快猫短视频

AS A SPECIES, Cambridge physicists are not renowned for being excitable.
Popular imagery has them leading solitary existences in small, stuffy rooms,
where they ponder the finer points of life, the Universe and everything. For
relaxation, there are the mugs of stewed tea, over which endless debates take
place about the number of currants in the canteen buns.

But in the spring of 1989, the occupants of Cambridge University鈥檚 Cavendish
Laboratory were jolted from their deep deliberations on both cosmos and
currants. The atmosphere glowed with speculation about a trio of researchers who
had been spotted in hushed conversation. Rumours abounded of secret experiments,
and there were ever-so-slightly envious whispers about the 鈥淧鈥�
word鈥攑atent. Something extraordinary seemed to have happened, and the
question on everyone鈥檚 lips was鈥攚hat have they found?

At face value, something unremarkable. By chance, Jeremy Burroughes, Donal
Bradley (now at the University of Sheffield) and Richard Friend had discovered
that if you slapped a voltage across an ultrathin film of an exotic plastic
known as poly(p-phenylene vinylene), or PPV, it glowed a pale
yellow-green. 鈥淚 was about six feet away, facing in the other direction, when I
noticed this bright green light in the corner of my eye,鈥� recalls Burroughes.
鈥淚t was just pouring light out all over the place!鈥�

So what was the big deal? After all, you can make almost anything glow with
enough volts, although you will probably fry it in the process. But this was no
inadvertent example of small-scale arc-welding. The plastic was turning the
energy of the electric current directly into light. This is just what you would
need to make, say, a wafer-thin television screen. 鈥淚t hit me straight away,鈥�
says Friend, 鈥渢hat here was a technology that could potentially go all the
飞补测.鈥�

Today, barely a decade later, with financial support from a behemoth of 1970s
rock music and a lot of hard work, these plastic wonders are about to see the
commercial light of day. They鈥檙e in demand because they鈥檙e bendy, thrifty with
power and easy to make. In the next few months, glowing polymers will make their
debut in mobile phones, but this is just the beginning. With their unique
mechanical properties and a dash of imagination, they could well spark an era of
bright, flexible electronic devices, such as rollable, foldable TVs and even
鈥渢ele-visual clothing鈥濃�攚earable multimedia.

To most people, the word 鈥減lastic鈥� conjures up visions of detergent bottles,
insulating tape and those annoying bits of transparent film within which so many
foods come vacuum-packed. Plastics tend to excel in such roles because of their
inertness, their capacity for doing nothing whatsoever. But some plastics break
the rules, and do so spectacularly well. A case in point is the 鈥渃onjugated
polymers鈥�, a vast family of plastics whose unusual structure gives them the
ability to conduct electricity鈥攁nd turn it into light.

Electronic timeshare

The simplest member of the family, polyacetylene, is nothing more than a
string of carbon atoms鈥攖he polymer 鈥渂ackbone鈥濃�攅ach with an attendant
hydrogen atom (see Diagram).
In many organic molecules, each of a carbon
atom鈥檚 four outer electrons are shared with different neighbours, forming four
single bonds. But in polyacetylene, every carbon shares two of these electrons
with just one of its neighbours, creating a backbone of alternating single and
double bonds.

But things are a little more complex than this neat picture suggests.
Although each carbon has a favoured partner, its other carbon neighbour is not
neglected. It receives a partial loan of the floating electron that makes up the
double bond鈥攁 kind of electronic timeshare. This combination of
alternating bonds and sharing of electrons gives conjugated polymers their
unusual properties.

Add more electrons to the chain, or remove them to leave positively charged
鈥渉oles鈥�, and the excess charge zips along the backbone with very little
resistance, flipping the double bonds between atoms as it goes. If enough charge
is added to or removed from the chain, the polymers can even conduct electricity
as well as a metal.

Back in the 1970s, researchers hoped to use this principle to develop a
plastic replacement for metal conductors. It didn鈥檛 work, however, largely
because the best polymer conductors turned out to be terribly unstable.
Nevertheless, more stable relatives of polyacetylene may yet find use as
anti-static coatings.

In the 1980s, attention turned from metal-like polymers to their more poorly
conducting brethren, semiconductors. The idea was to develop a plastic
equivalent of silicon microelectronics: not to replace it, but to provide a
complementary technology of cheap, flexible, disposable electronics. This avenue
is proving more fruitful, and plastic circuitry could soon be widely use, for
example, in smart cards. But it wasn鈥檛 till the end of the 1980s that, as Friend
puts it, 鈥渨e saw the light鈥�.

The polymers pump out light because of the unusual arrangement of energy
levels that their electrons can occupy. Put simply, there鈥檚 a two-tier system in
which the lower level is chock-a-block with electrons that are barely able to
move and an empty upper tier. It takes a certain amount of energy for an
electron to move up a level, but it鈥檚 worth it: an elevated electron can zip
along the polymer chain. Similarly, the hole left behind in the lower tier when
an electron is removed can also move around.

So sandwich these polymers between a pair of electrodes and apply a voltage.
Electrons will then move one way through the material and holes the other.
Should a pair of these opposite charges meet, the electron will 鈥渄rop鈥� back into
the hole, releasing its extra energy.

In many semiconductors, such as polyacetylene and even silicon, this energy
is converted into heat and absorbed by the material. But in some semiconductors,
the energy is converted directly into light鈥攖he colour of which depends on
just how big the electron-hole energy difference is.

There are a number of materials already in use that produce light in this
way. Light-emitting diodes made of inorganic semiconductors such as gallium
arsenide, for example, are the basis of a thriving technology. These LEDs use
little power and, unlike domestic light bulbs, they last a long, long time. This
is why they鈥檙e widely used in simple displays, and why the bulbs in traffic
lights are increasingly being replaced by dense arrays of LEDs.

Given the immense success of the crystalline inorganics, why go plastic?
Well, on the one hand, the polymers do not need the ultra-clean and costly
fabrication conditions that inorganic semiconductors do. What鈥檚 more, they start
off as treacle-like liquids that can be easily cast, painted or spun as thin
films over electrodes as large as you like. (At least one of the electrodes must
be transparent so light can escape.) Inorganic LEDs, by contrast, can be made
only as small, discrete units, and many have to be assembled and connected to
make a glowing panel.

Just as importantly, the conjugated polymers are not limited like their
inorganic counterparts in the colours they produce. Here, chemists can really
work their magic. The electron-hole energy difference鈥攁nd hence the colour
of the light emitted鈥攊s largely determined by the extent to which the
floating electrons are shared along the chain. Replace the dangling hydrogen
atoms with more exotic molecules that grasp the electrons more tightly, or more
loosely, and the polymers can be tailored to emit red or yellow, green or blue,
and anything in between.

Feeble glow

With so much potential, it is little wonder that the discoverers went running
to the Patent Office. But when Burroughes and co initially touted their wares,
the response from potential industrial partners was one familiar to budding
inventors: indifference. The catch was that most of the early devices were
lousy. Only a tiny fraction of the electrons pumped into the material鈥攍ess
than 0.05 per cent鈥攜ielded photons, and they needed impractically high
voltages to obtain even the feeblest of glows.

To make matters worse, the early devices were extremely short-lived, often
dying after just a few minutes of operation. This didn鈥檛 detract from the
academic interest in the phenomenon, but the extensive development needed to
turn glowing plastics from an intellectual curiosity to a viable display
technology fell far outside the remit鈥攁nd resources鈥攐f a university
laboratory. 鈥淎nd we could think of plenty of reasons why it wouldn鈥檛 work,鈥� adds
Friend, so the caution of industry was understandable.

Enter, stage left, the forward-looking rock musicians (or 鈥渂usiness angels鈥�
as Friend describes them) who decided to invest some of their cash with our
budding technologists, giving them the chance to start up their own company:
Cambridge Display Technology. 鈥淭heir investment was absolutely critical,鈥� says
Friend, 鈥渁nd meant that we could start employing people.鈥� And not before time.
Despite the conservatism of industry as a whole, the potential of the plastics
had not gone unnoticed, and other small companies were springing up to exploit
them.

The past decade has yielded truly astonishing results. The present generation
of polymer LEDs are no longer the power-hungry, feebly glowing, fickle devices
of yore. With electron-to-photon conversion efficiencies now approaching several
per cent, they can be too bright to look at directly. In terms of light produced
for power used, they are snapping at the heels of inorganic LEDs
(see Diagram).
The devices also last for tens of thousands of hours. According to
Burroughes, these plastics now offer lower-cost flat-panel displays than any
other technology out there.FIG-mg21944801.JPG

The first commercial application of polymer LEDs will be pretty crude, little
more than glowing panels that provide back-lighting for liquid-crystal displays.
In portable devices, back lights are one of the biggest drains on battery life,
and their support electronics contribute significantly to the weight. The
plastics offer a lighter, less power-hungry alternative, and the first products
using this technology鈥攎obile phones made by Philips鈥攕hould be on the
shelves within a few months.

In a year鈥檚 time, we can expect a bit more sophistication: the electrodes
driving the glowing plastic panels will be divided into arrays of independently
addressable pixels to create single-colour displays that are free from liquid
crystals. And beyond that, things get more interesting still.

Glowing plastics, like all other display technologies, aspire to full colour.
And there is no problem making them produce the red, green and blue light needed
for full-colour images. But there is one difficulty. Because the materials are
prepared from solution, it鈥檚 easy to deposit large, uniform layers for
single-colour displays, but not so easy for assembling intricate arrays of
different-coloured pixels.

Here researchers have borrowed a trick from the neo-Impressionists and turned
to 鈥減lastic pointillism鈥濃�攑ainting the plastics on dot by dot. They use
ink-jet printers to squirt tiny blobs of red, green and blue-glowing materials
onto a patterned electrode array. And because the resolution of these printers
is now so high, we can expect pixels of polymer ink just 30 micrometres across,
says Friend. That is easily enough for a high-resolution display that would have
Seurat glowing a pale yellow-green with envy.

The glowing material need not be the only flexible component in these
displays. The electrodes, the electronics, and even the supporting material can
be made from similarly flexible substances, plastic or otherwise. So in addition
to being light, flat and efficient, the plastic TV screen can also be bendy.
Just think, you could one day have in your hand a television that to all intents
and purposes handles like a floppy sheet of cellophane. You could watch it in
the bath, clip it into your Filofax and, when you鈥檝e finished, roll it up and
put it in your pocket.

Dancing lights

But why stop there? 鈥淪ometimes you just have to suspend disbelief and take a
`what if鈥� view,鈥� says Friend. Detergent bottles with animated ads might be going
a bit too far, but television as a fashion accessory? Now there鈥檚 an exciting
possibility.

We could start with accessories such as handbags, wallets, backpacks and
diaries. Many of these are covered by plastic, with garish designs and vivid
colours providing much of the fashion appeal. So why not go for a glowing
alternative? What could be more vivid than colours that literally shine from the
surface? And rather than settling for static designs, the images could dance
across the surface.

The same, of course, applies to clothing. If you are the sort of extrovert
who likes to be noticed in clubs, just imagine the impression you would make if
your vinyl bodystocking, with its discreetly placed power pack, provided its own
cinema show, vying for attention with the dance-floor lighting. Unbelievable?
鈥淲e鈥檝e had fashion people from Paris talking to us about it,鈥� says
Burroughes.

But such applications could be serious, too. In many situations, to be seen
is to be safe. The extra visibility afforded by light-emitting plastics could be
a real boon for the emergency services, for example. Glowing suits could make it
easier for firemen to keep track of each other in a dense smoke.

So what started off as a seemingly obscure physical effect might soon be
revolutionising the very way we perceive, use and interact with visual media.
The future is bright, the future is plastic. And which rock band had the
financial and technological foresight to invest in all this, you ask? Not any
old Phil, Mick and Tony, but those early proponents of cod classicism,
extravagant light shows and narcissistic costumery: Genesis.