Biomedical Engineering Reference
In-Depth Information
In this chapter, we review the recent advances in employing CNT as electrically
conductive substrates for neuronal growth. Emphasis is given to the use of CNTs as
electrical interfaces to individual neurons and, in particular, to the modeling of the
CNT-neuron junction properties.
2
Carbon Nanotube Electronics
CNTs are cylindrically shaped nanostructures constituted by sheets of graphene
rolled up to form hollow tubes. Historically, multi-walled carbon nanotubes (MWNTs)
were characterized in 1991, (Iijima
1991
) followed, later in 1993, by their single-
walled analogous (Iijima and Ichihashi
1993
; Bethune et al.
1993
) . However, evi-
dences for nano-sized carbon tubes trace back to the 1952, when two Russian
scientists, Radushkevich and Lukyanovich, published TEM images of nano-sized
hollow carbon fi laments. Since the original article was written in Russian, that can
explain why this work was unrecognized by most of the scientists in the rest of the
world. For details on this issue, please consult a recent editorial report about the full
history of the nanotubes' discovery (Monthioux and Kuznetsov
2006
) .
Single-walled carbon nanotubes (SWNTs) possess the simplest geometry, i.e., a
rolled-up graphene sheet that is closed by fullerene-like caps. Their diameter is in the
order of 0.7-2 nm while their length can reach up to several millimeters. Depending
on the orientation of the tube axis with respect to the hexagonal lattice, the structure
of a nanotube can be completely specifi ed through its chiral vector, which is denoted
by the chiral indices (
n
,
m
) (Fig.
1
). When the chiral indices
n
=
m
nanotubes are metal-
lic and are considered quasi metallic (with a tiny band gap) when
n
−
m
is divisible by
3. All other tubes are semiconducting with band gaps of the order of 0.5 eV. SWNTs
can be either metallic or semiconducting depending on their geometry; CNTs are to
date the only material known to have this unique property (Saito et al.
1992
) . The ratio
of metallic-semiconducting CNTs in a raw sample is about 1/3 of metallic and 2/3 of
semiconducting. On the contrary, MWNTs present metallic properties because they
are constituted of multiple layers of graphene rolled on themselves.
The large aspect ratio, the electrical properties, and the chemical and thermal
stability of individual nanotubes make CNTs promising materials for molecular
electronics (Léonard and Talin
2006
; Avouris and Chen
2006
; Avouris et al.
2007
) .
In SWNT, electrons propagate along the tube axis. In particular, electrical transport
in metallic SWNT was found to be ballistic over micron lengths at low biases
(Bachtold et al.
2000
) while in semiconducting tubes the mean free path is about
700 nm (Fuhrer et al.
2001
). Individual metallic SWNTs are able to carry currents
with a density exceeding 109 A cm
−2
(Yao et al.
2000
). Because of this high current
density and also because of their mechanical properties, CNTs (i.e., metallic SWNT
and MWNT) have been suggested as interconnect materials in silicon nanoelectron-
ics (Graham et al.
2005
) .
On the other hand, semiconducting SWNTs represent promising materials in
electronics to replace (or in combination with) silicon-based devices. While the
