Observation of a giant two-dimensional band-piezoelectric effect on biaxial-strained graphene

Piezoelectric materials used in the development of nanoscale mechanical sensors, actuators and energy harvesters have received much attention. More recently, devices made of graphene are of particular interest because of graphene’s intriguing electronic and mechanical properties. Intrinsic graphene has long been considered devoid of the piezoelectric effect, although flexoelectricity has been exploited to demonstrate piezoelectricity in functionalized graphene and graphene nanoribbons. The perceived lack of this property has restricted graphene’s use in nanoelectromechanical systems (NEMS) for electromechanical coupling purposes. Here an unprecedented two-dimensional (2D) piezoelectric effect on a strained/unstrained graphene junction is reported. In stark contrast to the bulk piezoelectric effect that results from the occurrence of electric dipole moments in solids, the 2D piezoelectric effect arises from the charge transfer along a work function gradient introduced by the biaxial-strain-engineered band structure. The observed effect, termed the band-piezoelectric effect, exhibits an enormous magnitude due to the ultrathin structure of graphene. On the basis of the band-piezoelectric effect, a graphene nanogenerator and a pressure gauge were fabricated. The results not only provide a versatile NEMS platform for sensing, actuating and energy harvesting, but also pave the way for efficiently modulating graphene via strain engineering. Researchers in China have observed two-dimensional piezoelectricity in graphene by subjecting it to an in-plane biaxial strain. This is highly significant as intrinsic graphene has long been considered to be devoid of piezoelectricity and hence has found limited use in nanoelectromechanical systems (NEMS) for electromechanical coupling. The effect is considered to originate from charge transfer along the work-function gradient arising from the modified band structure induced by the biaxial strain. The extreme thinness of graphene means that its piezoelectric coefficient is about two orders of magnitude higher than that of bulk piezoelectric materials. The researchers demonstrated the effect by fabricating a graphene-based piezoelectric nanogenerator and high-sensitivity pressure gauge. This finding opens the possibility of realizing novel two-dimensional electromechanical coupling and graphene strain engineering as well as using graphene-based NEMS for sensing, actuating and energy harvesting. Graphene is an emerging material for nanoelectromechanical systems (NEMS) due to its intriguing electronic properties and promising mechanical character. However, intrinsic graphene has long been considered devoid of piezoelectric effect which restricts its electromechanical coupling ability. We report on a giant two-dimensional (2D) piezoelectric effect on an intrinsic graphene-based NEMS platform, which results from dynamical adjustment of band structure-induced polarization instead of occurrence of electric dipoles at the molecular level. These findings not only open an avenue for dynamical strain-engineered 2D electronics, but also pave the way for low-cost sensing and energy harvesting applications.