Unlocking the potential properties of graphene is proving difficult. However, according to US researchers, bacteria could be the key to turning the much-hyped material into a useful semiconductor.
Graphene is a very good conductor of electricity, but because of its two-dimensional atomic structure a sheet of graphene tends to conduct electricity randomly across its whole surface, which is not very useful, particularly in electronics; this depends on digital signals, which are either conducting electricity across a defined gap or not at all.
One way to influence the way that graphene conducts electricity is to shape the sheet, which is the research focus of Vikas Berry’s laboratory at the University of Illinois in Chicago. One line of research that might prove particularly fruitful is focusing on wrinkling graphene sheets in defined ways, with the help of bacteria.
Wrinkling a graphene sheet creates ridges and channels along which electrons can travel because the resistance is lower along the channel than across the rippled surface, but it has proven difficult to control how the ridges form. Berry’s team has found a way to do this in a way similar to vacuum-moulding, using bacteria as a mould. The team used a rod-shaped bacterium called Bacillus subtilis, which has two useful properties: first, it can be induced to line up in an ordered array by using electrostatics; and second, when dehydrated the surface of the bacteria wrinkle, forming ridges along the axis of the rod-shaped structure about 33nm apart.
Berry’s team produced an array of the bacteria in a nutrient solution on a silicon chip with electrodes at either end, draped a sheet of graphene on top, then cooked the chip and graphene in a vacuum chamber at 250°C. This dried out the nutrient solution, wrinkling the bacteria and pulling the graphene sheet down onto the wrinkles to create what Berry calls ‘nano-corduroy’. This is in effect an entirely new allotrope of carbon, he noted; a series of half-nanotubes. “The structure is different and the fundamental electronics properties are new.” Potentially, the technique could be used to form the bacteria into the shapes of electronic circuitry and then impress these shapes into graphene.
Berry also believes that the vacuum-moulding process itself has applications, as the graphene will let water out of the bacteria but not back in; this creates a nanoscopic valve effect, he said, that could potentially be exploited in microfluidics. The team describes the research in the journal ACS Nano.