Data-driven discovery of high performance layered van der Waals piezoelectric NbOI 2 .

Using high-throughput first-principles calculations to search for layered van der Waals materials with the largest piezoelectric stress coefficients, we discover NbOI 2 to be the one among 2940 monolayers screened. The piezoelectric performance of NbOI 2 is independent of thickness, and its electromechanical coupling factor of near unity is a hallmark of optimal interconversion between electrical and mechanical energy. Laser scanning vibrometer studies on bulk and few-layer NbOI 2 crystals verify their huge piezoelectric responses, which exceed internal references such as In 2 Se 3 and CuInP 2 S 6 . Furthermore, we provide insights into the atomic origins of anti-correlated piezoelectric and ferroelectric responses in NbOX 2 (X = Cl, Br, I), based on bond covalency and structural distortions in these materials. Our discovery that NbOI 2 has the largest piezoelectric stress coefficients among 2D materials calls for the development of NbOI 2 -based flexible nanoscale piezoelectric devices.