Bugs in the system
Bacteria may be the key to turning graphene into a semiconductor
SINCE its discovery in 2004 graphene, a form of carbon made of sheets a single atom thick, has been an invention in search of an application. In particular, it has fired engineers’ imaginations with the possibility of making thin, flexible, semi-transparent electronics. But it has always promised more than it has delivered because, although it is an excellent conductor of electricity, its other electronic properties are lacklustre. First, instead of being easily channelled, electric current moves across a graphene sheet randomly and in all directions. Second, graphene does not have a bandgap—a property needed to create the distinct “on” and “off” electronic states that transistors rely on to work, and which is induced in a material by disrupting the way its electrons are distributed.
One way to open up a bandgap is to introduce atoms of other elements into a substance. For graphene, however, this reduces the conductivity that is one of its attractive features. Another approach is to modify the atomic sheets’ shapes by, for example, wrinkling them—but existing methods of doing this do not control where the wrinkles form or how they are oriented.
That is about to change, and in a quite surprising way: by employing bacteria as templates. A team of researchers led by Vikas Berry of the University of Illinois, in Chicago, has found out how to produce wrinkles controllably in graphene, using a bacterium called Bacillus subtilis.
Bacillus subtilis cells are normally short, plump cylinders with smooth surfaces. If they get dehydrated, though, they shrink. That makes them wrinkle up, much like a grape shrivelling into a raisin. These wrinkles, Dr Berry and his team report in ACS Nano, can be patterned on to graphene.
The researchers started by placing a droplet of nutrient solution containing Bacillus subtilisonto a chip made of silica that had electrodes at either end. Running a current between the electrodes caused the bacteria themselves to become charged (positive at one end of the cylinder and negative at the other), and thus to line up parallel with the flow of current.
Next, they placed a sheet of graphene on top of the aligned bacteria and cooked the lot in a vacuum chamber heated to 250°C. This caused the bacteria to dehydrate and shrink, dragging the graphene sheet with them so that it took on the wrinkle patterns of the cells underneath it.
Crucially, bacteria do not wrinkle at random. Bacillus subtilis cells form wrinkles about 33 nanometres (billionths of a metre) apart—so that was the separation of the ridges imposed on the graphene. Unfortunately, this is too far apart to create a significant bandgap. The ridges do not disrupt graphene’s electronic structure enough. To do that, they would have to be less than five nanometres apart. But Dr Berry thinks such distances might be achieved by using another species of bacterium, one with stronger cell walls—or, perhaps, different sorts of cells altogether.
Even the 33-nanometre wrinkles, though, give graphene some interesting properties. Instead of zipping randomly across it, electrons traversing a sheet of wrinkly graphene are channelled between the ridges. This suggests that, if the bandgap problem can be resolved, then placing bacteria in preset arrays to create complex channel patterns would be the equivalent of etching a silicon chip. Components like the logic gates which form the basis of computing could thus be created.
Before that happens, though, two other problems need to be resolved. One is removing the bacteria and releasing the wrinkled graphene. That will mean finding the right chemical to do the loosening. The other is reproducibility. Individual bacteria differ slightly, not least because they are often of different ages, so any product that used unsorted cells as templates would be unreliable. This might be dealt with by cell-sorting techniques, or even by synthesising artificial scaffolds that behave similarly to cells. These, though, are details. The important thing is that Dr Berry has managed to push graphene towards semiconductivity in a novel and intriguing manner. The search for an application for the stuff has taken a step forward.