Donald Fitzmaurice and Declan Ryan

There are two ways to build a house.  Starting from a large concrete structure, two-storeys high, one could begin to carve out each window, room and doorway.  Alternatively, one could start with individual bricks that are layered in an organised manner so that each window, room and doorway are present by design.  The latter approach is thankfully the one used in everyday life and, for obvious reasons, too.  In particular, this approach – let’s call it the bottom-up approach – is efficient, versatile and fast.

Similarly, there are two ways to build an electronic circuit.  The most widely accepted method uses light to define patterns in semiconducting materials like, for example, silicon.  This approach – often called the top-down approach – has been successful in developing all of today’s microtechnology - from the electronic watch to the Pentium processor.  It does, however, suffer from a number of limiting drawbacks: firstly, the number of transistors that can be placed on a circuit is limited by the ability to define features on the circuit – this limit is determined by the wavelength of light used and is currently restricted to 190 nm; secondly, at that dimension various physical effects related to silicon interfere with the circuit function – these are expected to become more serious as efforts to reduce dimensions improve.

It is therefore obvious that new methods are required so that smaller circuits can be fabricated without suffering from the above drawbacks.  Among these new methods is one currently being pursued in the fields of chemistry, biochemistry and materials science – the bottom-up approach.  This approach uses nanocrystals – tiny particles of conducting, semiconducting and non-conducting material whose dimensions range from 1-100 nm – as ‘bricks’ so that organised assemblies of nanocrystals can be fabricated.  These nanocrystals need to be glued together in very specific ways if we are to make useful nanoscale circuits.  To control how nanocrystals assemble we need to choose very good ‘cement’.  Recently, biomolecules like, for example, DNA strands attached to separate nanocrystals have been used to assemble nanoscale architecture.  This works quite well – individual strands of DNA can be designed so that each recognises the other and binds to it in very specific ways.  This assembly method has many advantages – it is at once efficient: there is no wastage; versatile: each strand can be attached to any number of different materials; and fast: DNA assembly can take place in a matter of minutes.

 

The above image represents this popular approach to building functional nanocrystal architecture – commonly referred to as the ‘Factory of the Future.’  Each beaker contains different types of nanocrystals each programmed to recognise and bind to another so that, when mixed, they form regular structures that can eventually be positioned at a useful substrate for device fabrication.  With the bottom-up approach to synthesising nanoscale architecture currently being developed, it is reasonable to suggest that existing microtechnology fabrication methods will be displaced by new molecular nanotechnology fabrication methods in the coming decades.

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Prof Donald Fitzmaurice is Associate Professor of Physical Chemistry at the University College Dublin, Ireland.  His research interests include, among others, synthesis of novel recognition motifs for use in directing the assembly and organisation of nanoscale matter and subsequent characterisation of this nanoscale architecture.

Mr. Declan Ryan is a graduate of Industrial Chemistry from the University of Limerick.  He is currently undertaking a PhD in nanotechnology under the supervision of Professor Donald Fitzmaurice at the University College Dubiln.

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Links:

http://nanotech.about.com/science/nanotech/msubintro.htm A good central

source of information with lots of links.

http://www.mitre.org/research/nanotech/intronano.html A good tutorial source

on nanoelectronics with some links to more information.

http://www.almaden.ibm.com/vis/stm/stm.html A collection of images of

materials whose dimensions are on the atomic scale.

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