NANOSCIENCE: THE FORERUNNER TO NANOTECHNOLOGY
A briefing document prepared for the Royal Society
and the Association of British Science Writers
by Peter Evans
December 1994
Introduction In his preliminary comments chairman Richard Friend
welcomed the fact that this press briefing's title Nanoscience: the
forerunner to nanotechnology lets researchers off the hook. Although
scientists and media alike have been exciting each other (and the
public) with some of the futuristic ambitions of nano-scale
technology - especially in the domain of micro-miniature electronic
devices - this remains still an area with more promise than
realisation. Later Fraser Stoddart made the same point "Don't let's
raise too many hopes too quickly" he warned. "The fundamental
science needs to be put in place first."
Dr Friend also pointed out that this is an area in which a number of
parallel lines are advancing simultaneously:
1. Synthetic chemistry: trying to make molecules and materials
with interesting electronic properties, borrowing from biology
self-assembly and replication strategies.
2. Solid state physics: smaller and smaller devices such as
transistors and computer gates which will ultimately shrink down to
molecular sizes.
3. Atomic scale imaging: using the Scanning Tunnelling Microscope
(STM) to look at individual atoms and even move them around one at a
time.
He reminded us that the EPSRC Molecular Electronics Committee makes
a distinction between manipulating lots of molecules for say
electronic devices and trying to produce circuit elements on single
molecules. Both though qualify for the handle 'nanotechnology'.
Definitions Just to remind you of the scale at which this science
operates. A nanometre is 10-9m of a metre. Measurement is done in
nanometres or sometimes angstroms - 10-10m. More familiarly, we are
talking about the sorts of units you need to describe the sizes of
individual atoms/molecules.
Professor J. Fraser Stoddart of the School of Chemistry in the
University of Birmingham How do you construct molecules that might
function as information processing machines? Nanochemist Professor
Stoddart believes that we are reaching the end of the road for the
'top-down' approach to miniaturising devices. So far we have
progressed from valves to transistors to etched silicon and, in the
process, achieved extraordinary reductions in size. But the ultimate
miniaturised computer will have to be built bottom up - by
assembling a processor molecule-by-molecule. The biological world
gives the chemist lots of examples and models, from insulin to
haemoglobin and, of course, that superb information processor - DNA.
Can the chemist imitate biological self-organization, self-assembly
and self-replication? This is a quantum leap in synthetic chemistry
akin to the technological jump from the arched bridge of the Romans
to modern cantilever and suspension bridges.
Think of atoms as the equivalent to letters, molecules as words,
assemblages of molecules as sentences and supramolecular arrays as
paragraphs. The Birmingham work with catenanes and rotaxanes takes
us up to the word level - indeed up to hyphenated words - but
ultimately the aim is to construct whole chapters and full-length
novels.
One way to get ordered molecular systems to self-assemble is to take
inexpensive and readily-available compounds such as the weedkiller
paraquat and build mechanically interlocked molecules called
catenanes (from the Latin catena 'chain'). These are basically
interlocking rings. A variant is the dumbbell shaped rotaxane in
which bulky groups of atoms at each end of the rod stop a ring from
slipping off.
These are novel molecules because they are stitched together both
chemically - using familiar co-valent bonding - and mechanically.
They are built up firstly by a stopper attaching to a linear
component (a bell to a bar). This switches on a recognition site for
the cyclic molecule on the linear, rodlike component. The cyclic
unit binds to this site and is then trapped in place by another end
stopper. As this cap is attached it switches on a second recognition
site for the cyclic component within the assembly. Thus a thread, a
bead and two stoppers are put in place by alternate chemical
(covalent) and non-chemical bonding.
When you have one self-built molecule you can have a second and so
on - each one highly ordered and stacked in a predetermined array.
The next step is to do something useful with these molecules. By
passing a current into them you can get the bead/ring component to
jump rapidly along the thread or rod. So rapidly in fact that there
is here a possible switching device. Multiply your switches millions
of times and you have - in theory still - a "molecular computer".
Here then is intelligence, cheaply built to the highest levels of
precision, using synthetic chemistry that mimics the way the
biological world constructs its massive arrays.
Professor Stoddart offers some take-home messages:
1. It is possible to borrow from the biological world the
technique of self-assembling molecular and supermolecular arrays on
the nanoscale with high precision.
2. The building blocks are cheap - dirt cheap - chemicals. But
synthetic chemists will need to learn as quickly as possible how to
transfer the concepts of biology into the chemical laboratory.
3. These novel building blocks can hold information -
'intelligence'.
4. In the last century unnatural products were synthesised in
industries such as dyemaking. This century it is pharmaceuticals. In
the 21st century we could make the leap into nanoscale machines with
intelligence. "The molecular computer? It's not whether but when we
bring it about."
Dr Michael Flanagan of the Department of Electronic and Electrical
Engineering at University College London Dr Flanagan addressed the
topic of "biosensors and bioelectronics" carrying on the theme of
what the life sciences have to offer nanoscience, in particular the
design of electronic devices.
Think of a cell. Proteins are being transported from outside, across
membranes to the inside. When say sodium crosses a cell membrane
there is a change in electrical potential to allow the channel to
open and then to close. Here is a system that might be harnessed by
the electronic engineer, except that proteins are a mere 10
nanometres across and wiring up such systems to the outside world
looks pretty tricky at present.
Nevertheless, there is a lot of interest in the operations carried
out by cellular machinery which currently can only be mimicked by
much larger complex circuitry. Biological sensors are extremely
sensitive. The snake can detect infra red radiation, the platypus
electric fields. Current developments link biosensors capable of
detecting a complex organic molecule with standard electronic
components such as transistors.
Some examples. There are biosensors under development for monitoring
blood sugar in diabetics. At present these are located outside the
body but in the future, with more miniaturisation of electronic
components and better understanding of biocompatibility problems,
one can envisage such a sensor being implanted in the patient's
body. Rolling on from there, biosensors could regulate the
controlled release of drugs or hormones in phase with the body's
changing demands.
Another marriage of biological molecules and electronics centres on
an unusually stable protein - bacteriorhodopsin. This can absorb
photons, so it acts as an efficient photoreceptor. It can form the
basis of devices that are not just optical detectors - electronic
eyes - but also optical memories with very high information storage
capacities.
In the near future though most bioelectronics will be biomimetic -
using organic chemistry as much as biochemistry to copy natural
systems as opposed to harnessing them directly. This gets round the
two big problems already touched on: the instability of biomolecules
and the difficulty of wiring them up to the outside world.
Comparison between the growing complexity of the interconnections
between transistors with increasing silicon circuit density and the
connectivity between cells in the human cortex clearly indicates
that we do have a great deal to learn from biology.
Neural networks have arisen from attempts to mimic the architecture
of the human brain but are implemented using standard electronic and
optoelectronic components and work in this area is now diverging
from the study of real neural networks. A re-examination of this
interface between biology and electronics, in the context of the
problems of connectivity may be equally fruitful.
Among the more exciting discoveries recently is the fact that some
microorganisms act as natural factories for nanometre-scale
semiconducting crystals called 'quantum dots'. They do this in
response to potentially lethal concentrations of cadmium by
synthesising crystalline spheres of cadmium sulphide coated with a
peptide molecule. Cadmium sulphide is a class of semiconductor that
is becoming increasingly important in optoelectronics but quantum
dot size crystals are hard to make by conventional techniques.
If, using biotechnology, microorganisms could do the job, here would
be a source of components for ultra-small lasers and optical
computers of the future.
Professor Roy Sambles of the Department of Physics at the University
of Exeter Professor Sambles discussed his successes in Exeter in
fabricating molecular rectifiers. A rectifier is a widely-used
device that allows electrical current to flow preferentially in one
direction.
He began with the longstanding difficulties of bridging the gap
between inorganic electronics, dominated by silicon and other simple
inorganic compounds and "organic ionics" - life. Interfacing the two
is a major challenge. If it is to come about, the feeling has been
for many years that an organic or molecular rectifier, equivalent to
the silicon diode, would be essential.
Because a rectifier passes current more in one direction than
another, it is unbalanced. In other words, in its conducting
properties, an asymmetric molecule is needed. Such molecules have
been synthesised by chemistry colleagues. Then, by using the
technique of Langmuir-Blodgett deposition, it is possible to build
up multilayer, molecular structures layer-by-layer with the right
conducting properties.
At Exeter such structures have been fabricated into rectifying
devices. This is "a major breakthrough" says Professor Sambles but
so far it has been the practicality of the fabrication technique
rather than the usefulness of the molecular device that has been
demonstrated. They only carry "abysmally low" currents - a million
times lower than those needed for useful working rectifiers. But
these are early days. The current loads will increase and, as well
as rectifiers, molecular capacitors and, possibly, molecular
amplifiers may be made.
The next step will be to assemble more elaborate molecules into some
kind of structure on a suitable substrate (again using perhaps
self-assembly techniques) and then interfacing this with a detection
system. This problem of connecting active molecules and the
intelligent user has so far received hardly any attention.
Looking ahead, complex organic molecules might well be made that
show not just classical electrical behaviour (rectifying, amplifying
etc) but possibly even quantum effects. How to make these, what the
effects might be and how they might be used are all for tomorrow.
Professor John Barker of the Nanoelectronic Research Centre at the
University of Glasgow A dominant word in this presentation on the
opportunities for nanoscale electronics was "play".
Professor Barker touched on the work colleagues are doing on
micro-motors and other machines, molecular abacuses and cages and
showed how new capabilities are emerging from this scientific
playground.
His talk focused on single electron devices and the possibilities
for designer molecules of the kind described by Fraser Stoddart
earlier. The 'jewel in the crown' of present day lithography, he
contends, is the production of metal-insulator structures that
enable you to manipulate and perform logic with single electrons:
one bit on one electron.
This would open up a new domain of atomic scale instrumentation - a
nano-electrometer for example that enables you to determine the
distribution of electric charge in a given molecule. Such machines
need to control single electrons by getting them to tunnel between
ultra-tiny electrodes. There is an analogy here between the movement
of electrons and the highly correlated pattern of traffic on a
motorway.
It may be possible to use such devices for testing some of the
predictions of quantum physics. A single-electron accelerator could
run experiments inaccessible to high energy physics.
The same technology may also be the route to the perennial problem
of interfacing molecular arrays to electronics. Designer molecules
being generated by supramolecular chemistry (such as rotaxanes)
would function as elements in ultra dense integrated circuits if
they could be attached into networks on inorganic nanoscale
electrodes. Research funded by EPSRC and ESPRIT and carried out at
Glasgow and Mainz is directed towards new techniques for the
controlled attachment, visualisation and verification of designer
molecules on structured substrates.
"This is a completely new scientific area" says John Barker. "We've
very little idea how molecules really work and behave."
Dr Mark Welland of the Department of Engineering in the University
of Cambridge It is now possible to observe, manipulate and measure
single atoms, as well as study the local physics of the structure of
atoms and molecules using the STM - Scanning Tunnelling Microscope.
The STM is a probe for controlling and measuring with high precision
all sorts of surface phenomena, including those of single atoms and
electrons.
The tip of the microscope interacts with the surface it is scanning.
Originally this interaction consisted of the tunnelling of electrons
between tip and surface. But now other interactions are being
generated - magnetic, optical, mechanical and thermal.
Measurements have been made of the magnetic structure of the surface
of cobalt- palladium multi-layer film.
Optical images come from a tip that is a sharpened optical fibre.
Light is pumped down, couples with the surface and reflects back up
the fibre.
The STM has been used to observe the mechanical properties of liquid
molecules squashed between two mica surfaces. The mechanical
stiffness of the molecules has been measured - both single molecules
and ensembles.
The response of the surface of silicon molecules subjected to
electric currents has also been observed using the STM.
Watching the behaviour of electrons jumping on and off defects in
crystals is another successful experiment in this field.
It is tempting to call the STM a "laboratory on a tip".
As well as observation and measurement there is also the possibility
of manipulating atoms with the STM. The famous front page of Nature
illustrated with an atomic IBM comes to mind. This is the 'ultimate
device' potentially, but STM manipulation is still much too slow to
be a practical way of fabricating atomic scale structures or
devices.
But this method could be used as a way of making a "template" for
the chemist to work on.
And finally ... According to Fraser Stoddart there is a problem in
developing nanoscience and nanotechnology to yield its undoubted
benefits - and that problem is an educational one. "You do not
nurture creative minds on a diet of A level mathematics, physics and
chemistry. You need to ensure that early training is much broader so
that art, culture and language all play their respective parts in
producing creative minds.
The scientist of the 21st Century is not only going to need highly
developed analytical skills, but will also have to be an artist
capable of expressing himself or herself like a sculptor, painter or
musician."
Speakers
Dr Richard H. Friend
Department of Physics
University of Cambridge
Madingley Road
Cambridge
CB3 0HE tel: 0223 337218
fax: 0223 350226
Professor J. Fraser Stoddart
School of Chemistry
University of Birmingham
Edgbaston
Birmingham
B15 2TT tel: 021 414 4362
fax: 021 414 3531
Dr Michael T. Flanagan
Department of Electronic and
Electrical Engineering
University College London
Torrington Place
London
WC1E 7JE tel: 0171 387 7050
fax: 0171 388 9325
Professor J. Roy Sambles
Department of Physics
University of Exeter
Stocker Road
Exeter
EX4 4QL tel: 0392 264103
fax: 0392 264111
Professor John R. Barker
Nanoelectronic Research Centre
Department of Electronics and
Electrical Engineering
University of Glasgow
Glasgow
G12 8QQ tel: 041 339 8855
fax: 041 330 4907
Dr Mark E. Welland
Department of Engineering
University of Cambridge
Trumpington Street
Cambridge
CB2 1PZ tel: 0223 332676
fax: 0223 332662
Main players in the area of nanoscience research
Dr Dennis Robinson
EPSRC Nanotechnology Research Coordinator tel: 0225 866544
fax: 0225 868993
Professor E. Southern F.R.S.
University of Oxford tel: 0865 275280
fax: 0865 275283
Area of research: solid state devices for applications in molecular
genetics
Mr Richard Coombs
Hammersmith Hospital tel: 0181 734 2030
fax: 0181 742 9202
Area of research: the relationship of nanotechnology to biomaterials
Professor C. Stirling
University of Sheffield tel: 0742 824483
fax: 0742 738673
Area of research: the formation, structure and behaviour of
self-assembled monolayers of organic compounds
Professor S. Davies
University of Nottingham tel: 0602 515121
fax: 0602 515122
Area of research: biomedical applications of nanotechnology
particularly drug targeting
Dr G. Attard
University of Southampton tel: 0703 593019
fax: 0703 593781
Area of research: self-assembled nanostructures; synthesis of
nanoceramics in liquid crystals and nanoparticles for synthetic
vaccines
Dr D. Tolferee
Daresbury Laboratory tel: 0925 603607
fax: 0925 603196
Area of research: microfabrication of deep structures using
synchrotron radiation
Professor C. Wilkinson
University of Glasgow tel: 041 330 5219
fax: 041 330 4907
Area of research: applications of nanotechnology in biology and
medicine and nanofabrication of structures on a very fine scale
NANOSCIENCE: THE FORERUNNER TO NANOTECHNOLOGY
On 30 March 1994 a press briefing on nanoscience, organized jointly
by the Royal Society and the Association of British Science Writers,
was held at the Royal Society as part of a programme to further the
public understanding of science. The attached document was prepared
after the briefing to summarize key issues raised by the speakers at
the time and also to provide a list of relevant contacts for future
reference. This document does not necessarily constitute the views
of the Royal Society, and views expressed in it should not be
attributed to the Society.
The document is free of copyright and may be used without reference
to source.
Ref: PR 67 (94)
Enquiries to: INFORMATION NOTE
Miss Anna Link
Science Promotion Section
The Royal Society
6 Carlton House Terrace
London SW1Y 5AG
Telephone: 0171-839 5561 ext 315 13 December 1994