Uses of Graphene in everyday products
At this day in age, pencils are so commonly found that society doesn’t second guess this piece of technology. Every time a pencil is used, the phenomenon behind it is simply taken for granted. The lead of a pencil could never be converted into more advanced technology like in electronics, right? Graphene comes from graphite, the lead in a pencil: a kind of pure carbon formed from flat, stacked layers of atoms.1,2,3 Graphene possesses many qualities that are favorable in electronics. Could these carbon sheets hold value in our society?
Graphene is made up entirely of carbon atoms bound together in a network of repeating hexagons. In a single plane just one atom thick, the large assembly of benzene rings resembles chicken wire, as seen in Figure 1 at left.1 Graphene has always been before our eyes, but no one ever tried to look,” says Andre Geim, a physicist at the University of Manchester in England.3 However mundane the stuff may be, physicists have long predicted that if it were possible to isolate single graphene sheets, they would be sturdier than diamond and would have almost preternatural abilities to manipulate electrons.3 That could make graphene a better material than silicon for making computer chips.2,3 Until recently, though, no one had been able to isolate graphene sheets, let alone do anything useful with them. In 2004, Geim and his collaborators startled the physics community by announcing they had had peeled graphene layers off graphite using common adhesive tape.1,3 In order to isolate this layer of graphene, researchers started with bits of debri left over after splitting graphite by brute force(like rubbing a pencil across a piece of paper). 1,2 A flake of graphite debris was then stuck onto plastic adhesive tape, folded the sticky side of the top of the flake and then pulled the tape apart, cleaving the flake in two. As the experimenters repeated the process, the resulting fragments grew thinner.1,2,3 The discovery raised a buzz in the physics community similar to that of the excitement of carbon nanotubes a decade ago.3
Other graphitic forms are built up out of graphene. Buckyballs and the many other non-tubular fullerenes can be thought of as graphene sheets wrapped up into atomic scale spheres, elongated spheroids, and the like.1 This concept is more fully represented in Figure 2 below. Carbon nanotubes are essentially graphene sheets rolled into minute cylinders. Carbon nanotubes combine unusual properties such as chemical, electronic, mechanical, optical and thermal that have inspired a wide variety of innovative potential applications.1 Those innovations include materials that might replace silicon in microchips and fibers that might be woven into lightweight, ultra-strong cables.1
Two features of graphene make it an exceptional material. First, despite the relatively crude ways it is still being made, graphene exhibits remarkably high quality resulting form a combination of the purity of its carbon content and the orderliness of the lattice into which its carbon atoms are arranged.1,2 The newly identified bits of graphene turned out to have high crystal quality and to be chemically stable even at room temperature. Not only is it the thinnest of all possible materials, it is also extremely strong and stiff.1 Investigators have so far failed to find a single atomic defect in graphene. That perfect crystalline order seems to stem from the strong yet highly flexible inter-atomic bonds which create a substance harder than diamond yet allow the planes to bend when mechanical force is applied.1 The flexibility enables the structure to accommodate a good deal of deformation before its atoms must reshuffle to adjust to the strain. The quality of its crystal lattice is also responsible for the remarkably high electrical conductivity of graphene.1,2 Its electrons can travel without being scattered off course by lattice imperfections and foreign atoms. Even the jostling from the surrounding carbon atoms, which elections in graphene must endure at room temperature, is relatively small because of the high strength of the inter-atomic bonds.
The second exceptional feature of graphene is its conduction of electrons. In its pure form it conducts electrons faster at room temperature than any other substance.1 , Besides traveling largely unimpeded through the lattice, electrons move much faster and as if they had far less mass than do the electrons that wander about through ordinary metals and semiconductors.1,2 Graphene comes in sheets; it may be possible to etch graphene circuits, just as circuits are now etched into silicon wafers. Forming circuits from one sheet of graphene could be much easier than assembling then from nanotube pieces.2,3 “We want to be able to use the essential properties of carbon nanotubes in a material that can be patterned easily,” says Walt de Heer of the Georgia Institute of Technology in Atlanta. “It could realize the dream people had of carbon-nanotube electronics.”3 Graphene circuits could in principle work efficiently even with components measuring only a few atoms across- scales that can’t be achieved with ordinary semiconductors.
In recent months, scientists have learned how to make graphene-based transistors and diodes-the basic elements of computer chips.2,3 They have also begun trying to connect graphene to other materials, including carbon nanotubes. If graphene is to replace silicon once day, scientists and engineers will have to figure out how to manufacture large numbers of circuits with nearly atomic precision. Geim’s adhesive tape stratagem could hardly be the basis for a new chip-fabrication plant, but it continues to be researchers’ favorite way of making graphene for experimentation.3
Because electrons in graphene move at high speeds, graphene-based transistors could in principle switch currents on and off faster than semiconductor-based transistors do. Like carbon nanotubes, graphene is an excellent conductor of heat, so graphene chips could stay cooler than silicon chips. But the feature that makes graphene most appealing to scientists is its toughness.2,3 “The graphitic bond-the carbon-to-carbon bond – is the strongest in nature,” even stronger than the bonds between carbon atoms in diamond, says Heer.3 That strength gives graphene its remarkable stability, and means that graphene circuits could in principle be miniaturized to sizes of a few nanometers without falling apart.1,2,3 By contrast, molecular-scale circuits made of silicon of other materials would quickly fail. “All other material oxidize, decompose, move around, or melt,” Geim says.3
Furthermore, conventional transistors are made from silicon or another semiconductor that has been “doped” to modify its electronic properties. In negative doping, addition of a small amount of another element increase the number of current-carrying electrons.3 In positive doping, addition of a different element creates gaps in the electron distribution, which move around like positively charged carriers of currents.3 At nanometer scales, it becomes almost impossible to dope a material uniformly because the dopant atoms are so few and far between.2,3 These limitations mean that individual features in silicon chips, already as small as 65 n and with 45 nm technology in the offing, will probably reach their smallest possible size 10 to 15 years.3
Whatever the future has in store for graphene, it will almost certainly remain in the limelight for decades to come. Engineers will continue to work to bring its innovative by-products to market. The basic research in graphene has made remarkable strides in just over 2 years. Geim says that the new research field is here to stay, “It’s not a blip on the screen.”3 What is truly astonishing is the realization that all its complexity had for centuries laid hidden in nearly every ordinary pencil mark.
1. Geim, Andre. Kim, Philip. “Carbon Wonderland”. Scientific American. April 2008:90-97
2. Gruner, George. “Carbon Nanonets: Spark New Electronics”. Scientific American. May 2007:76-83
3. Castelvecchi, Davide. “Electron Superhighway”. Science News. 29 September 2007:200-201