NANOSCALE DEVICES: APPLICATIONS AND MODELING - Bio-Inspired and Nanoscale Integrated Computing - page 36

describing a microscopic or macroscopic piece of the same material. In this sense,
nanoparticles are at the intermediate level between bulk materials and the atomic
world.
2.2.3. Nanowires
We recall from geometry that an object can be zero-dimensional (0D: a point),
one-dimensional (1D: a line), two-dimensional (2D: a plane) or three-dimensional
(3D: a volume). In analogy, nanoparticles are like points. One important category
of nanomaterials is 1D objects or ''lines'' at the nanoscale, namely nanowires. A
typical nanowire has a thickness of a few nanometers to a few tens of nanometers;
its length is much more than a nanometer, giving it a very high aspect ratio (like a
line). Nanowires have been made of many materials, such as silicon, germanium,
zinc oxide, and gallium nitride [2-5]. Nanowires are extremely important due to
their one-dimensionality. In particular, the electronic properties in these 1D
materials can be drastically different from their bulk counterparts due to the fact
that charge carriers (electrons) are confined to a small space (only a few
nanometers) in the lateral dimensions. This leads to very strong quantum
mechanical effects and can result in outstanding electronic transport characteristics
for devices such as transistors and switches needed in ultra-high-performance
electronic circuits and chips.
2.2.4. Nanotubes
One particularly important form of nanowires is what we call a nanotube. The
name explains it all: It is a hollow cylinder with a diameter on the order of 1 nm.
The most common nanotubes are those made of carbon, i.e., carbon nanotubes.
Graphite is a well known form of carbon. Pencil lead is made out of graphite.
Graphite has an interesting structure, in that it is composed of one-atom-thick
layers stacked on top of each other. Each layer, called graphene, is a sheet of
carbon atoms arranged in a hexagonal lattice structure, like a chicken wire. A
carbon nanotube can be thought of as a single graphene sheet rolled along a
certain direction into a nanometer-diameter tube that is hollow inside. Carbon
nanotubes can be extremely long, up to several centimeters, and that means they
have aspect ratios on the order of 10 7 . Thus, they represent highly 1D systems.
They can be single-walled (Fig. 2.1), i.e., composed of only one layer, or multi-
walled, composed of several co-axial, single-walled nanotubes. Carbon nanotubes
were discovered in 1991 by Sumio Iijima [6] and have since attracted much
attention in the world of nanotechnology due to characteristics such as out-
standing electronic properties arising from their 1D nature, as well as extreme
mechanical strength due to the strong carbon-carbon SP 2 bond.
One fundamental difference between nanotubes and other forms of nanowires
is that a nanotube is by itself a perfect structure. In other words, most nanowires
can be thought of as a very thin wire carved out of a 3D bulk. As such, there could
be many unsatisfied and dangling bonds on the surface of these structures. They