Biomedical Engineering Reference
In-Depth Information
12.2
Carbon Nanotubes
12.2.1
Introduction
Carbon nanotubes are a class of new, 1D carbon nanomaterials discovered by Iijima in
1991 [4]. The extraordinary mechanical properties and unique electrical properties of
CNTs have stimulated extensive research activities worldwide, resulting in many special
issues of journals [5-7] and several topics [8-11]. Driven by potential applications includ-
ing nanocomposite materials, nanoelectrode materials, field emitters, nanoelectronics, and
nanoscale sensors, significant progress has been made in CNT synthesis and characteriza-
tion in the past ten years.
Structures comprising one cylindrical tube are called single-walled carbon nanotubes
(SWCNTs). SWCNTs have a relatively smaller diameter, as small as 0.4 nm, and can be
metallic or semiconducting, depending on their structure. The SWCNT structure is charac-
terized by a vector (
m, n
) designating the orientation of the graphene layer relative to the
tube axis: armchair (
n
0), or chiral (any other
n
and
m
). Examples
are shown in Figure 12.1A. All armchair SWNTs are metals; those with
n
m
), zigzag (
n
0 or
m
3
k
, where
k
is a nonzero integer, are semiconductors with a tiny band gap; all others are semiconductors
with a band gap that inversely depends on the nanotube diameter [12]. While semicon-
ducting SWCNTs (S-SWCNTs) can be used to build high-performance FETs and sensors,
metallic SWCNTs (M-SWCNTs) might be useful for interconnects.
Structures that contain a concentric set of cylinders with a constant interlayer separa-
tion of 0.34 nm are called multi-walled carbon nanotubes (MWCNTs) (Figure 12.1B).
MWCNTs have relatively large diameter, ranging from a few nanometers to several tens
of nanometers, and are conducting materials. The electronic properties of perfect
MWCNTs are rather similar to those of perfect SWCNTs because the coupling between
the cylinders is weak in MWCNTs.
Carbon nanotubes exhibit various extraordinary properties including mechanical,
electrical, thermal, and chemical properties. Their Young's modulus is over 1 TPa and the
tensile strength is about 200 GPa. The thermal conductivity can be as high as 3,000 W/mK.
m
(a)
(b)
(c)
FIGURE 12.1
(See color insert following page 330)
A. Schematic illustrations of the structures
of (a) armchair, (b) zigzag, and (c) chiral
SWNTs (Reproduced with permission from
Baughman, R. H., Zakhidov, A. A., de Heer, W. A.
(2002). Carbon Nanotubes—The Route Toward
Applications.
Science
, 297, 787-792.). B. Structure
of a multi-walled carbon nanotube made up of
three shells of differing chirality. (Reproduced
with permission from Balasubramanian, K.,
Burghard, M. (2005). Chemically Functionalized
Carbon Nanotubes.
Small
, 1, 180-192.)
(A)
(B)
