Biomedical Engineering Reference
In-Depth Information
Carbon Film
Carbon film electrodes are generally prepared from conducting or semiconducting materi-
als deposited either chemically or in vacuo on an appropriate substrate such as a glass slide
[8]. The carbon films thus obtained can also be referred to as hard carbon. Film electrodes
were developed for use in spectroelectrochemical studies where a beam of light is passed
through an optically transparent film [8]. Besides this, the planarity of films deposited on
an optically flat glass substrate makes them suitable for optical reflection spectroscopy.
Unlike GC, carbon film is not hard and it has a density lower than that of graphite. In con-
trast to PG, carbon film possesses up to 10% sp
3
-hybridized carbon atom. Carbon film with
improved hardness can be obtained by the plasma deposition from CH
4
or CH
4
/H
2
atmo-
sphere [8]. Carbon films resulted from plasma method has enhanced hydrogen content.
The presence of hydrogen in the carbon film was reported to result in improved hardness
and lower density than nonhydrogenated carbon films.
Diamond-Like Carbon
In recent years, DLC has emerged as a potential material because of its high hardness, low
frictional coefficient, high wear and corrosion resistance, chemical inertness, high elec-
trical resistivity, infrared transparency, high refractive index, and excellent smoothness.
Although it is called “diamond-like,” DLC is in fact not like crystalline diamond and also
not as hard and is virtually amorphous. Hydrogen is frequently present in amounts up
to 40%, at occupying regions of low electron density in the matrix. Its presence strongly
influences the mechanical and tribological behavior of DLC coatings. In addition, the
microstructure of DLC allows the incorporation of other species such as nitrogen, silicon,
tungsten, silver, and sulfur. DLC comprises a family of such materials, the properties of
which can be tailored far more readily than those of diamond. Furthermore, DLC film
comprises a mixture of sp
2
and sp
3
carbon bonds and is deposited by using high-energy
carbon species. DLC films can be synthesized by a variety of techniques such as ion beam
deposition [28], radio frequency plasma-enhanced CVD [28], filtered cathodic vacuum arc
[29], ion plating [30], plasma immersion ion implantation and deposition [31], magnetron
sputtering [32], ion beam sputtering [33], pulsed laser deposition [34], and mass-selected
ion beam deposition [35]. The hydrogen content in DLC films varies up to 40%. Because
of its amorphous structure, DLC films can be easily doped and alloyed with different ele-
ments. This leads to a wide range of properties depending on its sp
3
, sp
2
, and hydrogen
content together with element incorporation.
Carbon Nanotubes
Since the discovery of CNTs, it has revolutionized the biomedical research as they can
show superior performance because of their impressive structural, mechanical, and elec-
tronic properties such as small size and mass, high strength, higher electrical, and thermal
conductivity [36-38]. They are hexagonal networks of carbon atoms possessing a diameter
of about 1 nm and 1-100 nm in length and can essentially be thought of as a layer of graph-
ite rolled up into a cylinder [39]. Furthermore, CNTs are of two types: single-walled nano-
tubes (SWNTs) and multiwalled nanotubes, and they differ in the arrangement of their
graphene cylinders. SWNTs have only one single layer of graphene cylinders, whereas
multiwalled nanotubes have several layers (approximately 50), as shown in Figure 3.3a
[40]. Furthermore, the films of synthesized CNTs can be aligned or random in nature [36].
© 2011 by Taylor & Francis Group, LLC
