Nano what?..The tiny technology of tomorrow

TWI Bulletin, March - April 2007

An emerging science of nanoscale proportions takes centre stage

by Amir Bahrami

Amir Bahrami is a senior project leader in the Advanced Materials and Processes group at TWI. He gained an honours degree in materials science and engineering from Queen Mary University of London. After spending time in the business consultancy world and then in the medical materials community he joined TWI in 2005. He has managed a range of small and large projects. He is doing a PhD research degree at the University of Cambridge in the field of carbon nanotubes and is a professional member of the Institute of Nanotechnology and a fellow of the Royal Commission. Amir has now taken on the task of developing and road mapping TWI's activities and business in the field of nanotechnology.

The apparently new phenomenon of nanotechnology or as it is usually abbreviated nano is thought to have an impact on all areas of science and technology. Much of its potential is yet to be realized. As Amir Bahrami reports the commitment to nanoscale research by the US and Japan and other industrialised nations, indicates the global perception of nano as a major driver of technological and economic change.

As an organization devoted to serving its Members, TWI has started developing expertise in nanotechnology so that it will be able to provide high standards of research, innovation, advice and industrial solution to the growing and future needs in this field.

What is nanotechnology?

This revolutionary field of science is defined as the art of producing, manipulating, controlling and exploiting properties of matter at almost atomic scale to produce materials, systems and structures with unique and desirable properties.

Taking this definition, one can envisage that such technology is not so new considering the invention of Indian ink, soap and many other materials that we have used for centuries. However the difference arises from the fact that it has only just been realised that we can take advantage of materials by deliberately manipulating them at very small scales. This is due to the development of instruments such as electronic microscopes. Seeing is believing. So there is potential for designing the building blocks of matter so that it will do precisely what we want it to do at a larger scale. This field of research can be divided into two interdependent categories:

Nanoscience, which is the study of the phenomena of nano-sized objects where properties differ significantly from those on a larger scale.

Nanotechnology, which is the design, characterisation, production and application of nano-scaled materials and systems by controlling their structure at the nano-scale.

The two are inseparable. So we refer to nanotechnology as a generic term encompassing both.

So how small is nano?

A nanometer is 10 ^-9 metre, but nano in terms of nanotechnology is defined as the size range from atomic dimensions ~0.3nm up to 100nm. Figure 1 below puts the scale of nano in context.

Physics at nanoscale

The properties of nano-size objects differ significantly from those of larger scale materials. Two principal factors cause this significant change; one is the increased relative surface area, and the other is related to quantum effects. These factors can enhance properties such as reactivity, strength and electrical characteristics.  

As the size of a particle decreases, its surface area per unit mass increases. For example, a particle of size 30nm has 5% of its atoms on its surface, this increases to 20% at 10nm and 50% for a 3nm particle. Therefore nanoparticles have a much greater surface area per unit mass compared with larger particles. As growth and catalytic chemical reactions occur at surfaces, this means that a given mass of material in nano-particulate form will be much more reactive than the same mass of material made up from larger particles.

In tandem with surface area effects, quantum effects can begin to dominate the properties of matter as size is reduced to the nanoscale. These can affect the optical, electrical and magnetic behaviour of materials, particularly as the structure or particle size approaches the smaller end of the nanoscale, for example electrical conductivity in a perfect nano-wire can be very high as the flow of charge is not disturbed through scattering by vacancies or lattice vibrations giving rise to ballistic conduction.

For materials such as crystalline solids, as the size of their structural components decreases there is a much greater interfacial area within the material; this can greatly affect the mechanical properties. For example, most metals are made up of small crystalline grains; the boundaries between the grains slow down or arrest the propagation of defects when the material is subjected to an applied load, thus increasing its yield strength. If these grains can bemade very small or even nanoscale in size, the interfacial area within the material greatly increases, which enhances its strength. For example, nanocrystalline nickel is as strong as hardened steel. Understanding surfaces and interfaces as well as the quantum effects is a key challenge for those working on nanomaterials and it is an area where the development of new imaging and analysis instruments with better resolution and accuracy is vital.

What are the common approaches to nanotechnology?

Nanotechnology encompasses precision engineering as well as electronics; electromechanical systems ( eg 'lab-on-a-chip' devices) and also mainstream biomedical applications in areas as diverse as gene therapy, drug delivery and novel drug discovery techniques. There are two fundamentally different approaches to nanotechnology, which are graphically termed 'top down' and 'bottom up'. 'Top-down' refers to making nano-scale structures by machining or etching techniques, whereas 'bottom-up', applies to building organic and inorganic structures atom-by-atom, or molecule-by-molecule. Figure 2 shows diagrammatic examples of the two approaches.

Market facts and figures

To compete in an increasingly competitive global market there is a drive for all industries to move towards improved technology and higher added value products. Nanotechnology is a promising candidate as an enabling technology to achieve this objective with key drivers being:

• Novel properties and functionalities
• Scalability - miniaturisation
• Reduced materials demand
• Reduced power requirements
• Easier recycling
• Reduced life cycle costs

As a result there has been unprecedented investment by both governments and industries globally to support research and development in this field. Table 1 shows the current spending and market values with projections to the near future.

Table 1 Predicted global nanotechnology market by 2015

Current activities and technology gaps

Generally each of the current activities in nanotechnology belong to one of the following categories:

• Nano materials
• Nano scale surface and interface science and simulation
• Nano devices
• Molecular self-assembly
• Metrology
• Nano-electronics
• Bio-nano-technology and nano medicine

Although all of the above are very important areas and are interdependent, the majority of the current technology gaps and the resultant delay in commercialisation arises from insufficient knowledge of how materials behave atnanoscale. Some of these knowledge gaps are as follows:

• Understanding of nanoscale materials (eg carbon nanotubes) and their properties
• Processing issues including: reproducible products in-line monitoring and control
• Development of techniques to allow processing on traditional machinery
• Modelling: lack of predictive modelling at the nano level
• Health and safety: determine the risk for nano-particulates and develop monitoring methods
• Recycling and sustainability

TWI activities in nanotechnology

Nanotechnology can only achieve all its potential if it is backed up by a sound understanding of the underpinning and fundamental science. TWI has therefore started a series of in-depth research activities involving carbonnanotubes. These mainly focus on exploiting the physical and mechanical properties of carbon nanotubes for industrial applications. Carbon nanotubes are a true example of nanotechnology and are the hottest topic of research in physics.These materials are considered for use across all industries due to their unique physical and mechanical properties. Carbon nanotubes are on the verge of bulk production, with companies hoping to produce tonnes of this material peryear to supply the growing industrial and research demand.

So what are carbon nanotubes?

Carbon nanotubes are molecular scale carbon fibres made of honeycomb lattices rolled into a cylinder. They can be thought of as a layer of a graphite sheet (graphene) rolled up into a cylindrical shape with diameters of 1 to 70nmdepending on the structure, and length of up to a micron. Such structures can consist of, a single wall tube, two concentric or double wall tubes or many concentric or multiwall tubes. Figure 3 shows different types of carbon nanotubes.

b) Schematic of a single wall nanotube

Although it was the work of Iijima in 1991 which shone light on the existence of carbon nanotubes as we know them today, reports of tubular carbon deposit on iron and nickel from carbon monoxide date back to 1955 in the paper by LJEHofer et al. Figure 4 shows in-situ growth of carbon nanotubes in high voltage TEM.

What makes these structures unique?

Carbon nanotubes have physical and mechanical properties which open the door to a whole new generation of devices. For example their sp2 bonding structure is much stronger than the sp3 bonds found in diamond, providing the molecules with unique strength. Their stiffness is in the order of one TPa with tensile strength of 200 GPa and specific strength 500 times greater than that of aluminium. Although like any other structure and material, they can have defects such as vacancies or non hexagonal lattices which subsequently can compromise their properties.

Another important property of carbon nanotubes is their transport behaviour; they are superb conductors of electricity. In this case they can be metallic or semi-conducting depending on the twist in the tube along the tube axis. The conductivity in metallic tubes is of ballistic nature where the transport of electrons along the length of the tube is not disturbed by lattice vibrations or vacancies in perfect tubes. They also have a very high thermal conductivity in the order of 3000 W/mK which is three times higher than that of diamond. This is due to the large mean free path for the lattice vibration quantum, where the conduction of heat by phonons is not disturbed over short distances.

How are carbon nanotubes made?

Carbon nanotubes are made by evaporating carbon with an electric arc or a laser, or by condensing a vapour of carbon atoms under the right conditions. The resultant tubes have key characteristics depending on the method of production and the process condition. These characteristics include: structure (SWCNT or MWCNT), purity, crystalline quality, entanglements and surface chemistry. Figure 5 shows a carpet of multiwall nanotubes grown by the chemical vapour deposition method.

What are the current applications of carbon nanotubes?

Carbon nanotubes are used as additives to polymeric matrices due to their high specific strength and stiffness. An application of this exists in modern tennis rackets which incorporate carbon nanotubes. Another example is the use of these tubes in laser equipment as mode-lucks. A mode-luck acts like a sieve and is wavelength and intensity selective. Usually molecular epitaxies have been used as mode-lucks but these are only for one wavelength.

Carbon nanotubes absorption band gap depends on their diameter and therefore use of a range of nanotubes with different diameters in a mode-luck will allow selection of a range of wavelengths.

They are also used as ultra capacitors. This is because the storage capacity in an ultra capacitor is proportional to the surface area of the electrodes and commonly these electrodes are made of activated carbon, which is extremely porous and therefore has a large surface area. However the pores in these electrodes are irregular in size and shape, which reduces efficiency. Nanotubes on the other hand have a regular shape and a size that is only several atomic diameters in width and therefore has significantly greater effective surface area. This results in long life and high power characteristics of a commercial ultra capacitor combined with the higher energy storage density of a chemical battery. Other applications under development include drug delivery systems and field emission (FE) electron sources, for applications such as flat panel displays for which prototypes have already been built.

In conclusion nanotechnology with its considerable potential is here to stay and steer the world of research and industry. As the demand for better and higher performance materials increases, and as the need for more energy efficient technologies prevails the possibility of manipulating matter at a nanometer scale becomes more and more attractive. TWI has taken a proactive approach to understand the underpinning science of carbon nanotubes, one of the most prominent fields in nanotechnology, so that we can continue providing excellent standards of industrial advice and solutions to our Members today and tomorrow.