Introduction to Nanoscience and Nanotechnology E-Book

Chris Binns

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Serie: Wiley Survival Guides in Engineering and Science
• Sprache: Englisch

Seitenzahl: 551

Veröffentlichungsjahr: 2010

Beschreibung

Introduction to Nanoscience and Nanotechnology explains nanotechnology to an audience that does not necessarily have a scientific background. It covers all aspects, including the new areas of biomedical applications and the use of nanotechnology to probe the "quantum vacuum." After discussing the present state of the art in nanotechnology, the book makes estimates of where these technologies are going and what will be possible in the future.

Leseprobe

Table of Contents

Series Page

Title Page

Copyright

Preface

Acknowledgments

Nanotechnology Time Line

Introduction

0.1 Incremental Nanotechnology

0.2 Evolutionary Nanotechnology

0.3 Radical Nanotechnology

0.4 Bottom-Up/Top-Down Nanotechnology

Chapter 1: Size Matters

1.1 The Fundamental Importance of Size

1.2 The Magnetic Behavior of Nanoparticles

1.3 The Mechanical Properties of Nanostructured Materials

1.4 The Chemical Properties of Nanoparticles

1.5 Nanoparticles Interacting with Living Systems

Chapter 2: Nanoparticles Everywhere

2.1 Nanoparticles in the Atmosphere

2.2 Atmospheric Nanoparticles and Health

2.3 Nanoparticles and Climate

2.4 Marine Aerosol

2.5 Nanoparticles in Space

Chapter 3: Carbon Nanostructures: Bucky Balls and Nanotubes

3.1 Why Carbon?

3.2 Discovery of the First Fullerene: C60

3.3 Structural Symmetry of the Closed Fullerenes

3.4 Smaller Fullerenes and “Shrink-Wrapping” Atoms

3.5 Larger Fullerenes

3.6 Electronic Properties of Individual Fullerenes

3.7 Materials Produced by Assembling Fullerenes (Fullerites and Fullerides)

3.8 Discovery of Carbon Nanotubes

3.9 Structure of SWNTs

3.10 Electronic Properties of SWNTs

3.11 Electronic Transport in Carbon Nanotubes

3.12 Mechanical Properties of Nanotubes

3.13 Thermal Conductivity of Nanotubes

3.14 Carbon Nanohorns

3.15 Carbon Nanobuds and Pea Pods

Chapter 4: The Nanotechnology Toolkit

4.1 Making Nanostructures Using Bottom-Up Methods

4.2 Making Nanostructures Using Top-Down Methods

4.3 Combining Bottom-Up and Top-Down Nanostructures

4.4 Imaging, Probing, and Manipulating Nanostructures

Chapter 5: Single-Nanoparticles Devices

5.1 Data Storage on Magnetic Nanoparticles

5.2 Quantum Dots

5.3 Nanoparticles as Transistors

5.4 Carbon NanoElectronics

Chapter 6: Magic Beacons and Magic Bullets: The Medical Applications of Functional Nanoparticles

6.1 Nanoparticles Interacting with Living Organisms

6.2 Treatment of Tumors by Hyperthermia

6.3 Medical Diagnosis and “Theranostics” Using Nanomaterials

Chapter 7: Radical Nanotechnology

7.1 Locomotion for Nanobots and Nanofactories

7.2 On-Board Processing for Nanomachines

7.3 Medical Nanobots

7.4 Molecular Assembly

Chapter 8: Prodding the Cosmic Fabric

8.1 Zero-Point Energy of Space

8.2 The Casimir Force

8.3 The Casimir Force in Nanomachines

Glossary

Index

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Introduction to Nanoscience and Nanotechnology Chris Binns

Copyright © 2010 by John Wiley & Sons, Inc. All rights reserved.

Published by John Wiley & Sons, Inc., Hoboken, New Jersey

Published simultaneously in Canada

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Library of Congress Cataloging-in-Publication Data:

Binns, Chris, 1954-

Introduction to nanoscience and nanotechnology / Chris Binns.

p. cm. – (Wiley survival guides in engineering and science)

Includes bibliographical references.

ISBN 978-0-471-77647-5 (cloth)

1. Nanoscience–Popular works. 2. Nanotechnology–Popular works. I. Title.

QC176.8.N35B56 2010

620′.5–dc22

2009045886

Preface

This book has been a long time in the making. I was taken aback recently when I looked at the original proposal and found that it was written in Spring 2005. At the time there did not appear to be any books that covered the entire field of nanotechnology in a holistic manner written for the layman. To me, one of the most exciting aspects of Nanoscience and Nanotechnology is that they transcend the barriers between the mainstream scientific disciplines of Physics, Chemistry, Biology and Engineering. Thus they provide new insights into the nature of matter and dazzling possibilities for new technology. So full of enthusiasm I put hand to keyboard and embarked on my project to fill this gap, with an original intention to finish in 18 months. Of course the entire sweep of the topic, including, as it does, all the mainstream sciences, is incredibly broad and the intention was to cover it with a relatively light touch to give the reader the ‘feel’ of what is exciting about the subject. Nanotechnology has a way of sucking you in however and there were so many things that I just couldn't resist including that the touch soon began to get heavier. At some stage in this process of increasing depth I decided to go a stage further and increase the academic level by including ‘Advanced Reading Boxes’ and some worked problems. This was so that the book could be used as an introductory text for University courses on Nanotechnology, which are becoming increasingly common. Indeed much of the material has been foisted on our own undergraduates at the University of Leicester where we run such a course. The book is still written so that the subject material is covered if one never ventures into these boxes but they provide additional depth for the serious student.

So, four and a half years later, here we are with a longer and more detailed book than I originally intended but in which, I hope, the original intention of giving the reader a holistic feel of the subject has not been lost. There are now many excellent books on Nanotechnology available at a range of different academic levels but I believe that this one still has the widest scope. Despite that, there are still holes, for example, the important areas of Nanomechanics and Nanofluidics, which I hope to fill in future editions. Whatever your use for the book I hope you enjoy it and get from it the excitement that is fundamental to the topic.

Chris Binns

December 2009

Acknowledgments

I don't think it is possible for a person, at least, a person with a family, to write a book without a good deal of support. In my case I have had massive support from my wife, Angela who has also been a constant source of inspiration and indeed practical help by proof-reading the book. I dedicate this book to her.

I thank my extended family accrued from two marriages, that is, Callum, Rory, Connor, Edward, Tamsyn and Sophie for bringing inspiration and joy to my life as well as the inevitable problems. I would also like to thank my first wife Nerissa for supporting me in my early career and shouldering a large part of the burden of looking after our children.

Finally I would like to thank my parents who with meager resources supported me in pursuit of higher education despite their feeling that I should get a proper job.

Nanotechnology Time Line

1931 Ernst Ruska and Max Knoll build the first Transmission Electron Microscope (TEM) [1]. See chapter 4, section 4.4.6

1959 Lecture by Richard Feynman entitled “There is plenty of room at the bottom, an invitation to enter a new field of physics” [2]. In it he said:

“…I am not afraid to consider the final question as to whether, ultimately-in the great future-we can arrange the atoms the way we want; the very atoms, all the way down! What would happen if we could arrange the atoms one by one the way we want them….” See chapter 4 section 4.4.2

1968 Development of Molecular Beam Epitaxy (MBE) by Arthur and Cho that enables materials to be grown an atomic layer at a time.

1974 N. Taniguchi generally credited for using the word ‘Nanotechnology’ for the first time [3].

1981 Development of Scanning Tunneling Microscope (STM) by Rohrer and Binnig enables atomic resolution images of surfaces [4]. See chapter 4, section 4.4.1.

1985 Discovery of C60 and other fullerenes by Harry Kroto, Richard Smalley and Robert Curl, Jr. [5]. See chapter 3, section 3.2.

1985 Tom Newman wrote the first page of Charles Dickens' novel A tale of two cities with a reduction factor of 25 000 using Electron Beam Lithography (EBL) [6] thus winning a prize of $1,000 offered by Richard Feynman after his 1959 speech. See chapter 4, section 4.2.1.

1986 Development of Atomic Force Microscope (AFM) by Binnig and co-workers [7]. See chapter 4, section 4.4.4.

1987 Development of Magnetic Force Microscope (MFM) by Martin and Wickramasinghe [8]. See chapter 4, section 4.4.4.

1990 D. M. Eigler and E. K. Schweizer use an STM to demonstrate atomic-scale positioning of individual Xe atoms on a Ni surface at low temperature (4K) to write “IBM” [9]. This is the first step towards the realization of the Feynman dream set out in the highlighted statement statement from his lecture above. See chapter 4, section 4.4.2.

1991 Sumio Iijima discovers carbon nanotubes [10]. See chapter 3, section 3.8

1993 Iijima and Ichihashi grow single-wall carbon nanotubes [11]. See chapter 3, section 3.8.

1995 Takahashi and co-workers demonstrate single-electron transistor operating at room temperature [12]. See chapter 5, section 5.3

1996 Cuberes, Schlittler and Gimzewski demonstrate room temperature positioning of individual C60 fullerenes with an STM to produce the “C60 abacus” [13]. See chapter 4, section 4.4.2.

1997 Steve Lamoreaux measures the Casimir force at sub-micron distances [14]. See chapter 8, section 8.2

1998 Umar Mohideen and Anushree Roy use AFM used to measure the Casimir force at distance scales down to 90 nm [15]. See chapter 8, section 8.2

2001 Postma and co-workers, demonstrate single-electron transistor operation in a carbon nanotube [16]. See chapter 5, section 5.4.

2002 Regression of tumour in mouse achieved using magnetic nanoparticle hyperthermia achieved by Brusentsov and co-workers [17]. See chapter 6, section 6.2.2.

2007 Johanssen and co-workers conduct first human clinical trials of magnetic nanoparticle hyperthermia treatment of cancer [18]. See chapter 6, section 6.2.2.

References

1. See http://www.microscopy.ethz.ch/history.htm.

2. R. P. Feynman, There is plenty of room at the bottom, an invitation to enter a new field of physics, Engineering Science Magazine23 (1960) 143.

3. N. Taniguchi, On the basic concept of nano-technology, Proceedings of the International Conference of Production Engineering (Tokyo) Japanese Society of Precision Engineering Part II (1974) 245.

4. G. Binnig and H. Rohrer, Scanning Tunneling Microscopy, IBM Journal of Research and Development30 (1986) 355–369.

5. H. W. Kroto, J. R. Heath, S. C. O'Brien, R. F. Curl and R. E. Smalley, C60: Buckminsterfullerene Nature318 (1985) 162.

6. T. Newman, Tiny tale gets grand, Journal of Engineering Science49 (1986) 24.

7. G. Binnig, C. F. Quate and Ch. Gerber, Atomic force microscope, Physical Review Letters56 (1986) 930–933.

8. Y. Martin and H. K. Wickramasinghe, Magnetic imaging by “force microscopy” with 1000 resolution Applied Physics Letters50 (1987) 455.

9. D. M. Eigler and E. K. Shweizer, Positioning single atoms with a Scanning Tunneling Microscope, Nature344 (1990) 524–526.

10. S. Iijima, Helical microtubules of graphitic carbon, Nature354 (1991) 56–58.

11. S. Iijima and T. Ichihashi Single-shell carbon nanotubes of 1nm diameter, Nature363 (1993) 603–605.

12. Y. Takahashi, M. Nagase, H. Namatsu, K. Kurihara, K. Iwdate, Y. Nakajima, S. Horiguchi, K. Murase and M. Tabe, Fabrication technique for Si single-electron transistor operating at room temperature, Electronics Letters31 (1995) 136–137.

13. M. T. Cuberes, R. R. Schlittler and J. K. Gimzewski, Room-temperature repositioning of individual C60 molecules at Cu steps: Operation of a molecular counting device, Applied Physics Letters69 (1996) 3016.

14. S. K. Larmoreaux, Demonstration of the Casimir force in the 0.6 µm to 6 µm range, Physical Review Letters78 (1997) 5–8.

15. Umar Mohideen and Anushree Roy, Precision measurement of the Casimir force from 0.1 to 0.9µm, Physical Review Letters21 (1998) 4549.

16. H. W. Ch. Postma, T. Teepen, Z. Yao, M. Grifoni and C. Dekker, Carbon nanotube single-electron transistors at room temperature. Science293 (2001) 76–79.

17. N. A. Brusentsov, L. V. Nikitin, T. N. Brusentsova, A. A. Kuznetsov, F. S. Bayburtskiy, L. I. Shumakov and N. Y. Jurchenko, Journal of Magnetism and Magnetic Materials252 (2002) 378–380.

18. M. Johanssen, U. Gneveckow, K. Taymoorian, B. Thiesen, N. Waldöfner, R. Scholz, K. Jung, A. Jordan, P. Wust and S. A. Loening, Morbidity and quality of life during thermotherapy using magnetic nanoparticles in locally recurrent prostate cancer: Results of a prospective phase I trial, International Journal of Hyperthermia, 23 (2007) 315–323.

Introduction

Research in nanotechnology is a growth industry, with worldwide government-funded research spending running at over four billion dollars per year and growing at an annual rate of about 20% [1]. Industry is also willing to spend vast sums on investigating nanotechnology, with, for example, major cosmetics companies announcing big increases in their annual Research and Development budgets for the field. It is clear that nanotechnology is expected to have a significant impact on our lives, so what is it and what does it do? These simple direct questions, unfortunately, do not have simple direct answers, and it very much depends on who you ask. There are thousands of researchers in nanotechnology in the world, and one suspects that one would get thousands of different responses. A definition that would probably offend the smallest number of researchers is that nanotechnology is the study and the manipulation of matter at length scales of the order of a few nanometers (100 atoms or so) to produce useful materials and devices.

This still leaves a lot of room for maneuver. A nanotechnologist working on suspensions of particles might tell you that it is achieving better control of tiny particles a few nanometers across (nanoparticles) so that face creams can penetrate the epidermis (outer skin layer). A scientist working at the so-called “life sciences interface” would say that it is finding ways of attaching antibodies to magnetic nanoparticles to develop revolutionary cancer treatments. A researcher working on “molecular electronics” will tell you that it is creating self-ordered assemblies of nanoparticles to produce electronic circuits in which the active components are a thousand times smaller than a single transistor on a Pentium IV chip. Some nanotechnologists (a small minority) would tell you that it is finding ways to build tiny robots whose components are the size of molecules (nanobots).

We will talk in detail about size scales in Chapter 1; but for the moment consider Fig. 0.1, which shows, schematically, the size scale of interest in nanotechnology (the nanoworld). For reasons that will become clear in Chapter 1, the upper edge of the nanoworld is set at about 100 nm. Even though this is hundreds of times smaller than the tiniest mote you can see with your eyes and is smaller than anything that can be resolved by the most powerful optical microscope, a chunk of matter this size or bigger can be considered to be a “chip off the old block”—that is, a very tiny piece of ordinary material. If we were to assemble pieces of copper or iron this big into a large chunk, the resulting block would behave exactly as we would expect for the bulk material. Thus nanotechnology does not consider pieces of matter larger than about 100 nm to be useful building blocks.

Figure 0.1The nanoworld. The size range of interest in nanotechnology and some representative objects.

As shown in Fig. 0.1, viruses are small enough to be inhabitants of the nanoworld whereas bacteria are much larger, being typically over 10 µm (10,000 nm) in size, though they are packed with “machinery” that falls into the size range of the nanoworld (see Chapter 6, Section 6.1.3). Going down in size, the figure shows typical sizes of metal particles, containing ∼1000 atoms that can be used to produce advanced materials. The properties of these (per atom) deviate significantly from the bulk material, and so assembling these into macroscopic chunks produces materials with novel behavior.

Finally, the lower edge of the nanoworld is defined by the size of single atoms, whose diameters vary from 0.1 nm (hydrogen atom) to about 0.4 nm (uranium atom). We cannot build materials or devices with building blocks smaller than atoms, and so these represent the smallest structures that can be used in nanotechnology.

There are so many aspects to nanotechnology that one of the difficulties in writing about it is finding ways to organize the text into a coherent structure. This book will largely follow a classification scheme introduced by Richard Jones in his book Soft Machines Nanotechnology and Life [2] that helps to categorise nanotechnology into a logical framework. He defines three categories in order of increasing sophistication—that is, incremental, evolutionary, and radical nanotechnology. These are described in detail below.

0.1 Incremental Nanotechnology

All substances, even solid chunks of metal, have a grain structure, and controlling this grain structure allows one to produce higher performance materials. This could mean stronger metals, magnetic films with a very high magnetization, suspensions of nanoparticles with tailored properties, and so on. Actually, some aspects of incremental nanotechnology can be considered to date back to the ancients. For example, the invention of Indian ink, probably in China around 2700 b.c., relies on producing carbon nanoparticles in water. Also medieval potters in Europe knew how to produce a lustre on pots by coating them with copper and silver nanoparticles [3], a process that can be traced back to 9th century a.d. Mesopotamia. Figure 0.2 shows an electron microscope image of the glaze of a 16th-century Italian pot, whose luster derives from the coating by 5-nm-diameter copper particles.

Figure 0.2Ancient Incremental Nanotechnology. Copper nanocrystals on a 10th century pot of about 10 nm diameter used to produce a surface luster. The inset shows an increased magnification image of a single 7 nm diameter particle with atomic planes visible revealing its crystallinity. Reproduced with the permission of Elsevier Science from I. Borgia et al. [3].

Most modern nanotechnologists would be proud of the size control of the particles in this picture. Whereas these days a process that involved nanoparticles such as this would be proudly claimed to be nanotechnology and thus open the door to research funding, spin-off companies, and so on, the ancients were developing processes that did something invisible to the materials but nevertheless allowed them to achieve certain results. In this sense, a lot of incremental nanotechnology can sometimes be considered to be a re-branding of other, more traditional lines of research such as materials science and chemistry. The nanotechnology title is still useful, however, since nanotechnology is, by its nature, multidisciplinary and it encourages cross-disciplinary communication between researchers.

The aspect of incremental nanotechnology that has really changed in the modern world is the development of instruments (see Chapter 4) that can probe at the nanoscale and image the particles within materials or devices. Researchers can actually observe what is happening to the particles or grains in response to changes in processing. This not only makes development of new processes more efficient but also leads to the discovery of completely new structures that were not known to exist and hence new applications. Nature is full of surprises when one studies sufficiently small pieces of matter, as will become clear throughout this book.

0.2 Evolutionary Nanotechnology

Whereas incremental nanotechnology is the business of assembling vast numbers of very tiny particles to produce novel substances, evolutionary nanotechnology attempts to build nanoparticles that individually perform some kind of useful function. They may need to be assembled in vast numbers to form a macroscopic array in order to produce a device, but a functionality is built into each one. Such nanoparticles are necessarily more complex than those used in incremental nanotechnology. A simple example is a magnetic nanoparticle that is used to store a single bit of information by defining the direction of its magnetization. If one wants to do this using nanoparticles with diameters smaller than about 6 nm at ambient temperature, simple elemental nanoparticles made, for example, of pure Fe will not work because thermal vibrations will instantly change their direction of magnetization. To produce a particle that doesn't lose its “memory” without cooling to very low temperatures, each one has to be made with more than one material and formed either as a uniform alloy or as a core-shell particle (see Fig. 0.3a)—that is, a kind of nanoscale chocolate peanut with a core consisting of one material surrounded by a shell of a different substance. As stated above, these have to be assembled in vast numbers into some sort of ordered array (the big unsolved problem with this technology) to produce a useful device, but each particle has within it the capacity to store a data bit. If and when this technology succeeds, it would represent a storage density about 1000 times greater than existing hard disks on computers. For example, the bottom image in Fig. 0.3a shows an array of core-shell nanoparticles used to store the word “nanotechnology” in ASCII code. The storage density represented would allow about two million books or a large library to be written in an area the size of a postage stamp.

Figure 0.3Functionalized nanoparticles. (a) A “core-shell” magnetic nanoparticles consisting of a ferromagnetic metal core (e.g. iron or cobalt) surrounded by an antiferromagnetic shell, e.g. cobalt-oxide or manganese) that is specialized to record a single bit of information encoded by the direction of the core magnetization (see Chapter 5, Section 5.1). The lower picture shows an array storing the word ‘nanotechnology’ in ASCII format with blue representing down magnetization or ‘0’ and red representing up magnetization or ‘1’. Writing at this density would enable the storage of about two million books (a large University library) on an area the size of a postage stamp. (b) Gold nanoparticles with attached thiol molecules. Each nanoparticle can behave as a transistor and the thiols can bond onto other thiol-coated gold nanoparticles via electrically resistive or capacitive links to build circuits with a component density 1000 times greater than existing devices (see Chapter 5, Section 5.3). Reproduced with the permission of Dr. Mark Everard from [4]. (c) A core-shell magnetic nanoparticle (as in (a)) with a second shell of gold that makes it easy to attach biological molecules such as proteins or antibodies, or drugs. The magnetic core of the particle can be utilized to steer the attached molecule to specific areas of the body by external fields for targeted drug delivery. Alternatively, the attached biological molecule could be used to target specific cells (e.g. tumor cells) that could then be heated and killed by a weak external radio-frequency magnetic field that is harmless to healthy tissue (see Chapter 6, Section 6.2.2).

An example of a more sophisticated “functionalized” nanoparticle, shown in Fig. 0.3b, is a gold nanoparticle about 2 nm across with attached molecules called thiols. If wires could be attached to this nanoparticle, it could be made to act like a transistor by a process called Coulomb blockade (see Chapter 5, Section 5.3). It turns out that the thiols, of which there are many types, can act as wires if they come together in the right way. A slight change in the bonding produces a change in the resistance of the link or makes it capacitive (insulating). In other words, an entire circuit network consisting of transistors, capacitors, and resistors can be produced by placing an array of thiol-coated nanoparticles in the correct positions. This, of course, is the unsolved problem but is a tantalizing one because the density of components in such an array means that about 1000 nanoparticle transistors could be placed in the space occupied by a single silicon-based transistor on a Pentium IV chip.

No one can fail to be impressed by the huge increases in performance and density of components/memory elements in devices made by the electronics and magnetic recording industries in the last few decades. The above two examples illustrate, however, that there is still a long way to go, nicely reinforcing a lecture on nanotechnology given by the visionary Nobel laureate Richard Feynman and entitled There's Plenty of Room at the Bottom. The amazing thing about this lecture was that it was given in 1959.

Continuing the trend toward complexity of individual nanoparticles, Fig. 0.3c shows a combination of the types in Figs. 0.3a and 0.3b consisting of a magnetic core-shell particle, with controlled properties, coated with a second shell of gold that facilitates its attachment to complex biological molecules—for example, drugs, proteins, or antibodies. The magnetic core of the particle can be utilized to steer the attached molecule to specific areas of the body by external fields for targeted drug delivery. Alternatively, the attached biological molecule could be used to target specific cells (e.g., tumor cells) that could then be heated and killed by a weak external radio-frequency field that is harmless to healthy tissue (see Chapter 6, Section 6.2.2).

As a rough guide to where we are with evolutionary nanotechnology, the functional nanoparticles shown in Fig. 0.3 can be routinely manufactured, but their use in technologies such as those described above awaits the solution to enormously difficult technological problems such as controlling their self-assembly into arrays.

0.3 Radical Nanotechnology

Finally, the most far-reaching version of nanotechnology, described as radical nanotechnology by Richard Jones, is the construction of machines whose mechanical components are the size of molecules. The field has bifurcated into two distinct branches, that of (a) molecular manufacturing in which macroscopic structures and devices are built by assembling their constituent atoms, and (b) nanorobots or nanobots, which are invisibly small mobile machines. Molecular manufacturing was originally proposed by the Nobel laureate Richard Feynman in his famous lecture in 1959 and was subsequently advocated with much enthusiasm by Eric Drexler [5]. In 1990 the IBM research laboratories in Zurich demonstrated that they could move and position individual atoms using a scanning tunneling microscope (STM—see Chapter 4, Section 4.4.2), lending support to the idea that molecular manufacturing may, at least in principle, be possible. The problems and the emergence of some enabling technologies for molecular manufacturing are presented in Chapter 7.

Nanobots have generated a good deal of controversy, especially ones that can play atomic Lego and build anything out of atoms lying around. If this were possible, then one could, in principle, build a nanobot that moved around exploring the surface it occupied. If it were equipped with an assembler that could assemble atoms and molecules, it could make a copy of itself by rooting around and finding the atoms it needed to reproduce. Obviously, this kind of activity would need on-board intelligence, and this could be provided by either a mechanical computer, again with molecular-sized components, or something more akin to a the molecular electronic-type circuit shown in Fig. 0.3b. Since each nanobot could make multiple copies of itself, the population could increase exponentially and would quickly produce a sufficiently vast army to build macroscopic objects. Drexler himself pointed out the doomsday scenario where the nanobots multiply out of control like a virus and eventually exist in such vast numbers that they could rearrange the atoms of the planet to produce a kind of “gray goo.” Unfortunately, this scenario has tended to hijack discussions on radical nanotechnology; and since the two branches of radical nanotechnology have been melded together in the public debate, there is a general feeling that all radical nanotechnology is dangerous. The reality is that exponentially self-replicating machines are not required for molecular manufacturing [6] and nanobots do not need to be built with assemblers to self-replicate in order to perform useful functions.

There is a scientific debate about whether this technology is feasible, even in the long term, or indeed desirable, but the discussion has moved on from generalities to a consideration of the detailed processes required for molecular manufacturing (see Chapter 7 and the references therein). A frequently proposed argument in favor of radical nanotechnology is that it already exists in all living things. Biological cells are filled with what may be regarded as nanomachines and molecular assemblers. Biology, however, is very different to the nanoscale process-engineering path envisaged by radical nanotechnologists, as explained in Soft Machines Nanotechnology and Life [2]. It is fair to say that both the feasibility and timescale of Radical Nanotechnology divides the community. The point is that while incremental nanotechnology exists and evolutionary nanotechnology is close (∼10 years), radical nanotechnology, if feasible, is probably decades away. Whatever the twists and turns of the debate, once we get away from the argument over nanobots, there is no doubt that the ability to produce nanomachines and achieve safe nonexponential molecular manufacturing will reap enormous benefits.

It is possible that the solution to some of the more difficult technological problems involved with radical nanotechnology may arise from a better fundamental understanding of the true nature of empty space. The quantum description of our universe predicts new types of force at very short distance scales (nanometers) arising directly out of the properties of vacuum. Although we can only detect these forces with very sensitive instruments (the tools of nanotechnology in fact—see Chapters 4 and 8), to a nanoscale machine whose components are within nanometers of each other, these forces will be as natural a part of their environment as gravity is to us. Research on these forces and how to utilize them in nanotechnology is already being undertaken by several research groups worldwide. This may be the missing link between biology and radical nanotechnology—that is, natural systems, whose inner workings happen on the same scale as nanomachines have evolved over billions of years and must have utilized all available forces including the exotic ones.

0.4 Bottom-Up/Top-Down Nanotechnology

Finally in this introduction it is worth mentioning another way of categorizing nanotechnology, that is, bottom-up and top-down approaches. Everything discussed so far has been part of a bottom-up approach in which the building block (nanoparticle, molecular machine component, etc.) is identified and produced naturally and then assembled to produce the material or device required. In the top-down approach, you start with a block of some material and machine a device or structure out of it. This is akin to conventional engineering using lathes and millers to machine a shape out of a solid block. The modern tools of nanotechnology, however, are able to machine structures with sizes of a few nanometers, so the size of components made with a top-down approach is not much different from the building blocks of the bottom-up approach. The flexibility of top-down tools—in particular, focused ion beam systems (FIBs)—is further enhanced by their ability to deposit material to produce nanoscale features as well as to remove it. This is beautifully illustrated in Fig. 0.4, which shows an example of a “wineglass” with a cup diameter 20 times smaller than the width of a human hair produced by depositing carbon. Although this is a rather big structure on the scale of nanometers, the smallest feature size that can be produced by a modern FIB is less than 100 nm.

Figure 0.4The smallest wineglass in the world (authorized by Guinness World Records). Wine glass whose cup diameter is 20 times smaller than the width of a human hair produced by deposition of carbon using a focused ion beam (FIB) machine. The structure arose from a Joint development by SII NanoTechnology, NEC, and the University of Hyogo, Japan. Although this is a rather big structure on the scale of nanometers, the smallest feature size that can be produced by a modern FIB is less than 100 nanometers (see Chapter 4, Section 4.2.2).

The two approaches (top-down and bottom-up) are complementary, and some of the most exciting research arises out of combining them. For example, if one wants to measure the electrical or magnetic properties of an individual nanoparticle, the fantastic precision of a modern top-down tool enables the production of electrodes that can attach to it. In general terms, the bottom-up/top-down categorization can be applied separately to each of the incremental, evolutionary, and radical nanotechnology categories.

The above is an attempt at a lightning tour of nanotechnology with generic descriptions and without addressing details. The rest of the book looks in detail at these and other aspects of nanotechnology. Chapter 1 aims to instill a feeling of how small the nanometer length scale is in comparison to macroscopic objects and why it is special. It discusses the basic conception of the discrete nature of matter starting from the original philosophical ideas of Leucippus and Democritus of ancient Greece to the modern view of atomic structure. It also describes why the properties of pieces of matter with a size in the nanometer range (nanoparticles) deviate significantly from the bulk material and how these special properties may be used to produce high-performance materials and devices. In Chapter 2 the discussion is broadened to include naturally occurring nanoparticles, both in the Earth's atmosphere and in space. Chapter 3 is dedicated to nanoparticles composed of carbon, and the justification for devoting a chapter to a single element is the rich variety of nanostructures produced by carbon and their importance in the rest of nanotechnology. Chapter 4 presents the tools of nanotechnology that can build, image, and manipulate nanostructures to build materials and devices using a bottom-up approach. It also describes top-down manufacturing methods that are capable of shaping nanostructures and perhaps one of the most exciting aspects of the field—that is, combining bottom-up and top-down approaches so that individual nanostructures can be probed. Chapter 5 is about artificially produced nanostructures that have a built-in functionality. Examples presented include magnetic nanoparticles that can store a data bit, nanoparticles that can function as transistors and quantum dots, which behave as “artificial atoms” with novel optical and electronic properties. Chapter 6 shows how combining advances in the production of nanoparticles and in biotechnology makes it possible to produce biologically active nanoparticles that can interact with specific cells in the body. These can then be used as nanoscale-amplifiers in biological images or they can be used to destroy their attached cells under the application of external stimulation, such as microwave or infrared radiation leading to powerful new treatments for cancer. Chapter 7 presents radical nanotechnology and discusses the potential for building autonomous machines with nanoscale components. Chapter 8 discusses how the tools of nanotechnology can be exploited to study the basic nature of vacuum itself via the Casimir effect. This is a strange “force from nothing” that arises from the zero-point energy density of empty space. These experiments may one day uncover a deeper level to our universe that underlies the observable universe consisting of all particles and all normal energy that we can sense or detect with instruments. The Casimir force may also be an important phenomenon for the practical implementation of nanomachines as a method to transmit force without contact.

The three-tier classification scheme divides among the chapters as follows. Chapters 1 and 3 deal with incremental nanotechnology, Chapters 5 and 6 deal with evolutionary nanotechnology, and Chapter 7 deals with radical nanotechnology. Where appropriate, a separate bottom-up/top-down categorization is introduced. Chapter 2 deals with naturally occurring nanoparticles, Chapter 4 deals with the tools of nanotechnology, and Chapter 8 deals with the Casmir force, so these stand outside the broad classification scheme.

References

1. Nanoscience and Nanotechnologies: Opportunities and Uncertainties, Report by the Royal Society and Royal Academy of Engineering, published July 29, 2004. Available from http://www.nanotec.org.uk/finalReport.htm.

2. R. A. L. Jones, Soft Machines Nanotechnology and Life, Oxford University Press, Oxford, 2004.

3. I. Borgia, B. Brunetti, I. Mariani, A. Sgamelotti, F. Cariati, P. Fermo, M. Mellini, C. Viti, and G. Padeletti, Heterogeneous distribution of metal nanocrystals in glazes of historial pottery, Appl. Surface Sci.185 (2002), 206–216.

4. M. J. Everard, X-ray scattering studies of self-assembled nanostructures, PhD thesis, 2003, University of Leicester.

5. E. Drexler, Engines of Creation, Garden City, NY, 1986 and Fourth Estate, London, 1990.

6. C. Pheonix and E. Drexler, Safe exponential manufacturing, Nanotechnology15 (2004), 869–872.

Chapter 1

Size Matters

1.1 The Fundamental Importance of Size

The aim of this chapter is to instill an intuitive feel for the smallness of the structures that are used in nanotechnology and what is special about the size range involved. As we will see, the importance of the nanoscale is wrapped up with fundamental questions about the nature of matter and space that were first pondered by the ancient Greeks. Three thousand years ago, they led the philosophers Leucippus and Democritus to propose the concept of the atom. These ideas will come around full circle at the end of the book when we will see that modern answers (or at least partial answers—the issue is still hot) are very much wrapped up in nanotechnology.

First of all, let us remind ourselves how small nanostructures really are. This is a useful exercise even for professionals working in the field. The standard unit of length in the metric system is the meter, originally calibrated from a platinum–iridium alloy bar kept in Paris but since 1983 has been defined as the distance that light travels in 1/299,792,458 seconds. For convenience, so that we are not constantly writing very small or very large numbers, the metric system introduces a new prefix every time we multiply or divide the standard units by 1000. Thus a thousandth of a meter is a milli-meter or mm (from the Roman word mille meaning 1000); a thousandth of a millimeter (or a millionth of a meter) is a micro-meter or µm (from the Greek word mikros meaning small). Similarly, a thousandth of a micrometer (or a billionth of a meter) is a nanometer or nm (from the Greek word nanos meaning dwarf). As shown in Fig. 0.1 (“the nanoworld”), we will be dealing in building blocks that vary from 100 nm across down to atoms, which are 0.1 to 0.4 nm across.

Nowadays, instruments can directly image nanostructures; for example, Fig. 1.1 shows a scanning tunneling microscope (STM—see Chapter 4, Section 4.4.1) image of a few manganese nanoparticles with a diameter of about 3 nm deposited onto a bed of carbon-60 molecules (“bucky balls”—see Chapter 3) with a diameter of 0.7 nm on silicon. It thus displays two different nanoparticles of interest in the same image. This could just as easily be a picture of snowballs on a bed of marbles, and it is easy to lose sight of the difference in scale between our world and that of the nanoparticles. To get some idea of the scale, take a sharp pencil and gently tap it onto a piece of paper with just enough force to get a mark that is barely visible. This will typically be about 100 μm or 100,000 nm across. If the frame in Fig. 1.1 were this large, it would contain about 100,000,000 of the Mn nanoparticles and 20,000,000,000 of the carbon-60 nanoparticles, which is about three times the population of the planet.

Figure 1.1Manganese nanoparticles on bucky balls. STM image (see Chapter 4, Section 4.4.1) of a few manganese nanoparticles with a diameter of about 3 nm (i.e. each one contains a few hundred manganese atoms) deposited onto a bed of carbon-60 nanoparticles with a diameter of 0.7 nm on a silicon surface. Reproduced with the permission of the American Institute of Physics from M. D. Upward et al. [1].

In fact, distance scales used in science go to much smaller than nanometers and much larger than meters, but the title of this book suggests that there is something special about the nanoeter scale—so what is it? The answer to this lies in philosophical questions that were originally posed regarding the fundamental nature of matter and space by Democritus and his contemporaries that still resonate today. Democritus (Fig. 1.2) was born in Abdera, Northern Greece in about 460 B.C. to a wealthy noble family, and he spent his considerable inheritance (millions of dollars in today's currency) traveling to every corner of the globe learning everything he could. Known as the laughing philosopher, he lived to over 100 years old, so it appears to have been a good life. He wrote more than 75 books about almost everything from magnets to spiders and their webs. Only fragments of his work survived, with most of his books being destroyed in the third and fifth centuries. His lasting impact on modern science was to propose, with his teacher Leucippus, the concept of the atom.

Figure 1.2Democritus. Marble bust of Democritus, Victoria and Albert Museum, London. Reproduced with permission from Carlos Parada, Greek Mythology Link: (http://homepage.mac.com/cparada/GML).

All our experience in the macroscopic world suggests that matter is continuous; and thus with nothing but our eyes for sensors, the original suggestion that matter is made from continuous basic elements such as earth, fire, air, and water seems reasonable. This, however, leads to a paradox because if matter were a continuum, it could be cut into smaller and smaller pieces without end. If one were able to keep cutting a piece of matter in two, each of those pieces into two, and so on ad infinitum, one could, at least in principle, cut it out of existence into pieces of nothing that could not be reassembled. This led Leucippus and Democritus to propose that there must be a smallest indivisible piece of substance, the a-tomon (i.e., uncuttable), from which the modern word atom is derived. They suggested that the different substances in the world were composed of atoms of different shapes and sizes, which is not an unrecognisable description of modern chemistry. Once you propose atoms, however, you automatically require a “void” in which they move; and the void is a concept that also produces dilemmas, which were the subject of much debate three thousand years ago. For example, is the void a “something” or a “nothing” and is it a continuum or does it also have a smallest uncuttable piece? Whereas atoms are a familiar part of the scientific world, the true nature of the void between them is something that is still not fully understood and many scientists believe that the void question lies at the heart of all the “big” questions about the universe and the nature of reality. It is a question that nanotechnology can address, and we will return to this discussion in Chapter 8.

Meanwhile returning to atoms, one could argue that the basic philosophy outlined above, which led to their being proposed, means that you should really attribute the a-tomon to more fundamental constituents of atoms, such as electrons and quarks. There is a good reason to stick with atoms, however, since we are talking about constituents of materials; and if we pick on a particular material, say copper, the smallest indivisible unit of “copperness” is the copper atom. If we divide a copper atom in two, we get two atoms of different materials.

So what has all this got to do with nanotechnology? Nowadays we can carry out, in practice, the Democritus mind experiment and study pieces of matter of smaller and smaller size right down to the atom. The important result is that the properties of the pieces start to change at sizes much bigger than a single atom. When the size of the material crosses into the “nanoworld” (Fig. 0.1), its fundamental properties start to change and become dependent on the size of the piece. This is, in itself, a strange thing because we take it for granted that, for example, copper will behave like copper whether the piece is a meter across or a centimeter across. This is not the case in the nanoworld, and the onset of this strange behavior first shows up at the large end of the nanoworld scale with the magnetic properties of metals such as iron. It is worth spending a little time on this because it is a clear illustration of how the behavior of a piece of material can become critically dependent on its size.

1.2 The Magnetic Behavior of Nanoparticles

It is widely known that iron is a magnetic material, but in fact a piece of pure (or “soft”) iron is not magnetized. This is easy to prove by taking a piece of soft Fe and seeing that it does not attract a ball bearing (Fig. 1.3a). In contrast, a permanent magnet, which is an alloy, such as neodymium–iron–boron that is permanently magnetized, strongly attracts the ball bearing (Fig. 1.3b). A simple and illustrative experiment is to sandwich the ball bearing between the permanent magnet and piece of soft iron and then pull the magnet and the pure iron apart (Fig. 1.3c). Oddly, while the ball bearing shows no attraction to the soft iron on its own, in the presence of the magnet it stays glued firmly to the piece of soft iron as it is pulled away, showing that it is magnetized to a greater degree than the actual magnet. Beyond a certain distance from the magnet, the soft iron reverts to its demagnetized state and the ball bearing comes loose (Fig. 1.3d).

Figure 1.3Simple experiment to demonstrate magnetic domains. (a) Soft iron does not attract the ball bearing. (b) A conventional magnet does, however (c) when the soft iron is magnetized by being in the presence of the magnet it becomes more magnetic than the magnet and the ball bearing stays with the soft iron in preference to the magnet. (d) The situation persists until the magnet is far enough away that the soft iron reverts to its domain structure and externally generates no magnetic field.

The source of magnetism in materials is their constituent atoms, which consist of tiny permanent dipolar magnets whose strength is given by the magnetic moment1 of the atom (see Advanced Reading Box 1.1). In a material such as iron, there is a strong interaction (the exchange interaction) between the atoms that lines up the atomic magnets to produce a macroscopic magnetization. Note that the exchange interaction is a quantum mechanical effect and is not the normal interaction that you would see between two bar magnets, for example. For one thing the interaction between bar magnets aligns them in opposite directions, and for another the exchange interaction is thousands of times stronger than the direct magnetic interaction.

Advanced Reading Box 1.1—Atomic Magnetic Moments and The Exchange Interaction

The individual atoms of most elements have a permanent magnetic moment, so they generate a dipolar magnetic field similar to a simple bar magnet. The source of the atomic magnetic moment is twofold. It arises from the orbital motion of the electrons around the nucleus, which can be considered to constitute a simple current loop, and also from the intrinsic angular momentum (spin) of the electrons. These two contributions generate an orbital and a spin magnetic moment; and for the elements Fe, Co, and Ni, the two contributions are simply added to obtain the total magnetic moment. The exchange interaction that acts between neighboring atoms arises from the Pauli Exclusion Principle. This tends to keep electrons apart if they have the same spins so that the Coulomb repulsion energy between the outermost electrons of neighboring atoms is reduced if the electrons align their spins in the same direction. This appears as a very strong magnetic interaction trying to align the spin magnetic moments, but it is an electrostatic effect produced by the quantum nature of the electrons. It is typically 3–4 orders of magnitude stronger that the direct magnetic interaction of the atomic magnetic moments taken to be simple bar magnets.

So in a magnetic material the powerful exchange interaction tries to line up all the microscopic atomic magnets to lie in the same direction. This, however, is not necessarily the preferred configuration because the uniformly magnetized state generates a magnetic field that passes through the material and the magnetization finds itself pointing the wrong way in its own magnetic field; that is, it has the maximum magnetostatic energy.2 Of course, reversing the magnetization is of no use because the generated field reverses and again the sample magnetization and the generated field are aligned in the least favorable direction to minimize energy. The exchange interaction and the magnetostatic energy are thus competing, which at first glance does not appear to be much of a competition considering that the exchange energy per atom between nearest neighbors is 3–4 orders of magnitude stronger than magnetostatic one. The magnetostatic interaction, however, is long range while the exchange interaction only operates between atomic neighbors. There is thus a compromise that will minimize the energy relative to the totally magnetized state by organizing the magnetization into so-called domains with opposite alignment (Fig. 1.4a). If these domains have the right size, the reduction in magnetostatic energy is greater than the increased exchange energy from the atoms along the boundaries that are neighbors and have their magnetization pointing in opposite directions. In the minimum energy state the material does whatever is necessary to produce no external magnetic field, and this is what has happened in Fig. 1.3a. The magnetization of the soft iron has organized itself into domains, and externally it is as magnetically dead as a piece of copper. The actual magnet has been treated to prevent the domains forming so that it stays magnetized (Fig. 1.3b). When we bring the piece of soft iron into the field of the magnet, its domains are all aligned in the same direction and it has a greater magnetization than the magnet so that when we pull the two apart, the ball bearing stays stuck firmly to the soft iron. This continues until the soft iron is far enough away from the magnet to revert to its domain structure and become magnetically dead externally.

Figure 1.4Single-domain particles. Domain formation in iron to minimize energy. Below a critical size (approx. 100 nm), the energy balance favors just a single domain and the piece of iron stays permanently and fully magnetized.

The phrase “If these domains have the right size” in the previous paragraph encapsulates the critical point. If we do the Democritus experiment and start chopping the piece of soft iron into smaller and smaller pieces, the number of domains within the material decreases (Fig. 1.4b). There must come a size, below which the energy balance that forms domains simply does not work any more and the particle maintains a uniform magnetization in which all the atomic magnets are pointing the same way (Fig. 1.4c). So what size is this? It turns out to be about 100 nm—that is, the upper edge of the nanoworld. Any iron particle that is smaller than this is a single domain and is fully magnetized. This may seem like a subtle size effect, but it has profound consequences. Fully magnetized iron is a much more powerful magnet than any actual magnet as shown in Fig. 1.3. The reason is that a permanent magnet must contain some nonmagnetic material to prevent the process of domain formation so that its magnetization is diluted compared to the pure material. A world in which every piece of iron or steel was fully magnetized would be very different from our familiar one. Every steel object would attract or repel every other one with enormous force. Cars with their magnetization in opposite directions would be very difficult to separate if they came into contact.

Nature makes good use of this magnetic size effect. Bacteria, such as the one shown in Fig. 1.5, have evolved, which use strings of magnetic nanoparticles to orient their body along the local magnetic field lines of the Earth. The strain shown in the figure, which is found in northern Germany, lives in water and feeds off sediments at the bottom. For a tiny floating life form such as this, knowing up and down is not trivial. If the local field lines have a large angle to the horizontal, as they do in Northern Europe, then the string of magnetic nanoparticles makes the body point downwards and all the bacterium has to do is to swim knowing that it will eventually find the bottom.

Figure 1.5Magnetic bacterium using single-domain particles. The Magnetic bacterium (magnetospirillum gryphiswaldense) from river sediments in Northern Germany. The lines of (permanently magnetized) single-domain magnetic nanoparticles, appearing as dark dots, align the body of the bacterium along the local direction of the Earth's magnetic field, which in Germany is inclined at 55° from horizontal. This means that the bacterium will always swim downwards towards the sediments where it feeds. Reproduced with With kind permission of Springer Science and Business Media from D. Schüler [2].

The intelligence of evolution is highlighted here. If the particles are single-domain particles, then they will stay magnetized forever, so forming a string of these ensures that the navigation system will naturally work. If the bacterium formed a single piece of the material the same size as the chain of particles, then a domain structure would form and it would become magnetically dead. The nanoparticles are composed of magnetite (Fe3O4) rather than pure iron, but the argument is the same. There is currently research devoted to persuading the bacteria to modify the composition of the nanoparticles by feeding them with cobalt-containing minerals as a method of high-quality nanoparticle synthesis (see Chapter 4, Section 4.1.8).

Interestingly, chains of magnetic nanoparticles with a similar structure have been found on a piece of meteorite known to have come from Mars [3]. Since the only known way of producing this mineral is biological, the observation is evidence that there was once life on Mars, though this analysis remains controversial. In fact, Mars no longer has a significant planetary magnetic field, which disappeared in the distant past, but supporters of the Martian bacteria proposal argue that there could be localized magnetic fields around magnetic minerals on the surface.

Formation of single-domain particles is only the onset of size effects in the nanoworld. If we continue the Democritus experiment and continue to cut the particles into smaller pieces, other size effects start to become apparent (Fig. 1.6). In atoms the electrons occupy discrete energy levels, whereas in a bulk metal the outermost electrons occupy energy bands, in which the energy, for all normal considerations, is a continuum. For nanoparticles smaller than 10 nm, containing about 50,000 atoms, the energy levels of the outermost electrons in the atoms start to display their discrete energies. In other words, the quantum nature of the particles starts to become apparent. In this size range, a lot of the novel and size-dependent behavior can be understood simply in terms of the enhanced proportion of the atoms at the surface of the particles. In a macroscopic piece of metal—for example, a sphere 2 cm across—only a tiny proportion of the atoms, less than 1 in 10 million, are on the surface atomic layer. A 10-nm-diameter particle, however, has 10% of its constituent atoms making up the surface layer, and this proportion increases to 50% for a 2-nm particle. Surface atoms are in a chemical environment that is different from that of the interior and are either exposed to vacuum or interacting with atoms of a matrix in which the nanoparticle is embedded. Novel behavior of atoms at the surfaces of metals has been known for decades; thus, for example, the atomic structure at the surface is often different from that of a layer in the interior of a bulk crystal. When such a high proportion of atoms comprise the surface, their novel behavior can distort the properties of the whole nanoparticle.

Figure 1.6Size-dependent behavior in nanoparticles. For particles smaller than 10 nm, quantum effects start to become apparent. In this size range the proportion of atoms that constitute the surface layer starts to become significant reaching 50% in 2 nm diameter particles. Below about 3 nm the strength of magnetism per atom starts to increase.

Returning to magnetism, a well-known effect in sufficiently small particles is that not only are they single domains but also the strength of their magnetism per atom is enhanced. A method for measuring the strength of magnetism (or the magnetic moment) in small free particles is to form a beam of them (see Chapter 4, Section 4.1.1 for a description of nanoparticle sources) and pass them through a nonuniform magnetic field as shown in Fig. 1.7. The amount the beam is deflected from its original path is a measure of the nanoparticle magnetic moment; and if the number of atoms in the particles is known, then one obtains the magnetic moment per atom.

Figure 1.7Measuring the magnetic moment in free nanoparticles. The magnetic moment in free nanoparticles can be measured by passing a beam of them through a non-uniform magnetic field and measuring the deflection in their path.

Magnetic moments of atoms are measured in units called Bohr magnetons3 or μB (after the Nobel laureate Neils Bohr), and the number of Bohr magnetons specifies the strength of the magnetism of a particular type of atom. For example, the magnetic moments of iron, cobalt, nickel, and rhodium atoms within their bulk materials are 2.2μB, 1.7μB, 0.6μB, and 0μB (rhodium is a nonmagnetic metal), respectively. Figure 1.8 shows measurements of the magnetic moment per atom in nanoparticles of the above four metals as a function of the number of atoms in the particle. In the case of iron, cobalt, and nickel, a significant increase in the magnetic moment per atom over the bulk value is observed for particles containing less than about 600 atoms. Perhaps most surprisingly, sufficiently small particles (containing less than about 100 atoms) of the nonmagnetic metal rhodium become magnetic.

Figure 1.8Measured magnetic moments per atom in magnetic nanoparticles. Experimental measurements of the magnetic moment per atom in iron, cobalt, nickel and rhodium (a non-magnetic metal in the bulk) nanoparticles as a function of the number of atoms in the particle. For iron, cobalt and nickel, there is a significant increase in the magnetic moment per atom over the bulk value for particles containing less than about 600 atoms. Rhodium becomes magnetic in particles containing less than about 100 atoms. Note the very dramatic change in the magnetic moment of iron particles in going from a 12-atom particle to a 13-atom particle. Reproduced with the permission of the American Association for the Advancement of Science (AAAS) from I. M. L. Billas et al. [4], permission of the American Physical Society from A. J. Cox et al. [5] and S. Apset et al. [6], Copyright 1994 and 1996 and permission of Elsevier Science from M. B. Knickelbein [7].

Throughout the whole size range in Fig. 1.8, the fundamental magnetic behavior of the particles is size-dependent. Do not lose sight of how strange a property this is and how it runs counter to our experience in the macroscopic world. It is as strange as a piece of metal changing color if we cut it in half (something else that happens in nanoparticles). If Democritus were doing his chopping experiment on iron, when he reached a piece 100 nm across, which would be invisible in even the most powerful optical microscope, he would say that he had not yet reached the atomon