Uncovering piezoelectric effect in polycrystalline diamond membranes.

Diamonds have been regarded as nonpiezoelectric materials for more than one century. Here, we uncover a notable piezoelectric effect in ultrathin and ultraflexible polycrystalline diamond membranes. Our experiments show that the piezoelectricity depends on membrane thickness, with peak response (exhibiting a piezoelectric voltage coefficient of 82.2 millivolt meters per newton that surpasses many conventional piezoelectric materials) seen in 5-micrometer-thick membranes. First-principles calculations reveal that the unexpected piezoelectricity in polycrystalline membranes originates from local asymmetry induced by grain boundaries, which alter the electric polarization within a finite region near the boundary during deformation. This finding together with the outstanding properties of our membranes is expected to catalyze the exploration and development of diamond-based applications in energy harvesting, intelligent sensing, and wearable electronics.