SCIENCE IN

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SMALL SCIENCE – BIG IM

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the future fornanoscience and nanotechnology

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In the past millennium, we have learnt to make machineson ever smaller scales, progressing from the delicateclockwork, say, of a fine wristwatch to the complexmicrostructure of the latest electronic chip.

In the past 25 years, new methods of probing and controllingthe physical shape and chemical behaviour of matter right down tothe molecular and atomic level have become available. The result isthat physicists, chemists, biologists and technologists are allworking together to create materials and devices with a structuraland functional complexity on the scale of a billionth of a metre, ornanometre. This is the burgeoning field of nanoscience andnanotechnology.

One obvious aim is to miniaturise the workings of systems suchas electronic processors, sensors and even motors, or to combinethe properties of various nano-scale components to make high-performance materials. Such approaches introduce usefulproperties such as lightness, strength, and chemical reactivity and also selectivity. They save on raw materials and energy,producing cleaner, cheaper, more efficient, sustainabletechnologies for economic and environmentalbenefits – both so important in today's world.

New properties Just as exciting is the prospect of discoveringand exploiting the novel properties that emergeat the nano-level. It is already well-known thatmatter made of nano-sized particles, or innano-thin films, can look and behave quitedifferently from the corresponding bulkmaterial. One crucial difference is their largesurface-area to volume ratio – which leads tosignificantly increased chemical reactivity.

In a more exotic vein, quantum effectsemerge, leading to unusual electronic, optical,and magnetic phenomena. These stem from

the wave-like behaviour of matter which manifests itself at thenano level. The discoveries emerging in these areas are some of themost exhilarating aspects of nanoscience and nanotechnologyresearch. These properties might one day be exploited to createnovel technologies such as very fast 'quantum' computing.

The principle of the self-assembly of molecules has also becomeimportant in nanoscience. Electrostatic and chemical forces,combined with the vibrations and random motion of individualmolecular units, cause them to self-assemble into nano-systemswith specific characteristics – as seen in biology with the formationof large molecules such as proteins. By complex manipulation ofexperimental conditions, researchers can already mimic somesimple biologically self-assembled systems. This will be ofenormous future benefit not only to healthcare but also indeveloping new approaches to nano-scale engineering.

Areas of application • HIGH-PERFORMANCE MATERIALS – paints, coatings, inks, ceramics, composites, complex fluids –

used in construction, aerospace, transport, defence, sport, household materials, cosmetics and food• 'GREEN' OR SUSTAINABLE CHEMISTRY – catalysts, nano-porous membranes, nano-structured solvents• ENERGY PRODUCTION AND STORAGE – solar cells, fuel cells, hydrogen storage• INFORMATION PROCESSING, STORAGE AND DISPLAYS, AND TELECOMMUNICATIONS – nanocomputers,

electronic, photonic and spintronic devices • MINIATURISED ENGINEERING – components, sensors, mobile analytical equipment, spacecraft• HEALTHCARE – drug delivery, gene therapy, implants, biosensors, prosthetics, tissue repair, medical

diagnostics and gene analysis, cancer treatment• ENVIRONMENT – land remediation and clean-up

Controlling matter at the smallest scales

Humans have always manipulatedtheir environment to fashion newmaterials, creating tools and devicesto improve their lives

An electron micrographof a 30-nanometre gold

dot on a silicon substrateTop to bottom:

A scanning near-fieldoptical micrograph ofhexagonally arranged

30-nanometre aluminium particles

Scanning tunnellingmicrograph of

nanoscale ‘gold fingers’

Scanning tunnellingmicrograph of an iron

silicide crystal

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Then and nowThe exploitation of nano-scale phenomena is not new. In the 17thcentury, Andreas Cassius chemically prepared solutions of goldnano-particles, which were used to impart a deep red colour toglass and ceramics. Such 'colloidal' suspensions have beenemployed throughout history as dyes and pigments – andrepresent an early, albeit unwitting application of nanotechnology.

Modern chemistry is largely about increasing structuralcomplexity at the molecular, nano-level via strategies involving aseries of highly selective chemical reactions. However, it was thephysicist Richard Feynman who first suggested in 1959 that itmight be possible to manipulate atoms and molecules individuallyto assemble minute machines and circuits.

By combining these ideas and approaches, nanotechnology isevolving. Although solidly based in the traditional scientificdisciplines, it involves a new multidisciplinary way of working.Many universities are setting up real or virtual nanotechnologycentres to encourage chemists, physicists, biologists and engineersto pool their expertise.

Making the nanoworld To create nano-sized structural features, scientists have extendedthe standard lithographic techniques applied in microelectronics,using, for example, extreme ultraviolet light, X-rays and electronbeams to etch patterns in thin films.

More direct methods evolved with the development of scanning probe microscopes in the 1980s. These extraordinaryinstruments sense atoms and molecules by passing a fine tip overtheir surfaces and turning the measurements made into images.They can also be used to manipulate single atoms, so have hugepotential as fabrication tools as well as devices. These are so-calledtop-down strategies.

The alternative is the bottom-up approach whereby atoms andmolecules self-organise into nano-sized crystals or more complexmolecular assemblies. Life itself is based on such biomolecularmechanisms. Chemists have for several decades been exploitingthese same ideas to construct complex materials.

To create complexity, researchers are increasingly opting forhybrid strategies that involve using pre-shaped or chemicallyprepared surfaces or particles on which to build nano-structures.For example, electron beam lithography can be used to etch aminiature pattern onto a master 'stamp'. When coated with asuitable chemical, the pattern can be printed on a reactive surfaceto make an electronic device or sensor. Similarly nano-sizedparticles can be coated in a polymer or metal and then dissolved toleave behind a pore-like structure.

HUMAN HAIR BLOOD CELL BACTERIUM VIRUS TYPICAL NANOPARTICLE C60

200,000 nanometres 20,000 nanometres 1000 – 2000 nanometres 20 – 250 nanometres 1 – 100 nanometres 1 nanometre

Economic prospects Nanotechnology is likely to seep into every industry sooneror later, and will have profound effects on manufacturingindustry and also on medicine. American forecasters predictthat products made using nanotechnological processes willrise from the current 0.1 per cent of global manufacturingoutput to 15 per cent in 2014 and will be worth 2.6 billionbillion dollars – larger than the combined sales of the IT and telecomms market. Fifty per cent of electronicmanufacturing will involve nanotechnology, with furtherinput into medical applications – pharmaceuticals andmedical devices – rising to 16 per cent following extensivetrials. One area of focus is cancer research: the US is investing$144.3m over five years in nanotechnologies for diagnosingand treating cancer.

S C I E N C E I N F O C U S > > N A N O S C I E N C E A N D N A N O T E C H N O L O G Y)

‘Nanoman’ createdby a focused

electron beam on thetip of a scanning

tunnelling microscopewith a precision of

10 nanometres

1 N A N O M E T R E I S O N E B I L L I O N T H O F A M E T R E ( 1 0 - 9 M E T R E S )

10-2 m

10-3 m

10-4 m

10-5 m

10-6 m

10-7 m

10-8 m

10-9 m

10-10 m

10mm

1mm =

100μm

10μm

1μm =

0.1μm = 100 nm

0.01μm = 10 nm

1 nm

0.1 nm

1,000

,000 n

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1,000

nano

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s = 1

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Nano-particles and structures It is the special properties of nano-structured matter that mostinterest scientists and technologists. Nano-sized particles may lookand behave differently from the bulk material because of their highsurface area compared to volume. A gold wedding ring, of course,looks gold-coloured – but 60-nanometre gold particles look redbecause of the way their surfaces interact with light.

Similarly, the opaque white pigment titanium dioxide, whichabsorbs ultraviolet light, is transparent in its nano-form and isincluded in sun-screen lotions. Ceramic nanoparticles are alsobeing incorporated into plastics to create nanostructuredcomposites that are more robust. For example, car-tyre rubber canbe reinforced with carbon and silica nanoparticles.

High surface areas can also make materials more chemicallyreactive. Catalysts, which accelerate chemical reactions, do so byforming transient intermediates at active centres on their surfaces.They are increasingly being used in the nano-structured form toimprove efficiency and selectivity. When in the nano-form,otherwise poorly-performing catalysts can replacemore expensive catalyst materials, for example,nano-scale nickel can replace platinum incleaning up car-fuel exhaust.

Zeolites – minerals with a porous structure – have beenemployed for many years as catalysts that not only speed upchemical reactions but selectively produce specific compounds,(particularly to make chemicals from petroleum). The size andshape of the pores constrain the type of molecule that can form atthe reactive catalytic sites within the zeolite structure. One recentdevelopment is to disperse a nanocatalyst on a porous supportsuch as a zeolite to give improved catalytic activity.

Complex nanofluids Metallic, ceramic and polymer nanoparticles can also be incorporatedinto soft materials for specific functions such as enhancing thermaland electrical conductivity or controlling viscosity. For example, afluid suspension of magnetic iron-oxide nanoparticles can be madeto thicken suddenly by applying a magnetic field. Such magneto-rheological fluids are used in shock absorbers.

Some of the most technologically important nanofluids containlong-chain molecules called surfactants which self-assemble into

spherical particles. Their rolled-up layered structure resemblescell-membranes, and they can be designed to encapsulate

drug molecules, selectively delivering them to, tumourcells, for example.

Nano-icons 1 - the carbon nanotubeCarbon nanotubes – rolled-up cylinders of graphite-like carbon – are roughly 1 nanometre in size; theirversatility has meant that they have become an icon of nanotechnology.• They were first discovered in 1991 – in soot produced during an arc discharge between graphite

electrodes. Scientists have been exploring their properties and potential uses ever since. • They are extremely strong and can be made into fibres and film. They are already being incorporated

into fabrics for wear-resistant clothes and plastics, and to make stronger, lighter sports equipment. • They display intriguing electronic properties and could be used in miniaturised computer circuits, as

wires, and in displays and solar cells. • Their tube-like structure is hollow so that other molecules can be captured inside them. They can

therefore be used as pollution filters, and as potential drug-delivery vehicles.

Inset top:An atomic force micrograph of a carbon

nanotube ring placed over gold electrodesBottom:

The world's smallest test-tube: DavidBritz and colleagues at the University

of Oxford have inserted fullerenemolecules into a nanotube where

they react, linking into a long chain

Making the most of the nano-world

Nano-materials are already being used in everyday life

Oxonica, a spin-out company fromthe University of Oxford, producestitanium dioxide nanoparticles foruse in sunscreens. They have beenmodified with a small amount ofmanganese to prevent the generationof skin-damaging free radicals

A carbon nanotube

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Towards nano-electronics Over the past 25 years, electronic devices haveshrunk incredibly, and today's chips are alreadyentering the nano-world. A decade ago, thetransistors that make up integrated circuits werebeing built at a scale of 500 nanometres; today,the latest PCs contain Intel chips with transistorsonly tens of nanometres across. IBM is alreadyplanning to print circuitry on the 30-nanometrescale using new ultraviolet lithography.

Similarly, the amount of data that can be storedon a disk drive is getting ever denser, havingincreased 50 million times in 50 years. Computerdrives can store data at densities of 6 billion bits ofdata per square inch, using nanometre-thicklayered films. Magnetic multilayers are also being used to construct tiny logic and memoryelements, and sensors, based on the magneticproperties or 'spins' of electrons (spintronics). The magnetic properties of single moleculescontaining metal atoms could be the basis offuture molecular spintronic devices.

Ultimately, devices may be on the scale of individualatoms or clusters of atoms and molecules, and chemists are already devising ingenious molecular structures that can act as transistors. A transistor made of semiconducting carbonnanotubes has been constructed but there is some way to gobefore this can be connected to other components to makesmaller, faster devices. Molecules that can change their electronicstates can also be used as switches.

Quantum devices At the nano-scale, quantum effects become influential. Forexample, silver is not normally magnetic but becomes so whensilver atoms are grouped into small clusters, due to subtleelectronic interactions. Tiny magnetic cobalt rings less than 100nanometres across, in which the magnetic force is confined insidethe ring due to quantum effects, could be the basis of highly stablecomputer memories.

Physicists are also investigating how to exploit other exoticquantum behaviour such as tunnelling through energy barriers,quantisation of electron behaviour and quantum interferenceeffects caused by electron or light waves.

Photonics – using photons of light to carry and processinformation – is a growing area, and may lead to an all-opticalcomputer. Scientists are looking to fabricate 'holey' structures onthe nano-scale that will guide light into tiny opto-electronic devices.

Whether future devices depend on electronics, spintronics orphotonics, they will all be working at the nano-level.

Nano-icons 2 - the quantum dotThe archetypal quantum device is an exotic structure called a quantum dot. It is a nano-sized (usually) semiconductor structure which can imprison electrons. The electrons canoccupy different energy levels as in an atom, and quantum dots are sometimes calledartificial atoms. The energy levels can be tuned to absorb or emit different frequencies oflight by changing the quantum dot's size and shape. • Quantum dots have lots of potential applications. Because they can be brightly coloured,

they are replacing traditional fluorescent dyes used in biological sensors. • Quantum dot blue lasers are the data-reading devices in the next generation of high-

definition DVDs and the new PlayStation 3.• Quantum dots could also be the basis of much higher efficiency solar cells, since they can

emit as many as three electrons for every photon of sunlightcaptured, compared with, at most one electron perphoton in current photovoltaic cells.

• They are candidates for one of the most excitinggoals of nanotechnology – quantumcomputing which takes advantage ofquantum behaviour to carry out muchfaster calculations. A quantum dot couldbe made to operate as the element (a qubit) of a quantum computer.

Magnetic cobalt nano-rings

S C I E N C E I N F O C U S > > N A N O S C I E N C E A N D N A N O T E C H N O L O G Y)

An electron micrograph of agermanium/silicon quantum

dot in a silicon matrix

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NanocomputersWhile many researchers are working hard on innovative ways ofmaking nano-structures and investigating their often remarkableproperties, others are already exploring the next stage ofdevelopment – to link devices, such as logic gates, memory circuitsand actuators, so that they can be interlinked to makenanocomputers.

The 'bottom-up' approach of self-assembly is being used toorganise metallic and semiconductor nanoparticles, as well aselectronically-active molecular structures, into arrays on supportsor fixed in membrane pores. For example, organic molecules canbe chemically bonded to an active surface; these are then linked bycurrent-carrying nanowires or nanotubes.

One approach to making a computing device is to arrange alayer of parallel nanowires so that they criss-cross over another set.The two layers are then connected by molecules that can switchcurrent on or off. The goal is to create a computer memory chipwith a density of 100 billion bits per square centimetre.

Nanomachines Scientists are also devising nano-electromechanical systems(NEMS) to convert mechanical energy into electrical or opticalsignals, or vice versa. Molecules shaped like wheels, propellers andbelts, which rotate, untwist or move along an axle, are candidatesfor motors, actuators and pumps. As molecules tend to jiggle aboutrandomly, two of the major challenges are how to harness thismotion so that it goes in the required direction, and how to powerit using chemical energy or light.

NEMS could form components of the next generation of 'labs ona chip' which integrate microscopic pumps, separators andanalysers for carrying out chemical and biochemical analysis.Potential applications include medical diagnostics, gene analysis,drug discovery and remote chemical analysis, for example in space.Methods of patterning nano-sized channels for transporting fluidon such chips are now being developed.

Researchers are starting toengineer complex structuresat the nano-level

Future visions for nanotechnology

DNA - the ultimate nano toolThe molecule that carries our genetic blueprint is incrediblyversatile and is a natural tool for nanotechnology: It is easy tomake; its structure is well-understood; it is chemically versatilewith complex but predictable dynamic properties; consisting of a combination of four bases, it is programmable; and, of course,DNA does replicate in the right biological environment, offeringthe tantalising if very speculative possibility that machines could clone themselves.

Pieces of the double helix can be arranged into cubes andcylinders that could be the building blocks of nanomachines todiagnose and treat disease. DNA 'tiles' can be slotted togetherinto stripes like bar codes to be the read-out elements of a DNAcomputer. DNA scaffolding could also provide a template toorganise other useful units such as proteins and nanoparticlesin self-assembled arrays.

A molecular shuttlecock

Andrew Turberfield atthe University ofOxford is working onsynthetic DNA motors

A nano-electromechanical system (NEMS)IBM is already working on a NEMS nano-drive called Millipede. It consists of an array ofthousands of silicon cantilevers with sharp tips (like those in a scanning probe microscope) whichwrite, read and erase information using a combination of electrically-induced mechanical forceand heat to create nano-sized dents in a plastic substrate. It will be able to record and store anentire CD collection, or several feature films, on a chip the size of a postage stamp.

As Nature does itThe ultimate nanotechnologist is Nature, which creates molecularassemblies that function autonomously. Not surprisingly, scientistshave been inspired by biochemical systems, mimicking their structureand the way they work, or actually harnessing the nanostructuresthemselves – assemblies of proteins – to create devices.

Several enzymes found in all cells act as biomolecular motorsfuelled by electrochemical energy. Researchers are integratingthem with metal and organic nanostructures to build devices.Other proteins found in cell membranes, which act as pumps andtriggers, are also being explored. Optically active biomolecularunits that can harvest light and turn it into chemical energy couldalso be the basis for new biosensors.

One of the most practical and immediateapplications of bio-inspired nano-structures is intissue engineering. Researchers are nowmimicking the hierarchical structure of naturalmaterials like bone, for use in implants.Nanofibre structures with appropriate bioactiveregions can encourage and guide the growth ofnew cells such as neurons.

The future The examples given in this booklet show that nanotechnology hasbeen around for a long time – living organisms operate at thenano-level. However, it is the availability of new imaging, analyticaland fabrication techniques that has accelerated the evolution ofthe field. Many novel scientific concepts are being explored and westill do not know which of the more speculative ideas willeventually bear commercial fruit.

There are still many technical problems to overcome such ashow to deal with the 'stickiness' of molecular-sized devices or howto ensure they are robust. Many of the methods of fabrication areexpensive and not easy to commercialise.

Nevertheless, it is inevitable that developments in technologywill tend towards increasing miniaturisation, probably involvingthe integration of chemical and biologically-based processes withfundamental physical phenomena at the quantum level. This willinvolve scientists of all disciplines working together.

In terms of nanoscience, there are huge benefits to be gainedfrom understanding the world at a scale just above that of atomsand molecules – even something as simple as how the behaviourof nano-sized tannin particles affects the taste of red wine.

The potential for improving our way of life as well as increasingour understanding of the natural world through nanoscience andnanotechnology is incalculable.

Nanotechnology and societyAs with all developing technologies, society as a whole has to consider how best to exploit thepossibilities, whilst anticipating potential problems. The toxicity of nano-sized materials may bedifferent from their bulk analogues and needs to be identified.

Concerns have been raised on the health and environmental impacts of using productscontaining nanoscale particles, which are highly reactive and may be able to traverse cellmembranes. It is well known that nano-sized air pollutants can cause respiratory problems, andthere have been recent reports that carbon nanoparticles (C60 and nanotubes) are toxic to humanskin cells, although chemically modifying the nanotubes has been shown to reduce their harmfuleffects. Not enough is known about the toxicity of nanoparticles used in cosmetics, for example.

There are no reports of clinical toxicity in humans so far; nevertheless, more researchurgently needs to be done. Further studies are also needed on how nano-particles could

spread in the atmosphere and accumulate in the environment.Governments around the world, together with industry and research communities,

must establish global protocols for testing the safety of nanotechnological products,and engage with the public in discussing any ensuing risk/benefit issues. Only in thisway will society benefit fully from this powerful technology.

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S C I E N C E I N F O C U S > > N A N O S C I E N C E A N D N A N O T E C H N O L O G Y)Tissue engineering:osteoblast (bone-making)cells cultured on a surfaceof carbon nanotubes –approximately 70nanometres in diameter

Drugs could bedelivered to tissues

via nanoparticlescalled micelles

The membrane protein F-ATPase acts as a rotary

biomolecular motordriving the production of

chemical energy in cells

PAGE 2Aluminium particlesBristol SPM group

Nanoscale ‘gold fingers’Q. Guo / Nanoscale PhysicsResearch Laboratory, University ofBirmingham

Iron silicide crystalMark Welland / University ofCambridge

Gold dot on a siliconsubstrateWeiss group / Pennsylvania StateUniversity

Applications imagesAccelrys, Nokia, PhotoDisc, ScottPaints

PAGE 3NanomanR.E. Palmer / Nanoscale PhysicsResearch Laboratory, University ofBirmingham

Nanometre scale imagesMary Ann Moran / University ofGeorgia, Chris Ewels /www.ewels.info

PAGE 4Carbon nanotubeGeoffrey R. Hutchinson /http://geoffhutchison.net/gallery/molecules/Nanotube.png.html

Carbon nanotube ringIBM

World's smallest test-tubeDavid A. Britz and Simon C.Benjamin / Royal Society ofChemistry

PAGE 5Quantum dotDiana Zhi, Paul Midgley, RafalDunin-Borkowski / University ofCambridge / Don Pashley, BruceJoyce / Imperial College London

Magnetic cobalt nano-ringsA. Wei et al, Angew. Chem. Int.Ed., 2003, 42, 5591

PAGE 6MillipedeIBM

Molecular shuttlecockAccelrys

DNA motorsAndrew Turberfield / University of Oxford

PAGE 7Osteoblast cells Julian George, Milo Shaffer andMolly Stevens / Imperial collegeLondon

F-ATPase molecular structureRichard Berry / University ofOxford

MicellesAccelrys

BACK COVERInset figure of eight machineDamian Gregory Allis(damian@somewhereville.com)

WriterNina Hall

DesignSpaced / pete@spaced-out.biz

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