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.