crystalline gallium phosphide nanowires with mean diameter 40 nm and length
up to 300 mm via sublimation of ball-milled gallium phosphide powder. Lee and
coworkers [293d] have synthesized large quantities of Si nanowires (6–28 nm diameter, @1 mm in length) by the simple sublimation of SiO powder. The thermal
sublimation of SiO powders produced SiO vapor, which underwent a disproportionation reaction, was transported and deposited at @930
C to form nanowires
containing a crystalline Si core and an amorphous SiO 2 sheath. The axis of the Si
nanowires is approximately along the [211] direction. This method has the advantage over the laser-assisted catalytic growth method because it can produce highpurity Si nanowires without any metal contamination. Recently, Ma et al. [294b]
have prepared small diameter (1–7 nm) Si nanowires using the above oxideassisted procedure. They obtained stable, faceted Si nanowire surfaces terminated
with hydrogen after removing the SiO 2 sheath by dipping these nanowires in hydrofluoric acid. Scanning tunneling microscopy (STM) of these Si nanowires
showed atomically resolved images with two types of nanowire surfaces which they
interpreted as hydrogen-terminated Si (111)-(1 Â 1) and Si (001)-(1 Â 1) surfaces
corresponding to SiH 3 on Si (111) and SiH 2 on Si (001), respectively. Interestingly
these hydrogen terminated Si nanowire surfaces are more oxidation-resistant than
similarly treated Si wafer surfaces. The scanning tunneling spectroscopy (STS)
measurements showed that the electronic energy gaps were found to increase with
decreasing Si nanowire diameter from 1.4 eV for 7 nm to 3.5 eV for 1.3 nm. Wang
et al. reported the synthesis of other nanostructures such as oxide nanobelts by
simply evaporating the commercial metal oxide powders at high temperatures
[296–298].
Carbothermal Reactions It is noteworthy that a variety of oxides, nitrides and elemental nanowires can be synthesized by carbothermal reactions. For example,
carbon (activated carbon or carbon nanotubes) in mixture with an oxide produces
oxide or suboxide vapor species, which react with other reactants (O 2 , N 2 or NH 3 )
to produce the desired nanowires. Thus GaN nanowires are produced by heating a
mixture of Ga 2 O 3 and carbon in N 2 or NH 3 . Silicon nanowires can also be made by
heating SiO 2 with carbon in a suitable atmosphere.
Yang et al. reported the synthesis of MgO, Al 2 O 3 , ZnO, SnO 2 nanowires via
a carbothermal reduction process [299, 300]. Gundiah et al. [301] employed an indirect vapor–phase method via a carbothermal reduction processes, for the synthesis of silicon carbide, silicon oxynitride and silicon nitride nanowires. The simplest method to obtain b-SiC nanowires involves heating silica gel with activated
carbon at 1360
C in H 2 or NH 3 (Figure 8.30). The same reaction, if carried out
in the presence of catalytic iron particles, at 1200
C gives a-Si 3 N 4 nanowires and
Si 2 N 2 O nanowires at 1100
C. Another method to obtain Si 3 N 4 nanowires is to
heat MWNTs with silica gel at 1360
C in an atmosphere of NH 3 . In the presence
of catalytic Fe particles, this method yields Si 3 N 4 nanowires in pure form. The
formation of carbide follows two steps (steps I and II shown below), initially carbon reduces the SiO 2 to the volatile suboxide of silicon and then the formation of
carbide follows.
8 Nanotubes and Nanowires
260
up to 300 mm via sublimation of ball-milled gallium phosphide powder. Lee and
coworkers [293d] have synthesized large quantities of Si nanowires (6–28 nm diameter, @1 mm in length) by the simple sublimation of SiO powder. The thermal
sublimation of SiO powders produced SiO vapor, which underwent a disproportionation reaction, was transported and deposited at @930
C to form nanowires
containing a crystalline Si core and an amorphous SiO 2 sheath. The axis of the Si
nanowires is approximately along the [211] direction. This method has the advantage over the laser-assisted catalytic growth method because it can produce highpurity Si nanowires without any metal contamination. Recently, Ma et al. [294b]
have prepared small diameter (1–7 nm) Si nanowires using the above oxideassisted procedure. They obtained stable, faceted Si nanowire surfaces terminated
with hydrogen after removing the SiO 2 sheath by dipping these nanowires in hydrofluoric acid. Scanning tunneling microscopy (STM) of these Si nanowires
showed atomically resolved images with two types of nanowire surfaces which they
interpreted as hydrogen-terminated Si (111)-(1 Â 1) and Si (001)-(1 Â 1) surfaces
corresponding to SiH 3 on Si (111) and SiH 2 on Si (001), respectively. Interestingly
these hydrogen terminated Si nanowire surfaces are more oxidation-resistant than
similarly treated Si wafer surfaces. The scanning tunneling spectroscopy (STS)
measurements showed that the electronic energy gaps were found to increase with
decreasing Si nanowire diameter from 1.4 eV for 7 nm to 3.5 eV for 1.3 nm. Wang
et al. reported the synthesis of other nanostructures such as oxide nanobelts by
simply evaporating the commercial metal oxide powders at high temperatures
[296–298].
Carbothermal Reactions It is noteworthy that a variety of oxides, nitrides and elemental nanowires can be synthesized by carbothermal reactions. For example,
carbon (activated carbon or carbon nanotubes) in mixture with an oxide produces
oxide or suboxide vapor species, which react with other reactants (O 2 , N 2 or NH 3 )
to produce the desired nanowires. Thus GaN nanowires are produced by heating a
mixture of Ga 2 O 3 and carbon in N 2 or NH 3 . Silicon nanowires can also be made by
heating SiO 2 with carbon in a suitable atmosphere.
Yang et al. reported the synthesis of MgO, Al 2 O 3 , ZnO, SnO 2 nanowires via
a carbothermal reduction process [299, 300]. Gundiah et al. [301] employed an indirect vapor–phase method via a carbothermal reduction processes, for the synthesis of silicon carbide, silicon oxynitride and silicon nitride nanowires. The simplest method to obtain b-SiC nanowires involves heating silica gel with activated
carbon at 1360
C in H 2 or NH 3 (Figure 8.30). The same reaction, if carried out
in the presence of catalytic iron particles, at 1200
C gives a-Si 3 N 4 nanowires and
Si 2 N 2 O nanowires at 1100
C. Another method to obtain Si 3 N 4 nanowires is to
heat MWNTs with silica gel at 1360
C in an atmosphere of NH 3 . In the presence
of catalytic Fe particles, this method yields Si 3 N 4 nanowires in pure form. The
formation of carbide follows two steps (steps I and II shown below), initially carbon reduces the SiO 2 to the volatile suboxide of silicon and then the formation of
carbide follows.
8 Nanotubes and Nanowires
260