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.