A Primer on Carbon Nanotubes – Part 1

Part 1 | Introduction, Discovery, Structure & Properties

Nanotube starburst

Introduction 

Since their discovery over two decades ago carbon nanotubes have been heralded as a material that will come to provide foundations for a wave of major technological advancements during the 21st century.

Built from cylindrical layers of carbon, the apparent simplicity of nanotubes belies their extraordinary nature. With diameters just several billionths of a meter across, or nanometers (nm), nanotubes are the focus of engineering at some of the smallest scales currently being explored.

In part due to their infinitesimal size and features of their atomic structure, nanotubes boast a diverse set of exotic and valuable properties. Nanotubes are stronger than steel and harder than diamond but can be as flexible as cotton; they’re light-weight, transparent and hold unparalleled capacity for conducting electrical or thermal energy.

Such versatility has driven research seeking to take nanotubes in many directions. Scientists the world over are working to harness the unique physical characteristics of nanotubes with a view to applying them to not only advance existing technologies but construct new wholly materials and technologies too.

Plucking carbon nanotubes from a forest.

Drawing carbon nanotubes from a dense ‘forest’ of the material is a common way to isolate individual nanotubes before preparing them for particular tests or further fabrication processes.

In consequence to such efforts, with increasing frequency nanotubes are featuring at the centre of breakthrough materials and technologies. From bullet-proof linen and super efficient solar cells to flexible electronic devices and advanced energy storage systems, nanotubes are proving an invaluable material to engineer with. In such ways nanotubes are exemplifying the power of nanotechnology, the science of manipulating objects at nanoscale.

But in spite any indications that nanotubes are themselves the latest incarnation of our ability to engineer on the atomic level they’re actually far from new. They were first seen in Japan, in 1991. Frustratingly however, a combination of persistent challenges interjected by said demonstrations of  novel applications alongside steady progress in fabrication methods has kept them as a state of the art technology, seemingly always just around the corner.

“Nanotechnology is an enabling technology that will change the nature of almost every human-made object in the next century.” (National Science and Technology Council, USA)

It’s a favourable cusp to find ourselves on nevertheless. The promise of nanotubes is exceptional largely because of the sheer range of domains in which they are sure to have a massive impact. Name an industry or field of technologies and there’s fair chance nanotubes will feature in the envelope of its cutting edge research.

Reaching the day where nanotube based materials and devices are commonplace is proving a long road however. Precisely why this is the case we address later in this primer; sufficed to say that there are fundamental challenges relating to their manufacture that must be overcome before we might see their successful application.

Green Energy: A CNT forest recolured to resemble blades of grass. ASIN.

An award-winning photograph of carbon nanotubes, recoloured. ASIN.

To greater and lesser extents though, there are technologies already making use of nanotubes in the world today. The applications and areas of research to consider in this context highlights the extraordinary power and flexibility of nanotubes. In looking at some of them we gain an exciting glimpse into how pervasive nanotubes may become given their wholesale emergence into the world beyond niche applications.

But a glimpse is all it can be, for there remains a great deal to be understood about nanotubes and the science surrounding them. Fundamental research of nanotubes, which seeks to provide such an understanding, has come a long way since 1991 to be sure. But while this has brought about a greater understanding of nanotubes – how to control and manipulate them – it has borne yet more avenues through which we should anticipate the material bearing utility.

At the same time nanotechnology as a whole proceeds at pace, uncovering fantastic new ways to engineer devices and interact with the molecular world that amounts to a radically new form of science and technology. Altogether it’s therefore very hard to imagine how far reaching the significance of nanotubes will be in our lives, for technology and even for our understanding of the natural world.

In this primer we take a look at the super material; consider what carbon nanotubes are, what gives rise to their properties, how they might be applied in the world, and what stands in the way of a proliferation of nanotube technologies.

 

Carbon, Graphene & Graphite

To provide an appropriate introduction on carbon nanotubes we must consider their base material – the element carbon. The fourth most abundant element in the universe, here on Earth carbon is not just ubiquitous, it’s the fundamental chemical basis for life as we understand it.

In the natural world carbon is present in many forms that differ dramatically in their physical appearance. Termed carbon ‘allotropes’, these variations of carbon include diamond, charcoal and graphite (the ‘lead’ in pencils).

Diamond and graphite are useful examples to consider because they exemplify a key feature of allotropes – that although composed of the same element, their atomic structures are different to produce materials with starkly different physical properties.

Diamond is the hardest of naturally occurring solids; graphite is soft and brittle. Diamond is highly transparent; graphite is dark and opaque. Diamond is a very poor conductor of electricity; but graphite a very good one.

It’s easy to imagine that in the right circumstances these properties would be immensely valuable – and indeed they are. Diamonds are perfect for industrial drilling and cutting; while graphite finds itself with equal utility in a humble pencil or as a heat-resistant material protecting space ships.

diamond graphite

Diamond and graphite – two carbon allotropes as disparate in their appearance and physical properties as they are in their applications.

Although arising in the natural world, allotropes can also be engineered and manipulated. A prominent example of this is graphene – an artificial carbon allotrope composed of a two-dimensional layer of carbon atoms arranged in a hexagonal honeycomb lattice.

Graphene is hugely significant in its own rights on account of its extraordinary properties which are being exhaustively researched for use in many of tomorrow’s technologies. Graphene is also the basic structural element of other allotropes. Layering graphene produces graphite; while rolling it into tubes produces carbon nanotubes (often denoted as CNTs).

 

Discovering Carbon Nanotubes

In something of a chance discovery by researcher Sumio Iijima, the first direct observation of carbon nanotubes took place in Japan, June 1991.

Iijima had spent much of his career developing high resolution electron microscopy techniques which he applied in the course of his researching the structures of nanometer-sized crystals. Working at NEC in June 1991, Iijima was synthesising carbon fullerenes (a class of allotrope) using a technique called arc discharge. This work involves passing a current between two graphite electrodes in an atmosphere of evaporated carbon. The chemical reactions that ensue produce carbon residues in the form of soot and as deposits on the electrodes. In the course of examining his carbon samples Iijima found not only the spherical fullerenes he sought after, but also needle-like filaments.

It transpired that what Iijima had actually produced were cylindrical structures of carbon with walls just one atom thick. Each tube also contained several smaller sized ones; each one nested within an outer one. Their two-dimensional structure resembling graphene earned nanotubes their now often used description as tubes of rolled graphene.

In the fullness of time Iijima named the new molecules carbon nanotubes. Acknowledging the shelled structure of the new molecules, he termed the variety he first discovered as multi-walled carbon nanotubes. It was not for a further two years that Iijima and a colleague would be the first to produce the more simple, single-walled nanotube (1993). Iijima’s website describes his discovery in full.

Sumio Iijima is credited with discovery of carbon nanotubes in 1991.

Sumio Iijima is credited with discovery of carbon nanotubes in 1991.

The subsequent years saw many more labs begin to investigate nanotubes. Across the world groups were making predictions of their properties and developing techniques for fabricating them, including and beyond the arc discharge technique (which to this day remains a principal method for producing nanotubes; we return to this topic in ‘Fabrication’.)

By the end of the decade, much of what had been theorised concerning the electrical and structural characteristics of nanotubes had been proven and new avenues of research were opening up, often with an eye to applications.

From those earliest of days of nanotube research it was clear that nanotubes held enormous potential across a wide range of industries, from materials and electronics to health and imaging but also for nanotechnology itself which was taking off at pace through the 1990s.

 

Structure of Carbon Nanotubes

Nanotubes exist as allotropic macro-molecules of carbon, with a structure of rolled graphene. Useful simplicity of that commonly used analogy aside, it’s important to realise that nanotubes aren’t actually ‘rolled’ sheets of graphene. Instead, a nanotube forms as a result of chemical reaction that leads to carbon atoms bonding together to form a cylinder.

An animation of a single-walled carbon nanotube.

The prototypical cylindrical lattice structure of a single-walled carbon nanotube.

Because of the way nanotubes assemble themselves, gradually lengthening or growing as carbon bonds accumulate at their base, scientists often talk about nanotube ‘growth’. We’ll return to the issue of carbon bonding, growth and nanotube fabrication later.

Like graphene though, the walls of these cylindrical nanostructures are comprised solely of carbon atoms arranged as hexagons in a lattice, with each atom covalently bonded to three others. This arrangement of atoms confers onto nanotubes a two-dimensional structure. Typically nanotubes are closed off, or capped, at their ends via carbon atoms bonded into pentagons (rather than hexagons).

As carbon macro-molecules, nanotubes belong to the fullerene family – a class of carbon allotrope formed from carbon atoms arranged as hexagons and/or pentagons which collectively form a shape, for instance a sphere, ellipsoid, or in the case of nanotubes, a cylinder.

jij

The major structural forms of nanotubes. Left: Armchair, Zigzag and Chiral lattice arrangements. Centre: two single-walled carbon nanotubes. Right: a double-walled carbon nanotube. Click to enlarge. Image adapted from Rice University researchers.

While all nanotubes are built from a cylindrical lattice of carbon atoms, the lattice structure can vary to define the overall form of the nanotube.

Lattice variation depends on the position of carbon atoms relative to one another – something that is determined during nanotube growth. The arrangement of atoms necessarily effects the alignment of hexagons, such that they align with or at some angle against one another.

To picture this, imagine the difference between rolling a sheet of lined paper from its edge compared to rolling it from a corner, or some other angle – all three methods produce a tube, but the lines will fall at different angles.

In nanotubes, the alignment of hexagons throughout the lattice are described by indices ’n’ and ‘m’. Together these infer the nanotube’s chiral vector, or chirality. This is an important point because together with diameter, chirality translates to determine which of three known forms the nanotube will take: Zigzag, Armchair, or Chiral.

Armchair type nanotubes are defined by having hexagons in straight alignment (where m=n). (Armchair type nanotubes are always metallic and as such conduct electricity extremely well, almost without loss of any power at all.)

If the hexagons are turned through 30 degrees, then the nanotubes take on what’s called a Zigzag form (m=0). (Zigzag type nanotubes have semiconducting properties.)

In a Chiral arrangement, equivalent atoms of each hexagon are aligned on a spiral with hexagons angled at less than 30 degrees to one another (m≠n).

Beyond the structure of the nanotube wall, there is a second notable aspect to nanotube structure. There are single-walled nanotubes (SWNTs), with just one layer; and multi-walled nanotubes (MWNTs), which manifest as multiple nanotubes nested within one another – like a series of concentric cylinders each progressively narrower than its outside neighbour.

 

Properties of Carbon Nanotubes

In material sciences the physical and chemical characteristics a material expresses, either in isolation or when brought into contact with some form of energy such as heat or electricity, are described by its properties. It’s a material’s properties which render them suitable to a particular application.

When considering the properties of nanotubes we find them to be a truly unique material. In several contexts nanotube performance far surpasses that of other materials exhibiting the same characteristics. Because of the way in which nanotube properties emerge during growth, there’s great potential for tuning them in a way that optimises their utility in a particular application. Some applications are mentioned beneath alongside the related property, but the topic is discussed at greater length in part 2, ‘Applications’.

Concluding this first part of the primer we consider the mechanical properties of nanotubes, their size and strength, and their electrical and thermal conductance properties.

Carbon nanotubes

Carbon nanotubes often resemble sponges if left untreated after fabrication due to the variability in their direction of growth.

 

Size 

Nanotubes are measured on the nanoscale, with dimensions measured by just several billionths of a meter (or a millionth of a millimeter). Aside from the single atom thickness of their walls, a nanotube’s diameter is its smallest dimension, generally varying from less than 1nm to around 40nm. For comparison’s sake: a nanometer is about one ten-thousandth the thickness of a human hair; while DNA strands are 2nm thick and a red blood cell about 5000nm across.

MWNTs may have slightly wider diameters, but even here there is usually only 0.3-0.6nm separating the tubes.

While nanotubes are exquisitely narrow their length can be many many times larger. Ten years ago it was already possible to create nanotubes with a length-to-diameter ratio greater than 40 million. Such massive length-to-diameter ratio is key to the varied utility of nanotubes, and is unparalleled in material science.

While nanotube lengths are commonly measured in the scale of microns (where a micron, µm, is a millionth of a meter), advances have led to much longer nanotubes being constructed. One of the longest single nanotubes was 55cm (Tsinghua University in Beijing, published in ACS Nano, June 27 2013).

Having a nano-scale structure leaves nanotubes vulnerable to crystallographic defects that can lessen the tensile strength of nanotubes as well as undermine its electrical and thermal properties.

A single carbon nanotube is removed from a forest. This high-precision process is necessary to secure and prepare individual nanotubes for particular applications.

A single carbon nanotube is removed from a forest. A high-precision process that underlines the minuscule scales involved in engineering with nanotubes.

 

Strength

Like graphene, nanotubes are exceptionally strong, with tensile strength up to six times greater than that of high-carbon steel. Tensile strength refers to the amount of tensile stress (stretching) that a material can withstand before breaking. Critically though nanotubes are strong in spite of having a very low mass, or weight.

Liquid droplets on cnt

Droplets of liquid suspended by a mesh constructed from nanotubes. The image conveys the exceptionally high strength-to-weight ratio of nanotubes – here, the droplets have mass tens of thousands of times greater than the nanotubes. Image rights: UTD NanoTech Institute.

The bonds between carbon atoms provide nanotubes their strength, and they are stronger even than those found in diamond. Nanotubes are also elastic and can bend before returning to their original shape.

Of course there are limits to nanotube strength and elasticity. They are only as strong as their weakest link and structural irregularities in the lattice. Crystallographic defects such as atomic vacancies (missing atoms) or odd arrangements between atoms arising during nanotube growth, have a major impact on the nanotube’s final strength. Equally, nanotubes can become deformed if subjected to enough pressure.

Having said all this, while a single nanotube does have exceptional mechanical properties, for many practical purposes they will have to be bundled together into larger fibers in which nanotubes are well aligned and very compact for them to serve well.

Each of these pillars contains a million aligned nanotubes.

Each of these customised pillars contains over a million aligned nanotubes. Designing nanotube structures in this way is a common practice often required in order to exploit properties for a particular application. Image rights: Nanobliss.

The strength of nanotubes make them excellent candidates to fulfil functions within composite materials. Moreover as carbon polymers, nanotubes can be mixed together with other polymers (of which there’s a huge abundance) to form ultra strong, versatile compound materials.

 

Electrical Properties

Nanotubes exist as either a metallic or semiconducting material. In their metallic form nanotubes conduct electricity with unparalleled efficiency. In theory they are some one thousand times more conductive than copper. At Rice University in 2014 nanotube-based fibers were proven to have greater capacity to carry electrical current than copper cables of the same mass. There remains debate over whether nanotubes may exhibit superconductivity, the phenomenon of zero electrical resistance and expulsion of magnetic fields in a cooled material.

A carbon nanotube filament conduct electricity, and generates light in the same manner as a traditional lightbulb. Image rights: UTD NanoTech Institute. http://nanotech.utdallas.edu/research/.

A carbon nanotube filament conducts electricity and generates light in the same manner as a traditional lightbulb. Image rights: UTD NanoTech Institute.

Nanotubes have the highest current density of any known material, measured as high as 109 A/cm2 (amperes per square centimetre).

Alternatively nanotubes can exhibit a semiconducting characteristic – conducting electricity under some conditions but not others.

The electrical properties of nanotubes derive in large part from their one-dimensional structure. Crucially, having only one dimension means electrons can travel only forward or backward along the nanotube’s axis. Under such circumstances, only ‘back-scattering’ can produce electrical resistance; leaving nanotubes with a very low electrical resistance.

These peculiar electronic characteristics have been known of since the early 1990s, but they remain the subject of intense research.

We know that whether a nanotube is metallic or semiconducting depends on its structure, in particular its chirality and diameter. Understanding these determining factors in order that we learn of how to tune electronic properties of nanotubes is therefore the subject of much investigation. This is especially the case for those interested in the electrical applications of nanotubes where having an impure mix of both metallic and semi-conducting nanotubes when the application requires one or the other is simply not an option.

 

Thermal Properties 

Thermal conductivity refers to the rate of heat transfer through a material in steady state. The thermal conductivity of nanotubes is the highest of any known material, conducting heat extremely well along their length. Conversely, they are very good insulators to thermal energy lateral to their tube axis. Thermal conductivity of nanotubes has been demonstrated as being twice that of diamond, which prior to the discovery of nanotubes was the best known thermal conductor.

When compared to copper wires, a common thermal conductor, carbon nanotubes can transmit over 15 times the amount of watts per meter per Kelvin (W/m·K). Theoretical calculations suggest that the thermal conductivity for individual SWCNTs could be as high as 6000 W/m·K in the axial direction but with very small values in the radial direction. MWNTs are less thermally conductive, but have shown 3000 W/m·K.

The high strength bonds of carbon nanotubes also allows nanotubes to withstand very high temperatures, and they remain thermally stable up to 2800 degrees celsius in vacuum.

(A useful source for information on nanotube properties – CNT Composites)

 

Multi-walled CNTs

The inner tubes of MWNTs have notable telescopic properties, which make them strong candidates to perform operations in nanomechanical devices. More specifically, an inner nanotube may glide easily within its outer shell – almost without friction. This situation leaves nanotubes well suited to performing certain mechanic tasks – for example it has already been put to use in a rotational motor.

Image rights: Nanobliss.

Up close with the ends of two nanotubes fibers. Image rights: Nanobliss.

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A Primer on Carbon Nanotubes continues in part 2 covering Fabrication, Application & Challenges