IT’S LIKE walking into a bank vault. Pass codes secure the doors. The walls and floor are made of reinforced concrete up to 2 metres thick – all built on solid sandstone. The ventilation ducts have automatic shut-offs. Not even cellphone signals can sneak in.
All this might seem fitting given that the place houses diamonds by the hundred. Yet this is no vault. It’s a lab in the at the University of Bristol, UK, and the diamonds stored here are each no bigger than a speck of dust. Diamonds this size might not interest a bank robber, but they are turning out to be a physicist’s best friend.
And it’s not just diamonds. Gold and silver, too, are acquiring new allure in the lab. These materials’ superlative hardness, lustre and resistance to corrosion have been prized for centuries, but reduce this stuff to the nanoscale and other characteristics emerge; valuable properties which promise to transform the way we build electrical gadgets of every kind. Welcome to the shiny new world of “blingtronics”.
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Unravelling the remarkable riches of this nano-world takes an exceptionally steady hand – which is why the Bristol lab is so solidly built. Here physicist Neil Fox spends his day manipulating delicate films of diamond as thin as a human hair. The experiments are so sensitive that even the faintest vibration could spell failure.
Fox aims to turn these diamond films into , one that generates electricity by absorbing heat rather than visible-light wavelengths. He is exploiting ““, the propensity of some materials to spit out electrons when heated, and it turns out that ultrathin diamond is better at this than most. Fox plans to use a reflective dish to focus sunlight onto a device made from two thin films of diamond that are separated by a vacuum a few hundred micrometres thick. As sunlight heats the outer film, the hottest, most energetic electrons fly off and are collected by the other film, generating current (see diagram).
Conventional devices for capturing the sun’s heat do it by focusing sunlight onto tubes containing oil or water. The heated fluid can then be used to produce steam to drive a turbine and generate electricity. With no moving parts, a diamond solar cell should be more efficient, says Fox. Nor must the technology rely on the sun: the cells could also be used to harvest waste heat from power stations, industrial plants or vehicle exhausts.
To make the diamond films work effectively, Fox must first implant lithium atoms into them. These atoms form positive charges near the film surface and this helps hot electrons leave. Unfortunately, the very arrangement of carbon atoms that gives diamond its hardness makes it devilishly difficult to insert alien atoms. Lithium atoms will seep in slowly if the diamond film is red hot, but they end up in clumps, where they are ineffective. So Fox has turned his attention to lithium ions, which he believes will diffuse more easily throughout the structure. “It’s a bit of a game trying to get these things where you want them,” he says.
Nano-diamonds could also offer an alternative to the silicon circuitry used in microchips, if a project led by the (DARPA) succeeds. It aims to replace silicon-based electronic circuits with microscopic mechanical components made from diamond. DARPA engineers believe that such devices will offer significant advantages over a swathe of electronic components, particularly if they can be built out of (UNCD), a material developed by the Argonne National Laboratory in Chicago.
“Nano-diamonds could offer an alternative to the silicon circuitry used in microchips”
UNCD can be etched away to form nanoscale cantilevers or vibrating membranes that are able to operate over a broader range of frequencies than conventional electronic switches and oscillators. And thanks to the fact that UNCD can be layered onto silicon, these components can be directly integrated into silicon chips, making them cheap to build. “Diamond is a very unique material,” says Jan Isberg, an engineer at Uppsala University, Sweden, who is studying its uses in electronics.
The DARPA-funded researchers hope to use their diamond components to create a military radio that operates at broadband speeds, akin to a souped-up smartphone. And UNCD is just the stuff for the job, they say, thanks to its toughness and resistance to corrosion.
While diamond offers new tricks for manipulating electrons, other forms of bling could allow us to replace electrons altogether, using photons. Unlike electrons, which are subject to collisions and interference as they travel through a circuit, photons can whizz round optical fibres without interfering with each other. This means photons can be packed together at higher densities than is possible with electrons, so optical circuits should be able to carry more data.
Finding a way to control these photons remains a big challenge. One solution is to use , which can be thought of as light waves trapped on the surface of a metal by the sea of electrons inside. Unlike photons, plasmons can easily be manipulated with electric fields or even beams of light. A team at the Electronics and Telecommunications Research Institute in Daejeon, South Korea, recently transferred data between computer chips using plasmons to channel a broadband light signal along gold wires. Some manufacturers, including Intel, are beginning to use connections of this type to replace conventional wiring in personal computers.
The ultimate aim, though, is to have light itself perform the processing in every microchip. Part of the trick here lies in the ability to generate pulses of light and switch them on and off at high speed, all in a tiny space. The smallest conventional lasers measure several hundred nanometres across and so are simply too big for the task. To compete with transistors, a laser would need to be less than 50 nanometres across, an impossibility with conventional designs.
Then last year teams of physicists in the US and China created the first examples of a device known as a spaser, which gets its name from the fact that it amplifies surface plasmons in a similar way to how a laser boosts light. The spaser has a gold core wrapped in silica and dye molecules. When it is switched on – using an external light source at present, though the goal is to use an electric current – the gold core ripples with plasmons. These excite the dye molecules, which emit light. This light in turn creates more plasmons. The result is a beam of light from a device tens of nanometres wide.
Inside your body
It will be years before engineers can use such nano-bling to build an optical computer. In the meantime, nanoparticles of gold and silver have other gifts to offer. Injected into human tissue and exposed to light, gold nanoparticles can generate plasmons that then emit light of a different wavelength. This can be used to analyse the chemistry of cells spectroscopically, which could play a useful role in medical diagnostics or, if the wavelengths emitted are in the infrared, kill cancerous cells.
As for silver nanoparticles, they can help make LEDs more efficient. Much used in consumer electronics, LEDs produce light when electrons and “holes” – gaps in a semiconductor where electrons should be – recombine. It turns out that adding silver nanoparticles to LEDs can (Applied Physics Letters, ). This could ultimately lead to new types of low-power display screens or lighting. “It’s a very large area of research that’s really just taking off now,” says Teri Odom, who works in nanotechnology at Northwestern University in Evanston, Illinois.
Yet the most marketable bling technology might be wrapped into something that you take with you everywhere. It could transform your favourite gadgets, including cellphones and music players – by incorporating them into your clothing. “Rather than carrying your iPod, the whole electronic system could be embedded in your jacket,” says , a materials scientist at the University of Illinois at Urbana-Champaign.
“Bling could transform your favourite gadgets by incorporating them into your clothing”
Lewis is working on making blingtronics wearable. Last year, her group found a way to print tiny micrometre-sized wires in much the same way as an inkjet printer makes an image on paper. Using an electrically conducting ink containing silver nanoparticles, they were able to print wires onto a variety of materials, including glass and plastic (). Lewis was also keen to discover if her printing technique would work with flexible materials like fabric, but here she hit a snag.
To make the silver nanoparticles, Lewis precipitates them gradually from a solution of silver salts, adding a polymer “capping” agent which stops the particles growing beyond the required size. The polymer wraps around the particles, preventing any more silver from sticking. The problem is how to remove the polymer once the printing process is complete, since the polymer is an insulator and reduces the conductivity of the wires. Heating does the trick. Unfortunately it turned out that Lewis’s team could only get rid of the polymer at temperatures above 100°C – not conditions that are kind to delicate fabrics.
Lewis’s most recent work, to be published later this year, suggests an answer. Her group has found that it can minimise the insulating effect of the polymer by carefully adjusting the size of the nanoparticles. Using ink containing particles of this ideal size, they can print wires whose conductivity is one-tenth that of ordinary silver, with no heating required. “We’ve made quite a lot of progress, but there’s more work to do,” she says.
Lewis’s ultimate goal is to print all the components and circuitry of a phone or music player onto fabric. Most of these components, she points out, are already printed onto circuit boards, by depositing a layer of conductor or semiconductor and then etching away everything except the pattern required. In principle, she says, there’s no reason why the circuitry for any electronic device you’d care to name couldn’t be printed onto your clothes, all thanks to nanoparticle ink.
Whether or not the cuff of your next coat or cardigan comes with its own circuitry, it seems certain that there’s a bright future for electronics made using gold, silver and diamonds. And even if this bling isn’t your style, never fear – how can you look gauche when the stuff is too small to see?
