Depositing lithium ions on one face of the carbon monolayer material graphene can endow it with piezoelectric properties, allowing it to generate a voltage when deformed or to deform when a voltage is applied to the material. New research from Stanford University points the way to using graphene as an engineering control material in microelectromechanical systems (MEMS), their nano counterparts, NEMS, and other applications.
“The physical deformations we can create are directly proportional to the electric field applied. This represents a fundamentally new way to control electronics at the nanoscale,” said Mitchell Ong, a post-doctoral researcher in the Materials Computation and Theory Group at Stanford and lead author of the study.
The Stanford team, led by Evan Reed, has added a dynamic dimension to “straintronics” in graphene [ACS Nano (2012) 6(2), 1387–1394; doi 10.1021/nn204198g]. “Piezoelectric graphene could provide an unparalleled degree of electrical, optical or mechanical control for applications ranging from touchscreens to nanoscale transistors,” explains Ong.
Doping one face of a graphene sheet could make the carbon sheets piezoelectric.
To arrive at the optimal coating for graphene, the team modeled different approaches to deposition and simulated the piezoelectric effect. The researchers used density functional theory (DFT) to model graphene doped with lithium, hydrogen, potassium, and fluorine, as well as combinations of hydrogen and fluorine and lithium and fluorine on either face of the graphene lattice. Given that some doped forms of graphene have been fabricated, the team is confident that their findings will be demonstrated experimentally.
The process of doping breaks physical symmetry and allows the piezoelectric effect to become manifest. “We thought the piezoelectric effect would be present, but relatively small. Yet, we were able to achieve piezoelectric levels comparable to some traditional three-dimensional materials,” says team leader Evan Reed. “It was pretty significant.”
The team's approach allows them to control where and how much the graphene is deformed by an applied electric field. Moreover, the same doping technique might be applicable to graphene's tubular chemical cousins – carbon nanotubes, which have also attracted a great deal of interest for electronics, photonics, sensor, and energy harvesting applications. Conventionally, the piezoelectric effect has been considered an intrinsic property of a given mineral phase, in wurtzite, for instance. The effect arises because the phase has a non-centrosymmetric point group, in other words it is a material without an inversion centre. This new research opens up the concept of nanoscale engineering of this property into two-dimensional materials not limited to doped graphene.
