Scanning the properties of nanotubes
Excerpted from Nanotechnology For Dummies by Richard Booker and Earl Boysen
One property of nanotubes is that they’re really, really strong. Tensile strength
is a measure of the amount of force an object can withstand without tearing
apart. The tensile strength of carbon nanotubes is approximately 100 times
greater than that of steel of the same diameter.
There are two things that account for this strength. The first is the strength
provided by the interlocking carbon-to-carbon covalent bonds. The second
is the fact that each carbon nanotube is one large molecule. This means it doesn’t have the weak spots found in other materials, such as the boundaries
between the crystalline grains that form steel.
Nanotubes are strong but are also elastic. This means it takes a lot of force to
bend a nanotube, but the little guy will spring right back to its original shape
when you release it, just like a rubber band does. Researchers have used
atomic force microscopes to physically push nanotubes around and observe
their elastic properties. Evaluations with transmission electron microscopes —
the kind of microscope sensitive enough to give you a peek at atomic shapes —
show that the bonds in the atomic lattice don’t break when you bend or compress
Young’s modulus for carbon nanotubes, a measurement of how much force it
takes to bend a material, is about 5 times higher than for steel, so if you were
thinking of going out and bending a nanotube, think again. The fact is, there’s
not another element with a lattice structure in the whole periodic table that
bonds to itself with as much strength as carbon atoms. And, since carbon
nanotubes have such a perfect structure, they avoid the degradation of
strength that you get with other materials.
In addition to being strong and elastic, carbon nanotubes are also lightweight,
with a density about one quarter that of steel.
As if that weren’t enough, carbon nanotubes also conduct heat and cold
really well (they have a high thermal conductivity); some researchers predict
a thermal conductivity more than 10 times that of silver — and if you’ve ever
picked up a fork from a hot stove, you know silver and other metals are
pretty darn good conductors of heat. While metals depend upon the movement
of electrons to conduct heat, carbon nanotubes conduct heat by the
vibration of the covalent bonds holding the carbon atoms together; the
atoms themselves are wiggling around and transmitting the heat through
the material. The stiffness of the carbon bond helps transmit this vibration
throughout the nanotube, providing very good thermal conductivity.
A diamond, which is also a lattice of carbon atoms covalently bonded, uses
the same method to conduct heat, so it’s also an excellent thermal conductor,
as well as a stunning piece of jewelry.
Carbon nanotubes are a little bit sticky, as well. The electron clouds on the
surface of each nanotube provide a mild attractive force between the nanotubes.
This attraction is called van der Waals’ force (which we discuss in
Chapter 3). This involves forces between nonpolar molecules (a molecule
without a positive end and a negative end). A carbon nanotube just happens
to be a nonpolar molecule.the hardest material found in nature (and reportedly a girl’s
But not all nanotubes are exactly alike. Armchair nanotubes all have electrical
properties like metals — but only about a third of all zigzag and chiral
nanotubes have electrical properties like metal; the rest (roughly two thirds)
have electrical properties like semiconductors. (For more about the difference,
see the “Metallic or semiconducting?” sidebar, later in this chapter.) A
metallic carbon nanotube conducts electricity when you connect different
voltages to each end, just like a wire. Applying a negative voltage at one end
and a positive voltage at the other end causes electrons to flow towards the
positive voltage. To get electrons to flow through a semiconducting carbon
nanotube, you also have to add some energy. (You could shine light on the
nanotube, for example.)
Carbon nanotubes conduct electricity better than metals. When electrons
travel through metal there is some resistance to their movement. This resistance
happens when electrons bump into metal atoms. When an electron
travels through a carbon nanotube, it’s traveling under the rules of quantum
mechanicals, and so it behaves like a wave traveling down a smooth channel
with no atoms to bump into. This quantum movement of an electron within
nanotubes is called ballistic transport.
Carbon atoms in nanotubes, like those in buckyballs, have the ability to covalently
bond to other atoms or molecules creating a new molecule with customized
properties. Bonding an atom or molecule to a nanotube to change its
properties is called functionalization.
Metallic or semiconducting?
The diameter of a carbon nanotube and the
amount of twist in its lattice determines whether
it’s metallic or semiconducting. Electrons in
carbon nanotubes can only be at certain energy
levels, just like electrons in atoms. A nanotube is
metallic if the energy level that allows delocalized
electrons to flow between atoms throughout
the nanotube (referred to as the conduction
band) is right above the energy level used by
electrons attached to atoms (the valance band).
In a metallic nanotube, electrons can easily
move to the conduction band. A nanotube is
semiconducting if the energy level of the conduction
band is high enough so that there is an
energy gap between it and the valance band. In
this case, additional energy, such as light, is
needed for an electron to jump that gap to move
to the conduction band. While there is no gap
between the valance and conduction bands for
armchair nanotubes (which makes them metallic),
an energy gap does exist between the
valance and conduction bands in about two
thirds of zigzag and chiral nanotubes — which
makes them semiconducting.
Obviously, it’s important to be able to control
what type of nanotube you are growing. Most
current production processes for nanotubes
create both metallic and semiconductor nanotubes.
Researchers at Rice University have hit
on a way to control this process. They take short
lengths of nanotubes of the type they want and
attach nanocatalyst particles (typically a metal
such as nickel) to one end. These nanotubes are
placed in the reaction chamber and act like
seeds. New, long, nanotubes are grown from
these seeds, kind of like nanotube cloning.
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