Graphene shows its colours

GRAPHENE, a form of carbon that comes in sheets a single atom thick, has gained a reputation as a wonder material. It is the best conductor yet discovered of heat at room temperature and is 40 times stronger than steel. It is also a semiconductor whose electrical conductivity is 1,000 times better than silicon’s. This means it could be used to make devices far more sensitive than is possible now, leading some to predict that it will one day become the material of choice for computer chips. There was little surprise, therefore, when Andre Geim (pictured above) and Konstantin Novoselov, two physicists who were investigating graphene’s structure, won the 2010 Nobel prize for their work.
Actually converting the wonders of graphene into products has been tough. But Frank Koppens and his colleagues at the Institute of Photonic Sciences in Barcelona think they have found a way to do so. As they describe in Nature Nanotechnology, they believe graphene can be used to make ultra-sensitive, low-cost photodetectors.
Photodetectors are devices which convert light into electricity. They are used in digital cameras, night-vision gear, biomedical imagers, pollution sensors and telecommunications. A typical photodetector is made of a silicon chip a few millimetres across onto which light is focused by a small lens. Light striking the chip knocks electrons free from some of the silicon atoms, producing a signal that the chip’s electronics convert into a picture or other useful information.
Silicon photodetectors suffer, though, from a handicap: they are inflexible. Nor are they particularly cheap. And they are not that sensitive. They absorb only 10-20% of the light that falls on to them. For years, therefore, engineers have been on the lookout for a cheap, bendable, sensitive photodetector. Such a device could have many novel applications—wearable electronics, for example. With a little clever engineering, graphene seems to fit the bill.
By itself, graphene is worse than silicon at absorbing light. According to Dr Koppens only 2.7% of the photons falling on it are captured. But he and his colleague Gerasimos Konstantatos have managed to increase this to more than 50% by spraying tiny crystals of lead sulphide onto the surface of the material.
These crystals are so small (three to ten nanometres across, a nanometre being a billionth of a metre) that they are known as quantum dots, because at dimensions measured in nanometres the weird effects of quantum mechanics start to manifest themselves. One such is that the size of a quantum dot affects the colour of the light it best absorbs. The larger the dot, the redder that light; the smaller, conversely, the bluer. This allows Dr Koppens and Dr Konstantatos to span all wavelengths from ultraviolet to infra-red, greatly increasing the utility of any photodetector that might emerge. Infra-red, for example, is important in telecoms and night-vision applications. Visible wavelengths, by contrast, are needed for cameras and solar cells.
According to Dr Koppens, the interaction between the dots and the graphene works because graphene has so many mobile electrons in its structure. (This is the reason it is such a good conductor of both heat and electricity.) This abundance of free electrons makes it particularly sensitive to the changes induced in a quantum dot when it absorbs a photon of light: each incident photon mobilises about 100m electrons. In the jargon of electronic engineering, therefore, the quantum dot-graphene hybrid has enormously high “gain”. And that means the material might have even wider applications than snazzy cameras and smart clothing. For what Dr Koppens and Dr Konstantatos have actually done is to create the guts of a transistor that is regulated by light.
Ordinary transistors are switches in which one electric current (usually a weak one) is used to regulate the passage of another (usually much stronger). Any signal carried by the weak current is thus amplified into one carried by the strong current—a high-gain system.
Such transistors are the workhorses of conventional electronics. But optoelectronic transistors, particularly those with high gain, are much harder to make. Which is a pity, for they are greatly in demand in the world’s telecoms networks, in which signals are processed locally as electrons but are transmitted long-distance as light.
At the moment Dr Koppens and his colleagues say their goal is to create “the thinnest and most flexible detector in the world”. It is notable, however, that they actually deposited their experimental quantum dot-graphene photodetector onto a piece of silicon. Their purpose in doing so was to show that the technology meshes with the standard silicon-processing techniques used to make computer chips.
Many have tried to overcome the barriers that stand in the way of the seamless integration of information technologies that are based on electrons with those based on photons, and none has unequivocally succeeded. Whether Dr Koppens is the man to do it remains to be seen. But if he is, then he will certainly have justified the brouhaha that graphene has stirred up.