Page 107 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanomaterials Fabrication 93

Figure 3.47 SEM image of a “carpet” of SWNTs grown perpendicular to the
surface of the support.

smooth sides of the SWNTs forming bundles or ropes with a van der
Waals binding energy of approximately 500 eV per
m of tube contact
[191]. The van der Waals force between the tubes is so great that it takes
tremendous energy to pry them apart. The insolubility of nanotubes
makes it very difficult to make combinations of nanotubes with other
materials, such as in composite applications. The functionalization of
nanotubes—that is, the attachment of “chemical functional groups”—
provides a strategy for overcoming those barriers. Functionalization can
improve solubility and processibility, and will be able to link the unique
properties of nanotubes to those of other materials. Through the chem-
ical functional groups, nanotubes might take the interaction with other
entities, such as solvents, polymer, nanoparticles, and other nanotubes.
In functionalization of SWNTs, a distinction should be made between
covalent and noncovalent functionalization. Covalent functionalization
shows covalent linkage of functional groups onto the surface of nan-
otubes, either the sidewall or the cap of nanotubes. It is important to
note that covalent functionalization methods have one problem in
common: extensive covalent functionalization modifies SWNT proper-
ties by disrupting the continuous –system of SWNTs.
Current methods for solubilizing nanotubes without covalent function-
alization include highly aromatic solvents, super acids [192], DNA [193],