Figure 5. Low-resolution scanning electron micrographs of the graphene-carbon nanotube hybrid material for the production of fiber-reinforced polymer composites.
The high-resolution scanning electron micrographs of the graphene-carbon nanotube hybrid material are illustrated in Figure 6 for the production of fiber-reinforced polymer composites. A key difficulty of using graphene-carbon nanotubes in many applications is their poor adhesion to the substrate which can give rise to reliability issues and also compromise good electrical contacts. In chemical vapor deposition of both graphene and carbon nanotube, a metal catalyst is used which is susceptible to environmental poisoning, such as oxidation, prior to the growth process and hence degrades the material properties. The poisoned catalyst may further poison underneath materials. Furthermore, many in-situ graphene and carbon nanotubes-based device fabrication processes involve patterning where etching is performed. In a buried catalyst arrangement, the catalyst is also attacked by etchants during the etching process. Protection of the catalyst film from etchants attack, process poisoning, and growth of reliably attached material with the substrate is highly favorable for the applications of graphene-carbon nanotubes in various areas. The techniques may include chemically doping the cleaned carbon nanotube-graphene hybrid film to increase conductivity. A carbon nanotube film can be a mixture of semiconducting and metallic carbon nanotubes. The doping permanently increases the charge concentration in semiconducting carbon nanotubes present in the film, thereby decreasing the sheet resistance of the network. The doping step also increases the electrical performance of the film. Doping the nanotube-graphene hybrid film can include using a solution doping technique. Carbon nanotubes can be doped in solution before getting deposited over the substrate. Similarly, solution suspended graphene oxide flakes can be doped before getting deposited over carbon nanotubes. The dopants can be acid solutions such as nitric acid and sulfuric acid, or the dopants can be metal-organic compounds which can form charge-transfer complexes with the bonded carbon atoms in carbon nanotube and graphene. The resultant structure can appear as nanotubes scattered over or under a single or multiple large area graphene sheet reducing the sheet resistance of graphene. Doping is preferably conducted in solution phase, although gas phase doping is also feasible. For solution processes, organic solvents such as dichlorobenzene, dichloromethane, ethanol, acetonitrile, chloroform, methanol, butanol, among others, are suitable. Doping can be accomplished via charge transfer from the dopants to the nano-components, for example, interaction of the lone electron pairs of doping molecules with the quantum confined orbitals of semiconductor nanowires and nanocrystals which affects the concentration of carriers involved in charge transport.