turn indicates the existence of intramolecular junctions in the carbon nanotubes.
The spectrum of interesting findings discussed above opens up the possibility of
assembling carbon nanotubes, possessing such novel device-like properties [157,
161, 162, 172] into multi-functional circuits, and ultimately towards the realization
of a carbon-nanotube-based computer chip. Rueckes et al. [166] have recently described the concept of carbon-nanotube-based nonvolatile random access memory
for molecular computing. The viability of the concept has also been demonstrated.
Integrated nanotube devices involving two nanotube transistors have been reported
[163, 166], providing visions of large-scale integration. Patterned growth of SWNTs
on a 4 in (10 cm) silicon wafer [173] may prove an important step toward integrated nanotube electronics. IBM expects that nanotube electronics will be realized
in about a decade [174].
Sensors and Probes Possible chemical sensor applications of nonmetallic nanotubes are interesting, because nanotube electronic transport and thermopower
(voltages between junctions caused by interjunction temperature differences) are
very sensitive to substances that affect the amount of injected charge [175, 176].
The main advantages are the minute size of the nanotube sensing element and the
correspondingly small amount of material required for a response. Major challenges remain, however, in making devices that differentiate between absorbed
species in complex mixtures and provide rapid forward and reverse responses.
Nishijima et al. [177] have developed such a novel microprocess incorporated in a
SEM, to attach individual nanotubes to scanning probe microscope tips, which are
later used as probes to image biological and industrial specimens. Carbon nanotube scanning probe tips for atomic probe microscopes are commercially available.
The cylindrical shape and small tube diameter enable imaging in narrow, deep
crevices and improve resolution in comparison to conventional nanoprobes, especially for high sample feature heights [178, 179]. Covalently modifying the nanotube tips, such as by adding biologically responsive ligands, enables the mapping
of chemical and biological functions [180]. Nanoscopic tweezers have been made
by Kim and Lieber that are driven by the electrostatic interaction between two
nanotubes on a probe tip [181]. They attached carbon nanotubes to electrodes fabricated on pulled glass micropipettes. Voltages are applied to the electrodes to
achieve closing and opening of the free ends of the nanotubes, to facilitate the
grabbing and manipulation of submicron clusters and nanowires. They may be
used as nanoprobes for assembly. These uses may not have the business impact of
other applications, but they increase the value of measurement systems for characterization and manipulation on the nanometer scale. Arie et al. [182] use Ni 3 Cfilled MWNTs as probes in a magnetic force microscope (MFM) and to image
magnetic recording media. The resolution, however, needs improvement through
the optimization of ferromagnetic particle size, trace height etc. Karl and Tomanek
[183] propose a molecular pump based on carbon nanotubes for the transport of
atoms. Using SWNTs attached to an AFM cantilever as the probe and atomically
flat titanium surfaces on an a-Al 2 O 3 substrate, Cooper et al. [184] demonstrate an
areal data storage density of the order of terabits per square inch. This method
8.2 Carbon Nanotubes 233
The spectrum of interesting findings discussed above opens up the possibility of
assembling carbon nanotubes, possessing such novel device-like properties [157,
161, 162, 172] into multi-functional circuits, and ultimately towards the realization
of a carbon-nanotube-based computer chip. Rueckes et al. [166] have recently described the concept of carbon-nanotube-based nonvolatile random access memory
for molecular computing. The viability of the concept has also been demonstrated.
Integrated nanotube devices involving two nanotube transistors have been reported
[163, 166], providing visions of large-scale integration. Patterned growth of SWNTs
on a 4 in (10 cm) silicon wafer [173] may prove an important step toward integrated nanotube electronics. IBM expects that nanotube electronics will be realized
in about a decade [174].
Sensors and Probes Possible chemical sensor applications of nonmetallic nanotubes are interesting, because nanotube electronic transport and thermopower
(voltages between junctions caused by interjunction temperature differences) are
very sensitive to substances that affect the amount of injected charge [175, 176].
The main advantages are the minute size of the nanotube sensing element and the
correspondingly small amount of material required for a response. Major challenges remain, however, in making devices that differentiate between absorbed
species in complex mixtures and provide rapid forward and reverse responses.
Nishijima et al. [177] have developed such a novel microprocess incorporated in a
SEM, to attach individual nanotubes to scanning probe microscope tips, which are
later used as probes to image biological and industrial specimens. Carbon nanotube scanning probe tips for atomic probe microscopes are commercially available.
The cylindrical shape and small tube diameter enable imaging in narrow, deep
crevices and improve resolution in comparison to conventional nanoprobes, especially for high sample feature heights [178, 179]. Covalently modifying the nanotube tips, such as by adding biologically responsive ligands, enables the mapping
of chemical and biological functions [180]. Nanoscopic tweezers have been made
by Kim and Lieber that are driven by the electrostatic interaction between two
nanotubes on a probe tip [181]. They attached carbon nanotubes to electrodes fabricated on pulled glass micropipettes. Voltages are applied to the electrodes to
achieve closing and opening of the free ends of the nanotubes, to facilitate the
grabbing and manipulation of submicron clusters and nanowires. They may be
used as nanoprobes for assembly. These uses may not have the business impact of
other applications, but they increase the value of measurement systems for characterization and manipulation on the nanometer scale. Arie et al. [182] use Ni 3 Cfilled MWNTs as probes in a magnetic force microscope (MFM) and to image
magnetic recording media. The resolution, however, needs improvement through
the optimization of ferromagnetic particle size, trace height etc. Karl and Tomanek
[183] propose a molecular pump based on carbon nanotubes for the transport of
atoms. Using SWNTs attached to an AFM cantilever as the probe and atomically
flat titanium surfaces on an a-Al 2 O 3 substrate, Cooper et al. [184] demonstrate an
areal data storage density of the order of terabits per square inch. This method
8.2 Carbon Nanotubes 233