Carbon Nanotubes

by • March 22, 2012 • FeaturedComments (2)1021



The history of carbon nanotubes is not entirely clear even for those who work in the field. As with many great discoveries before them, there were several serendipitous events which went into the development of carbon nanotubes. Therefore giving proper credit to the person that invented the carbon nanotube is a rather difficult task. It is much more appropriate to simply identify who contributed along the way to making this field what it is today.

The first event worthy of note occurred in 1952 when two obscure Russian scientists, Radushkevich and Lukyanovich published a paper in the Soviet Journal of Physical Chemistry showing hollow graphitic carbon fibers with a diameter of 50 nanometers. Some identify this as the official discovery however others claim there was knowledge of their existence previous to this, for instance a patent from 1889 discusses the use of “carbon filaments” produced by the combustion of methane in light bulbs. The credit for Radushkevich and Lukyanovich’s discovery is still up for debate though, as these were certainly the first pictures ever taken of multiple walled carbon nanotubes taken by a Tunneling Electron Microscope (TEM) indicating an interest any previous discoverers did not have. In fact many of the scientists who studied carbon nanotubes were actually interested in avoiding their formation. They were typically materials scientists who wanted to find out how to prevent the creation of carbon filament impurities during the production of steel. Though these first few were unable to see the potential of carbon nanotubes there were many soon to come who would.


Examples of fullerenes, (left) buckminsterfullerene (right) carbon nanotube

Over the following years, work with carbon nanotubes or “carbon filaments” as they were often called continued in indirect ways, that is there wasn’t really anyone who was solely researching their properties and applications. Instead there were many small events in which scientists, researching other technologies, made notes relating to carbon nanotubes. For instance in 1960 two physicists, W. Bollmann and J. Spreadborough published a paper in the Nature discussing the friction properties of carbon due to rolling sheets of graphene. Also in 1976 Oberlin, Endo and Koyama reported that they had grown nanometer-scaled carbon fibers by a process known as Chemical Vapor Deposition. One very noteworthy occasion during this time was the official discovery of fullerenes in 1985. A fullerene is defined as any molecule which is completely composed of the element carbon. These molecules may come in the form of a sphere, known as a buckyball, and ellipsoid or a tube. The discovery of fullerenes was attributed to Richard Smalley, Robert Curl, James Heath, Sean O’Brien, and Harold Kroto who attributed much to the development of their study as well.


The discovery of fullerenes began a steady increase of interest in the field of nanomaterials in the scientific community. Following soon after this boom of concentration in the field came what some actually claim to be the official discovery of carbon nanotubes by the Japanese physicist Sumio Iijima in 1991. In this paper Iijima’s observations were of multiple walled carbon nanotubes, however two years later he was without argument the first to discover single walled carbon nanotubes in a paper he published in Nature. Particular note should be taken of Iijima’s work as it propelled nanotechnology to the forefront of much of the scientific community’s agenda at that time. Since Iijima and the realization of their potential, there has been a huge effort to learn all that we can about carbon nanotubes and we have come a long way, and the doors it has opened for us have been incredible.



Through many years of work with carbon nanotubes there have been numerous different types synthesized in the search for new properties and applications. Some were found by accident while others were an intentional blending of previous structures. No matter their origin they are all continuously being studied in the hope that one day one will provide the next big breakthrough.

The simplest structure is the single walled carbon nanotube (SWCNT). However even this basic structure has several permutations, the first of which is known as the armchair nanotubes. In order to differentiate between these structures we must define a description based on the characteristics of the nanotubes. The description used by many in the field of carbon nanotubes is to define (n,m) for the nanotube where n and m are the number of unit vectors along two directions in the honeycomb crystal lattice.  This description of SWCNTs is particularly advantageous as it allows us to directly calculate, or at least approximate, the diameter of an ideal nanotube with unit vectors (n,m). This is given by the formula:  where a=0.246nm. By this method, armchair nanotubes are characterized by m=n or (n,n) indicating an equal number of unit vectors in the two directions. The next type of SWCNT is the zigzag nanotube which is characterized by m=0 or (n,0). Every other SWCNT which does not fall into either of these categories is known as a chiral nanotube.

Left: Armchair Middle: ZigZag Right: Chiral

As one might guess, there are also multiple walled carbon nanotubes (MWCNT). These consist of multiple layers of rolled graphene (single sheets of sp2 carbon). Primarily, there are only two models currently used to describe the structure of MWCNTs. First, there is the more commonly utilized Russian Doll model, in which sheets of graphite are arranged in concentric cylinders. Basically this method consists of several SWCNTs inside of one another. Next there is the Parchment model, in which a single sheet of graphite is rolled in around itself. To visualize this method, one may think of a rollup up newspaper or as the name suggests a parchment. The interlayer distance in MWCNTs is close to the distance between graphene layers in graphite, approximately 3.4 Å.

A particular interest has been taken by many in the most basic subsection of MWCNTs, namely double walled nanotubes. These can be thought of simply as two SWCNTs placed inside of one another and so one may be inclined to think there should be nothing particularly special about them however, as large amounts of research have shown this assumption is false. While it is true that many of the physical properties of double walled nanotubes are basically a superposition of the properties of the two constituting concentric tubes, the exciting and promising difference is in their resistance to chemicals which is significantly improved. This property is of extreme importance when attempting to graft chemical functions at the surface of the nanotubes, this adding new properties to them. We may also note that current production methods, including catalytical CVD (chemical vapor deposition) have become highly efficient and accurate in creating double walled nanotubes and thus their study continues to grow at a great pace.

The remaining structures of carbon nanotubes receive much less attention than those mentioned previously and in fact some are actually completely theoretical. For instance a nanotorus is a theoretical carbon nanotube bent into a torus (doughnut shape). They are predicted to have many unique properties, such as magnetic moments one thousand times larger than expected for other types on nanotubes. Another newly created structure is carbon nanobuds. These combine two previously discovered allotropes of carbon, carbon nanotubes and fullerenes in fullerene-like “buds” covalently bonded to the outer sidewalls of the underlying carbon nanotube. Like double walled carbon nanotubes they display a combination of the materials which make them up as well as other advantageous properties. Another new hybrid material like the nanobuds are graphenated carbon nanotubes. These consist of graphitic foliates grown along the sidewalls of MWCNTs. While research with these materials has been limited, there have been reports of its use for enhanced supercapacitor performance. The final modern structure we will consider is the peapod. Another hybrid structure, the peapod consists of fullerenes trapped inside of a carbon nanotube. These have been noted to possess interesting magnetic properties with heating and irradiating. The fact that these hybrid structures have brought to light new materials with useful properties has led to the discovery of numerous other structures, the research of which is performed every day all over the world and the breakthroughs begotten from them have been incredible.




Carbon Nanobud
















As noted previously the study of carbon nanotubes began as an attempt to avoid their creation and now they are among the most highly researched topics in modern science for their applications. Obviously somewhere along the way we must have discovered them to have attractive properties to warrant such attention. In fact, carbon nanotubes display such incredible physical, electrical and optical properties that they are being integrated into numerous, diverse fields and allowing incredible advancements to take place wherever they are used.

To begin, we will look at the physical properties of carbon nanotubes. It is in this area that carbon nanotubes hold a valuable title, and that is that they are the strongest, in terms of tensile strength, and stiffest, in terms of elastic modulus, materials ever discovered. The strength of carbon nanotubes comes from the fact that the carbon atoms in the hexagonal lattice of graphite are held together by sp2 hybridized covalent bonds. This makes them stronger than diamond, a similar product of carbon however made up of weaker sp3 hybridized bonds. Also since carbon nanotubes have a very low density, 1.3-1.4 g/cm3, they also have the best specific strength of any known material. SWCNTs have also been experimentally shown to withstand pressures of up to 55GPa and corresponding calculations estimate a bulk modulus from 462 to 546 GPa, greater than that of diamond (420GPa). As wonderful as this may sound it does not tell the whole story as the strength of nanotubes against compression (force along the central axis) is weak and they will buckle at relatively small loads. We may also make note here of the experimentally found physical extremes of carbon nanotubes to date. The longest carbon nanotubes grown were 18.5cm which will certainly help the application of nanotubes in more macro settings. Also the thinnest carbon nanotube was a (2,2) armchair with a diameter of 3 Å.

The electronic properties of carbon nanotubes are another frontier which brought incredible discoveries and possibilities for modern technology. Early on in the development of carbon nanotubes, physicists theorized that their electronic properties were strongly dependent on their structural characteristics. In particular, “the diameter and the helicity of carbon atoms in the nanotube shells are believed to determine whether the nanotube is metallic or a semiconductor” (T. W. Ebbesen, H. J. Lezec, H. Hiura, J. W. Bennett, H. F. Ghaemi & T. Thio from Nature, 1996). As research methods and technology improved, these theories were proven true. It is now known that for a (n,m) carbon nanotube, if n-m is an integer multiple of three then the nanotube displays metallic behavior in terms of conduction. Consequently all armchair nanotubes (n,n) are metallic. When n-m is not an integer multiple of three the nanotube displays semiconducting characteristics. Thus zigzag and chiral nanotubes may be semiconducting or metallic. However this is not an absolute rule. To see where it fails we must note that in a metallic material the fundamental energy gap is approximately 0 eV. We may also note that the formula describing the fundamental energy gap is given by  where  is the C-C tight bonding overlap energy (2.7±0.1 eV), acc is the nearest neighbor C-C distance (0.142 nm), and d is the diameter of the nanotube. As we can see when considering a nanotube with a very small diameter the fundamental energy gap becomes larger, thus it is possible to have a zigzag or chiral nanotube which ought to display metallic characteristics but in actuality is semiconducting. One of the most important experimentally shown results of MWCNTs electrical properties is that they become superconducting at a relatively high temperature of 12 K. This property could be extremely useful for applications as reaching the more extreme low or high temperatures needed for other materials to become superconducting is very expensive and difficult to maintain. Also, when considering metallic nanotubes, modern theory holds that an electric current density of up to 4×109 A/cm2 can be carried, more than one thousand times greater than the capabilities of copper.


Van Hove singularities

The optical properties of carbon nanotubes are highly connected with the electronic properties. In particular they come from electronic transitions within the one-dimensional density of states. Because carbon nanotubes are one-dimensional materials, they do not display the characteristic continuous density of states as a function of energy. Instead the density of states decreases gradually and then increases at discontinuous spikes. These spikes are known as Van Hove singularities and because these spikes are so sharp, the optical transitions are also sharp and strong, thus it is relatively easy to excite all nanotubes with a particular (n,m), as well as to detect optical signals from individual nanotubes. This has been demonstrated experimentally and is incredibly useful. Also we may note that since the Van Hove singularities differ between metallic and semiconducting materials, it is possible to alter the optical properties of a carbon nanotube by varying its structure (i.e. n,m values) which is also a useful tool. These optoelectronic transitions also tie into the optical absorption properties of the nanotubes. Since the absorption originates from the electronic transitions, it follows the same trend of sharp peaks at certain energies and again fine tuning is possible by varying the structure. The most extraordinary aspect of the optical properties is without doubt the fact that it has been experimentally shown that vertically aligned “forests” of SWCNTs have an emissivity of 0.98-0.99 making it to a good approximation a black body. Needless to say this makes carbon nanotubes a prime candidate for research in solar energy. One final aspect of the optical properties of carbon nanotubes which must be observed is the process of photoluminescence. Photoluminescence occurs when a valence electron in a carbon nanotube

is excited into the conduction band via absorption of some outside energy. After it is excited it begins to

The process of Photoluminescence

“relax” back into its original position in the valence shell of the carbon atom. However it cannot simply return without removing the energy it absorbed and so it emits a photon, or light particle, with the proper energy (wavelength) it needs to remove. Thus if the nanotube is constantly being excited it will produce a stream of light. One may point out that this doesn’t exactly make sense since this whole process is not instantaneous, however it has been shown that while this is true, the whole process takes approximately 100 picoseconds, much faster than the human eye can perceive. The true beauty of this process comes from DeBroglie’s second law E=hf, or an emitted photon with frequency f has energy hf where h is Planck’s constant. By this law and the relationship between frequency and wavelength it is possible to emit light of any color if the electron is excited with      the correct amount of energy. It should be noted that this process only takes place in semiconducting nanotubes, as in metallic nanotubes, when the electron was excited, another electron would simply move to take its place and thus the excited electron would not be able to relax back.


Growth Techniques

As discussed earlier the formation of carbon nanotubes was at first a mistake, and really a nuisance to researchers. Now the creation of high-quality carbon nanotubes is the goal of researchers all over the world. Thus far the most productive techniques are arc discharge, laser ablation, and chemical vapor deposition (CVD).

Arc discharge and laser ablation were the first methods that allowed synthesis of SWCNTs in relatively large amounts (grams). These methods are performed by allowing the condensation of hot gaseous carbon atoms which were generated by the evaporation of solid carbon. In Arc discharge two pieces of carbon are used, one attached to a positive electrode and the other to a negative one. Both electrodes are given a large current and the carbon attached to the negative electrode sublimate because of the high discharge temperatures, however a metal catalyst was required for the actual creation of nanotubes. In the laser ablation method or laser oven, a laser vaporizes a piece of carbon and the gaseous atoms are collected on a cooler surface in the oven and form nanotubes. The problems with these methods include the expensive equipment requirements and large amount of energy consumed by them. Also these methods are not capable of producing ordered nanotubes on substrates which is the output most researches would like to have. Also there is no control over the nanotubes created, you are simply left with a powdered sample consisting of SWCNTs and MWCNTs. Despite these issues both arc discharge and laser ablation are widely used and produce nanotubes with relatively few defects.


Arc Discharge

Laser Ablation













The chemical vapor deposition method involves the decomposition of a carbon containing gas which is catalyzed by metallic nanoparticles on a flat substrate. More precisely, it is performed by preparing a substrate with a layer of metal catalyst nanoparticles, such as nickel, cobalt or iron. The substrate is heated and the growth of the nanotubes is initiated by the application of two gases which bled in the reactor: the first a process gas such as ammonia, nitrogen or hydrogen and the second a carbon containing gas such as acetylene, ethylene, ethanol or methane. Nanotubes grow wherever there are metal nanoparticles by the following process: the carbon containing gas is broken apart at the surface of the catalyst particle, and the carbon is transported to the edges of the particle, where it forms the nanotube. These nanoparticles are what determine the diameter of the nanotubes; therefore much research goes into their accurate development. Some methods currently in use to create them are patterned deposition of the metal, annealing, or by plasma etching of a metal layer. CVD is used the most out of any


Typical CVD system, carbon containing gas: ethanol, process gas: Nitrogen, Hydrogen







methods on the commercial level as it can easily be scaled up to industrial production amounts. Both SWCNTs and MWCNTs have been well developed using CVD and it has the particular advantage that it allows more control over the structure of the produced nanotubes. With this method we can produce well separated individual nanotubes on a large scale. Another advantageous result sought by researchers is for vertically aligned nanotubes as a field of nanotubes going off in all different directions is difficult to work with. In response to this desire a new method of CVD was created known as plasma enhanced chemical vapor deposition (PECVD). In this process, the PECVD reactor is pumped down to a low base pressure and a high voltage is applied causing the ionization of the gases which creates plasma during the growth of the nanotubes. The type of plasma created varies according to the experimental setup. Some examples of types of plasmas created are direct current (DC), radio-frequency (RF) and inductively-coupled (ICP) plasma sources. The process may use one or a combination of many types of plasma sources. This plasma is used to align the nanotubes as they will follow the electric field in the plasma which may be aligned perpendicular to the substrate thus creating vertically aligned nanotubes which is very useful in the applications of the nanotubes.

Following the physical creation of a large amount of carbon nanotubes, researchers must find a way to select only those nanotubes with the structure they are studying. The process by which they do this is known as “purification”. This is a very important stage in nanotube research as the final output of the current production methods used cannot guarantee only SWCNTs or only MWCNTs. Also the raw products are rarely without non-nanotube impurities such as amorphous carbon, catalysts, carbon nanoparticles and catalyst supports. Purification methods can be separated into two catagories: dry methods and wet methods. Dry methods are those that can selectively remove, through gas-phase oxidation, amorphous carbon species due to their higher reactivity compared to the carbon nanotubes. It was found however that a large amount of the usable sample was also destroyed using this method however a solution to this came by increasing the temperature of the air, for instance many research reports have shown that optimal temperatures for oxidation purification of SWCNTs synthesized by arc discharge is between 350oC and 470oC. The wet methods treat nanotubes in solution for purification purposes. These can be done alone or together with dry methods and it is very often that dry methods are followed by a step of acid treatment to dissolve metal oxides formed during the oxidation. Nitric acid (HNO3) is the most common solution used in wet methods as it is cheap and very effective in removing and amorphous carbon from large quantities of raw product. Other less used techniques include chromatography and centrifugation.

Despite the fact that there has been great progress made in selectively synthesizing carbon nanotubes of specific structure and electronic type, all methods currently in use produce mixtures of different types of carbon nanotubes. Because no highly effective method has been perfected yet, a final step is employed following purification and that is the sorting of these mixtures into specific structures or types. The parameters for the sorting process may include electronic type (metallic or superconducting), structure (length, diameter, chirality) or even specific pair indices (n,m). Unfortunately finding methods for selecting these parameters is also a rather difficult task and more often partial sorting is used in which samples are produced with a higher proportion of a particular type. The following is a list of selective forces used in sorting (in order from weaker to stronger): gravity (sedimentation), inertial forces (centrifugation), electric field (dielectrophoresis), selective noncovalent adsorbtion (by van der Waals forces, charge transfer), or selective elimination by a chemical (etching) or physical (electrical breakdown) process.



Over the past few years we have seen a huge amount of interest from the scientific community of carbon nanotubes and of course this would not be the case if there weren’t applications for this new technology. In this regard, carbon nanotubes certainly do not disappoint. The potential applications for carbon nanotubes grows every day as we discover more and more about them. Even today the accomplishments they have provided us with are astounding and we will undoubtedly see many more in the coming years.


Scanning electron micrograph of a carbon nanotube yarn symmetrically twist-spun from a MWCNT forest, length = 3.8 μm, twist: α= 37°

The first currently researched application for carbon nanotubes we will discuss is in the field of artificial muscles. Linear and bending carbon nanotube artificial muscles are well known. These may be powered by electricity, fuels, light, or heat and can generate stress 100 times that achievable by natural muscles. The issue is that there has yet to be found an artificial muscle of any type that provides large torsional rotation per muscle length, especially at high rotation rates. Recent research has suggested that strong, highly conducting yarns which are twist-spun from forests of MWCNTs may provide the solution to this problem. Experiments have shown that this yarn is able to make 41 full turns and return back to its

original state undamaged. The maximum start-up torque was at least τ =  = 10 nN·m, which is 1.85 N·m per kilogram of actuating yarn mass. This specific

torque is similar to that achieved by large commercial electric motors, which

ranges from 2.5 to 6 N·m per kilogram. Of course natural muscles are far stronger

than this as a result of complex evolution over the course of a billion years which can produce about 200 N·m per kilogram of active protein and a peak torque of 3700 pN·nm. Tests have also shown that the nanotubes are capable of producing a maximum power output of 61W per kilogram. However this power density is less than the 300 W per kilogram achieved by large, high-power electric motors. Despite the fact that these twist-spun yarns may achieve lower power output than other artificial muscles currently in use, the added functionality from thier rotational abilities makes them a promising product in modern medicine.





Sub-10nm carbon nanotube transistor electron microscope images. Approximate channel length (Lch) 9nm.

The next exciting application for carbon nanotubes is in electric transistors. For quite some time carbon nanotube transistors have been promoted as a replacement for silicon technology. The issue with the silicon transistors is that within the next decade many believe computing technology will require transistors with channel lengths below 10nm. However in silicon transistors, when channel length decreases, the ability to effectively control the electrical current in the transistor diminishes. This is called the short-channel effect and is a well known issue. Carbon nanotubes have been suggested as a replacement because of their superior electrical properties and ultra thin structures. This ultra thin body is what many researchers believe will eliminate the short-channel effect in transistors. Unfortunately there isn’t much theoretical work and neither have there been many experimental reports on how nanotubes will perform at sub-10 nm channel lengths. Not only were the theoretical works few, but those that were done did not provide promising results. Theoretical projections suggested

that a severe increase in inverse subthreshold slope would occur at

channel lengths of less than 15nm in carbon nanotube transistors.

Increased inverse subthreshold slope would mean a loss of gate control, which is a result of weakened electrostatics in the scaled channel. Also anticipated from theory is a loss of drain current saturation in the output characteristics. However a recently published paper provided experimental results which went against both of these theories. The researchers fabricated several transistors on the same nanotube which showed switching behavior comparable to the best silicon devices. Furthermore, the sub-10 nm carbon nanotube transistor provided low voltage operation that was superior to any similarly scaled device to date. The report concluded that more theoretical work was required in order to make progress in this field however their hope was that given their results new interest would be put into the purity and placement of nanotubes in transistor structures.


Modern Supercapacitor using MWCNTs, source: Vendum Batteries inc.

One particularly marketable application of carbon nanotubes is their use in energy-storage systems. Carbon nanotubes are seen as a preferred alternative to current systems because of their good chemical stability, high electrical conductivity and highly accessible surface area. In particular nanotubes are believed to show great promise in supercapacitors, lithium ion batteries and solar cells. Supercapacitors have been extensively researched over the past number of years because of their incredible useful ability to store and deliver energy rapidly and efficiently with a long lifetime. Also their wide range of power delivery makes it possible to hybridize them with other energy-storage devices. The main setback in supercapacitor technology is their inherent low energy density. This is where carbon nanotubes are believed to provide an answer. Supercapacitors with carbon nanotube electrodes have been experimentally shown to increase their energy density. The widespread use of supercapacitors in electronics, automobiles and several other areas means that any improvements nanotubes can make will have an outstanding impact on technology. Nanotube’s high-potential in lithium ion batteries has been suggested because of their large reversible capacity. The absence of a voltage plateau and the large hysteresis observed in regular carbon restricted its use in lithium ion batteries; however carbon nanotubes are currently being commercialized as the anode material in the batteries. The uniform spread of carbon atoms in the nanotubes allows it to act as a mechanically strong electrode. Some producers have reported as much as doubled energy efficiency of lithium ion batteries when incorporating carbon nanotubes. Alternative energy has become one of the hottest new topics in modern technology as we search for new methods to preserve our planet. Silicon based photovoltaic cells largely in use now have reached solar energy conversion efficiency of up to 25%. However these products require environmentally hazardous processing, defeating their original purpose, as well as being far from cost efficient in replacing current nonrenewable energy sources. Carbon nanotubes in the form of bulk heterojunctions in organic thin film solar cells could provide an alternative to these silicon cells. As one-dimensional nanostructures, nanotubes are ideal for electron transport as it means that the electron will have to travel a shorter distance before reaching the electrode. These cells are less costly to create and are much safer in their production. Currently, there is a huge amount of research into this application as any viable renewable energy source would be an incredible breakthrough and generate huge profits for those involved, not to mention the positive environmental impact.


Sandwich structure of bulk heterojunction organic thin film photovoltaic cell. Glass or PET substrate is first coated with indium tin oxide (ITO), polystyrenesulfonate (PSS) is then spin coated on top of the ITO to promote hole mobility into the electrode. The nanotube composite is then layered onto the device and an aluminum electrode is then thermally evaporated on top.




The final applications we will consider are those in which carbon nanotubes’ impressive mechanical properties are put to use. These hold the potential to be the biggest large-scale application for the material. Nanotubes have fifty times the specific strength of steel making them excellent load bearing reinforcements in composite materials. Even today they are already being used in light weight, high strength composites such as baseball bats and tennis rackets to spacecraft and aircraft body parts. Also the fact that nanotube’s mechanical properties such as flexibility depend on their structure which can very effectively be controlled is extremely useful in possible applications. Despite the seemingly perfect characteristics nanotubes have for mechanical applications, significant progress has not been made in developing nanotube-based composites that outperform the best carbon-fiber composites. The main problem is in creating a good interface between nanotubes and the polymer matrix and getting good load transfer between the two. The reason for this difficulty is that by most growth methods used nanotubes are made smooth and this causes a weak interface between the nanotubes and the polymer matrix. Also these methods produce aggregates of SWCNTs and MWCNTs which behave differently in response to a load, thus making it impossible to guarantee good load transfer.


Final Thoughts/Future

The progress we have made in the field of carbon nanotubes over the past decade has been truly astounding. They seem like such a simple material, made of only one element and so small one who is less informed may think them useless. Yet these simple tubes display astonishing characteristics and have allowed for exponential growth in modern technology. With so much effort being put into their perfection it is easy to believe that these trends of astounding forward progress will continue. Their potential seems limitless and we intend to find out just how far they can take us.

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