книги / Nanotechnology Read and Discuss

of awards, including the Williamson Prize (for being the top Engineering student in his undergraduate studies at the University of Hong Kong), Thomas Turner Research Prize (for the quality of his PhD thesis at the University of Birmingham), Outstanding Young Researcher Award at the University of Hong Kong, and in 2007 was awarded the Rosenhain Medal of the Institute of Materials, Minerals and Mining. He also held visiting professorship appointments at Nanjing University and the Central Iron and Steel Research Institute in Beijing, and in 2003, he was also awarded the Universitas 21 Fellowship to visit the University of Auckland. He is active in conference organization and journal editorial work.

Professor A. H.W. Ngan: career ladder

He is active in _________________________ and

_____________________________ work.

In 2007 he _______________________ the Rosenhain Medal of the Institute of Materials

Did research in dislocation theory, ____________

________________________ and nanomechanics

Lecturer in _________________________________, at the Department of Mechanical Engineering

Postdoctoral research at Oxford University on

__________________________

PhD on ______________________________ in 1992

Received the Williamson Prize (for being _______

__________________ in his undergraduate studies

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? What are their most impressive achievements? Do you think you would like to have a career in the academic sphere? What would you like to achieve?

Materials for nanotechnology

Reading 2

1. What do you think was the main reasons for nanotechnology to appear? Read the following introduction to a chapter about nanomaterials, taken from a course book ‘Physical Metallurgy and Advanced Materials’ by R. E. Smallman and A. H.W. Ngan (2007), and compare with your ideas.

Introduction

Based on the success of microtechnology (integrated circuits, etc.) since the 1970s, scientists began in the 1980s to enquire about the possibility of developing technologies in the next level down, at the nanometer length scale. The outcome was nanotechnology, involving materials and structures of dimensions in the range of about 1–100 nm. Apart from a desire to further miniaturize from microtechnology, a strong motivation for nanotechnol-

ogy is to exploit the various new physical phenomena and novel properties of materials when their sizes shrink to below 1 micron. The develop-

ment of nanotechnology has been greatly facilitated by continuous breakthrough along two major fronts beginning from the 1980s, namely the invention of novel microscopy techniques such as STM and AFM, that enable the observation, characterization, as well as manipulation of nanometer-sized materials and structures, and the development of new synthetic techniques that allow nanometer-sized materials and structures to be made. In this section the focus is on the materials aspects.

Nanotechnology is based on building blocks including individual atoms, groups of atoms or molecules, ultra-small solid shapes or bulk solids that are nanostructured.

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Examples include fullerenes, nanotubes and nanowires, colloids, confined atomic clusters such as nanometer-sized islands on supporting substrates, nanocrystalline sol-

ids, etc. To discover new nanomaterials and to develop methods to make them is an ongoing, intensive research field, and so a complete account is not possible at this stage. However, many common nanosized materials are fabricated

using either the ‘top-down’ or ‘bottom-up’ approach. A top-down method is one that evolves from a microfabrication technique such as lithography, nanoimprinting, etc. Microlithography methods (fig. 1) make use of radiation and so the manufactured feature size is limited by the wavelength of the radiation.

Fig. 1. Metallic zinc nanocrystals produced by thermal evaporation. The crystals adopt overall hexagonal shapes based

on the hcp crystal structure of zinc, an indication of the strong anisotropy of surface energy (Ng, Muley, Ngan and co-workers, 2006)

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However, by exploiting the interference of light (laser), for example, sub-wavelength periodic features can be produced. The bottom-up approach relies primarily on a material’s ability to self-assemble to form nano-sized shapes. A common self-assembly technique is known as heteroepitaxy, in which a semiconductor thin film deposited on a slightly mismatching substrate relaxes to form a periodic array of islands upon annealing. While this technique is material dependent, a certain degree of control is possible through the choice of the substrate, controlling the temperature and/or straining the substrate during the self-assembly process.

2. Match the sentence halves, check with the underlined phrases in the text above.

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Nanoparticles

Nanoparticles from a few to a few dozen nanometers are an important form of materials for nanotechnology. Sufficiently small particles may exhibit the ‘quantum confinement’ effect in which the electronic states and the associated properties of the particle are altered relative to the bulk situation. In ad-

dition to this, nanometersized particles often behave differently from their bulk counterparts as a result of their much higher fraction of surface atoms present, since many physical and chemical properties of matter are due to the surface atoms. A currently much exploited nanotechnology is based on the photocatalytic action of TiO2, which produces disinfection and deodorization effects. Under the action of sunlight, TiO2 acts as a catalyst for the decomposition of water molecules

in air into hydroxyl radicals and superoxide anions. The latter two are strong oxidation reagents and can in turn decompose toxic organic substances through oxidation, thus achieving disinfection effects. Although such a photocatalytic property of TiO2 is well known in the past, using the TiO2 in nanoparticle form greatly improves the catalytic efficiency, since catalysis is a surface reaction effect. In therapeutic applications, silver nanoparticles (of the order of 10 nm in size) have also been found to be effective in wound healing, exhibit anti-HIV activities and are ef-

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fective against proliferation of different types of bacteria. There are numerous methods, both chemical and physical, for producing nanoparticles in different shapes and sizes. Metallic nanoparticles can be routinely prepared by physical vapor deposition methods, such as thermal evaporation, magnetron sputtering, etc. Fig. 2 shows metallic Zn nanocrystals prepared by thermal evaporation. The shapes of the crystals in this case are heavily affected by the crystallographic anisotropy of the surface energy, again an indication of the dominating effects of surface atoms. Because of the energetic surface atoms, nanocrystals are less stable and exhibit fluctuations even at relatively low temperatures.

Fig. 2. Sequence of grain coalescence recorded from an Ni–25 at.% Al sputter-deposited film with low density of nanocrystals, during in situ TEM annealing at 800◦C (Ng and Ngan, 2002, courtesy

of Materials Research Society)

Figure 12.20 shows a series of snapshots of the coalescence process between two nano-sized Ni–25

at.% Al grains recorded during in situ heating in the TEM at 800◦C. The change in the bright-field contrast of the two participating grains from (b)–(d) to (f)–( j) indicates thermal fluctuations of the granular orientations before final coalescence in (k)–(l). Nano-silver

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__________ atoms present, since many properties of matter are

__________ the surface atoms. A currently _________________

nanotechnology is based on the photocatalytic action of TiO2. Using the TiO2 in nanoparticle form greatly improves the catalytic

____________, since catalysis is a surface reaction effect. In therapeutic applications, silver nanoparticles have also been found to be effective in

____________. They are also effective ___________________of different types of bacteria. There are numerous methods, both chemical and physical, for producing nanoparticles: physical vapor

______________ methods, such as thermal evaporation, magnetron

__________, or through a chemical _____________ such as a

_____________ reaction.

8. Imagine you were asked to contribute a weekly column to the Institute of Physics Nanotechweb site. This week the topic is ‘Nanoparticles’ Write a short article about their significance (150 –

200words). Reading 4

9.You are going to read the second part of a chapter about nanomaterials taken from a course book ‘Physical Metallurgy and Advanced Materials’ by R. E. Smallman and A. H.W. Ngan (2007). Before you read it, define the terms ‘nanotube’ and ‘fullerene’, then read and details to your definitions.

Prompts

A nanotube is a hollow cylindrical or toroidal molecule made of one element, usually carbon. Nanotubes are being investigated as semiconductors and for uses in nanotechnology.

Buckminsterfullerene is a form of carbon that contains molecules having 60 carbon atoms arranged at the vertices of a polyhedron with hexagonal and pentagonal faces. It is produced in carbon arcs and occurs naturally in small amounts in certain minerals. Also called fullerene. [named after (Richard) Buckminster Fuller (1895–1983), US architect and engineer]

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Fullerenes and nanotubes

The various forms of fullerene-based molecules and carbon nanotubes (CNTs) can be introduced as a recently discovered form of carbon. Fullerenes and in par-

ticular the CNTs have attracted a lot of attention as an important class of building blocks for nanotechnology, because their properties can be engineered via chemical or physical manipulation. The C60 molecules can be made to bond to one another

to form an fcc crystal, with each lattice site occupied by a C60. Such a crystalline state is an insulator, but upon doping with alkaki atoms such as potassium it becomes an electrical conductor. The electronic properties of CNTs depend on their diameter as well as their chirality.

Armchair CNTs tend to be metallic, and for a given chirality, smaller tubes tend to be semiconducting, while larger tubes tend to be metallic. CNTs are also known to exhibit fieldemission effects, when a small electric field applied along the tube axis causes an intensive emission of electrons from the end of the tube. This enables CNTs to be considered for applications such as electron sources for flat-panel displays. Bridging two metallic electrodes by a CNT also forms a nanoscale fieldeffect transistor, in which a small change in voltage applied across the electrodes can cause a huge change in the current through the CNT by several orders of magnitude. Apart from finding potential applications in future high-speed computers, the I–V characteristics of a CNT fieldeffect transistor are also strongly affected by the gaseous environment in which the transistor is situated, allowing potential applications as gas sensors. CNTs are also potential lithiumand hydrogen-storage materials for fuel cell applications.

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