Biomedical Engineering Reference
In-Depth Information
films treated by N
2
plasma, the emission current density and fluorescent
photos at 6.4 V/
m clearly demonstrated the influence of the treatment time
on field emission properties (Fig. 14.11).
The improvement of the field emission properties was attributed to the
change in CNT morphology after plasma treatment with the formation of
nano-protuberances, i.e. 'multi-tip' CNTs. The dependence of the emission
current with treatment duration showed an optimum after 20 minutes of
treatment. Prolonged treatment was shown to reduce the field emission
properties that were associated with the destruction of CNTs and nano-
protuberances.
The N
2
RF plasma treatment was also used to improve the Pt catalyst
deposition at
μ
the CNT surface, resulting in an enhancement of
the
electrochemical activity (Kim et al., 2008).
Bystrzejewski et al. reported the treatment of single-walled CNTs in a DC
hollow cathode glow discharge (HCGD) (Bystrzejewski et al., 2009).
Various gases (N
2
,H
2
O, N
2
+H
2
O and NH
3
+H
2
O) were used to
functionalize CNTs. The N
2
and/or H
2
O plasma treatment resulted in the
presence of amorphous carbon species at the CNT surface while the use of
NH
3
+H
2
O yielded a very clean product consisting of functionalized
CNTs. In-situ optical emission studies showed that the functionalization
occurs via radical addition channels with the initial assistance of N
2
+
radical ions. The N
2
+
bombardment breaks the C-C bonds on the CNT
surface, after which other chemical radicals are subsequently added and
quenched (Fig. 14.12).
An NH
3
μ
-wave plasma treatment has been also used to enhance the
solubility of CNTs (Wu et al., 2007). The introduction of polar functional
groups increases the hydrophilicity of CNTs, making the application of
CNTs in the immobilization of biomolecules and construction of biosensors
more convenient.
The studies cited above demonstrate the wide range of plasma processes
that can be used to modify the CNT surface. The excited species such as
electrons, ions and radicals within the plasma interact with the surface of
CNTs and break the C=C bonds, which promotes the creation of active sites
to bind functional groups as well as some physical modification of the CNT
surface. Moreover, it was also reported that UV photons interact with
CNTs and create active sites on their sidewalls, however, they can at the
same time promote the defunctionalization of moieties grafted onto CNTs
(Khare et al., 2002). Due to the interaction of the different plasma reactive
species with CNTs, the grafting of functional groups can therefore occur
through different simultaneous reactions. This wide range of interactions
involves a lack of control on the grafting of wanted chemical function and
makes it difficult to determine the weight of each functional mechanism.
Moreover, prolonged plasma treatments promote damage of CNTs,
