Ferroelectricity in untwisted heterobilayers of transition metal dichalcogenides Two-dimensional materials with out-of-plane (OOP) ferroelectric and piezoelectric properties are highly desirable for the realization of ultrathin ferro- and piezoelectronic devices. We demonstrate unexpected OOP ferroelectricity and piezoelectricity in untwisted, commensurate, and epitaxial MoS 2 /WS 2 heterobilayers synthesized by scalable one-step chemical vapor deposition. We show d 33 piezoelectric constants o...