Utilizing Shear Piezoelectricity of Chiral Lead-Free Metal Halides for Electromechanical Sensing

Chiral hybrid metal halides have emerged as a promising class of piezoelectric materials owing to their ease of synthesis, low acoustic impedance, and solution processibility. However, many known chiral halides crystalize in structures with only shear piezoelectricity which is less facile to be utilized compared with the longitudinal piezoelectricity, largely preventing their practical utility. In addressing this problem, chiral lead-free S-MPBiCl5 and R-MPBiCl5 (MP = 2-methylpiperazinium) with shear piezoelectricity are synthesized and the thin films orientating along the polarization direction are fabricated. These orientated films, characterized by numerous grains and compact grain boundaries, can effectively facilitate the grain boundary sliding to convert normal stress into shear stress, thus facilely activating the shear piezoelectricity. The resulting devices can accurately identify the position and type of ultrasound sources without the need for a complex acoustic impedance matching layer. Moreover, these devices can successfully sense delicate variations in momentum caused by table tennis balls. The findings open up a new pathway to utilize shear piezoelectricity that is normally considered unuseful of metal halides and other molecular piezoelectrics. Chiral metal halides with shear piezoelectricity are synthesized by introducing molecular chirality. This shear piezoelectricity, which is less facile to be utilized in practice, can be activated by constructing the oriented polycrystalline film to convert the normal stress to shear stress. The corresponding film devices exhibit prominent sensing capacities for assisted sports training and underwater ultrasound detection. image