Enhanced out-of-plane piezoelectricity of group-III(A) Janus hydrofluoride monolayers

Two-dimensional (2D) piezoelectric materials utilizing the unique electronic characteristic were widely needed for microelectronic sensors and energy conversion devices. However, the currently reported 2D piezoelectric films face thorny challenges in practical application due to lack of the mechanical durability or the strong vertical piezoelectric effect. Herein, based on first-principles calculations, we theoretically simulate the Janus semiconductor nanosheets, namely, $\mathrm{F}\text{\ensuremath{-}}M\text{\ensuremath{-}}H$ ($M=\mathrm{B}$, Al, and Ga) monolayers. The result shows that they have admirable dynamical and thermal stabilities to achieve strong mechanical durability and possess the desired vertical piezoelectric effect by applying uniaxial strain in the $z$-axis direction vertical to the nanosheet surface. For the three materials, F-B-H has the strongest stability and the most significant vertical piezoelectric effect. The out-of-plane piezoelectric stress coefficient ${e}_{31}$ of F-B-H is higher than other reported 2D piezoelectric SnOSe films by two orders of magnitude. Furthermore, we found an interesting relationship among these three nanosheets: the higher the electronegativity difference ratio, the higher the piezoelectric stress coefficient. Finally, this work not only deepens the understanding of 2D piezoelectric materials, but also provides a platform for fabricating cutting-edge piezoelectric equipment such as wearable electronic, medical blood pressure detectors and robotic bionic skin tactile sensors.