Boron-doped carbon nanotubes have been synthesized by carrying out the pyrolysis
of mixtures of acetylene and diborane and characterized by employing microscopic
and spectroscopic techniques [111]. The average composition of these nanotubes is
C 35 B. BaCaN nanotubes have been prepared by striking an arc between a graphite
anode filled with BaN and a pure graphite cathode in a helium atmosphere [112].
BaCaN nanotubes have also been obtained by laser ablation of a composite target
containing BaN, carbon, Ni and Co at 1000
C under flowing nitrogen [113]. Terrones et al. [114] pyrolyzed the addition compound, CH 3 CN:BCl 3 , over Co powder
at 1000
C to obtain BaCaN nanotubes. BaCaN as well as CaN nanotubes were
prepared by Sen et al. [115] by the pyrolysis of appropriate precursors. Pyrolysis of
aza-aromatics such as pyridine over Co catalysts gives CaN nanotubes (C 33 N on
average). Pyrolysis of the 1:1 addition compound of BH 3 with (CH 3 ) 3 N produces
BaCaN nanotubes. Typical TEM images of a few nanotubes are shown in Figure
8.11, exhibiting bamboo-shaped, nested cone-shaped cross sections as well as
unusual morphology, including coiled nanotubes. The composition of the BaCaN
nanotubes varies with the preparation. Furthermore, considerable variability exists
in the composition in any given batch of BaC, BaCaN or CaN nanotubes obtained
by the pyrolysis of precursors.
Aligned carbon nanotubes are considered ideal for field emission properties. The
availability of aligned bundles of doped carbon nanotubes may provide further
improvement in field-emission characteristics. Nath et al. [116a] have obtained
aligned carbon–nitrogen nanotube bundles by the pyrolysis of pyridine over sol–
gel derived iron/silica or cobalt/silica substrates. Employing anodic alumina, Sung
et al. [116b] synthesized CaN nanotubes by electron cyclotron resonance CVD,
using C 2 H 2 and N 2 . Suenaga et al. [117] carried out CVD of Ni-phthalocyanine to
obtain aligned CaN nanotubes.
Goldberg et al. [118] have employed a method wherein SWNTs were thermally
treated with boron trioxide in a nitrogen atmosphere to obtain boron or boron and
nitrogen doped SWNTs. EELS analysis showed the boron content to be @10 at.% in
BaC nanotubes. An interesting aspect of the BaCaN nanostructures is that phase
separation occurs in which the BC 3 islands segregate in the graphene sheets. Tunnelling conductance measurements of doped nanotubes demonstrate acceptor-like
states, near the Fermi level, arising out of the BC 3 islands [119]. Efsarjani et al.
[120] propose that a nanotube with donor atoms at one side and acceptor atoms on
the other can function as a nano-diode. An experimental situation near to this effect is the observation of rectification in a SWNT [121]. The presence of an impurity in one of the segments of a SWNT influences its nonlinear transport behavior.
8.2.4.2 Opening, Filling and Functionalizing Nanotubes
Multi-walled nanotubes are generally closed at either end, the closure being made
possible by the presence of five-membered rings. MWNTs can be uncapped by oxidation with carbon dioxide or oxygen at elevated temperatures [74, 122, 123]. High
yields of uncapped MWNTs are, however, obtained by boiling them in concentrated
HNO 3 . Filling the opened nanotubes with metals has been accomplished. The
well-known method [124] involves the treatment of the nanotubes with boiling
8.2 Carbon Nanotubes 225
of mixtures of acetylene and diborane and characterized by employing microscopic
and spectroscopic techniques [111]. The average composition of these nanotubes is
C 35 B. BaCaN nanotubes have been prepared by striking an arc between a graphite
anode filled with BaN and a pure graphite cathode in a helium atmosphere [112].
BaCaN nanotubes have also been obtained by laser ablation of a composite target
containing BaN, carbon, Ni and Co at 1000
C under flowing nitrogen [113]. Terrones et al. [114] pyrolyzed the addition compound, CH 3 CN:BCl 3 , over Co powder
at 1000
C to obtain BaCaN nanotubes. BaCaN as well as CaN nanotubes were
prepared by Sen et al. [115] by the pyrolysis of appropriate precursors. Pyrolysis of
aza-aromatics such as pyridine over Co catalysts gives CaN nanotubes (C 33 N on
average). Pyrolysis of the 1:1 addition compound of BH 3 with (CH 3 ) 3 N produces
BaCaN nanotubes. Typical TEM images of a few nanotubes are shown in Figure
8.11, exhibiting bamboo-shaped, nested cone-shaped cross sections as well as
unusual morphology, including coiled nanotubes. The composition of the BaCaN
nanotubes varies with the preparation. Furthermore, considerable variability exists
in the composition in any given batch of BaC, BaCaN or CaN nanotubes obtained
by the pyrolysis of precursors.
Aligned carbon nanotubes are considered ideal for field emission properties. The
availability of aligned bundles of doped carbon nanotubes may provide further
improvement in field-emission characteristics. Nath et al. [116a] have obtained
aligned carbon–nitrogen nanotube bundles by the pyrolysis of pyridine over sol–
gel derived iron/silica or cobalt/silica substrates. Employing anodic alumina, Sung
et al. [116b] synthesized CaN nanotubes by electron cyclotron resonance CVD,
using C 2 H 2 and N 2 . Suenaga et al. [117] carried out CVD of Ni-phthalocyanine to
obtain aligned CaN nanotubes.
Goldberg et al. [118] have employed a method wherein SWNTs were thermally
treated with boron trioxide in a nitrogen atmosphere to obtain boron or boron and
nitrogen doped SWNTs. EELS analysis showed the boron content to be @10 at.% in
BaC nanotubes. An interesting aspect of the BaCaN nanostructures is that phase
separation occurs in which the BC 3 islands segregate in the graphene sheets. Tunnelling conductance measurements of doped nanotubes demonstrate acceptor-like
states, near the Fermi level, arising out of the BC 3 islands [119]. Efsarjani et al.
[120] propose that a nanotube with donor atoms at one side and acceptor atoms on
the other can function as a nano-diode. An experimental situation near to this effect is the observation of rectification in a SWNT [121]. The presence of an impurity in one of the segments of a SWNT influences its nonlinear transport behavior.
8.2.4.2 Opening, Filling and Functionalizing Nanotubes
Multi-walled nanotubes are generally closed at either end, the closure being made
possible by the presence of five-membered rings. MWNTs can be uncapped by oxidation with carbon dioxide or oxygen at elevated temperatures [74, 122, 123]. High
yields of uncapped MWNTs are, however, obtained by boiling them in concentrated
HNO 3 . Filling the opened nanotubes with metals has been accomplished. The
well-known method [124] involves the treatment of the nanotubes with boiling
8.2 Carbon Nanotubes 225