Screening transition metal-based polar pentagonal monolayers with large piezoelectricity and shift current.

Two-dimensional (2D) materials entirely composed of pentagon motifs are of interest for their wide applications. Here, we demonstrate that in-plane polar symmetry can exist in ternary pentagonal monolayers, where the induced electric polarization is not associated with specific conditions, such as ferroelectric phase transition, strain gradient, and layer stacking, but is an intrinsic structural property coming from the orderly arranged polar bonds. Based on the high-throughput screening method and first-principles calculations, we find eight stable 2D polar transition metal compounds with a number of intriguing properties. In particular, their piezoelectric coefficients are three orders of magnitude larger than those of 2D elemental and binary pentagonal structures, and their bulk photovaltaic shift current can reach up to 300 μA V⁻², superior to that of 2D conventional ferroelectric materials such as GeS. Our identified pentagonal monolayers not only expand the family of 2D pyroelectric materials, but also hold potential for energy conversions. [ABSTRACT FROM AUTHOR]