Electrochemical sensors based on carbon nanotubes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 462

15.2.2 Properties of CNTs
The carbon nanotubes possess such properties as high conductivity, excellent strength
and stiffness, and chemical inertness. CNTs also show unusual electronic characteris-
tics that are dependent on lattice helicity and elasticity. The density of SWNTs is esti-
mated to be smaller (0.6 g/cm 3 ) than graphite due to the presence of hollow channels
in the center of CNTs. As expected for nano-sized materials, the surface area of CNTs
is very large, e.g. 10-20 m 2 /g for MWNTs and the value of SWNTs is expected to be

an order of magnitude higher. Some detailed discussion of the mechanical, electronic,
and chemical properties of CNTs can be found in the following sections.

15.2.2.1 Mechanical properties
The strength of the C ß C covalent bond, which is one of the strongest in nature, makes
CNTs one of the strongest and stiffest materials. Treacy et al. estimated the elastic
modulus of CNTs to be in the terapascal range by measuring their thermal vibrational
amplitudes using TEM [20]. Because of the hollow structure and closed topology,
CNTs can sustain extreme strains (40%) in tension without showing plastic deforma-
tion of bond rupture [21]. Under strain, some local bonds are broken, but this local
defect is redistributed over the entire surface due to the mobility of these defects. This
process changes the helicity of CNTs and eventually affects its electronic property.

15.2.2.2 Electronic properties
The electronic properties of CNTs can be related to their structures. In particular, both
Wildöer et al. and Odom et al. explained the relationship between the structure and
electronic conductivity of SWNTs using STM/STS images and current voltage curves
obtained by tunneling spectroscopy on individual CNTs. Their studies indicate that
so-called armchair tubes are metallic, and the zigzag tubes and chiral tubes are either
metallic or semiconducting depending on the wrapping angle and the length of CNTs.
Nevertheless, the SWNT samples exhibit many different structures with no one species
dominating. Four-probe measurements of MWNTs reveal that the electrical conductiv-
ity of individual MWNTs is metallic, semiconducting or semimetallic [8]. The electri-
cal conductivity of MWNTs becomes metallic by doping with boron and nitrogen [22],
and the conductivity of SWNTs becomes an order of magnitude higher by intercalating
the CNT tubes with alkali and halogen dopants.

15.2.2.3 Chemical properties
It has been known that the basal graphite plane (graphene hexagon) is chemically inert.
However, CNTs are susceptive to some chemical reactions due to the

-orbital mis-
match in the curvature structures. Oxidation studies have revealed that the tips (caps)
of CNTs are more reactive than the cylindrical parts [8, 20]. Ab initio calculations
indicate that the average charge density of a pentagon (at the tips) is 3-4 times larger

π