A penta-BCP sheet with strong piezoelectricity and a record high positive Poisson's ratio

Two-dimensional structures composed solely of pentagonal motifs are of particular interest due to their unique geometries and novel properties. In particular, the broken centrosymmetry in many pentagon-based sheets results in piezoelectricity. Herein, we rationally design a penta-BCP sheet with large intrinsic polarization and weak mechanical strength. First-principles calculations reveal that the penta-BCP sheet is a dynamically, thermally, and mechanically stable semiconductor with an indirect band gap of 2.97 eV. More interestingly, the penta-BCP sheet is found to be the first penta-sheet with a positive Poisson's ratio larger than 1.00 (1.30), and possesses prominent spontaneous polarization (P-s = 4.66 x 10(-10) C m(-1)) and piezoelectricity (d(21) = -13.57 pm V-1, and d(22) = 6.33 pm V-1). The large positive Poisson's ratio coupled with its piezoelectricity leads to a significant overall piezoelectric response of 24 pm V-1. This study expands the family of ternary pentagon-based sheets with outstanding properties.