Graphene’s lightbulb moment
Can the “wonder material” live up to all the hype?
The researchers now have another distinction to their credit: their discovery is about to become a commercial product. A graphene-based lightbulb, said to be longer-lasting, more efficient and cheaper to make than today’s domestic LED lamps, will go on sale in a few months’ time. Though graphene flakes have already been incorporated into tennis racquets, skis and conductive ink, the new lightbulb is claimed by its manufacturer—Graphene Lighting Plc, a spin-out from the National Graphene Institute and Manchester University—to be the first commercially viable consumer product based on the material.
That may be splitting hairs. Even so, going from discovery to commercialisation in little more than a decade is quick. Many entrepreneurial companies find turning an invention into a successful innovation can take 20 years or more.
Graphene is composed of a single layer of carbon atoms arranged in the form of a hexagonal lattice. This means there is a world of difference between it and a three-dimensional crystalline structure like graphite. The electrons associated with carbon atoms in graphite can interact with other carbon atoms in the layers above and below them. In a sheet, this electron-coupling effect disappears, and the electrons are free to behave in entirely different ways.
The most striking consequence of this is that those electrons are thus able to move great distances at close to the speed of light, resulting in a material that has exceptionally low resistance. Because graphene can transport electricity 200 times faster than silicon, it seems a good candidate to replace that element as the semiconductor material used in computer chips.
Hype aside, graphene has not been called a “wonder material” for nothing. Apart from its remarkable electrical properties (and also, thermal and acoustical properties), it is the thinnest and lightest substance known, as well as being the strongest (more than 100 times stronger than high-strength steel). As if all that were not enough, graphene is also extremely flexible and almost totally transparent, absorbing only a minuscule amount of the light falling on it.
As such, potential applications of graphene appear myriad. Some of the more obvious ones include rapid-charging lithium-ion batteries, better solar cells, compact supercapacitors, printable electronics, foldable LED touchscreens, tunable sensors, ultrafast molecular sieves, improved DNA sequencers, corrosion-resistant coatings, a replacement for Kevlar, terahertz wave generators for extremely fast wireless communication, and, of course, more efficient lightbulbs. The list of proposals for future graphene products goes on and on, as researchers cozy up to potential sponsors.
There is little, save lack of money and consumer interest, to stop a good number of these suggestions reaching the market in the not-too-distant future. But the one graphene application that could turn the whole of electronics on its head—a replacement for silicon-based semiconductors—remains tantalisingly over the horizon.
Big computer, wireless and electronics firms—including such research powerhouses as IBM, Intel and Samsung—have been racing to create a field-effect transistor that uses graphene instead of silicon. They have a big incentive to do so. After 50 years of success, Moore’s Law (that the processing power of semiconductor devices doubles every 18 months or so) appears to be coming to an end, at least as far as silicon is concerned. Another material is needed to take its place.
Despite the billions poured into finding an alternative, attempts to make graphene work as a semiconductor have been disappointing. The problem is that the material has no “band gap”—the property that makes a solid an insulator (large band gap), a conductor (tiny or no band gap) or something in between—ie, a semiconductor (small band gap). Having no band gap at all is why graphene is such an excellent conductor. Making it into a semiconductor is tricky.
That is not all. A transistor works by flipping between two states—one insulating and the other conductive—in the presence of an electric field. These two states (off and on) represent the digital zeros and ones of computerspeak. By its nature, a graphene gate (switch) is on all the time. Getting it to turn off, let alone flip on and off billions of times a second, is the stumbling block.
There have been various attempts to open a band gap in graphene. One approach has been to dope it with compounds like silicon carbide or boron nitride that have matching crystalline lattice structures. Unfortunately, creating a band gap big enough to turn graphene into a usable semiconductor destroys the very properties—especially the high electron mobility—that made the material so attractive in the first place.
Older and wiser, researchers have turned to building hybrid chips that are fabricated, layer by layer, using conventional silicon epitaxy for everything except the final graphene transistor channels on the top of the device. These delicate structures are added at the end, so as not to get damaged during fabrication. So far, only analogue chips have been built this way. Even IBM has failed to create a band gap in graphene that would result in a digital device capable of challenging silicon’s preeminence. Transistors made entirely from graphene appear to be decades away.
So, where does that leave graphene’s prospects? While a replacement for silicon may be a long shot, many applications that do not rely on a band gap have a better chance of success. That said, not all graphene proposals being hyped at present can expect to survive the inevitable shake-out.
Had Gartner, an information-technology consultancy in Connecticut, included graphene-based processes as a stand-alone entry in its latest Emerging Technologies Hype Cycle, such processes would be over two-thirds the way up the slope to its “peak of inflated expectations”,which comes before the tip-over into the “trough of disillusionment” (see “Divining reality from hype”, August 27th 2014). Experience suggests that only those innovations which show genuine commercial value manage to crawl out of the trough and up the subsequent slope of enlightenment towards the plateau of productivity and market acceptance. Venture capitalists reckon no more than one in seven, at this stage of development, manages such a feat. It is too early to say whether graphene lightbulbs will be among them.
Readers with long memories may have noticed how the trajectory graphene is following resembles the one blazed by carbon fibre back in the 1960s. Then, as now, the new material was seen as a wonder product that would have numerous applications. Then, as now also, the British government felt it had a sacred duty to protect and promote what it perceived to be a home-grown invention—with the promise of jobs and exports.
If truth be told, the first hank of pyrolysed nylon (a carbon-fibre precursor) was snaffled from a Japanese textile factory and flown back to Britain in a diplomatic bag. At the time, British officials involved considered the super-strength material ideal for making gas centrifuges for enriching uranium. But samples that landed up at the Royal Aircraft Establishment in Farnborough led to the first carbon-fibre composite (Hyfil) being made available to select industrial partners, including the aeroengine manufacturer Rolls-Royce.
Of the many applications touted for carbon fibre, its promise to revolutionise air travel captured the most attention. With stronger, lighter fan blades, made from Hyfil instead of aluminium alloy or titanium, in a fan-jet’s first compressor stage, Rolls-Royce’s latest aircraft engine at the time, the RB211, would have had a significant weight-saving advantage—and thus better fuel economy—over rivals from General Electric and Pratt & Whitney.
The outcome was rather different. While turbine blades made from Hyfil had all the tensile strength, and more, to withstand the centrifugal forces of a big fan engine at full power, their shear strength left much to be desired. The story of how compressor blades shattered when a frozen chicken was fired at them to simulate bird impact contributed to carbon fibre’s fall from grace.
Meanwhile, the cost and delay involved in replacing the RB211’s Hyfil blades with titanium ones plunged Rolls-Royce into bankruptcy. Britain’s proudest engineering firm then had to be rescued at taxpayer expense. So much for governments picking winners. Hopefully, graphene is spared a similar fate.