Figure 7. Effect of temperature on the thermal conductivity of the monolayer graphene nanoribbon with different transverse edge termination states.
The effect of the number of layers on the thermal conductivity of the monolayer graphene nanoribbon is illustrated in Figure 8 with different transverse edge termination states. Graphene typically refers to a material having less than about 10 graphitic layers. The graphitic layers are characterized by an infinite two-dimensional basal plane having a hexagonal lattice structure and various edge functionalities, which may include, for example, carboxylic acid groups, hydroxyl groups, epoxide groups and ketone groups. Graphene nanoribbons are a special class of graphene, which are similarly characterized by a two-dimensional basal plane, but with a large aspect ratio of their length to their width. In this regard, graphene nanoribbons bear similarity to carbon nanotubes, which have a comparable large aspect ratio defined by one or more layers of graphene sheets rolled up to form a cylinder. The graphene nanoribbons can include various layers. For instance, the graphene nanoribbons may include a single layer. In some cases, the graphene nanoribbons may include a plurality of layers. In some cases, the graphene nanoribbons include from about one layer to about eight layers. In some cases, the graphene nanoribbons include from about second layers to about ten layers. In some cases, the graphene nanoribbon layers have interlayer spacings of more than about 0.2 nanometers. In some cases, the graphene nanoribbon layers have interlayer spacings of 0.34 nanometers or larger. The graphene nanoribbons may be derived from various carbon sources. For instance, the graphene nanoribbons may be derived from carbon nanotubes, such as multi-walled carbon nanotubes. In some cases, the graphene nanoribbons are derived through the longitudinal splitting of carbon nanotubes. Various methods may be used to split carbon nanotubes to form graphene nanoribbons. Carbon nanotubes may be split by exposure to potassium, sodium, lithium, alloys thereof, metals thereof, salts thereof, and combinations thereof. For instance, the splitting may occur by exposure of the carbon nanotubes to a mixture of sodium and potassium alloys, a mixture of potassium and naphthalene solutions, and combinations thereof. In some cases, the graphene nanoribbons are made by the longitudinal splitting of carbon nanotubes using oxidizing agents or by the longitudinal opening of carbon nanotubes. The thermally conductive materials may also utilize various graphene nanoribbons. For instance, the graphene nanoribbons include, without limitation, functionalized graphene nanoribbons, pristine graphene nanoribbons, doped graphene nanoribbons, graphene oxide nanoribbons, reduced graphene oxide nanoribbons, reduced graphene oxide flakes, and combinations thereof.