Electronic polymers revolution by George Marsh

Wrist watch size laptops and flat tele- visions 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 plas- tics could be conductors as well as insulators it turned conventional scien- tific thinking on its head.The scientific and commercial significance of the dis- covery 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 plas- tics' almost anomalous conductive abil- ity was made is as surprising as the revelation itself. It involves a chance

meeting, a coffee break and films.

When Japanese chemist Hideki Shirakawa, working in the early 1970s, mistakenly added a thousand times too much halogen catalyst to an experi- ment to synthesize polyacetylene, he was astonished to see on the inside of the reaction vessel a gleaming silvery film.At another temperature, a copper coloured film was formed. Varying combinations of temperature and cata- lyst concentrat ion 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 happened to meet, accidentally, during a coffee break. When the American-based scientist

heard about Shirakawa's discovery of

c : : ! i ! ;ilii iiiiii iilii ̧̧ ̧: i k : ; ¸¸¸ >

:i ~ ~?i ~ : ....... ii i i i~+i~i~? Y iy:!ii~iiiiiiiii~i iii!!iii~iiiiii+~!;~iii!iii!i:ii?:ii~iiiiiiiiiii+~ i~i!~i~i? C : ? ~' ~ ~, ~ i!i ~iiiill ii~

T h e w i n n e r s

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, who has been at the University of Pennsylvania for 43 years but received his PhD at University of Wisconsin in 1953, and at Cambridge Universit T in the UK, 1955. He was associate pro- fessor 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 cit- izen, 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 member 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 common 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, who 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 been a Professor there since 1982.

H H H H H H I I I I I I c c c c c ~ ~ c ~c ~ ~c ~ ~c ~ ~c ~ %c

I + I I J H H H H H

Polyacetylene

FIGURE 1: Polyacetylene polymer chain, showing alternate single and double bonds in the repeated acelylene hydrocarbon groups

4 Materials Today

an organic polymer that also gleamed

like silver, he invited Shirakawa to the

Universi ty of Pennsylvania in

Philadelphia. Here they set about mod-

ifying polyacetylene by oxidation with

iodine vapour. Shirakawa knew that

the optical propert ies changed in the

oxidat ion process and MacDiarmid

suggested that they ask Heeger to have

a look at the films. When one of

Heeger's students measured the con-

ductivity of the iodine-doped trans-

polyacetylene, all c o n c e r n e d were

astounded to find that it had increased

ten million times!

Heeger, MacDiarmid, Shirakawa and co-

workers published their discovery in

1977, in the article 'Synthesis of elec-

trically conduct ing organic polymers:

Halogen derivatives of polyacetylene

(CH) n, in The Journal of Chemical Society, Chemical Communications. Recognition has come almost a quarter

of a century later, now that we know

Oxidation with halogen (p-doping): [CHIn + 3x/2 t 2 ....... [CH]n x÷ + x 1 3-

Reduction with alkali metal (n-doping): [CH n + x Na ...... [CH]nX- + x Na +

FIGURE 2: Equations for the oxidation and reduction doping of polyacetylene.

Oonductivity

Insulators Semi-conductors

1'0 -14 1~_12 1; -10 1'0. 8 1'0 .6 110_4 1'0 -2 1~ 0

24,

Metals

1102 1 ~4 1~06 ] 108

g %°,

FIGURE 3: Conductivity spectrum showing that conductive polymers range from semiconductors to full conductors alongside copper, silver and other metals.

conductive polymers are commercially

important. But first, more of the mech-

anism involved.

S Y N T H E T I C M E T A L S

The Journal o f Electronic Polymers and Electronic Molecular Materials

This journal is an international medium for the rapid publication of orig- inal research papers, short communications and subject reviews dealing wi th research on and applications of electronic polymers and electronic molecular materials including novel carbon architectures. These func- t ional materials have the properties of metals, semiconductors, or mag- nets and are distinguishable from elemental and alloy/binary metals, semiconductors and magnets.

Materials considered to be wi th in 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 submis- sion. Original manuscripts on their chemical, electrochemical, electrical, photonic, and magnetic properties wil l 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.

Polymers are long-chain organic mole-

cules in wh ich hydrocarbon units

repeat themselves. Most of these plas-

tics are non-conductive. In fact a num-

ber make good insulators and are used

in cable sheathing and other applica-

tions.

Metals conduct because they have

electrons that are not bound to partic-

ular atoms and are free to move. For

plastics to imitate metals electrically,

they too need an excess of electrons -

alternatively, a deficit resulting in elec-

tron gaps or 'holes'. These are free to

move along the molecular structure

and hence to conduct. A class of plas-

tics whose structure is able to support

the presence 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 polymer-

ization of the hydrocarbon acetylene.

To become electrically conduct ive,

polyacetylene has to be doped to pro-

duce an electron surplus or deficiency.

Electrons are removed ( 'holes ' insert-

ed) by oxidizing the polymer with

halogen (p-doping), whilst electrons

are inserted by reduction of the mate-

rial with an alkali metal (n-doping).The

discovery that a thin film of polyacety-

lene could be oxidized with iodine

vapour and would then become highly

conductive was, of course, sensational;

and has led to a whole new science of

Materials Today 5

Electronic polymers revolution

A cross section of polymer light-emitting diode

Glass,

_ ~ Ca (electrode) ~pOxYAi ~ ~ ~ ' ~ ~ ~ ' ] electron ejector

] ~ ~ Conducting and Conjugated . . . . . . I I ~ ~ transparent polymer semi conductive ~ l (electrode polymers

I X ITO (indium tin oxide) { | (conducting and

transparent

Glass

Light

FIGURE 4: Cross section of polymer light emitting diode.

conducting polymers.The doped poly-

mer is a salt, with iodide or sodium

ions (Figure 2). Current is created

w h e n electrons released from the con-

jugated double bonds move along the

molecules of each polymer chain.

Conductivity is limited by the fact that

the electrons have to ' jump' from one

molecule to the next. Hence 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 reac-

tion 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 contribu-

t ion in packaging, TV and o ther

sectors, and seem set to revolutionize

electronics. Polythiophene derivatives

are of great commercial value as anti-

static t rea tment for photographic

films.They are also useful in supermar-

kets for marking products so that

checkouts can register trolley contents

automatically. Another material with

antistatic properties, doped polyani-

line, is used in carpets for offices and

operating theatres where static elec-

tricity is undesirable. It is also applied

to computer screens to protect users

from electromagnetic radiation, and as

a corrosion inhibitor.

Polydiall(ylfhiorenes can be found in

new colourTV screens and other video

devices. Materials such as

polyphenylenevinylene may soon be

used in mobile phone displays.

Much of the commercia l potent ial of

conduct ive and semiconduct ive poly-

mers rests on the fact that they can

be p roduced quickly and cheaply.

Electr ical c o m p o n e n t s based on

polymers and polymer-based integrat-

ed circuits have a promising future in

c o n s u m e r p roduc t s w h e r e low

cost is more important than high

speed.

Moreover, conductive polymers exhibit

electroluminescence, a phenomenon

well known in inorganic semiconduc-

tors 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 com-

prise a conductive polymer as a trans-

parent electrode on one side, then a cen-

tral 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 rela-

tively simple to produce large, thin lay-

ers 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 bound-

aries on size inherent in the physical

nature of conventional semiconduc-

tors. Polymer-based electronics could, in

theory, take the limits down to molecu-

lar scale.This could permit the develop-

ment of microprocessor and other

devices orders faster and with much

higher storage capacity than present

devices. Packing the processing power

of a current laptop into something the

size of a wrist watch could be a future

reality. We could truly be at the thresh-

old of a plastics and electronics revolu-

tion, which would in turn impact on

information technology.

6 Materials Today