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.