A History of Carbon Nanotubes

In 1991, Iijima discovered carbon nanotubes (CNTs). Since then, a new branch of knowledge in materials science has emerged – Nanoscience. In order to unveil the secrets of the innovative materials, hundreds of millions of dollars have been invested. Yet, it was all worth it since these functional nanoscale materials have diverse properties, which are unique, amazing and new. In fact, a fourth state of matter has been found to be water trapped inside a carbon nanotube, which does not act as gas, liquid or solid.

CNTs are 100 times more robust than steel on the molecular level. They are only one-sixth the weight of steel and have a high aspect ratio, making them valuable as a mechanical property that enhances filler material.

The Structure of Single Walled Carbon Nanotubes

A Representation of a Double Walled Carbon Nanotube

Carbon nanotubes are heat and electricity conductors just like copper. However, it has no oxidative concerns as long as they are well dispersed. Carbon nanotubes have been widely used commercially in the fields of polymers, displays, engineering plastics, thin films, coatings, anti-corrosion paints, transparent and non-transparent conductive electrodes, anti-static packaging and hydrophobic coatings. At the same time, active research is happening in the fields of batteries, solar cells, fuel cells, advanced devices, water desalination and optics, among others. Carbon nanotubes paved the way for graphene.

Allotropes of Carbon

Carbon nanotubes are tube-like materials made of carbon. They have a diameter measuring on the nanometer scale, which makes them revolutionary materials. A nanometer is about one-billionth of a meter that is approximately 10,000 thinner than a human hair. CNTs are one-of-a-kind because of their durable and sturdy inter-molecular bonds between the alternation 5 and 6 membered rings of carbon atoms. Van der Waals forces, which are the attraction of intermolecular forces between molecules, within carbon nanotubes make them at a greater risk of dispersion, agglomeration and reagglomeration. These can be challenging because of the forces, their high degree of entanglement with other CNTs and high aspect ratio.

Carbon nanotubes may have various structures, thickness, lengths and numerous layers. Carbon nanotubes are available in single-walled, double-walled and mutli-walled variants. You can best see the structure of a singled-walled carbon nanotube as the wrapper of a one-atom-thick layer of graphite dubbed as graphene into a unified, tube-like cylinder although they are designed to be a tube and not as a sheet that can be rolled up.

A structural pattern comes from the way a graphene sheet is rolled, which can be represented by a pair of indices (n,m). These integers (n,m) signify the number of unit vectors along two directions in the honeycomb crystal lattice of the carbon nanotubes. For instance, if m = 0, the nanotubes are pertained to as zigzag nanotubes. On the other hand, if n = m, the nanotubes are dubbed as armchair nanotubes. Or else, they are known as chiral. Double-walled carbon nanotubes are one concentric nanotube inside another.

The electronic features of nanotubes can vary depending on the chiral angle of nanotube as it was designed during synthesis, which causes it to act as either metallic or semiconducting material. Normally, they are grown and sold as mixed structures.

A Carbon Nanotube Density Gradient

Metallic and semiconducting single-walled carbon nanotubes can be effectively secluded by density differentiation. The process involves the use of chemicals to produce a density gradient. Then, the isolated carbon nanotubes collect in particular regions by type, which can be gathered as an isolated material.

It is best to envision the graphene layer that creates the nanotube as a rolled-up-chicken-wire-like structure that has alternating five and six membered hexagonal rings of carbon atoms. The specific synthesis conditions determine the structure. The synthesis conditions rarely produce a homogeneous product as they are normally mixtures of the different types of CNTs made in a specific reaction.

Carbon Nanotubes History

Many decades ago, in 1980, we knew of only three forms of carbon – graphite, diamond and amorphous carbon. Now, we know that there is an entire family of carbon forms. The first one that was discovered was the hollow, cage-like buckminsterfullerene molecule. It’s also dubbed as the buckyball or the C60 fullerene. Today, there are already thirty or more types of fullerenes and also an extended family of linear molecules known as carbon nanotubes. C60 is the first spherical carbon molecule, with carbon atoms organized in the shape of a soccer ball. In the structure, there are many five-membered rings isolated by six-membered rings and 60 carbon atoms.

The other one is a slightly elongated and spherical carbon molecule in the same group that is like a rugby ball. It has seventy carbon atoms, hence, called C70. Its stricture has extra six-membered carbon rings. Yet, there are also many other potential structures comprising of the same number of carbon atoms. Their specific shapes rely on whether five-membered ring are isolated or not. Also, they depend on whether seven-membered rings are existing. Various other forms of fullerenes up to and beyond C120 have been specified. It is likely to produce other fullerene structures with five-membered rings in numerous positions and sometimes, connecting each other.

A Graphical Representation of a Carbon Fullerene With 60 Carbon Atoms

One of the most important factors for nanotechnology is that important dopant atoms can be situated inside the carbon nanotube or hollow fullerene ball. The atoms inside the fullerene are known as endohedral. Actually, they can also be bonded to fullerenes outside the ball as salts, if the fullerene can acquire electrons.

Perhaps carbon nanotubes, which are associated with graphite, are more essential than fullerenes. The molecular structure of graphite is like stacked, one-atom-thick sheets of chicken wire – a planar network of interrelated hexagonal rings of carbon atoms. In the traditional graphite, the sheets of carbon are arranged on top of each other, letting them easily slide over one another. Due to this, graphite is not hard, but it also feels greasy. It can be utilized as a lubricant. Graphene sheets form CNTs when they are rolled into a cylinder and their edges are combined. However, only the tangents of the graphitic planes come into contact with each other, making their properties like those of a molecule.

The Structure of Several Endohedral Fullerenes

The type of metal atoms seized within the fullerene cages can produce endohedral fullerenes. A principle shows that the maximum electrical conductivity is expected from endohedral metal atoms, which will transfer three electrons to the fullerene. Fullerenes can be distributed on the surface as a monolayer. This means that there is only one layer of molecules. They are termed as mono dispersed. The given fullerenes can be positioned in very precise locations. They are aligned to create a fullerene wire. A recent demonstration of Teslaphoresis by the Rice University was conducted with the use of a Tesla coil to line up carbon nanotubes into a filament.

Systems with proper material inside the fullerene ball are conducting. They are of specific interest due to the fact that they can be deposited to create bead-like conducting circuits. When non-doped structures and endohedrally doped structures are combined, they change the actual composition of a fullerene wire so it can be adjusted in-situ during patterning. Thus, within a single wire, conducting and insulating regions may be exactly defined. One-dimensional junction engineering becomes accurate with fullerenes.

The Interior View of The Structure of a Single Walled Carbon Nanotube

Carbon nanotubes come in a wide array of diameters, functional group content and length. These can modify their use for particular uses. CNTs are used for industrial applications in bulk quantities up metric ton quantities. Many CNT producers have more than 100 tons per year production capacity for multi-walled nanotubes.

An SEM Image of our SW/DWNTs 99% Product

A nanotube contains one tube of interrelated graphite atoms, a one-atom thick single-walled nanotube or a number of concentric tubes dubbed as multi-walled nanotubes. Under a transmission electron microscope, these tubes look like planes. On the other hand, single-walled nanotubes look like two planes. In multi-walled nanotubes, more than two planes appear and can be viewed as a series of parallel lines. There are many forms of CNTs, because the graphitic sheets can be rolled in numerous ways. The three types of CNTs – Armchair, Chiral and Zigzag, are recognized by following the pattern across the diameter of the tubes. Also, they can be recognized as a series of parallel lines.

An SEM Image of Our Multi Walled Carbon Nanotubes

There are various types of CNTs, due to the fact that graphitic sheets can be rolled in various ways. Multi-walled nanotubes can come in an even more complicated range of forms, because each concentric single-walled nanotube can have various structures. Hence, there are a wide range of sequential arrangements. The simplest sequence happens when concentric layers are alike but different in diameter. Yet, mixed variations are possible, containing two or more forms of concentric CNTs organized in varying orders. These can have either random layering or regular layering. The structure of the nanotube affects its specifications – including thermal and electrical conductivity, lattice structure and density. Both diameter and type are vital. The broader the diameter of the nanotube, the more it acts like graphite. The slimmer the diameter of the nanotube, the more its basic properties relies on its specific type.

Multi-walled carbon nanotubes (MWNTs) consist of manifold nanotubes inside bigger nanotubes with the same and varying chiralities. You can even have metallic and semiconducting regions on the same individual nanotube structure. Two models best characterize the structure of multi-walled carbon nanotubes, the Parchment and Russian Doll models.

Russian Doll model carbon nanotubes are basically tubes inside larger tubes, which is like the famous children’s toy name would suggest. Parchment MWNTs feature a single sheet of graphite rolled around itself, similar to a parchment scroll or a rolled newspaper. The interlayer spacing is close to the space between the graphene layers in graphite, about 3.4 Å. The Russian Doll structure is more common.

A Vertically Aligned Carbon Nanotube Array

CCVD produce vertically aligned carbon nanotubes. They are attached to the 1 synthesis substrate, which is usually Si/SiO₂ or copper foils or stainless steel. They can be created by CCVD or PECVD in a top down or bottom up synthesis method. You can use the CNTs while on the array. They can be used free standing. Some uses such as super capacitors use a roller to flatten the line to make a conductive layer in the device.

This information has been sourced, reviewed and adapted from materials provided by Cheap Tubes Inc.

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