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.