Evaluating current nanotechnology

Transcription

1 11. 3fabrication Evaluating current nanotechnology methods The focus of this topic guide is to research details of a nanotechnology product that is currently being developed, but is not yet on the market. Having reviewed the different methods of nanofabrication that have been developed, you will use information about the product to suggest or justify the choice of nanofabrication technique that will be used to manufacture this product, considering safety and cost implications as you do so. This unit also reviews a variety of commercial applications of nanotechnology, including the use of polymers and organic molecules, and enables you to discuss the way in which your assigned product meets a commercial need. On successful completion of this topic you will: be able to evaluate current nanotechnology fabrication methods (LO3). To achieve a Pass in this unit you need to show that you can: carry out an assessment of different nanofabrication routes to an assigned device design (3.1) plan commercial nanofabrication routes for the assigned device (3.2) produce a report assessing cost, quality and safety of the planned route (3.3) present the findings and make recommendations (3.4). 1

2 1 Applications of nanofabrication methods Link This topic guide links back to the material on fabrication that you encountered in previous parts of the nanotechnology unit, particularly Topic guide These ideas are developed further in order to understand how particular types of nanotechnology products are fabricated, such as nanoscale integrated circuits, MEMS, quantum dots and nanotubes. Case studies are included to show how particular devices and materials are fabricated in many cases there are several possible fabrication routes to particular types of device and these case studies will help you evaluate the choices that are made in nanofabrication processes. Key terms Integrated circuit: A miniature complex of semiconductor devices (for example, transistors and diodes) and passive components (for example, transformers, capacitors, resistors and inducers) that are bonded to a substrate. Doping: Introducing carefully controlled amounts of impurities such as phosphorus or boron into a silicon crystal. Optical resolution: The size of the smallest features that can be displayed or detected by an imaging system. Link You will learn more about the advantages and disadvantages of developing nanoscale integrated circuits in Topic guide Overview of nanofabrication The scientific principles underlying these techniques were laid out in Topic guide In this topic guide, you will be evaluating the fabrication techniques used to manufacture several commercially important nanoproducts: nanoscale integrated circuits microelectromechanical systems (MEMS) quantum dots nanotubes and nanowires. Integrated circuits Integrated circuits commonly referred to by names such as microprocessors or silicon chips have reduced in scale dramatically since their first use in Modern circuits may include components with critical dimensions as small as 65 nm and so these components must be fabricated using nanotechnology. Integrated circuit manufacture makes use of lithography, as described in Topic guide Before lithography is used, a silicon wafer is covered by a thin layer of crystalline silicon (which may be doped with a suitable impurity to create the desired electronic property). As you will recall from the earlier material, in lithography, a pattern from a mask is transferred onto a substrate surface by selectively removing regions of a resist layer that has been exposed to light or UV radiation. Nanolithography To produce patterns on the nanoscale, the optical resolution of the techniques must be improved. The resolution is limited by the wavelength of the light being used, because if the gaps on the mask have a similar wavelength to the light passing through them, the light will undergo diffraction, which causes it to spread out and makes it impossible to produce sharp features on the substrate surface. The conventional light-based lithography can be adapted to produce patterns with features below 100 nm in width by using high frequency ultraviolet radiation, with a wavelength of around 200 nm, but only by making use of specialised techniques for creating the mask. 2

3 To increase the resolution of the technique, X-ray lithography is used (where the wavelength of the radiation is around 1 nm). However, most nanolithography makes use of even higher frequency radiation, for example, X-rays. X-rays have a wavelength of around 1 nm and can increase the resolution to about 15 nm. Although the resolution of these processes is limited to various extents, the speed and relatively low cost (compared to the electron beam methods described below) are significant advantages. Electron beam lithography Increasingly, nanolithography uses electron beam lithography. Electrons can be regarded as having wave properties and the electrons used in electron beam lithography have a wavelength of nm, so very high resolution is, in theory, possible. However, with this system a mask cannot be used. Instead, a beam of electrons is guided across the surface of the resist. The process is far slower than the maskbased processes above, and the cost of the equipment used in the lithography process is very high. Lithography is covered in detail in Introduction to Nanoscience (S.M. Lindsay, OUP, 2010), Chapter 5. Key terms Transducer: A device that converts energy from one form into another. Microactuator: A device, such as a relay or motor, that converts an input of energy into motion. Figure : An electrostatically actuated MEMS device that converts changes in voltage into the motion of a motor. A classic and much-quoted text that describes and evaluates nanofabrication techniques in commercial use is Fundamentals of Microfabrication and Nanotechnology: Manufacturing techniques for microfabrication and nanotechnology (Marc J. Madou, CRC Press, 2011). The book provides the necessary details to help establish the appropriate fabrication technique for any type of micro- or nanomachine. MEMS As well as being instrumental in the manufacture of integrated circuits (at both the nano- and microscale), the two processes of thin film deposition and lithography are used to manufacture microelectromechanical systems, known as MEMS. These are miniaturised mechanical and electromechanical devices and structures in other words, devices with some moving element. MEMS can be used as transducers in the form of: microsensors (e.g. for temperature, chemical species etc.) microactuators (such as gas control valves and optical switches), as shown in Figure Introductory material: MEMS, NEMS and their applications are introduced on the MEMSnet website: 3

4 Key terms Bulk micromachining: A process used during lithography to create 3-D structures within the substrate by selectively etching the substrate. Surface micromachining: A process used during lithography to create 3-D structures on top of the substrate by depositing thin layers on top of the substrate which are then selectively etched. MEMS can also be integrated with conventional integrated circuits onto a single integrated microchip to create smart products that combine the computational ability of microelectronics with the sensing and control properties of microsensors and microactuators. Manufacturing techniques If the MEMS incorporates an integrated circuit, then plainly the manufacture of the MEMS will begin with lithography and etching as described above. To fabricate the 3-D components of MEMS, techniques such as bulk micromachining and surface micromachining are used. These are types of etching processes that create structures either within the substrate or on top of it. Case study: Production of a cantilever by surface micromachining Surface micromachining could be used to create a simple cantilever that can be used as part of a MEMS. A pattern is transferred to an oxide resist on the surface of a silicon substrate by lithography techniques. A second layer, consisting of a silicon-based polymer, is then deposited on top of this. The resist is then dissolved using hydrofluoric acid, leaving a cantilever-like structure constructed from the polymeric material. Question: Oxide resist layers may typically be aluminium oxide or silicon oxide. Find out why hydrofluoric acid is a common choice as an etchant for these oxide layers. Case study: Production of a complex three-dimensional shape using bulk micromachining Bulk micromachining involves the etching of the substrate that has been exposed by the selective removal of the resist. Using a technique known as anisotropic wet chemical etching, the etched surface is removed at different rates, depending on the orientation of the crystal surfaces in the substrate. It makes use of etching agents such as potassium hydroxide solution, which is very effective at etching silica substrates. The result of this is that precise geometric structures, and therefore fine detail, can be produced in the substrate material. By etching from both the front and back of the substrate, complex three-dimensional structures can be formed, as shown in Figure Etched region Frontside mask Silica substrate Backside mask Figure : The fabrication of a complex three-dimensional shape by bulk micromachining. Question: Look at Figure What evidence is there that the frontside and backside mask are manufactured from a different crystalline form of silicon from that in the substrate? 4

5 Details of how several MEMS devices can be manufactured by micromachining processes can be read at Find an example of a simple MEMS such as an accelerometer. The applications of these are discussed in more detail in Topic guide 11.4, Section 1. Find out about the structure of the MEMS and discuss how it could be manufactured. A very useful introduction to this is a video available on YouTube ( watch?v=kzvgku6v808), which describes both the application and fabrication of a type of accelerometer found in most smartphones. 3-DOM materials 3-DOM stands for three-dimensional object modeller. This is essentially the process known as 3-D printing, in which a 3-D object is created by building it up layer by layer from polymer or resin material. When applied to nanomaterials, 3-D printing is a time-consuming, and therefore expensive, process. The technique of two-photon lithography appears to offer much promise for speeding up the process by a factor of 1000-fold. Rather than creating the object using individual layers of resin, lines of solid resin can be created by focusing narrow laser beams at particular locations in a liquid. If just two photons are absorbed by the resin, it will harden and this will occur only in the centre of the laser beam. Remarkable levels of detail on a nanoscale can be achieved using this method, as can be seen in Figure Figure : An object fabricated using 3-D nanoprinting. 5

6 Introductory material: More details of the process used to fabricate the racing car shown in Figure can be found at which includes a real time video showing how the object was built up layer-by-layer. Two-photon lithography is described in a detailed paper at Publications/Papers_Files/0112.pdf. Fabrication Engineer I work in a company developing new molecular-electronic devices. I am responsible for devising methods of fabrication of these devices the circuit design engineers will have already devised the necessary structural features of the device and I will work alongside chemists to select suitable materials for the device and then to fabricate a pilot device, using characterisation methods to analyse it for defects. The pilot fabrication method will then need to be automated to enable large scale production. The work is very team-oriented and I need to have a good knowledge of all aspects of the design process from the electronics and chemistry through to the characterisation techniques used in quality control. Use research to find an example of a 3-D product fabricated by (a) surface or bulk micromachining; (b) 3-D printing on a nanoscale. By comparing the benefits and drawbacks of each method and the nature of the product, give reasons for the choice of fabrication method for each product. Self-assembly techniques: Quantum dots, nanotubes and nanowires Self-assembly Key term Self-assembly: A process in which disordered components (such as molecules) form an organised structure. This is due to the innate interactions between the components rather than any external factor. Bottom-up techniques, especially self-assembly, are important in the formation of nanostructures with an ordered molecular architecture, such as nanotubes and nanowires. Different approaches are used depending on the dimensionality of the nanostructure. Quantum dots As you saw in Topic guide 11.2, nanoparticles can be fabricated by top-down approaches such as chemical vapour synthesis. However, this method does not allow close control of the size of the nanoparticle. In quantum dots, which you were introduced to in Topic guide 11.1, the size of the nanoparticle determines several key properties. Look back at Topic guide 11.1 to remind yourself about the nature of quantum dots. Explain why it is important to control the size of the quantum dot. 6

7 A more controllable method for the fabrication of quantum dots relies on layer-bylayer self-assembly. This was described in Topic guide 11.2 charged surfaces are alternately exposed to solutions of polyelectrolytes. The method can be extended to produce thin films of one semiconductor material on another, for example, indium arsenide (InAs) on gallium arsenide (GaAs). Rather than forming a simple two-dimensional film, islands of InAs form on the GaAs surface that will act as quantum dots. The various methods that can be used to fabricate quantum dots are described in a presentation available at Key term Nanowire: A nanostructure with a length:width ratio of 1000:1 or more; regarded as one-dimensional nanostructures. Nanowires and nanotubes (1-D nanostructures) Several techniques exist to fabricate these structures: By templating: Nanoscale cylindrical holes, such as those found in zeolite structures, can be filled with the required material, for example, gold. This process can produce nanowires by completely filling the cylinders, or nanotubes by just coating the cylinder, as shown in Figure Figure : The fabrication of nanowires and nanotubes by zeolite-based templating. Zeolite substrate (a) Nanowires (b) Nanotubes Figure : Vapour-liquidsolid synthesis techniques. By vapour-liquid-solid synthesis techniques shown in Figure This is often used to produce nanowires of semiconductors such as silicon. Nanoclusters of iron-silicon act as targets for precipitation. These are melted in a furnace and atoms of silicon condense on the target nanocluster. The silicon grows in one direction (anisotropically) and hence a nanowire is formed. Small amounts of phosphorus or boron are introduced to form doped semiconductors. Nanocluster target Nanowire array 7

8 A very detailed source of information about self-assembly is the text Nanochemistry (Ozin and Arsenault, 2005). Chapter 3 deals with self-assembly of 2-D nanostructures, Chapter 4 with 1-D nanostructures and Chapter 5 with 0-dimensional nanostructures. Key term Nanorod: Structure, usually cylindrical, in which all the dimensions are in the range nm and for which the length: width ratio is between 3:1 and 5:1. Solution-based synthesis: nanorods and colloidal gold As described in Section 1 of Topic guide 11.2, solution-based synthesis is described as a soft-fabrication technique as it occurs at ambient reaction conditions. It usually results in the formation of spherical nanoparticles, although under certain conditions nanorods can be formed. Nanorods: three-dimensional nanostructures Nanorods, used as components of MEMS or in LCD displays, can be fabricated by vapour deposition or solution-based synthesis techniques, as described in Topic guide For example, ZnO nanorods are fabricated by creating a vapour of ZnO; nanorods are deposited using the interface between a substrate and a metal catalyst, such as gold, at a very high temperature. However, gold nanorods, with possible applications in sensing and imaging technologies, are fabricated using an adapted version of the colloidal synthesis. HO O O O O Au COO O O O OH COO Figure : Citrate ions bonding to the surface of a growing gold nanoparticle. Case study: Gold nanoparticle synthesis (zero dimensional nanostructures) Gold nanoparticles (or colloidal gold) have been used for centuries in stained glass manufacture due to the intense red or purple colours of the colloidal suspension. Modern methods of fabricating gold nanoparticles use a variation on the method first used by Michael Faraday in 1857, which formed gold by the reduction of solutions containing the [AuCl 4 ] complex. Sodium citrate, or other reducing agents (such as NaBH 4 ) are added to a solution containing [AuCl 4 ] ions. If sodium citrate is used, the citrate ions act both as the reducing agent and also as a stabiliser, capping the surface of the growing nanoparticle with a layer of citrate ions and preventing further growth. The size of the gold nanoparticle depends on the concentration of the citrate ions and on the ph of the solution. Comment on the advantages of a colloidal method as described in this case study, compared with the vapour-phase method mentioned above. Use ideas from the case study to explain why the presence of citrate will produce smaller nanoparticles. Suggest the effect on the size of the nanoparticles of (a) increasing the citrate ion concentration; (b) decreasing the ph (hint: the COO groups in the citrate ions are basic). A variation on the citrate reduction method for forming gold nanoparticles is described in Inorganic Chemistry (5th edition) (Shriver and Atkins, OUP, 2010), p , along with other examples of solution-based synthesis. 8

9 Key term Cross-link: Strong bond (usually covalent in nature) formed between adjacent polymer chains and causing a modification in properties such as flexibility, melting point or solubility. 2 Polymers and organic molecules Applications in nanotechnology The range of applications of polymers Polymers play a role in several features of nanotechnology. For example: photoresists are made of polymeric material polyelectrolyte polymers are the basic building blocks used in layer-by-layer self-assembly certain types of polymers known as block co-polymers can self-assemble into complex nanostructures. Polymers as resists As explained in Topic guide 11.2, during lithography, a layer of photoresist is applied to a substrate prior to exposure to light from the lithography mask: the photoresist may be positive so that when exposed to light it becomes soluble in a suitable solvent it may be negative exposure to light makes it insoluble in the solvent used in the process. The process of increasing or decreasing solubility often involves the breaking or forming of cross-links between the polymer chains. Research some examples of positive and negative resists, and use them to explain how exposing polymers to ultraviolet light can change the solubility of the polymer in solvents. Suitable examples to use could be DNQ (positive resist) and SU-8 (negative resist). More details of other polymers used in photoresists can be found at ~chem421/polymod2.htm. Polymers in layer-by-layer self-assembly As previously explained in Topic guide 11.2, this is a nanofabrication process involving the building up of alternating layers of positively and negatively charged polyelectrolytes. The process involves creating a primer layer on the substrate surface. For example, if the substrate is gold, then molecules containing thiol (SH) groups can attach to the surface. If these molecules are modified to contain charged groups, such as amino or sulfonate groups, then a layer of polyelectrolyte will become strongly bound to it. Possible polyelectrolytes could be: negatively-charged polyelectrolyte: sodium salt of poly(styrene sulfonate) positively-charged polyelectrolyte: poly(allylamine hydrochloride). 9

10 Find the structures of these polyelectrolytes and hence sketch out a diagram to show the structure of a self-assembled structure based on a thiol-primed gold substrate alternately exposed to these two polyelectrolytes. Other examples: polymers as nanomaterials There are several emerging uses of polymers as nanomaterials. Some particularly interesting ones include: biocomposites organic-inorganic composites organic light-emitting diodes (OLEDs) printed electronic devices. Research some of these applications and find out more about the way that polymers are involved in these materials or devices. Fabricating polymer nanostructures Many of the polymers mentioned in the text in this section can be fabricated by self-assembly methods such as the block co-polymer self-assembly method described in Topic guide Supramolecular chemistry Supramolecular chemistry is concerned with the bringing together of molecules to form larger structures, held together by intermolecular forces such as hydrogen bonds or Van der Waals interactions. Processes of these sorts are important in biology, of course, notably in the interactions between enzymes and substrates, or the binding of signalling molecules to receptor sites. In the field of supramolecular chemistry, the molecules that interact to form supramolecular structures are known as host and guest ; supramolecular chemistry is therefore about the formation of host-guest complexes: the host molecule is a large molecule or other structure with a cavity able to act as a binding site the guest molecule(s) will bind to this host molecule in a particular orientation or arrangement. As well as processes similar to those occurring in biological systems supramolecular chemistry could involve: zeolites trapping smaller molecules crown ethers trapping metal ions. Supramolecular chemistry and nanotechnology Several of the processes that occur during the fabrication of nanoscale structures covered in the last two topic guides could be regarded as 10

11 non-covalent interaction between a host structure and guest molecules, for example, the synthesis of gold nanoparticles described in Section 1 of this topic guide. Exciting research is being carried out at the interface of supramolecular chemistry and nanoscience. Much supramolecular chemistry uses principles from biology in order to control the synthesis of supramolecular structures. Such methods for example making use of DNA as a template to direct the assembly of complex structures may have great future importance in nanotechnology. The use of processes derived from biology is known as biomimetics (see Topic guide 11.4). Look back over the fabrication methods covered in Topic guides 11.2 and Identify two of these that involve host-guest interactions and thus could be classified as supramolecular chemistry. Introductory material: The book Molecules: A very short introduction (Philip Ball, OUP, 2003), contains several good sections that introduce aspects of supramolecular chemistry in a nontechnical way (see, for example, Chapter 6 p , which deals with synthetic communication systems that mimic those found in biological systems). A more advanced text, Supramolecular Chemistry From Biological Inspiration to Biomedical Applications (Peter J. Cragg, 2010), Chapter 1, gives a relatively accessible introduction to the field, including an outline of the history of its development and a description of how it links with nanotechnology. Much of this chapter is viewable through Google Books. 3 Reviewing the technology Case study of a nanotechnology product You will be guided by your tutor to select a nanotechnology product or device that is currently being marketed or which is being prepared for marketing. To meet the assessment criteria for this section of the course, you will need to research four key aspects of the product or device and present them in an appropriate way. You will need to describe: the need that is being addressed by the product and how the product addresses this need the company fabricating and marketing the product and the target market for the product the fabrication method(s) used in manufacturing the product or device and the reasons for the selection of this method the factors affecting the cost and quality of the fabrication method and the health and safety implications. 11

12 Assessing the need You will need to find information relating to these questions: What are the properties of the device/product? How can these properties be explained by the structural features of the device/product? For what commercial application could these properties be useful? How far does the product/device meet the needs of this commercial application? Research a nanotechnology product. Good examples might be a product making use of quantum dots or a MEMS-based sensor. Write a report on the product, addressing the bullet points above. The company You will need to find information relating to these questions: What is the name of the company and where is it based? How long has it been in the nanotechnology market and what range of products does it market? Who is likely to be the target market for the product/device and how large might this market be? Are there other companies that market similar competing devices? Link You may need to use ideas from Topic guide 11.4, Section 1 (Commercial applications) in this task. Research a company specialising in nanoproducts. A list of large and small companies that are developing or using nanotechnology in their products is available from corporatewatch.org ( Choose a nanotechnology-based product produced by a named company and write a report covering the bullet points above. The fabrication methods You will need to use information about the device to answer these questions: What kind of product/device is this (for example, a MEMS/NEMS, nanowire, nanotube or spherical nanoparticle)? Can you find out details of its structure (for example, the components present, the atoms/molecules used in the structure, the key dimensions, etc.)? What nanofabrication method(s) could be used in the construction of the device or its components? What are the key features of this nanofabrication method? Why is the nanofabrication method particularly appropriate for the manufacture of the device/component (for example, in comparison with other methods)? 12

13 Choose a simple nanoproduct, for example, a MEMS microsensor. Research the questions above and write a report addressing these questions. You will need to use ideas from Topic guides 11.2, Section 1 and 11.3, Section 1 in this task. Cost, quality control and safety You will need to use information about the fabrication process to answer these questions: What kind of equipment will be necessary in the fabrication method? How expensive is this equipment? What other specialised conditions (for example, vacuum, controlled temperature) are necessary for the process? How significant will these be for the cost of the process? What issues might there be for quality control of the product (for example, controlling the size of key dimensions, ensuring accurate transfer of patterns onto a resist, controlling the micromachining process)? What techniques exist for quality control of these features? What are the hazards of the substances used in the process? How can the risk of exposure to these hazards be minimised? What other hazards are there in the process (for example, use of high-energy lasers, etc.)? How can the risk of exposure to these hazards be minimised? Choose a suitable manufacturing process, for example, the production of the MEMS microsensor from the previous activity. Write a report addressing the questions above. The government-sponsored site has a range of helpful information about the safety of nanotechnology. Detailed safety information about a range of fabrication processes is available in a 2004 HSE report: A parallel report considers the toxicity of a range of nanoparticles: 13

14 Case study: Which nanodevice? Which fabrication method? An interesting example of an application of nanotechnology, in which the same commercial need can be met by several types of device, is in the development of flat panel displays as used in televisions, computer monitors and mobile phone displays. Since the beginning of the century, the traditional cathode-ray tube formerly used for television screens and computer monitors has been completely superseded by flat panel displays, which, in turn, has enabled the development of smartphones and tablets with high definition yet physically tiny displays. Flat panel displays the options A variety of techniques exist that can produce a flat panel display. Some are already in use and some are in development. All of the technologies below make use of nanomaterials in some way: LCD (liquid crystal displays), backlit by carbon nanotubes organic light-emitting diodes (OLEDs) field emission displays (FEDs), using carbon nanotubes as electron emitters quantum dot LED displays (QD-LED) MEMS displays. Research the five technologies for flat panel displays mentioned in the case study above, Introductory commenting on material: the advantages A briefing and sheet disadvantages from the Observatory of each technology. NANO project The Observatory gives an introduction NANO to briefing these methods, sheet is a good and is place available to start at (see ). ObservatoryNANO%20Briefing%20No.9%20Nanotechnology%20for%20Flat%20Panel%20 Use information from this unit further research describe one possible method of fabricating Displays.pdf. the nanodevice used in each of these technologies. Portfolio activity (3.1, 3.2, 3.3, 3.4) Following discussion with your tutor, you will be assigned a suitable nanodevice to research. You will be required to research this device and the nanofabrication methods that will be used in its manufacture. You will then prepare a report and present your findings and recommendations. In your report you should: describe the possible nanofabrication routes to manufacture your assigned device assess these routes and describe the most suitable route assess the cost implications of the fabrication route and any significant issues of quality control describe the health and safety implications of the fabrication route assess the commercial viability of the product in terms of the likely market for the device. 14

15 Checklist At the end of this topic guide you should be familiar with the following ideas: there may be a choice of fabrication routes for particular types of nanodevices or nanomaterials MEMS fabrication may involve various types of micromachining fabrication of carbon nanotubes and nanowires uses templating and vapour deposition quantum dots may be fabricated using layer-by-layer self-assembly a range of polymer materials is employed in fabrication techniques, as resists or the building blocks of layers many fabrication methods involve supramolecular chemistry choice of fabrication route will involve consideration of safety implications and quality control issues as well as costs. Further reading This topic guide applies the principles of nanofabrication encountered in Topic guide 11.2, and so many of the suggestions there will also be helpful here. It is possible that more detailed sources of information may be needed to research particular nanofabrication techniques; the highly specialised text Fundamentals of Microfabrication and Nanotechnology: Manufacturing techniques for microfabrication and nanotechnology (Marc J. Madou, CRC Press, 2011) may be helpful to consult for specific information on a given technique. A very detailed source of information about self-assembly is the text Nanochemistry (Ozin and Arsenault, 2005). Chapter 3 deals with self-assembly of 2-D nanostructures, Chapter 4 with 1-D nanostructures and Chapter 5 with 0-dimensional nanostructures. Otherwise, there are many helpful websites that provide more information about the case studies used in this chapter, and similar related ones. For example: MEMS fabrication is covered on the MEMSnet and MEMS-exchange websites: Quantum dot fabrication techniques are compared in a presentation at nano/wp/quantum_dots_wp.pdf. The fast-growing and fascinating field of supramolecular chemistry is only dealt with briefly in this topic guide, but introductory and more detailed coverage can be found in Molecules: A very short introduction (Philip Ball, OUP, 2004) and Supramolecular Chemistry From Biological Inspiration to Biomedical Applications (Peter J. Cragg, 2010). Acknowledgements The publisher would like to thank the following for their kind permission to reproduce their photographs: Shutterstock.com: imredesiuk; Corbis: David Scharf / Science Faction 3; Vienna University of Technology 5 All other images Pearson Education We are grateful to the following for permission to reproduce copyright material: Illustration of shape of the etch profiles of a <100> oriented silicon substrate after immersion in an anisotropic wet etchant solution, produced by The MEMS and Nanotechnology Exchange. Reproduced with permission of the Corporation for National Research Initiatives. 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. 15