Biomedical Engineering Reference
In-Depth Information
mechanisms (Godfroid et al., 2005). The first is dissociation by direct
electronic impact, which is achieved by collision between a nitrogen
molecule and an electron:
N
2
X
þ
g
N
4
S
þ
N
4
S
e
þ
n
¼
0
!
e
þ
½
14
1
:
This reaction leads to the production of atomic nitrogen in the fundamental
state N(
4
S).
The second is called dissociative recombination and is achieved by
recombination between an electron and an ionized nitrogen molecule:
N
4
S
þ
N
4
S
N
2
!
e
þ
½
14
2
:
Finally, the set-up and parameters of the microwave discharge were studied
to find the higher nitrogen dissociation rate (Godfroid et al., 2003). The
most influential parameters proved to be the power supply pulse, the
dilution of N
2
in Ar and the total gas flow, which makes it possible to
achieve a high homolytic dissociation up to 40% of the molecular N
2
.
The CNTs were thus treated in the post-discharge of an Ar + N
2
microwave plasma sustained by a surface wave launched in a quartz tube via
a surfaguide supplied by a 2.45 GHz microwave generator, which can be
pulsed (Ruelle et al., 2007). In this plasma-induced functionalization, the
plasma was not used to directly interact with the surface of CNTs but as a
source of atomic nitrogen N
reactive flow. It is important to note that the
discharge tube and the post-discharge chamber are separated by a
diaphragm that consists of an aluminium ring with a circular aperture
whose diameter measures a few millimetres. This diaphragm plays two roles
in the post-discharge treatment. First, the separation between the plasma
set-up and the post-discharge protects the samples placed in the post-
discharge from irradiation of high-energy particles from
·
-wave plasma.
Moreover, the average distance of 40 cm between the end of the discharge
tube and the sample holder promotes the interaction of CNTs with atomic
nitrogen species that have enough mean lifetime to reach the sample holder,
in contrast to other reactive species.
High resolution photoelectron spectroscopy analysis showed that by
exposing multi-walled CNTs to atomic nitrogen N
μ
·
μ
-wave
plasma, nitrogen chemical groups were grafted onto the CNT surface,
altering the density of electronic states (Ruelle et al., 2008). High-resolution
TEM and SEM images revealed that the atomic nitrogen exposition did not
damage the surface of the CNTs (Fig. 14.14). The absence of damage in the
structure of carbon nanotubes after the plasma treatment also indicated that
nitrogen group grafting resulted only from chemical reaction between CNT
generated in the
