Properties at the. Unit 11: Nanotechnology

Transcription

1 11. 1nanoscale Properties at the dimension The birth of nanotechnology can be traced back over half a century, to the revolutionary thoughts of scientists such as Richard Feynman and, later, Eric Drexler. They envisaged a future in which structures are designed at the scale of molecules and atoms to solve problems in electronics, engineering and medicine. The development of techniques to image and characterise such structures set in train the rapid development of the field in the current century. This topic guide introduces the history and development of nanotechnology and looks at the current applications. Nanostructures that can be created from carbon, such as nanotubes, nanospheres and graphene, are examined, along with a consideration of how the control of dimensions in these nanoparticles in turn control the properties and applications of the materials. On successful completion of this topic you will: know how structure controls properties at the nanoscale dimension (LO1). To achieve a Pass in this unit you need to show that you can: describe the benefits of reducing a problem to the nanoscale (1.1) outline the definitions, history and current commercial applications of nanoscience (1.2) describe the control of properties by structure in the carbon allotropes (1.3) define the lengthscale controls of electronic properties (1.4). 1

2 1 Nanoscale and nanoscience Before you start Resources available to support the topic guides in this unit fall into two categories broad introductory material with few technical details or advanced material, with detailed technical content, often considerably above level 5. At present, material that bridges this gap is hard to find. In this topic guide a number of suitable introductory resources are highlighted in the Take it further features. Key terms Paradigm: A term coined by Thomas Kuhn in his book The Structure of Scientific Revolutions (1962) to describe a framework of ideas, theories and rules within which scientists work. In a scientific revolution a paradigm shift may occur in which one framework is replaced by a completely new one. Nanoscale: This term is used to describe structures or devices with dimensions of a few nanometres (or, more strictly, where at least one key dimension has a size less than about 100 nm). Some sources quote dimensions in angstroms (Å) where 1 Å = m, so 1 nm = 10 Å. The birth of nanoscience There s plenty of room at the bottom In 1959 Richard Feynman, the American theoretical physicist, delivered a lecture with the engaging title There s plenty of room at the bottom. In this visionary talk he described how, in theory, structures as small as single atoms could be manipulated and assembled. He imagined how such techniques could be used to store data, to produce miniature computers and to develop new methods of chemical synthesis. Feynman s lecture is often credited with giving birth to the nanoscale paradigm the realisation that manipulating matter to create nanoscale structures was possible, and that by doing so it would give rise to a completely new set of properties. Feynman s vision would now be described as a bottom-up approach to creating nanoscale structures; nanotechnology today also uses a top-down approach, as will be discussed later on. Nevertheless, it is on this paradigm that the science of nanotechnology is based. In fact it was not until the 1980s, when serious research began into some of these ideas, that it was realised that they had already been anticipated by Feynman some 30 or so years previously. Benefits of nanoscale solutions Feynman had begun in his talk to describe the benefits of reducing a problem to the nanoscale, a scale in which structures are made up of a small number of individual atoms. These included: the way in which quantum effects become important at the nanoscale, meaning that the individual atoms in nanoscale structures behave differently from atoms in larger, bulk structures the fact that structures can be copied at the nanoscale to be absolutely identical copies of each other the potential to be able to synthesise any chemical structure by manipulating the atoms individually. In later sections of this topic guide other specific advantages will be discussed such as: novel optical properties, enabling them to absorb or emit precise frequencies of light ability to act as semiconductors 2

3 high surface area to mass ratio, enhancing the ability of the structures to take part in chemical reactions high tensile strength due to the absence of dislocations between separate crystals the ability of such nanoscale systems to self-assemble the small physical size of the nanoparticle, allowing it to penetrate biological structures such as plasma membranes the ability to manipulate the dimensions of a structure, allowing the tuning of optical properties, for example. Activity A typical carbon nanotube may have a diameter of 10 nm and a density of 2100 kg m 3. Imagine a single carbon nanotube with a length of 1 mm. Calculate: a the volume of the nanotube b the mass of the nanotube c the external surface area of the nanotube and hence a value for the mass/surface area ratio. Assume the nanotube has a cylindrical shape. Compare this with the mass/surface area ratio of a macroscopic crystal of diamond (assume this to be a cube with side length = 1 mm). The density of diamond is 3520 kg m 3. Key terms Nanoparticles: Particles with at least one dimension smaller than 100 nm. Nanoscience: The study of nanoscale structures. Nanotechnology: The construction and deliberate manipulation of nanoscale structures to exploit novel or improved properties. Nanotechnology in history Remarkably, nanoparticles have been in use since ancient times; the coloured glasses used in Roman pottery have been shown to contain gold and silver nanoparticles, as do some of the pigments used in medieval stained glass windows. The deep black printers ink, used for centuries, contains carbon black, consisting of nanosized carbon granules; carbon black is also used on a large scale as a reinforcing agent in vehicle tyres. Modern nanotechnology began when it became possible to image these nanoparticles and to fabricate them in a controllable way. The development of modern nanoscience As noted above, Feynman s ideas remained undeveloped for several decades until several lines of research combined to create the fields of study that we now call nanoscience and nanotechnology. Some key events in the development of modern nanoscience: 1974: Norio Taniguchi coins the term nanotechnology. 1981: The scanning tunnelling microscope was developed by Gerd Binnig and Heinrich Rohre, enabling surfaces to be imaged at the atomic level. This was the precursor to the more powerful atomic force microscope (1986). The imaging of individual atoms was a crucial step towards the eventual manipulation of atoms in nanotechnological applications. 1985: Fullerenes, nanoparticles consisting entirely of carbon atoms, were discovered by Harry Kroto, Richard Smalley and Robert Curl. Their work on fullerenes eventually led to the discovery and synthesis of carbon nanotubes. 1986: K. Eric Drexler explored the possibilities of creating self-assembling nanoscale structures in his book The Engines of Creation. The following decade was marked by much research but commercial applications of nanotechnology did not begin to appear until the early years of the 21st century. 3

4 The National Nanotechnology Institute (NNI) in the United States was set up in 2000 to help to fund and coordinate nanotechnology research and development. Take it further Introductory level: The website of the NNI has a detailed and informative timeline of nanotechnology, tracing the origins back even before Feynman s seminal lecture, and providing some excellent illustrated examples of the research and development of commercial products that has been carried out in the past two decades ( Link Several of these commercial applications are discussed in greater detail in Topic guide This topic guide includes more details about the way in which nanotechnology allows specific commercial needs to be met. Key terms Photonics: The field of science relating to the emission, detection and processing of electromagnetic radiation, especially visible light. Optoelectronics: A field within photonics related to the study and application of electronic devices that emit, detect and control light. Nanocomposite: A material made of two or more constituent materials, in which one or more dimensions is less than 100 nm. Current commercial applications The range of commercial applications of nanotechnology is vast and evergrowing, but the following areas could be regarded as particularly important. Electronics and computing Computer chips can be made smaller (and hence faster) by using nanoscale components or by creating smaller nanoscale features. Photonics and optoelectronics Incorporating nanoparticles into LEDs increases their efficiency and allows the LEDs to be designed to emit specific wavelengths of light. The efficiency and tunability of the interaction between nanoparticles and radiation is also made use of in solar cells and light sensors. Nanomedicine Nanoparticles are used as drug delivery systems, by attaching drug molecules to nanoparticles or encapsulating them within nanoparticles. They are also being used as biosensors to detect specific biological molecules and as contrast media in imaging techniques such as MRI. Nanotechnology is used in regenerative medicine to produce scaffolds for the growth of new tissue and to create implant devices. Cosmetics, skin creams and sunscreens Manufacturing these products in the form of nanoparticles allows them to spread more evenly, penetrate skin more deeply or appear invisible against the skin. Textiles and sports equipment Nanomaterials can be incorporated into textiles to impart or improve properties such as abrasion resistance, water resistance or antibacterial properties. Carbon nanotubes and nanocomposites are used to increase strength in sports equipment, such as tennis racquets, or to improve other properties such as water resistance. Construction and engineering Inspired by the observed strength of biological materials such as bone or mollusc shells that contain nanocrystals, high-strength concrete is now being manufactured by incorporating nanocrystals of silica or calcium compounds. Incorporating nanoparticles into paints can improve their hardness, scratch resistance and anti-corrosion properties. 4

5 Nanoparticle additives in machine lubricants, made from zinc, titanium dioxide or even silver, have improved performance and reduced wear and friction between moving parts. Energy generation and storage Batteries using nanoscale lattices of silicon or carbon nanotubes have been shown to increase the rate at which energy can be supplied from a battery, reduce charging time and increase the shelf life of the battery. Pollution control Nanoporous fibres or nanocatalysts can be used to remove pollutants from car exhausts or from industrial emissions. Nanoscale features in ion exchange resins or semi-permeable membranes are increasing the efficiency and selectivity of water softening or water purification devices. Security Nanoscale lattices are being incorporated into banknotes in the new generation of anti-counterfeiting strategies; the lattice acts as a diffraction grating and produces a visible coloured pattern on the note that is extremely difficult to reproduce. Activity Choose one of these applications and find two websites that describe your chosen application in more detail. Summarise the most significant extra information that you find on these websites in a short report. Take it further Introductory level: A comprehensive inventory of the current commercial applications of nanotechnology-based products is available through the Project on Emerging Nanotechnologies: The ANEC/BEUC inventory of products containing nanotechnology ANEC-PT-2009-Nano-015.xls may be a useful additional source. Activity Using the NNI website ( as a starting point, choose some examples of nanotechnology products that have current commercial applications. Write a description of the product and the application for which it is used. Explain how using nanoscale technology allows it to be used in this way. Nanotechnology principles Take it further Introductory material: An excellent visual introduction to the principles and applications of nanotechnology is the 30-minute video Nano the next dimension produced by the European Commission in Researchers from all over Europe explain some of the key historical moments in the story of nanotechnology, including the introduction of the scanning tunnelling microscope, the formation of C60 and nanotubes, as well as describing some of the applications of nanotechnology they are developing. It is available at A similar (but shorter) presentation of some of the applications of nanoscience is available at 5

6 Key terms Top-down: Using conventional large-scale (bulk) techniques such as lithography (printing a thin film) or crystallisation, and scaling them down to nanoscale dimensions. Bottom-up: Assembling atoms or molecules in a controlled manner. To create nanoscale materials in a way that enables them to be used in a commercial context, two key problems must be overcome: There must be a technique for fabricating the nanostructure. This can be a top-down or bottom-up technique, or it may involve self-assembly, which is at the interface of these two techniques. The dimensions of the nanostructure must be able to be controlled, as it is these dimensions, which determine the key properties, on which commercial applications rely. Link Fabrication methods are discussed in detail in Topic guide Portfolio activity (1.1 and 1.2) Write a report outlining the importance of nanotechnology. In your answer you should: explain what is meant by nanotechnology describe the important stages in the development of modern day nanotechnology discuss how the use of nanoscale structures allows problems to be solved in a range of applications. Describe a range of examples to illustrate your answer. Figure : (a) A gold nanoparticle (zero dimensional), (b) a carbon nanotube (one-dimensional) and (c) a graphene film (two-dimensional) are examples of nanostructures with different numbers of macroscopic dimensions. 2 Control of properties: dimensions As described earlier, control of properties at the nanoscale is frequently achieved by the control of dimensions. This can be in terms of dimensionality the number of macroscopic dimensions possessed by the nanostructure or the actual size of the dimensions (both nanoscale and macroscopic). Nanostructures can be regarded as zero, one- or two-dimensional, as shown in Figure , although nanoparticles, such as the gold nanoclusters shown, are also sometimes described as three-dimensional. An excellent example of this is provided by a study of the allotropes of carbon. These include several different types of nanostructure with different dimensionality, including the fullerenes and carbon nanotubes, which were amongst the first nanoparticles to be discovered and characterised. (a) (b) (c) 6

7 Figure : In diamond, carbon atoms are arranged in a three-dimensional network; in graphite the atoms are arranged in two-dimensional layers that are packed on top of one another. Diamond and graphite Carbon is able to exist as several different allotropes. The most familiar naturally occurring allotropes are diamond and graphite. Both of these allotropes exist as macroscale or nanoscale forms. The three dimensional structures of diamond and graphite are shown in Figure Key terms Allotropes: Two different physical forms of the same element; they may differ in the structural arrangement of the atoms in a solid crystal or in the number of atoms in each discrete molecule. Macroscale: The scale at which objects are observable and measurable by the unaided eye. Link Hybridisation of atomic orbitals was introduced in Unit 5, Topic guide 5.4. Figure : Electron micrographs of (a) nanocrystalline diamond, showing smooth rounded crystals forming a thin film, and (b) graphene. Diamond can be regarded as a single molecule of carbon with macroscopic dimensions. The strong three dimensional network structure of this crystal explains the hardness and high melting point of the bulk structure. Both the bonding and the arrangement of the atoms in graphite differ from that of diamond. The carbon atoms in graphite display sp 2 hybridisation whereas those in diamond display sp 3 hybridisation. Hence in graphite, although it has a three dimensional structure, the carbon atoms form two-dimensional sheets, with the unhybridised p electrons delocalised over the sheet. Only weak Van der Waals forces exist between the sheets. The high electrical conductivity of graphite is related to the presence of delocalised electrons and the softness of the solid is related to the presence of only weak forces between the sheets. Graphene and nanocrystalline diamond Both diamond and graphite can be reduced to the nanoscale as shown in the electron micrographs in Figure (a) (b) 7

8 Nanocrystalline diamonds can be manufactured (for example, by decomposing methane by radio-frequency electromagnetic radiation). These nanocrystals show some differences from bulk diamond: they show greater chemical and biological activity they are coloured solids. However, they also display many of diamond s useful properties yet can be manufactured in the form of thin films and hence ultimately can be cheaper than using bulk diamonds. Graphene is, essentially, a single sheet of graphite and hence is a two-dimensional nanostructure. At this nanoscale, the presence of the delocalised electrons gives rise to several interesting properties: very high thermal conductivity hardness similar to diamond, but flexible and able to be stretched like a polymer able to adsorb a range of molecules and atoms. Take it further Introductory material: You can read more about the discovery and applications of graphene on the website developed by the team from the University of Manchester, which first isolated and studied graphene: Figure : The structure of Buckminster fullerene, consisting of 60 C atoms. Fullerenes and nanotubes Fullerenes In 1985, a team led by Harry Kroto, Richard Smalley and Bob Curl discovered that carbon atoms could be made to aggregate into clusters containing 60 (or sometimes 70) atoms. They deduced that the structure of the C 60 molecule was a hollow sphere, as shown in Figure , and it was named Buckminster fullerene (or, more affectionately, Buckyball) because of the resemblance of the structure to the architectural designs of the architect Buckminster Fuller. The structure, consisting of interlinked hexagons and pentagons, gives great structural stability to the molecule. In fact a range of other stable structures exists, based on the same structural principles, and these are known collectively as fullerenes. As discrete molecules, these simple fullerenes are classified as zero-dimensional nanostructures. Properties such as strength and conductivity are obviously not relevant to zerodimensional nanostructures, such as these fullerenes. Instead, their unusual chemical properties are most significant, for example: they can form complexes to metal atoms or ions, either by encapsulating the metal inside the structure or by the metal binding to the surface; organic groups can also be attached to the surface they are soluble in non-polar solvents such as methyl benzene. 8

9 Figure : The structure of some carbon nanotubes (a) shows a multiwalled carbon nanotube (MWCT) where several cylinders are arranged concentrically and (b) shows a very short nanotube with closed ends. Nanotubes Carbon nanotubes are also classified as fullerenes but form a distinct class of nanochemical. They were first discovered in 1991 using techniques developed from fullerene research. In a nanotube, a layer of sp 2 hybridised carbon atoms, similar to that in graphene, becomes rolled up into a cylinder, as shown in Figure (b) on page 6. Figure shows more complex versions. (a) (b) Nanotubes are classed as one-dimensional nanoparticles. Although the diameter of a typical nanotube may be as little as 1 nm, they may be several centimetres long. Figure : Nanotubes can be engineered with various degrees of twisting, as shown in this molecular model. Key properties include: very high tensile strength flexible and elastic high electrical conductivity. Small changes in the arrangement of carbon atoms in the nanotube can introduce twisting into the structure (see Figure ), and properties such as electrical conductivity depend on the degree to which the nanotube is twisted. Hence it is possible to engineer nanotubes to display either metallic-like conductivity or semiconductivity. Take it further More information about the electrical properties and applications of carbon nanotubes can be found at 9

10 Link Fabrication methods used to produce carbon nanotubes will be discussed in Topic guide Activity Research the applications of some of the types of nanoparticle discussed in this section graphene, nanocrystalline diamonds, fullerenes and nanotubes. Activity Find examples of zero-, one- and two-dimensional nanostructures in this section. Compare the properties of these structures and suggest how the dimensionality of the structures can explain some of these properties. Portfolio activity (1.3) Discuss the different properties of nanoscale carbon allotropes. In your answer you should: list carbon allotropes which can exist at a nanoscale describe the structure of the allotrope comment on any significant properties explain how these properties are controlled by the structure. Take it further Introductory material: A useful website that provides information about carbon-based nanoparticles at an introductory level is nanomaterials.html. There are details of current applications and links to websites of companies that are developing and marketing these applications. Key terms Quantum tunnelling: The ability of a particle, such as an electron, to penetrate a barrier (such as an insulating layer). (Nano) length scale: A length or distance of a particular order of magnitude. In the case of nano length scale, this will be approximately Length scale control of electronic properties By reducing one or more of the dimensions of semiconductors to nanoscale sizes, quantum effects can be observed. These quantum effects particularly the existence of discrete energy levels and the existence of a phenomenon known as quantum tunnelling can be made use of in electrical and photonic devices, although quantum tunnelling in particular is also a cause of significant problems in such devices. Depending on the number of dimensions that are on a nano length scale, they are known as quantum dots, quantum wires or quantum wells. In each of these cases, the actual size of the nanoscale dimension affects key properties of the device. In this section you will see the way in which controlling the number of nano length scale dimensions in a device affects the properties of the device and therefore its applications. Quantum dots These are nanoparticles of semiconducting material, such as cadmium, selenium, indium or even rare earth materials. Because of the small size, electrons are constrained in all three dimensions and therefore behave more like electrons in a discrete atom than in a crystal. This means that transitions between well-defined 10

11 energy levels occur, as in discrete atoms. Hence if electrons are excited to higher levels, they will emit light of a specific frequency when they drop back down to lower levels. The frequency of light is tuneable by adjusting the dimensions of the nanoparticle. In the reverse process, absorption of light of specific frequencies can excite electrons into the conduction band, increasing conductivity. Figure shows the difference in the relative sizes of the energy gap between occupied energy levels (the valence band) and unoccupied energy levels (the conduction band) of a quantum dot and a semiconductor. Figure : The energy gap between occupied and unoccupied energy levels in a quantum dot means that visible light energy is absorbed and emitted when electron transitions occur. Unoccupied energy levels e e Conduction band ΔEQD ΔESC Occupied energy levels e e e e Semiconductor Quantum dot Valence band The smaller the quantum dot, the larger the energy gap between the energy levels and hence the higher the frequency of light emitted. Figure shows light emitted by solutions containing different sizes of quantum dots. Figure : Quantum dots of different sizes emit different frequencies of light. Quantum dots could be used in solar cells or computer display screens. Quantum wires Electrical wires made on a macroscopic scale possess an electrical resistance which depends only on the length and area of the wire, according to the wellknown formula: R = l ρa where ρ is the resistivity of the material from which the wire is made, l is the length in m and A is the cross-sectional area of the wire in m 2. 11

12 However, if wires are made sufficiently thin, the electrons become constrained in the dimensions at right angles to the length of the wire and this causes their energies to be quantised. This quantisation is most noticeable in semiconductor materials as the gap between energy levels is greater than in metallic conductors. Figure : Quantum wells have a structure in which thin layers of a semiconductor such as gallium arsenide are sandwiched between layers of a different semiconductor. AIGaAs GaAs AIGaAs At present the applications of quantum wires are still being researched. Quantum wells In a quantum well, electrons are constrained in one dimension, while being able to move freely in the other two. A quantum well is formed by sandwiching a semiconductor material (for example, gallium arsenide) between two layers of a different semiconductor material with a greater band gap (for example, aluminium arsenide). Figure shows the structure of a quantum well. The significance of gallium arsenide is that its band gap (the energy difference between the valence band and the conduction band) is described as direct, which means that electrons can be excited to the conduction band (and vice versa) without needing to undergo a significant change in momentum. This makes the process much more efficient and explains why substances like gallium arsenide are used in optical devices such as lasers. Conventional lasers have used thicknesses of gallium arsenide of above 100 nm (hence just outside the nanoscale); quantum well lasers have thicknesses of around 10 nm. The quantum behaviour of these systems means that, as with quantum dots, the wavelength of light emitted can be tuned by adjusting the thickness of the layer. As a result, the use of quantum wells is widespread in high-powered diode lasers, which have applications in materials processing (for the welding of plastic), or some medical procedures (hair removal, surgery and dentistry). Lower-powered lasers based on quantum wells are used in CD and DVD readers. Portfolio activity (1.4) Discuss the importance of controlling the number of dimensions that are nano length scale in electronic devices properties. In your answer you should: describe some devices which have one, two and three nanoscale dimensions. Checklist At the end of this topic guide you should be familiar with the following ideas: reducing structures to the nanoscale results in significant changes in properties, due to the increasing importance of quantum effects these properties enable nanostructures to have applications in photonics, electronics, medicine and many other industries the growth of nanotechnology was dependent on the development of devices to image and manipulate nanostructures and to fabricate them on a large scale allotropes of carbon, such as fullerenes and nanotubes, were among the first nanostructures to be identified and synthesised the dimensions and dimensionality of nanostructures can be controlled to create structures with appropriate properties semiconductors can be fabricated as zero-, one- or two-dimensional nanoparticles to enable them to have a range of uses in the photonics and electronics industry. 12

13 Further reading Finding suitable further reading in the area of nanotechnology can present some difficulties; many of the scientific texts are aimed at advanced undergraduate or postgraduate students, while websites often deal with concepts in a very general way. Additionally, the field of nanotechnology is a very fast-moving one and any discussion of applications is likely to be out of date within a few years. Feynman s classic lecture can be found online or reprinted in several texts: try or in Introduction to Nanoscience (S.M. Lindsay, OUP, 2010). An interesting article by the science writer Philip Ball reviews Feynman s influence and is available at Lindsay s book is detailed and mathematical at times, but Chapter 1 is a very readable introduction to the field. Up-to-date textbooks of inorganic chemistry may contain useful chapters on nanomaterials, for example, Inorganic Chemistry (5th edition) (Shriver and Atkins, OUP, 2010). The first few pages of Chapter 25 provide a good overview of the key principles of nanotechnology. A really good source of material at a variety of levels is available through the Nanotechnology Applications and Career Knowledge network, aimed at US undergraduates and high-school students. At an introductory level, a range of PowerPoint presentations is available relevant to this topic guide, including the history and current state of nanotechnology, as well as descriptions of some of the applications in medicine and electronics. These are available to registered users of the network but registration is free and available to all. Go to to register. Other, more specific websites giving extra information for specific aspects of this topic guide have been indicated in the Take it further features scattered throughout the text, which also include several sources at an introductory level. Acknowledgements The publisher would like to thank the following for their kind permission to reproduce their photographs: (Key: b-bottom; c-centre; l-left; r-right; t-top) Shutterstock.com: imredesiuk; Fotolia.com: Tyler Boyes 6(c), molekuul.be 6(l), apops 7(tl); Science Photo Library Ltd: 11, Pasieka 6(r), 9(tl), Laguna Design 9(tr, b), Kenneth Eward 7(tr); DK Images: Andy Crawford & Tim Ridley/Dorling Kindersley 8; University of Bristol, UK: CVD Diamond Group/School of Chemistry 7(bl); Getty Images: Lawrence Berkeley National Laboratory / MCT 7(br) All other images Pearson Education Every effort has been made to trace the copyright holders and we apologise in advance for any unintentional omissions. We would be pleased to insert the appropriate acknowledgement in any subsequent edition of this publication. 13