Biomedical Engineering Reference
In-Depth Information
base-catalyzed hydrolysis reactions, and networks of O-Si-O linkages are formed
in subsequent condensation reactions. After this step, the treatment of the sol is
varied depending on the fi nal products desired. For example, spinning or dipping
techniques can create thin fi lm coating, and the exposure of the sol to a surfactant
can lead to powders. Depending on the water-alkoxide molar ratio
R
, the pH, the
temperature, and the type of solvent chosen, additional condensation steps can lead
to different polymeric structures, such as linear, entangled chains, clusters, and col-
loidal particles. In some cases, the resulting sol is cast into a mold and dried to
remove the solvent. This leads to the formation of a solid structure in the shape of
the mold (e.g., aerogels and xerogels) with large surface-to-volume ratios, high pore
connectivity, and narrow pore size distribution. They can be doped with a variety of
organic/inorganic materials during the mixing stage to target specifi c applications.
1.2.2
Chemical Vapor Deposition
The CVD process is now probably the most common of all bottom-up approaches. It
is used today to grow structures, like nanotubes, nanowires, and nanoparticles aided
by several different types of chambers and growth-enhancing methods. The process
consists of decomposing a gaseous precursor that adheres and accumulates onto a
substrate (i.e., a silicon wafer or a quartz slide) (Jensen
1989
). The presence of a cata-
lyst, either predeposited on the substrate or provided in the gas feedstock, activates
the chemical reaction between the substrate surface and the gaseous precursor. The
CVD reaction can be achieved either with temperature (thermal CVD) or with plasma
(PECVD). Plasmas can be obtained with DC electric fi elds, RF fi elds, or microwave
fi elds, and their presence allows decreasing signifi cantly the process temperature
compared to the thermal CVD process. The presence of plasma also enables a more
aligned or directional growth of the desired nanomaterials. A number of forms of
CVD systems are in wide use and are frequently referenced in the literature. Some
examples include atomic-layer CVD (ALCVD) in which two complementary pre-
cursors [e.g., Al(CH
3
)
3
and H
2
O] are alternatively introduced into the reaction cham-
ber; metal-organic CVD (MOCVD) in which metal-organic precursors are used to
obtain specifi c crystalline structures [e.g., tantalum ethoxide, Ta(OC
2
H
5
)
5
, to create
TaO nanostructures, and tetra dimethyl amino titanium (TDMAT) to create TiN];
laser-assisted CVD (LCVD); rapid thermal CVD (RTCVD) that uses heating lamps
or other methods to rapidly heat the wafer substrate; ultrahigh vacuum CVD
(UHVCVD); and more (Adams
1988
; Jensen
1989
) .
The CVD process is widely used to produce CNTs and semiconductor nano-
wires, such as Si, GaN, and ZnO. CNTs have received much attention in the recent
years for their potential application in several fi elds of bioengineering, from
enhanced cell growth to biosensing, biomanipulation, and drug delivery. The most
common synthesis routes for CNTs are CVD processes because they allow large-
scale production of CNTs with high purity and good yield. To clarify how the CVD
process works, we show a schematic diagram of a thermal CVD growth system used
for the synthesis of CNTs (Fig.
7
). In this example, a carbon-containing precursor is
