Figure 6. High-resolution scanning electron micrographs of the graphene
nanoribbons for research purposes of thermal transport characteristics.
The effect of temperature on the thermal conductivity of the monolayer
graphene nanoribbon is illustrated in Figure 7 with different transverse
edge termination states. Graphene is a two-dimensional carbon allotrope,
the electronic, and magnetic properties of which can be tuned by
engineering two-dimensional graphene sheets into one-dimensional
structures with confined width, known as graphene nanoribbons. The
properties of graphene nanoribbons are highly dependent on their width
and edge structure. Graphene nanoribbons possess a number of useful
properties, including, for example, beneficial electrical properties.
Unlike carbon nanotubes, which can be metallic, semi-metallic or
semiconducting depending on their chiral geometry and diameter, the
electrical properties of graphene nanoribbons are governed by their
width and their edge configurations and functionalization. For example,
graphene nanoribbons of less than about 10 nanometers in width are
semiconductors, whereas similar graphene nanoribbons having a width
greater than about 10 nanometers are metallic or semi-metallic
conductors. The edge configurations of graphene nanoribbons having an
armchair or zigzag arrangement of carbon atoms, along with the terminal
edge functional groups, are also calculated to affect the transmission
of electron carriers. Such armchair and zigzag arrangements are
analogous to those defined in carbon nanotubes. In addition to the
electrical properties, graphene nanoribbons maintain many of the
desirable mechanical properties that carbon nanotubes and graphene
sheets also possess. Various methods for making graphene sheets are
known, including, for example, adhesive tape exfoliation of individual
graphene layers from graphite, chemical-based exfoliation of graphene
layers from graphite, and chemical vapor deposition processes, each
process providing on the order of picogram quantities of graphene [41,
42]. Several lithographic and synthetic procedures have been developed
for producing minuscule amounts of graphene nanoribbons [43, 44].
Microscopic quantities of graphene nanoribbons have been produced by
partial encapsulation of carbon nanotubes in a polymer, followed by
plasma etching to longitudinally cut the carbon nanotubes [45, 46].
Macroscopic quantities of graphene nanoribbons have also been produced
by a chemical vapor deposition process [47, 48]. Graphene represents
an atomically thin layer of carbon in which the carbon atoms reside at
regular two-dimensional lattice positions within a single sheet or a few
stacked sheets of fused six-membered carbon rings. In its various forms,
this material has garnered widespread interest for use in a number of
applications, primarily due to its favorable combination of high
electrical and thermal conductivity values, good mechanical strength,
and unique electronic properties. However, an advantage of graphene over
carbon nanotubes is that graphene can generally be produced in bulk much
more inexpensively than can the latter, thereby addressing perceived
supply and cost issues that have been commonly associated with carbon
nanotubes. Despite the fact that graphene is generally synthesized more
easily than are carbon nanotubes, there remain issues with production of
graphene in quantities sufficient to support various commercial
operations. Scalability to produce large area graphene films represents
a particular problem. The most scalable processes developed to date for
making graphene films utilize chemical vapor deposition technology.
Graphene nanoribbons prepared by these processes are typically
characterized by multiple graphene layers with a kinked morphology and
irregular atomic structure. Graphene nanoribbons are a single or a few
layers of the well-known carbon allotrope graphitic carbon, which
possesses exceptional electrical and physical properties which may lead
to various applications. Graphene nanoribbons structurally have high
aspect ratio with length being much longer than the width or thickness.
Graphene nanoplatelets are similar to graphene nanoribbons except that
that the length is in the micron or sub-micron range and hence graphene
nanoplatelets lack the high aspect ratio of graphene nanoribbons.
Graphene nanoplatelets also possess many of the useful properties of
carbon nanotubes and graphene nanoribbons. Graphene nanoribbons are
prepared by chemical vapor deposition and from graphite using chemical
processes. Most typically, graphene nanoribbons are prepared from carbon
nanotubes by chemical unzipping and the quality of graphene nanoribbons
depends the purity of the carbon nanotube starting material.