For more than a century, diamond has been regarded as non-piezoelectric, meaning it should not generate an electrical voltage when mechanically deformed. Researchers at the University of Hong Kong (HKU) have now discovered that diamond can behave very differently when it is made extremely thin and flexible.
The team, led by Professor Zhiqin Chu and Professor Yuan Lin, used an edge-exfoliation technique to produce ultrathin polycrystalline diamond membranes. Making the material sufficiently thin allowed the normally rigid diamond to bend significantly. When the membranes were flexed, researchers recorded stable and repeatable voltage signals. Further mechanical cycling was used to rule out environmental interference and other effects that might have produced the electrical output.
The explanation appears to lie in the microscopic structure of the material. Polycrystalline diamond consists of many small diamond crystals separated by grain boundaries. First-principles calculations carried out by the researchers suggest that asymmetry at these boundaries creates the piezoelectric effect. As the membrane bends, electrical charge builds around the grain boundaries, creating a potential difference between its upper and lower surfaces.
The discovery could give diamond a new role in engineering. Its hardness, chemical stability, thermal conductivity and other properties already make it attractive for demanding applications, but adding an active electrical function could extend its use considerably. The researchers suggest flexible diamond membranes could eventually be used in self-powered sensors, miniature energy systems and implantable medical devices capable of generating power or detecting mechanical deformation.
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The research has been published in Science Advances.