Sliding ferroelectricity and alterpiezoelectricity in a two-dimensional heterobilayer from the in-plane hole of g-C3N4

It is well known that two-dimensional (2D) van der Waals (vdW) heterobilayers may undergo symmetry breaking in some specific stacking configurations, thus presenting three states (AA, AB, BA) with different out-of-plane (OOP) polarization, which is electrically switchable via interlayer sliding. Here, we find that in two vdW heterobilayers (S-g-C₃N₄/graphene and T-g-C₃N₄/graphene) formed by a g-C₃N₄ (S-g-C₃N₄ and T-g-C₃N₄) monolayer and graphene monolayer, interlayer sliding can not only switch the OOP polarization, but also change the in-plane (IP) piezoelectric properties of the T-g-C₃N₄/graphene heterobilayer, between positive and negative piezoelectricity, which we call alterpiezoelectricity. We show first-principles evidence that the strong charge transfer of graphene contributes to the charge redistribution in graphene induced by the in-plane holes of g-C₃N₄, so that stacking configurations with different piezoelectric properties appear in the switching process of interlayer sliding. In addition, we discover that for T-g-C₃N₄/graphene with alterpiezoelectric properties, the IP piezoelectric coefficient and band gap of different stacking configurations show opposite linear changes. The 2D heterobilayer ferroelectrics with unusual alterpiezoelectric characteristics will be ideal materials for realizing extraordinary piezoelectric electronic devices.