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 a nanotube.

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 best friend).

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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