but transferred allegiance to UniversiU" of Pennsylvania where he became associate professor a year later. He was a member of Penn's physics faculty and, for ...
Electronic polymers revolution by George Marsh
Wrist watch size laptops and flat televisions screens based on LED film aren't yet in the shops - but they might be soon thanks to an 'accidental' scientific discovery 30 years ago.When Alan MacDiarmid, Alan Heeger and Hideki Shirakawa discovered that plastics could be conductors as well as insulators it turned conventional scientific thinking on its head.The scientific and commercial significance of the discovery was recognized last year with the award of the Nobel Prize for Chemistry to the three researchers. George Marsh explores their sto W and its continuing impact on electronics technology. The way the discovery of some plastics' almost anomalous conductive ability was made is as surprising as the revelation itself. It involves a chance meeting, a coffee break and films. W h e n Japanese chemist Hideki Shirakawa, working in the early 1970s, mistakenly added a thousand times too m u c h halogen catalyst to an experim e n t to synthesize polyacetylene, he was astonished to see o n the inside of the reaction vessel a gleaming silvery film.At another temperature, a copper coloured film was formed. Varying combinations of temperature and catalyst concentration proved able to yield a range of different film characteristics. In another part of the world, chemist Alan MacDiarmid and physicist Alan Heeger were experimenting with a metallic-looking film of the inorganic polymer sulphur nitride (SN)x. At a seminar in Tokyo, MacDiarmid referred publicly to this work. Here the sto W might have ended, had not Shirakawa and MacDiarmid h a p p e n e d to meet, accidentally, during a coffee break. W h e n the American-based scientist heard about Shirakawa's discovery of
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Those responsible for this impending revolution, suitably recognized by the Royal Swedish Academy of Sciences with its Nobel Prize 'for the discovery and development of conductive polymers' are: Professor of Chemistry Alan G. MacDiarmid, 73, w h o has been at the University of Pennsylvania for 43 years but received his PhD at University of Wisconsin in 1953, and at Cambridge UniversitT in the UK, 1955. He was associate professor at University of Pennsylvania in 1956 and received a professorship there in 1964. Before spending most of his years in the USA and becoming a US citizen, he was born in Masterton, New Zealand. Alan J. Heeger, 64, received his PhD at University of California, Berkeley in 1961 but transferred allegiance to UniversiU" of Pennsylvania where he became associate professor a year later. He was a m e m b e r of Penn's physics faculty and, for 20 years from 1962, worked with the Laboratory for Research on the Structure of Matter. Between 1967 and '82 he had a professorship there. In '82 it was back to California where he became Professor of Physics at University of California, Santa Barbara and Director of the Institute for Polymers and Organic Solids. In 1990 he founded UNIAX, Corp where he is currently Chair of the Board. UNIAX is working (in c o m m o n with other companies including Philips, Cambridge Display Technology and Covion Organic Semiconductor GmbH) to develop light emitting diodes. For 20 years Prof Heeger edited the Elsevier Science journal Synthetic" Metals. Professor Hideki Shirakawa, 64, w h o was until recently regional editor for
Synthetic Metals, received his PhD at Tokyo Institute of Technology in 1966 and promptly became associate professor at the Institute of Materials Science at University of Tsukuba. He has b e e n a Professor there since 1982.
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FIGURE 1: Polyacetylenepolymer chain, showing alternate single and double bonds in the repeated acelylene hydrocarbon groups
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an organic polymer that also gleamed like silver, he invited Shirakawa to the University of Pennsylvania in Philadelphia. Here they set about modifying polyacetylene by oxidation with iodine vapour. Shirakawa k n e w that the optical properties changed in the oxidation p r o ces s and MacDiarmid suggested that they ask Heeger to have a look at the films. W h e n o n e of Heeger's students measured the conductivity of the iodine-doped transp o l y a c e t y l e n e , all c o n c e r n e d w e r e astounded to find that it had increased ten million times!
Oxidation with halogen (p-doping): [CHIn + 3x/2 t2 ....... [CH]nx÷ + x 13Reduction with alkali metal (n-doping): [CH n + x Na ...... [CH]nX- + x Na + FIGURE 2: Equations for the oxidation and reduction doping of polyacetylene.
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Heeger, MacDiarmid, Shirakawa and coworkers published their discovery in 1977, in the article 'Synthesis of electrically conducting organic polymers: Halogen derivatives of polyacetylene (CH) n, in The Journal of Chemical
Society, Chemical Communications. Recognition has c o m e almost a quarter of a century later, n o w that w e k n o w
conductive polymers are commercially important. But first, m o r e of the mechanism involved.
The Journal o f Electronic Polymers and Electronic Molecular Materials This journal is an international medium for the rapid publication of original research papers, short communications and subject reviews dealing with research on and applications of electronic polymers and electronic molecular materials including novel carbon architectures. These functional materials have the properties of metals, semiconductors, or magnets and are distinguishable from elemental and alloy/binary metals, semiconductors and magnets. Materials considered to be within the purview of this journal include: •
low-dimensional conductors and superconductors such as organic charge-transfer compounds and metal chain compounds
•
conducting and semiconducting polymers and molecular materials
•
fullerenes, carbon nanotubes and related novel carbon architectures
•
supramolecular conjugated architectures
•
nanoscale electronic molecular and electronic polymer materials
•
molecule- and polymer-based magnets
Experimental, theoretical and application papers on the chemistry, physics, and engineering of these materials are encouraged for submission. Original manuscripts on their chemical, electrochemical, electrical, photonic, and magnetic properties will be considered for publication. Papers on electronic, electroluminescent, lasing, solar cell, anticorrosion, sensor, actuator, biological and other potential device applications of these materials are encouraged.
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FIGURE 3: Conductivity spectrum showing that conductive polymers range from semiconductors to full conductors alongside copper, silver and other metals.
SYNTHETIC METALS
Materials
Semi-conductors
Polymers are long-chain organic molecules in w h i c h h y d r o c a r b o n units repeat themselves. Most of these plastics are non-conductive. In fact a number make good insulators and are used in cable sheathing and other applications. Metals c o n d u c t because they have electrons that are not b o u n d to particular atoms and are free to move. For plastics to imitate metals electrically, they too need an excess of electrons alternatively, a deficit resulting in electron gaps or 'holes'. These are free to move along the molecular structure and h e n c e to conduct. A class of plastics w h o s e structure is able to support the p r e s e n c e of free electrons is those having alternate single and double bonds, i.e. a conjugated system (Figure 1). One of these conjugated polymers is polyacetylene, prepared by polymerization of the hydrocarbon acetylene. To b e c o m e electrically co nduc t i ve , polyacetylene has to be d o p e d to produce an electron surplus or deficiency. Electrons are r em o v ed ('holes' inserted) by oxidizing the p o l y me r with halogen (p-doping), whilst electrons are inserted by reduction of the material with an alkali metal (n-doping).The discovery that a thin film of polyacetylene could be oxidized with iodine vapour and w o u l d then b e c o m e highly conductive was, of course, sensational; and has led to a w h o l e n e w science of
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Electronic polymers revolution
cost is m o r e i m p o r t a n t than high speed.
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conducting polymers.The d o p e d polyme r is a salt, with iodide or sodium ions (Figure 2). Current is created w h e n electrons released from the conjugated double bonds move along the m o l e c u l e s of e a c h p o l y m e r chain. Conductivity is limited by the fact that the electrons have to 'jump' from one molecule to the next. H e n c e those materials which, like polyacetylene, have chains densely packed in ordered rows, conduct best. ff a strong enough electrical field is applied, the iodide and sodium ions move either towards or away from the polymer. This means that the direction of the doping reaction can be controlled - consequently the conductive polymer can easily be switched on or off. But conductive polymers are far from being a scientific curiosity. They are already making a significant contribution in packaging, TV and o t h e r sectors, and seem set to revolutionize electronics. Polythiophene derivatives are of great commercial value as anti-
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static t r e a t m e n t for p h o t o g r a p h i c films.They are also useful in supermarkets for marking p r o d u c t s so that checkouts can register trolley contents automatically. Another material with antistatic properties, d o p ed polyaniline, is used in carpets for offices and operating theatres w h e r e static electricity is undesirable. It is also applied to c o m p u t e r screens to protect users from electromagnetic radiation, and as a corrosion inhibitor. Polydiall(ylfhiorenes can be found in n e w colourTV screens and other video devices. Materials such as polyphenylenevinylene may soon be used in mobile p h o n e displays. Much of the c o m m e r c i a l potential of c o n d u c t i v e and s e m i c o n d u c t i v e polymers rests on the fact that they can be p r o d u c e d quickly and cheaply. Electrical c o m p o n e n t s b ased on p o l y m e r s and polymer-based integrated circuits have a p r o m i si n g future in consumer products where low
Moreover, conductive polymers exhibit electroluminescence, a p h e n o m e n o n well known in inorganic semiconductors such as gallium phosphide. Ability to use polymer semiconductors instead promises much cheaper, more resilient and flexible devices. A light emitting diode (LED), for instance, would comprise a conductive polymer as a transparent electrode on one side, then a central semiconducting polymer layer and, at the other side, a thin metal foil as the other electrode (Figure 4). Applications for such brilliant plastic might, in a few years, include flat TV screens based on LED film, together with luminous traffic and information signs. Since it is relatively simple to produce large, thin layers of doped plastic, one might also see novel products such as light-emitting wallpaper or 'smart' windows that can exclude or admit light. But perhaps the most exciting prospect is the ability to transcend the boundaries on size inherent in the physical nature of conventional semiconductors. Polymer-based electronics could, in theory, take the limits d o w n to molecular scale.This could permit the developm e n t of m i c r o p r o c e s s o r and o t h e r devices orders faster and with much higher storage capacity than present devices. Packing the processing p o w e r of a current laptop into something the size of a wrist watch could be a future reality. We could truly be at the threshold of a plastics and electronics revolution, which would in turn impact on information technology.
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