particle. This is why the diameter of the metal particle determines the
diameter of the nanotube being grown.
There are two suggested growth mechanisms for nanotube growth from a
metal catalyst. The first method is root growth in which the nanotube
grows upward from the metal catalyst particle, which stays attached to the
substrate. The second suggested growth mechanism is tip growth in
which the nanotube grows from the catalyst downward into the substrate.
This pushes the catalyst up and detaches it from the substrate (Sinnot et
al., 1999). The two mechanisms are illustrated in Figure 9.24.
9.7 GRAPHENE
Carbon nanotubes are not the only important form of nanostructured
carbon. Networks of sp
2 carbon can also be assembled in large, planar
sheets that can be macroscopic dimension along two axes but are only
one atom deep, essentially a single layer of graphite. Such atomically thin
layers of carbon are called graphene. As electrons are confined along only
one axis, graphene is a 2D nanomaterial.
Graphene can be synthesized by vapor deposition techniques that are
similar to those used for carbon nanotubes, but on catalysts with a planar
surface geometry. It can also be produced by exfoliation, in which a single
layer is lifted off a piece of bulk graphite after adhesion to a substrate,
which can be as simple as a piece of cellophane tape. Finally, graphene
can also be synthesized in solution by reducing graphene oxide (produced from partial oxidization of graphite) or merging other types of
polycyclic aromatic hydrocarbons (Allen et al., 2010). Synthesis of
graphene from graphene oxide is shown in Figure 9.25.
Like carbon nanotubes, graphene has many potential uses. It has very
high tensile strength and controllable electrical and thermal conductivity.
Conductive varieties of graphene can be used to form thin, transparent
electrodes for semiconductor devices in place of typical inorganic transparent electrode materials such as indium tin oxide (ITO). Graphene
surfaces can also be functionalized with other compounds via partial
oxidization to graphene oxide. Graphene can also be formed into strong
but highly porous structures, making it useful as a dielectric for highenergy capacitors. It has also been examined as a gate material for
nanoscale field-effect transistors; an example of such a field-effect transistor produced by Kaner and coworkers (Tung et al., 2008) is shown in
Figure 9.26.
CHAPTER 9: Introduction to Functional Nanomaterials
376
diameter of the nanotube being grown.
There are two suggested growth mechanisms for nanotube growth from a
metal catalyst. The first method is root growth in which the nanotube
grows upward from the metal catalyst particle, which stays attached to the
substrate. The second suggested growth mechanism is tip growth in
which the nanotube grows from the catalyst downward into the substrate.
This pushes the catalyst up and detaches it from the substrate (Sinnot et
al., 1999). The two mechanisms are illustrated in Figure 9.24.
9.7 GRAPHENE
Carbon nanotubes are not the only important form of nanostructured
carbon. Networks of sp
2 carbon can also be assembled in large, planar
sheets that can be macroscopic dimension along two axes but are only
one atom deep, essentially a single layer of graphite. Such atomically thin
layers of carbon are called graphene. As electrons are confined along only
one axis, graphene is a 2D nanomaterial.
Graphene can be synthesized by vapor deposition techniques that are
similar to those used for carbon nanotubes, but on catalysts with a planar
surface geometry. It can also be produced by exfoliation, in which a single
layer is lifted off a piece of bulk graphite after adhesion to a substrate,
which can be as simple as a piece of cellophane tape. Finally, graphene
can also be synthesized in solution by reducing graphene oxide (produced from partial oxidization of graphite) or merging other types of
polycyclic aromatic hydrocarbons (Allen et al., 2010). Synthesis of
graphene from graphene oxide is shown in Figure 9.25.
Like carbon nanotubes, graphene has many potential uses. It has very
high tensile strength and controllable electrical and thermal conductivity.
Conductive varieties of graphene can be used to form thin, transparent
electrodes for semiconductor devices in place of typical inorganic transparent electrode materials such as indium tin oxide (ITO). Graphene
surfaces can also be functionalized with other compounds via partial
oxidization to graphene oxide. Graphene can also be formed into strong
but highly porous structures, making it useful as a dielectric for highenergy capacitors. It has also been examined as a gate material for
nanoscale field-effect transistors; an example of such a field-effect transistor produced by Kaner and coworkers (Tung et al., 2008) is shown in
Figure 9.26.
CHAPTER 9: Introduction to Functional Nanomaterials
376