Graphene has recently attracted a lot of researchers and scientists mainly for its unique ability to conduct electricity. This two-dimensional material is made up of single layer carbon atoms that are connected in a hexagonal chicken-wire pattern, and has caught the eye of several researchers for its phenomenal ability to conduct electricity, and among them are researchers from the University of Illinois at Chicago (UIC). They used rod-shaped bacteria which is accurately aligned in an electric field and was vacuum-shrunk using a graphene sheet in order to introduce nanoscale ripples in the material. The result of this experiment shows that it caused the electrons to conduct differently when in perpendicular directions.
The resulting material is like a graphene nano-corduroy which can be used to a silicone chip and may add to graphene’s already limitless vast potential in the field of electronics and nanotechnology.
Heading the research, Vikas Berry who is also UIC’s associate professor and interim head of chemical engineering, said that the current across the graphene wrinkles is surprisingly fewer as compared to the ones along them.
Berry added that the formation of the wrinkles is greatly attributed to the extreme flexibility of graphene at the nanometer scale, allowing formation of carbon nanotubes. He also said that the wrinkle opens a “V” in the electron cloud around each carbon atom, producing a dipole moment which can open an electronic band gap, something that a flat graphene does not possess.
Other researchers produced wrinkles in graphene employing other methods like stretching the sheet and allowing it to snap back. The wrinkles in this method however are not confined in microscale and cannot be directed towards a location on a micro-device.
Berry and several of his colleagues created a unique way to introduce circumscribed, guided, and regular graphene ripples by using bacillus bacteria. The graphene itself when used, act as a check-valve in order to change the volume of the cells.
They then put the bacteria in an electric field, which made them line up similar to hotdogs in repeating rows. The researchers then put a sheet of graphene over the top.
“Under vacuum, the graphene lifts, and lets water out,” Berry said. “But under pressure, graphene sits back down on the substrate and prevents water from re-entering the bacteria,” he added.
“It’s a nanoscopic valve that actuates unidirectional fluid flow in a microorganism,” He went on further. “Futuristically, this valve operation could be applied to microfluidic devices where we want flow in one direction but not the other.”
After the bacteria that were vacuum-shrunk, the graphene material reconforms but this time with wrinkles. When heat treatment was applied, the permanent ripples on top the bacteria were arranged in a line, with a height of approximately 7 to 10 nanometers with a wavelength of about 32 nm.
Researchers observed that the wrinkles using a field emission electronic microscopy must be done not only under high vacuum but also by atomic microscopy at atmospheric pressure.
“The [ripple] wavelength is proportional to the thickness of the material, and graphene is the thinnest material in the world,” Berry also said. “We envision that with graphene one could make the smallest wavelength wrinkles in the world — about 2 nanometers.”
Berry said that their next objective is to create processes which can further refine the ripples and vary not only their amplitude but also its wavelength and longitudinal length.
Shikai Deng, a graduate student of UIC and the lead author of the paper said that in order to measure the effect of the ripples’ orientation on the carrier transport, a plus-shaped device with bacteria aligned parallel to a pair of electrodes and perpendicular to another pair was created. Deng discovered that the rippled graphene’s conduction barrier was greater when it is in the transverse direction as compared to those in the longitudinal direction.
As head of the research team, Berry added that the introduction of the oriented ripples to graphene reveals a totally new material.
“Along with carbon nanotubes, graphene and fullerene, this is a new carbon allotrope — a half carbon nanotube linked to graphene,” the lead researcher said. “The structure is different, and the fundamental electronic properties are new.”
The findings of this research can be found in the journal ACS Nano, and funding the research are UIC and the National Science Foundation.
Also part of this excellent research are UIC’s Department of Chemical Engineering’s Sanjay Behura and Tsinghua University’s Enlai Gao, Yanlei Wang and Zhiping Xu. Completing the list of the co-authors are Soumyo Sen and Petr Kral of the UIC Department of Chemistry, and T.S. Sreeprasad of Clemson University.