modulus commercial ferroelectrics (@40 MPa). However, the ability to generate
stress is still >100 times lower than that predicted for nanotube fibers with the
modulus of the individual SWNTs. The success of actuator technology based on
carbon nanotubes will depend on improvements in the mechanical properties of
nanotube sheets and fibers with a high surface area by increasing nanotube alignment and the binding between nanotubes. The use of nanotubes as electrodes in
lithium batteries is a possibility because of the high reversible component of storage capacity at high discharge rates. The maximum reported reversible capacity is
1000 mA h g
À1 for SWNTs that are mechanically milled in order to enable the filling of nanotube cores, as compared to 372 mA h g
À1 for graphite [154] and 708
mA h g
À1 for ball-milled graphite [155].
Nanometer-Sized Electronic Devices The possible use of carbon nanotubes in
nanoelectronics has aroused considerable interest. Dramatic recent advances have
fueled speculation that nanotubes (SWNTs) will be useful for downsizing circuit
dimensions. Because of their unique electronic properties, SWNTs can be interfaced with other materials to form novel heterostructures [156]. The simplest device one can imagine with carbon nanotubes is that involving a bend or a kink,
arising from the presence of a diametrically opposite pentagon–heptagon pair. The
resultant junction connects two nanotubes of different chirality and hence of different electronic structure, leading to the realization of an intramolecular device.
Such a device in SWNTs is found to behave like a diode rectifier [157]. Silicon
nanowire–carbon nanotube heterojunctions do indeed exhibit a rectification behavior [158].
The current-induced electromigration causes conventional metal wire interconnects to fail when the wire diameter becomes too small. The covalently bonded
structure of carbon nanotubes militates against similar breakdown of nanotube
wires and, because of ballistic transport, the intrinsic resistance of the nanotube
should essentially vanish. Experimental results show that metallic SWNTs can
carry up to 10
9 A cm
À2 , whereas the maximum current densities for normal metals are 10
5 A cm
À2 [145, 159]. Unfortunately, the ballistic current carrying capability is less useful for presently envisioned applications because of necessarily large
contact resistances. An electronic circuit involving electrical leads to and from a
SWNT will have a resistance of at least h=4e
2 or 6.5 kW, where h is Planck’s constant and e is the charge of an electron [160]. Contacting all layers in a MWNT
could reduce this contact resistance, but it cannot be totally eliminated. In nanotube field effect transistors (NT-FETs), gating has been achieved by applying a voltage to a submerged gate beneath a SWNT (Figure 8.12(a) and (b)), which was
contacted at opposite nanotube ends by metal source and drain leads [161]. A
typical nanoelectronic device of NT-FET consists of a semiconducting nanotube,
which is on top of an insulating aluminum oxide layer, connected at both ends to a
gold electrode. The nanotube is switched by applying a potential to the aluminum
gate under the nanotube and aluminum oxide. The transistors were fabricated by
lithographically applying electrodes to the nanotubes that were either randomly
distributed on a silicon substrate or positioned on the substrate with an atomic
8.2 Carbon Nanotubes 229
stress is still >100 times lower than that predicted for nanotube fibers with the
modulus of the individual SWNTs. The success of actuator technology based on
carbon nanotubes will depend on improvements in the mechanical properties of
nanotube sheets and fibers with a high surface area by increasing nanotube alignment and the binding between nanotubes. The use of nanotubes as electrodes in
lithium batteries is a possibility because of the high reversible component of storage capacity at high discharge rates. The maximum reported reversible capacity is
1000 mA h g
À1 for SWNTs that are mechanically milled in order to enable the filling of nanotube cores, as compared to 372 mA h g
À1 for graphite [154] and 708
mA h g
À1 for ball-milled graphite [155].
Nanometer-Sized Electronic Devices The possible use of carbon nanotubes in
nanoelectronics has aroused considerable interest. Dramatic recent advances have
fueled speculation that nanotubes (SWNTs) will be useful for downsizing circuit
dimensions. Because of their unique electronic properties, SWNTs can be interfaced with other materials to form novel heterostructures [156]. The simplest device one can imagine with carbon nanotubes is that involving a bend or a kink,
arising from the presence of a diametrically opposite pentagon–heptagon pair. The
resultant junction connects two nanotubes of different chirality and hence of different electronic structure, leading to the realization of an intramolecular device.
Such a device in SWNTs is found to behave like a diode rectifier [157]. Silicon
nanowire–carbon nanotube heterojunctions do indeed exhibit a rectification behavior [158].
The current-induced electromigration causes conventional metal wire interconnects to fail when the wire diameter becomes too small. The covalently bonded
structure of carbon nanotubes militates against similar breakdown of nanotube
wires and, because of ballistic transport, the intrinsic resistance of the nanotube
should essentially vanish. Experimental results show that metallic SWNTs can
carry up to 10
9 A cm
À2 , whereas the maximum current densities for normal metals are 10
5 A cm
À2 [145, 159]. Unfortunately, the ballistic current carrying capability is less useful for presently envisioned applications because of necessarily large
contact resistances. An electronic circuit involving electrical leads to and from a
SWNT will have a resistance of at least h=4e
2 or 6.5 kW, where h is Planck’s constant and e is the charge of an electron [160]. Contacting all layers in a MWNT
could reduce this contact resistance, but it cannot be totally eliminated. In nanotube field effect transistors (NT-FETs), gating has been achieved by applying a voltage to a submerged gate beneath a SWNT (Figure 8.12(a) and (b)), which was
contacted at opposite nanotube ends by metal source and drain leads [161]. A
typical nanoelectronic device of NT-FET consists of a semiconducting nanotube,
which is on top of an insulating aluminum oxide layer, connected at both ends to a
gold electrode. The nanotube is switched by applying a potential to the aluminum
gate under the nanotube and aluminum oxide. The transistors were fabricated by
lithographically applying electrodes to the nanotubes that were either randomly
distributed on a silicon substrate or positioned on the substrate with an atomic
8.2 Carbon Nanotubes 229