Large in-plane negative piezoelectricity and giant nonlinear optical susceptibility in elementary ferroelectric monolayers

Negative piezoelectrics contract in the direction of an applied electric field, which is opposite to the behavior of normal piezoelectrics and rare in dielectric materials. The advent of low-dimensional ferroelectrics, with unconventional mechanisms of polarity, opens a fertile branch in the search for candidates with prominent negative piezoelectricity. Here, the distorted 𝛼-Bi monolayer, a newly identified elementary ferroelectric with a puckered black-phosphorus-like structure [Gou et al., Nature (London) 617, 67 (2023)], is computationally studied, which manifests a large negative in-plane piezoelectricity (with 𝑑33∼−26 pC/N). Its negative piezoelectricity originates from its unique buckling ferroelectric mechanism, namely intercolumn sliding. Consequently, a moderate tensile strain can significantly reduce its ferroelectric switching energy barrier, while compressive strain can significantly enhance its prominent nonlinear optical response. The physical mechanism of in-plane negative piezoelectricity also applies to other elementary ferroelectric monolayers.