Figure 4. Effect of nanoribbon length on the thermal conductivity of the
monolayer graphene nanoribbon with different transverse edge termination
states.
The low-resolution scanning electron micrographs of the graphene
nanoribbons are illustrated in Figure 5 for research purposes of thermal
transport characteristics. The graphene nanoribbons have aspect ratios
of at least 5 and the average aspect ratio of the graphene nanoribbons
in the plurality of nanoribbons is at least 10 and, in some cases, at
least 20. The graphene nanoribbons comprise two edges that run
substantially parallel along the length of the nanoribbon and they have
the armchair crystallographic direction of graphene running parallel to
the nanoribbon edges. They have carbon-carbon bonds running parallel to
the long nanoribbon axis. Because the graphene nanoribbons are grown,
rather than patterned lithographically from graphene sheets, they are
formed with atomically smooth edges. The degree of edge smoothness can
be characterized by the average root mean square roughness of the edges
of the nanoribbons in the array. The rms edge roughness along the length
of a nanoribbon can be measured using scanning tunneling microscopy. The
average rms edge roughness for the nanoribbons in a nanoribbon array
will vary since longer nanoribbons within the array will tend to have
rougher edges. Graphene nanoribbons outperform conventional materials
and lead to next-generation technologies. Graphene nanoribbons offer
tremendous promise for providing enhanced thermal transport performance.
However, the full potential of graphene nanoribbons in such applications
has not been realized. A major challenge facing graphene
nanoribbon-based devices is that scalable approaches to create
high-quality graphene nanoribbons with atomically-smooth edges are
lacking. Conventional, top-down techniques in which graphene nanoribbons
are etched from continuous graphene sheets result in structures with
rough, disordered edges that are riddled with defects, which
significantly degrade graphene's exceptional properties. This blunt
top-down etching can be avoided by synthesizing nanoribbons from the
bottom-up. For instance, organic synthesis can yield ribbons with smooth
edges, defined widths, and complex architectures. However, organic
synthesis forms short nanoribbons and is not adapted to technologically
relevant substrates, such as insulators or semiconductors, limiting its
potential for commercial development. Optionally, nucleation sites can
be introduced into or onto the germanium growth surface in order to
control the locations at which graphene nanoribbon growth originates and
the time at which graphene nanoribbon growth begins.