niques in a TEM, Bower et al. [128] observed alkali metal intercalation into the
SWNTs. Sealed tube reactions of SWNTs and metal salts are also known to yield
metal intercalated SWNTs [129]. In an effort to realize the conversion of sp
2 carbon of nanotubes into sp
3 , Hsu et al. [130] treated potassium intercalated MWNTs
with CCl 4 hydrothermally, and obtained crystallization of KCl inside the nanotubes
and within the tube walls. Possible ways of closing the nanotubes, opened by oxidants, have been examined [127]. Besides opening, filling and closing nanotubes,
highly functionalized MWNTs have been prepared by treatment with acids [127,
131]. SWNTs are readily opened by mild treatment with acids and filled with metals [45, 132]. Acid-treated nanotube surfaces can be decorated by nanoparticles of
metals such as Au, Ag or Pt [45, 133].
Chen et al. [134] have derivatized SWNT fragments with halogen and amine
moieties in order to dissolve them in organic solvents. Doping has been carried out
to modify their electronic properties in the solution phase. Fluorination of SWNTs
has been carried out by Mickelson et al. [135]. Fluorinated nanotubes can be solvated in alcohol media and precipitated back by reaction with hydrazine. STM
studies of fluorinated SWNTs reveal an interesting banded structure followed by
atomically resolved regions, indicating sidewall functionalization [136]. Starting
from fluorinated SWNTs, Boul et al. [137] have carried out alkylation by reaction with alkylmagnesium bromides or alkylithium. Individual SWNTs have been
deposited controllably on chemically functionalized nanolithographic templates
[138].
8.2.5
Electronic Structure, Properties and Devices
8.2.5.1 Electronic Structure and Properties
As with the fullerenes, the curvature of the graphitic sheets in the nanotubes
would be expected to influence the electronic structure. The electronic properties
of perfect MWNTs are rather similar to those of perfect SWNTs, because the coupling between the cylinders is weak in MWNTs. Calculations show that nanotubes
may be as good conductors as copper, although a combination of the degree of
helicity and the number of six-membered rings per turn around the tube can
tune the electronic properties in the metal–semiconductor range [139–141]. Lowtemperature STM and scanning tunneling spectroscopy (STS) studies of SWNTs
reveal the atomically resolved images of the graphene cylinders and their sizespecific transport properties [142, 143], in agreement with theoretical predictions.
Collins et al. [144] used STM to explore local electrical characteristics of SWNTs.
Well-defined positions where the current changes abruptly from a graphite-like response to a highly nonlinear response, were found, including near-perfect rectification. STM studies conducted in our laboratory in ultra-high vacuum (UHV)
show a variable conductivity and gap along the length of the nanotubes. Because of
the nearly one-dimensional electronic structure, electron transport in metallic
SWNTs and MWNTs occurs ballistically over long nanotube lengths, enabling
them to carry high currents with essentially no heating [145, 146]. Phonons also
8.2 Carbon Nanotubes 227
SWNTs. Sealed tube reactions of SWNTs and metal salts are also known to yield
metal intercalated SWNTs [129]. In an effort to realize the conversion of sp
2 carbon of nanotubes into sp
3 , Hsu et al. [130] treated potassium intercalated MWNTs
with CCl 4 hydrothermally, and obtained crystallization of KCl inside the nanotubes
and within the tube walls. Possible ways of closing the nanotubes, opened by oxidants, have been examined [127]. Besides opening, filling and closing nanotubes,
highly functionalized MWNTs have been prepared by treatment with acids [127,
131]. SWNTs are readily opened by mild treatment with acids and filled with metals [45, 132]. Acid-treated nanotube surfaces can be decorated by nanoparticles of
metals such as Au, Ag or Pt [45, 133].
Chen et al. [134] have derivatized SWNT fragments with halogen and amine
moieties in order to dissolve them in organic solvents. Doping has been carried out
to modify their electronic properties in the solution phase. Fluorination of SWNTs
has been carried out by Mickelson et al. [135]. Fluorinated nanotubes can be solvated in alcohol media and precipitated back by reaction with hydrazine. STM
studies of fluorinated SWNTs reveal an interesting banded structure followed by
atomically resolved regions, indicating sidewall functionalization [136]. Starting
from fluorinated SWNTs, Boul et al. [137] have carried out alkylation by reaction with alkylmagnesium bromides or alkylithium. Individual SWNTs have been
deposited controllably on chemically functionalized nanolithographic templates
[138].
8.2.5
Electronic Structure, Properties and Devices
8.2.5.1 Electronic Structure and Properties
As with the fullerenes, the curvature of the graphitic sheets in the nanotubes
would be expected to influence the electronic structure. The electronic properties
of perfect MWNTs are rather similar to those of perfect SWNTs, because the coupling between the cylinders is weak in MWNTs. Calculations show that nanotubes
may be as good conductors as copper, although a combination of the degree of
helicity and the number of six-membered rings per turn around the tube can
tune the electronic properties in the metal–semiconductor range [139–141]. Lowtemperature STM and scanning tunneling spectroscopy (STS) studies of SWNTs
reveal the atomically resolved images of the graphene cylinders and their sizespecific transport properties [142, 143], in agreement with theoretical predictions.
Collins et al. [144] used STM to explore local electrical characteristics of SWNTs.
Well-defined positions where the current changes abruptly from a graphite-like response to a highly nonlinear response, were found, including near-perfect rectification. STM studies conducted in our laboratory in ultra-high vacuum (UHV)
show a variable conductivity and gap along the length of the nanotubes. Because of
the nearly one-dimensional electronic structure, electron transport in metallic
SWNTs and MWNTs occurs ballistically over long nanotube lengths, enabling
them to carry high currents with essentially no heating [145, 146]. Phonons also
8.2 Carbon Nanotubes 227