Diamond has been classified as non-piezoelectric since the early twentieth century, a consequence of its highly symmetric crystal structure, which leaves no obvious route for mechanical deformation to generate charge separation. An interdisciplinary team spanning the Department of Electrical and Computer Engineering and the Department of Mechanical Engineering in the HKU Faculty of Engineering has reported a significant piezoelectric effect in ultrathin, flexible polycrystalline diamond membranes.
The result depended on being able to bend diamond at all. The membranes were produced using a recently developed edge-exfoliation method that allows this notoriously hard material to flex substantially rather than fracture. Bending produced stable voltage signals, and repeated mechanical cycling under controlled conditions gave consistent outputs, with the team ruling out environmental noise and triboelectric artefacts as explanations.
First-principles calculations attribute the effect mainly to asymmetry in the grain boundaries, where charge polarisation accumulates as deformation increases. That explanation is consistent with the classical picture rather than contradicting it: the single crystal remains symmetric, but a polycrystalline membrane is not, and at these thicknesses the boundaries dominate the response. The study was published in Science Advances.
Diamond is biocompatible and chemically stable, which makes a piezoelectric form of it interesting for implantable medical devices that must either harvest energy or sense their own deformation inside the body. Micro-energy systems and self-powered sensing are the other suggested directions. The broader lesson concerns how material classifications derived from ideal crystals can mislead once structure at the nanoscale becomes the controlling factor.