Study on the piezoelectric properties and the mechanism of strain-regulated piezoelectricity in flexible Janus monolayers Cr2X3Y3 (X/Y = Cl, Br, I)

Piezoelectric materials hold significant promise in piezoelectric electronics and piezoelectric optoelectronics. As a new member of this family, the 2D Janus structures characterized by central symmetry breaking have attracted much attention due to the out-of-plane piezoelectric effects. In this work, the mechanical, piezoelectric properties, and the strain regulation mechanism of Juans structure material (Cr2X3Y3, X/Y = Cl, Br, I) are systematically investigated by the first-principles methods. The calculated mechanical properties show that Cr2X3Y3 with a lower Young's modulus of 27.31∼29.76 N m−1 is more sensitive to applied stresses, theoretically exhibiting exceptional piezoelectric properties. The in-plane piezoelectric coefficients d 11 for Cr2Br3Cl3, Cr2I3Cl3, and Cr2I3Br3 are 4.92, 9.89, and 7.86 pm V−1, respectively; the out-of-plane piezoelectric coefficients d 31 are 1.13, 2.33, and 1.64 pm V−1, respectively. Cr2I3Cl3 has the highest values of d 11 and d 31 due to the large electronegativity difference between iodine and chlorine atoms. Based on the analysis, it can be deduced that Cr2X3Y3 demonstrates substantial piezoelectric responses in both in- and out-of-plane, with potential strain regulation effects. The d 31 values of Cr2I3Cl3 show an approximately linear relationship to strain in the range from −2% to 4% and remain consistently above 2.10 pm V−1 across a broader range of strain from −4% to 6%, underscoring its robustness to strain. Our study indicates that two-dimensional Janus Cr2X3Y3 monolayers would emerge as promising candidates for diverse applications in multifunctional electronic devices.