Biomedical Engineering Reference
In-Depth Information
Bonding Characterization of Amorphous Carbon
X-ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy (XPS) is a technique used to investigate the composition of
deposited films by ionizing surface atoms and measuring the energy of ejected photoelec-
trons. The method requires the sample of interest to be bombarded with low energy x-rays,
produced from an aluminum or magnesium source, with an energy of
hv
, where
h
is Planck's
constant (6.62 × 10
−34
Js) and
v
is the frequency (Hz) of the radiation. These x-rays cause elec-
trons to be ejected from either a valence or inner core electron shell. The kinetic energy of the
electron, KE, is given by
KE
=
hv
− BE, where BE is the binding energy of the atom. If the energy
of the ejected photoelectrons were measured, its binding energy, which is the energy required
to remove the electron from its atom, can be calculated. Some important facts about the sample
under investigation can be learned from the binding energy: (1) the elements from which it is
made, (2) the relative quantity of each element, (3) the chemical state of the elements present,
and (4) the lateral and depth distributions (profiles).
The change in chemical state (interatomic bonding) will cause a shift in the binding energy.
In a-C, there are two main hybridizations, namely sp
3
diamond and sp
2
graphite (as discussed
in Section 2.1.1). Since both are formed by bondings of carbon atoms, the chemical shift between
that of sp
3
and sp
2
is small. That is, the binding energy of the two as determined from XPS will
be relatively close to each other. Therefore, there will be a superpositioning of the C 1s core
level peak due to the contribution of the two hybridized carbons. The peak of amorphous
carbon is thus broadened. In order to determine the contribution of each hybridized carbon in
the film, the C 1s peak can be deconvoluted. It was found that the full-width at half-maximum
of both the sp
3
peak and the sp
2
peak tend to be wider than those from pure diamond and
graphite [24]. This is attributed to the environment within the a-C film with mixed sp
2
and sp
3
bondings. A deconvoluted C 1s peak is shown in Figure 2.5. This regime for the approximation
of the sp
2
and sp
3
concentration in the a-C is widely used [24-27].
Raman Spectroscopy
Raman spectroscopy is widely used, being a routine, nondestructive way to characterize
the structural quality of amorphous carbons. The Raman effect is possible because of the
Fitted curve
sp
3
curve
sp
2
curve
CO curve
300
296
292
288
284
280
Binding energy (eV)
FIGURE 2.5
Example of fits of C 1s peak from an XPS spectrum. (Reprinted with permission from Filik et al.,
Diamond Rel.
Mater.
, 12, 974, 2003.)
