite has a single Raman active mode, which is the
zone-center mode at 1581 cm
−1
of
E
2g
symmetry labeled “G” for “graphite.” The zone-center
optical modes in graphite can be decomposed into irreducible representation [28,29]:
Γ
= 2
B
2g
+ 2
E
2g
+
A
2u
+
E
1u
(2.11)
The
A
2u
and
E
1u
modes are infrared active, the
B
2g
modes are optically inactive, and the
E
2g
modes are Raman active. The symmetry of the
E
modes restricts the motion of the carbon
atoms. The two different
E
g
modes occur because adjacent planes can vibrate in phase or
with opposite phase [28,29]. The two in-plane Raman active
E
2g
modes are found to occur
at frequencies of 1581 cm
−1
(
E
2g2
) and 42 cm
−1
(
E
2g1
). The peak at 1581 cm
−1
from the Raman
active zone-center mode of graphite, assigned to the C
-
C stretching mode [28,29], is also
referred to as the G peak. When the long-range order of the graphite is disrupted (disor-
dered graphite), a second peak appears at about 1355 cm
−1
referred to as the D peak (“D”
for “disorder”). The D peak is associated with in-plane vibrations and results from struc-
tural imperfections. The Raman spectra of most disordered carbons remain dominated
Diamond
Graphite
D
G
μc-graphite
Glassy C
Sputtered a-C
a-C:H
ta-C
500
1000
1500
2000
Wavenumber (cm
-1
)
FIGURE 2.6
Comparison of typical Raman spectra of carbons. (Reprinted with permission from Robertson,
J., Mater. Sci.
Eng. R
, 37, 129, 2002.)
