Biomedical Engineering Reference
In-Depth Information
(a)
(b)
(c)
(d)
(g)
SWNT
(e)
CH
4
CH
4
(f)
Fe
2
O
3
Substrate
FIGURE 12.5
(a-f) TEM images of SWCNTs grown from discrete nanoparticles, showing particle-nanotube relationship. Scale
bars: 10 nm. (a-d) SWCNTs grown from discrete nanoparticles (dark dots at the bottom of the images). The
arrows point to the ends of the nanotubes. (e) Image of an ultrashort (
2
nm nanoparticle. (f) Image of a nanoparticle surrounded by a single graphitic shell. (g) A schematic model for
nanotube growth. (From Dai, H. (2002). Carbon Nanotubes: Synthesis, Integration, and Properties.
Acc. Chem.
Res
., 35, 1035-1044. )
4 nm) nanotube capsule grown from
particle, corresponding to f, e, and d in Figure 12.5, can be clearly seen. In fact, the CVD
reaction process involves decomposing a hydrocarbon gas such as methane to obtain car-
bon atoms, which then dissolve into the surface of the catalytic NP on the substrate to form
a carbon-metal solid-state solution. As the supersaturation increases, carbon atoms pre-
cipitate from the particle, leading to the growth of a nanotube (Figure 12.5g).
In the synthesis of SWCNTs, results have shown that S-SWCNTs and M-SWCNTs coex-
ist in all materials synthesized by arc discharge, laser ablation, and CVD methods. A key
challenge is how to separately obtain purely metallic and semiconducting nanotubes.
While chemical separation is a possible approach [31,32], preferential growth is also desir-
able. Recently, the preferential growth of S-SWCNTs (
90%) from Dai's group [33] was
obtained in a plasma-enhanced CVD process at 600
C, compared with S-SWCNTs of
30% for laser ablation. Similar observation of SWCNTs with uni-
form chirality produced from the CVD process has also been reported [34]. More recently,
Dai and coworkers [35] reported a correlation between the diameter of SWCNTs and the
relative fractions of metallic and semiconducting tubes. Small tubes (
d
60% for HiPCO and
1.1 nm) show
higher percentages of S-SWCNTs while large tubes (
d
1.1 nm) produce higher percent-
ages of M-SWCNTs. The preferential growth of small semiconducting tubes was attributed
