What Are Nanotubes – Properties, Definition, History and Structure - Simply All You Should Know about These Extraordinary Nanomaterials

What are nanotubes properties? Are you searching for an answer of that question? Would you like to learn more than just plain definitions?

If you are interested in nanotechnology and are eager to know more about this particular nanoproducts than go on and continue reading.

This article is a complex overview which main purpose is to provide you with all important facts and information about nanotubes properties and also to define their structure as clearly as possible.

It will also discuss their possible applications based on their amazing physical and chemical properties.

Moreover, you will get to know fascinating details of the history of nanotubes discovery that has established the whole new field of study.

“Research can be undertaken in any kind of environment, as long as you have the interest. I believe that true education means fostering the ability to be interested in something.”

Sumio Iijima

What Are Nanotubes – Properties that Have Drawn Attention of the Scientists All around the World

Before we get to the point, we should start from the very beginning. You might have heard about the much unexpected discovery of Buckminsterfullerene in 1985. Thanks to a confluence of events the scientists that were originally investigating something else discovered a molecule that was made purely of carbon. In that time the chemists thought there is nothing more to learn about this element.

In fact this discovery of buckyballs was an absolute breakthrough. From that moment the chemical, physical and also material worlds would never be the same.

That was in the 80s, what you should know is that the nanotubes (also known as buckytubes) had actually been discovered about 30 years earlier, but unfortunately they were not fully appreciated at that time.

Let’s have a closer look at the series of events that eventually led to the discovery of nanotubes.

A Brief History of Carbon Nanotubes until Its Discovery

• 1950s – There are three major events that had indicated the discovery of nanotubes. The first one happened already in 1952. Radushkevich and Lukyanovich published a paper that showed hollow graphitic carbon fibers that were 50 nm in diameter (the Soviet Journal of Physical Chemistry). In 1955 the trio scientists Hofer, Sterling and McCartney observed a growth of tubular carbon filaments of 10 – 200 nm in diameter. In 1958 Hillert and Lange made n-heptane decomposition on iron at 1000 °C and they also observed a growth of nanoscale tubular carbon filaments
• 1960s – Roger Bacon at Union Carbide, studied carbon under conditions that were near to its triple point. While doing so he found till that time unknown carbon fibre. In fact, he observed straight, hollow tubes of carbon. These tubes seemed to consist of graphitic layers of carbon. What was even more interesting was that these layers were separated by the same spacing as the planar sheets of graphite
• 1970s – Precisely in 1976 Morinobu Endo observed these tubes again. This time they were produced by the gas-phase process (Chemical Vapor Deposition). He could even recognize that some of these tubes consisted of only one layer of rolled-up graphite
• 1985 – Fullerenes were discovered by Richard Smalley, Harry Kroto and Robert Curl Jr. at Rice University in Texas. As it was mentioned before this was a major breakthrough that has established a base for a whole new era of nanotechnology science and tremendously increased the research of that matter
•  1991 – Sumio Iijima, Japanese researcher of NEC, observed multiwall nanotubes. They were formed in a carbon arc discharge. It is true that they were observed prior to Iijima’s experiments; however he could provide strong evidence of their existence. He was not only able to take picture of them, but he also clearly explained what they really are. Therefore he was credited with the discovery and is often cited as the “inventor” of carbon nanotubes. He is definitely an outstanding figure who deserves recognition at this point.

Who Is Sumio Iijima?

He was born 2^nd May 1939 in Saitama prefecture in Japan. He obtained a Bachelor of Engineering degree in 1963 at the University of Electro-Communication in Tokyo. In 1968 he received his Ph.D. in solid-state physics at Tohoku University in Sendai.

He concentrated on the research of crystalline materials and high-resolution electron microscopy at Arizona State University (1970 – 1982).

From 1982 – 1987 he worked for the Research Development Corporation of Japan, where he studied the ultra-fine particles. After this he joined NEC. He is still working there until today.

His work and research has had a huge impact on the materials science field of nanoscale science and electronics.

In 2002 his efforts were recognized and he was awarded by the Benjamin Franklin Medal in Physics for the discovery of single-wall and multi-wall carbon nanotubes and explanation of their structure and helical character.

His work has inspired the whole world. Thanks to his experiments and findings it is clear these nanomaterials have huge potential. They are extremely small, but very strong and show amazing electrical conductivity. These findings have triggered numerous researches which aim is to investigate their extraordinary properties that suggest many possible applications.

What Is Carbon Nanotubes Definition?

Carbon nanotubes (CNT) are a group of nanomaterials that are formed by two-dimensional hexagonal lattice of carbon atoms. These atoms are bent and joined in one direction. In other words, if you look at them, you would see a hollow cylindrical structure of carbon atoms. In this structure the hexagonal graphite molecules are attached at the edges.

CNTs are one of the two major families of fullerenes. They stand in between the buckyballs which have closed shells and graphene which structure is characterized by the flat sheets.

What Are Fullerenes?

Fullerenes are allotropes of carbon. This means that a fullerene molecule is comprised of carbon atoms. These atoms are connected by single or double bonds. They can form either closed or partially closed mesh. This mesh is formed by fused rings of 5 to 7 atoms.

Fullerene molecule can have a shape of a hollow sphere, ellipsoid and tube. Actually they can have many other shapes and sizes as well.

Nanotechnology and Nanomaterials Industry

The most famous fullerene is definitely Buckminsterfullerene (formula C₆₀). You already know it was the first discovered fullerene and it happened in 1985. This historical event has not only set the base for the new science of nanotechnology, it has also started a completely new industry. Soon after there was provided the evidence of buckyballs’ existence and thanks to Kratschmer-Huffman apparatus there were produced the first gram quantities of it in the lab. Everybody wanted to investigate this truly peculiar molecule. There was a high demand on fullerenes for sale despite the fact the price of 1 g could exceed 1000 $. A lot has changed since that time. Nowadays you can buy high quality fullerenes for much affordable prices. It is possible thanks to an advanced technology and production processes.

What Is Graphene?

To make it absolutely clear let’s define what graphene is. The same as fullerene also graphene is an allotrope of carbon. Its form is two-dimensional hexagonal lattice with one atom at each vertex. Considering its structure it could be seen as an extreme member of the fullerene family.

Graphene is the basic structural element of charcoal, graphite, fullerenes and carbon nanotubes.

Basic Characteristics of Carbon Nanotubes

• Size – they are very small. Actually they are usually only a few nanometers wide. But their length is something else. Just imagine that it can range from less than a micrometer to several millimeters
• Ends – can be both closed and opened (closed are more often). It might be interesting to know that in some cases the tube is getting narrower in diameter before closing-off
• Unique molecular structure – based on their structure we differentiate single-wall and multi-wall carbon nanotubes. Single-wall carbon nanotubes (SWCNTs) are one-dimensional carbon nanomaterials. They consist of sheets of graphene. These sheets are rolled up and form hollow tubes. Their walls are one atom thick. On the other hand multi-wall carbon nanotubes (MWCNTs) are form of carbon nanotubes where two or more single-wall nanotubes are nested inside one another. Thanks to their unique structure they have absolutely extraordinary properties such as tensile strength, high electrical conductivity, high heat conductivity, relative chemical inactivity and high ductility

There is no doubt the structure is the key to uncover their full potential. Therefore you will find more elaborate information about both the structure and particular properties in the upcoming chapters.

The Overview of Carbon Nanotubes Properties – Mechanical and Electrical

Richard Smalley once said: “These nanotubes are so beautiful that they must be useful for something.”

They certainly have a unique set of properties that are a subject of an intensive research.

Mechanical Properties of Carbon Nanotubes – Strength and Elasticity

Considering their tensile strength and elastic modulus the CNTs are the strongest and stiffest materials that have been discovered so far.

Of course their exceptional structure has a lot to do with it. If you look at the model of nanotubes what you can notice is that the carbon atoms of a single graphene sheet of graphite create a honeycomb lattice. Each atom of this lattice is bond to three adjacent atoms by a strong chemical bond. That is why the basal-plane elastic modulus of graphite is one of the biggest of any known material. That is already something and it gets even more interesting.

SWCNTs are in fact stiffer than steel and they are also very resistant to a damage that is caused by physical forces. Therefore scientists expect them to be the top high-strength fibers.

The Pressure Test

If there is a pressure on the tip of a nanotube, it will make it to bend, but the tip will not be damaged by such an action. Moreover, once the pressing forces are removed, the tip will go back to its original state.

The Strength

The source of their strength is their covalent sp² bonds that are present between the particular carbon atoms. There were also numerous experiments which aim was to measure their tensile strength.

• In 2000, the scientists tested the MWCNTs and the result was the tensile strength of 63 gigapascals. What does it mean? Basically it represents their ability to resist tension of a weight which is equivalent to 6,422 kilograms-force on a cable with a cross-section of 1 mm²
• In 2008, there was conducted further research investigating their strength. This research has revealed that the individual CNT shells have actually strengths of up to 100 gigapascals

But it is important to mention that despite the fact that the strength of individual CNT shells is extremely high, the shear interactions between the neighboring shells and tubes are weak. Therefore the effective strength of MWCNTs and also carbon nanotube bundles is significantly reduced down to only a few gigapascals.

If there is applied high-energy electron irradiation which goes through inner shells and tubes, it effectively increases the strengths of these materials (for double-walled carbon nanotubes bundles to 17 GPa and for MWCNTs to 60 GPa).

It Is Also Good to Know

However, if the CNTs are compressed they are not nearly as strong. In fact they tend to buckle. In science it is basically a state of an instability that leads to structural failure. This happens because of their high aspect ratio and hollow structure. So if they undergo compressive, bending or torsion stress buckling may occur.

If the MWCNTs break, their outermost layers break first.

Electrical Conductivity

First of all CNTs are either metallic or semiconducting along the tubular axis.

For a given carbon nanotube with particular combination of n and m (n and m are structural parameters which indicate how much the nanotube is twisted):

• If n = m the nanotube is metallic
• If n – m is a multiple of 3 and n ≠ m and nm ≠ 0, this means that the nanotube is quasi-metallic. In such a case there is a very small band gap
• Any other way the CNT is a moderate semiconductor.

Can They Also Be Semi-metallic?

If you are interested why carbon nanotubes are not semi-metallic, the reason for that is their degenerate point (degenerate point is where the π bonding band gets together with the π* anti-bonding band. The energy drops to zero at this point).

Their degenerate point is a little bit shifted away from the K point in the Brillouin zone (Brillouin zone is an important concept in solid state physics. It is a particular choice of the unit cell of the reciprocal lattice.)

This slight shift happens because of the curvature of the tube surface and it is the reason of hybridization between the σ* and π*anti-bonding bands. This results in modifying the band dispersion.

The Most Conductive Carbon Fibers Known

They have measured the conductivity and resistivity of single-wall carbon nanotubes by putting electrodes at different places of carbon nanotubes. What they have found out was that the resistivity of the SWCNT ropes was 10^–4 ohm-cm at 27 °C.

In other words it means that the single-wall carbon nanotube ropes are the most conductive carbon fibers man has known.

Considering the current density there was possible to achieve 10⁷ A/cm². But theoretically the SWCNT ropes should be able to sustain as high as 10¹³ A/cm² stable current densities.

More Interesting Facts

It might be also useful to know that it has been reported that SWCNTs have defects. But thanks to these defects SWCNTs can act as transistors.

How could it work? Transistor-like device could be formed by joining CNTs together. Moreover, a nanotube where there is a natural junction (a chiral semi-conducting section is connected to a straight metallic section) in fact behaves as a rectifying diode, which basically means that there is a half-transistor in a single molecule.

An American physicist Paul McEuen confirms that: “Carbon nanotubes are amazing because they're really good electrical conductors, yet they are only a few atoms in diameter. You can make transistors out of them in the same way you can with silicon. At Berkeley, we made the narrowest device anybody had ever made. It was basically a single molecule.”

It has also been recently announced SWCNTs can stream electrical signal at high speeds, precisely up to 10 GHz. This applies when used as interconnects on semi-conducting devices.

Thermal and Optical Properties of Nanotubes

The list of their extraordinary properties has not been finished yet. CNTs are also great thermal conductors and have very useful optical properties.

Thermal Conductivity and Expansion

Well, that is something really exciting. The recent studies and researches (University of Pennsylvania) suggest that carbon nanotubes may be the best heat-conducting material in the world.

Again their exceptional thermal properties are possible because of their unique structure and small size.

Carbon nanotubes exhibit ballistic conduction. It is a term known in mesoscopic physics. Ballistic conduction is in fact the transport of charge carriers in a medium where the medium are usually electrons. The electrons have insignificant electrical resistivity which is caused by scattering. If there was no scattering electrons would behave according to the Newton’s second law of motion at non-relativistic speeds.

An individual single-wall carbon nanotube has a room temperature thermal conductivity of approximately 3,500 W·m⁻¹·K⁻¹ (along its axis).  For example copper that is well known for its very good thermal conductivity transmits 385 W·m⁻¹·K⁻¹.

You should also know that if the room-temperature thermal conductivity is measured across the axis it is about 1.52 W·m⁻¹·K⁻¹. This means they are almost the same thermal conductive as soil.

Moreover, the ultra small single-wall carbon nanotubes exhibit superconductivity below 20 K which refers to – 253.15 °C.

It is all possible thanks to their very strong in-plane graphitic C-C bonds. These bonds make them unusually resistant against the axial strains. There is a huge inter-plane expansion, but almost zero in-plane thermal expansion. This indicates strong in-plane coupling and also high flexibility against nonaxial strains.

These extraordinary thermal properties propose many possible applications such as their use in sensing and actuating devices, in nanoscale molecular electronics or also as reinforcing additive fibers in functional composite materials.

Optical Properties

As it was mentioned above carbon nanotubes have also very interesting optical properties such as absorption, photoluminescence and Raman spectroscopy.

What is amazing is that the spectroscopic methods could quickly and non-destructively characterize quite large amounts of carbon nanotubes. If we look at it from the industrial point of view such a characterization is much desired. It means that various parameters of nanotube synthesis can be changed in order to modify the nanotube quality. It can happen both intentionally and unintentionally.

Based on these properties they could be used in optics and photonics, for instance in light-emitting diodes and photo-detectors.

You should know that their advantage is not their efficiency which is still relatively low. In fact it is the narrow selectivity in the wavelength emission and detection of light. There is also a possibility of light tuning through the CNT’s structure.

Let’s summarize what you have learned so far.

Basically all of these amazing properties would have not been possible, if it was not for their structure. Therefore it is time to reveal more fascinating facts about it.

Absolutely Amazing Carbon Nanotubes Structure

Carbon is one of a few elements that are actually known since antiquity. It is an element of special nature. If we look at the CNTs structure what we can see is a perfect combination of carbon’s nature and the molecular perfection of buckytubes. That is what endows them with the extraordinary properties which you have already learned about in details in previous chapters.

What Exactly Makes It So Special?

1. First of all there is no other element in the periodic table that would bond to itself in an extended lattice with the strength of the carbon-carbon bond
2. Secondly the pi-electron that was donated by each atom does not stay with its donor atom. In fact it is free to move around the whole structure. That is the origin of the metallic-type electrical conductivity
3. Last but not least the high-frequency carbon-carbon bond vibrations produce an inner thermal conductivity that is even higher than diamond.

Single-wall, Double-wall and Multi-wall Fullerene Nanotubes and Carbon Nanostructures

You are already able to differentiate between the particular forms of buckytubes as it was discussed in the beginning of this article.

We will go a bit more deep into this topic.

The Structure of Single-wall Carbon Nanotube

As it was stated before carbon nanotubes are formed by a curved graphene sheet. These sheets have a shape of seamless cylinders derived from a honeycomb lattice. This represents a single atomic layer of crystalline graphite.

The structure of SWCNT is one dimensional and defined by the vector:

v = n ∙ a₁ + m ∙ a₂

In this vector a₁ and a₂ represents unit vectors and n and m refers to integers. If a nanotube is built up this way, it is an (n, m) nanotube.

Types of Configurations

• Zigzag (m = 0) – a path that turns 60 degrees, changing left and right after it steps through each bond
• Armchair (n = m) – could be defined as a path that makes two left turns of 60 degrees and every four steps follow two right turns.

It means that an infinite nanotube that is of the armchair (zigzag) type of configuration is only formed by closed armchair (zigzag) paths which are connected to each other.

Hybridization of Carbon Nanotubes

Buckytubes and actually fullerenes in general are extremely strong thanks to their orbital hybridization. In chemistry it expresses the process of mixing atomic orbitals into new hybrid orbitals. These new orbitals have different shapes and energies than the component atomic orbitals and are suitable for pairing of electrons in order to form chemical bonds (valence bond theory).

This concept causes that the bonds between neighboring atoms of carbon are the sp² type.

Sp² bonds are stronger than sp³ bonds in diamond and alkenes.

What Are Possible Applications of Fullerenes and Carbon Nanotubes?

Fullerenes and carbon nanotubes have really long list of possible applications. Some of them are already successfully implemented into our lives while the others have been yet under development.

So there is no wonder their discovery has engendered much excitement.

Considering their huge potential nanotubes could be a real revolution in material science.

Their Unique Geometry, Composition and Properties Could Be Used in the Following Applications:

• Energy storage – they show the inner characteristics needed in materials that are used as electrodes in capacitors and batteries. Nowadays both of these technologies are in the spotlights and the importance of their development increases. CNTs have enormously high surface area; precisely we speak about 1000 m²/g! Moreover, thanks to the linear geometry their surface is accessible to the electrolyte. You should also know that according to the research they have the highest reversible capacity of any carbon material. This could be used in lithium-ion batteries. They could be used as fuel cell components and in gas diffusion layers etc
• Thermal materials – CNTs may be the best electron field-emitter possible. They are polymers of pure carbon. Thanks to them there is an opportunity to alter their structure. Therefore it is possible to optimize their solubility and dispersion. Their extraordinary thermal properties could be used in many applications where it is desired to move heat from one place to another. So their practical use is found in electronics, specifically in advanced computing (uncooled chips can exceed 100 °C)
• Molecular electronics – the idea of constructing electronic circuit out of molecules is actually a key component of nanotechnology and CNTs make the ideal candidates for the connections in molecular electronics
• Structural materials – their exceptional strength and stiffness could be used for example in advanced composites which require high value mechanical properties
• Electrical emitters – in fact carbon nanotubes are the best field emitters in the world. It is possible not only thanks to their great electrical conductivity, but also because they have unbelievable sharp tip. Because of the sharpness of the tip they are able to emit at very low voltage. This is a very important feature for constructing low-power electrical devices
• Fabrics and fibers – CNTs form super strong fibers. Therefore they could be used in the vehicle and body armor, woven fabrics and textiles or in transmission line cables
• Biomedical – there are numerous ongoing researches investigating their potential use in biomedicine. It is important to mention that they appear to be non-toxic for a human body as cells can grow on them. Their possible applications might be: coatings for prosthetics, vascular stents, neuron growth and regeneration
• Conductive plastics - plastics made a huge progress considering their structural applications. Therefore they are often used as a replacement of metals. However they are not electrically conductive, in fact they are great electrical insulators. In order to overcome such a deficiency plastics are loaded up with conductive fillers. CNTs are ideal solution, because they have the highest aspect ratio from of any carbon fiber. They can provide very long conductive pathways at ultra-low loadings
• Conductive adhesives – for instance potting compounds, coaxial cables, and other types of connectors.
• CNT ceramic materials – ceramic materials that have been reinforced with carbon nanotubes
• And other applications such as: CNTs air and water filtrations, catalysts support, nanoporous filters, solar collection and many more.

Let’s See What Are Nanotubes Made of - The Techniques of Synthesis

• • Arc discharge.

• • Laser Ablation.

• • Chemical vapor deposition (CVD) – this is a very popular method, because it produces high quantities (by using particular catalysts). It is also possible to control morphology, diameter and length. However, it is difficult to achieve the repeatability.

• • High pressure carbon monoxide disproportionation (HiPCO) – this method has advanced catalysis thanks to which it is possible to produce high purity single-wall carbon nanotubes in bigger quantities. It happens under a high temperature (900 – 1,100 °C) and also high pressure (30 – 50 bar). The source of carbon is carbon monoxide and the catalyst is nickel/iron penta carbonyl.

To sum it up the first three named are batch by batch processes while HiPCO is gas phase continuous process.

It is important to mention that there are always impurities (for instance other forms of carbon and non-carbonaceous impurities) which have to be removed in order to be able to use carbon nanotubes in applications.

The fast development of the production processes have led to a raise in quality and production quantities and also to a significant drop in price, which is definitely the good news.

Therefore nowadays you can buy carbon nanotubes for just a few dollars per gram.

What Are Fullerenes and Nanotubes Properties – The Summary

There is absolutely no doubt that the discovery of fullerenes and carbon nanotubes was a major breakthrough. Their perfect structure and exceptional properties have a potential to change the material science.

Successful implementation of the numerous possible applications could solve the most emerging problems in the world.

This article has pointed out the most important milestones in the history of carbon nanotubes.

Now you are able to identify their structure and its configurations. You also know that its structure is the reason for their extraordinary properties that have been discussed in details as well.

You could find a list of their possible applications and applications that are already being used.

Last but not least you got to know what the common production methods are.

Do you agree with the statement that fullerenes and nanotubes are going to change the world as we know it? They sure have the potential for it. Is that a prediction of a far future or we are already just a step ahead it? If you share this excitement or if you do not, tell us your opinion.