A research team from the University of Illinois Urbana-Champaign and the University of California has reported that the friction on a graphene surface can be dynamically tuned using external electric fields.
The team studied the friction at a single asperity nanoscale contact between the graphene surface of graphene FETs and an AFM tip in a dry nitrogen atmosphere, while the doping level of graphene was modulated in situ by changing the potential applied to the device’s back gate. In contrast to conducting or insulating contacts, graphene in contact with semiconducting tips exhibits an enhanced and tunable friction sensitive to the charge density in graphene.
Friction plays a key role in both natural and engineered systems, dictating the behavior of sliding contacts, affecting the wear of materials and influencing the flow of fluids across surfaces, among other effects. Friction can be controlled passively through the selection of design components, for example material and roughness. A more recent trend, however, has been to investigate systems whose frictional response can be dynamically tuned in situ, especially as micro- and nanoscale devices become more common. One of the more promising ways to achieve friction control is with external electric fields that can modulate the properties of lubricants and material surfaces as well as the interactions between them.
“Novel approaches to the design of interacting surfaces are necessary to move past the state of the art,” write the researchers, “and 2D materials are a new and excellent choice based on their high mechanical strength and chemical and thermal stability.”
Surfaces coated in graphene films generally exhibit very low friction, but the new results demonstrate that friction on graphene-coated surfaces can be “turned on” by exposing the surface to an electric field under the proper conditions. The system can then be controlled in this higher friction state before being switched back to lower friction, all without applying large electrical biases between the surfaces in contact.
“The work will be impactful in reducing energy consumption in nano- and micro-electromechanical systems, in addition to allowing dynamic control of friction while mitigating the enhanced wear and corrosion of sliding surfaces when direct bias is applied,” said University of Illinois Urbana-Champaign's Professor Rosa Espinosa-Marzal.