Figure 5. Low-resolution scanning electron micrographs of the graphene
nanoribbons for research purposes of thermal transport characteristics.
The high-resolution scanning electron micrographs of the graphene
nanoribbons are illustrated in Figure 6 for research purposes of thermal
transport characteristics. The methods for preparing graphene
nanoribbons described herein take place either in a liquid medium or on
a surface. Without being bound by theory or mechanism, it is though that
when a free-standing graphene sheet is in solution, the excess surface
energy may be stabilized by solvation energy such that folding into a
carbon nanotube becomes energetically unfavorable. As a result of the
solvation energy, the reverse process of longitudinally opening a carbon
nanotube into a graphene nanoribbon becomes energetically favorable in
an appropriate liquid medium. According to current understanding, the
oxidative longitudinal opening of carbon nanotubes appears to occur
along a line to afford predominantly straight-edged oxidized graphene
nanoribbons. Graphene nanoribbons may be incorporated into organic and
inorganic matrices such as, for example, polymer matrices. The polymer
matrices can include, without limitation, thermoplastic and
thermosetting polymer matrices. Incorporation of graphene nanoribbons
may improve mechanical properties of the polymer composites. In some
cases, polymer membranes including graphene nanoribbons may be prepared
which are useful for fluid separations, antistatic applications, or
electromagnetic shielding materials. The graphene nanoribbons may be
dispersed as individuals in the polymer matrices. The graphene
nanoribbons may also be aggregated together in two or more layers in the
polymer matrices. The graphene nanoribbons may be covalently bonded to
the polymer matrices. For example, carboxylic acid groups of graphene
nanoribbons may be utilized for making cross-linked polymer composites
in which the graphene nanoribbons are covalently bonded to the polymer
matrix. Other functional groups in the graphene nanoribbons may be
utilized as well for making cross-linked polymer composites. In other
cases, the graphene nanoribbons are not covalently bonded to the polymer
matrices. As a non-limiting example of composite materials, reinforced
rubber composites including graphene nanoribbons may be used to
manufacture gaskets and seals with improved tolerance to explosive
decompression. The graphene nanoribbons can be deposited from a mixture
of methane gas and hydrogen gas. By varying the composition of the
precursor gas mixture during growth, the duration of the growth time,
and the growth temperature, the graphene nanoribbon width, length, and
aspect ratio can be controlled. This control over the nanoribbon
structure makes it possible to tune the graphene properties. For
example, graphene undergoes a metallic-to-semiconducting transition as
the nanoribbon width decreases, wherein the induced bandgap is inversely
proportional to the nanoribbon width. Therefore, the present approach
makes it possible to control the width of the nanoribbons and,
therefore, to tailor their electronic structure. By tuning the precursor
composition and growth time, nanoribbons with widths below the current
lithography resolution can be achieved. Key parameters for realizing
anisotropic growth are the mole fractions of the precursor molecules and
the carrier molecules used in the chemical vapor deposition gas mixture,
where the mole fractions can be adjusted by adjusting the partial
pressures of the precursor and carrier gases. However, these parameters
are not independent, so the optimum value for one of the parameters will
depend on the others. The growth time also plays a role in determining
the dimensions of the chemical vapor deposition-grown graphene
nanoribbons. Generally, as growth time is decreased, narrower, shorter
nanoribbons are formed. Therefore, by tuning the duration of the growth
time and the ratio of precursor gas to carrier gas in the gas mixture,
nanoribbons with desired lengths and widths can be selectively grown
using bottom-up chemical vapor deposition growth. The optimum conditions
for achieving anisotropic graphene growth may vary somewhat depending
upon the laboratory conditions. For example, in a cleaner environment,
the growth rate at a given set of conditions would be expected to be
slower than in a dirtier environment. Therefore, to achieve the same low
growth rate observed under standard laboratory conditions in a cleaner
system, such as a clean room, a higher methane mole fraction and a lower
hydrogen mole fraction could be used.