Giant piezoelectricity in twisted hexagonal boron nitride

Superposed hexagonal boron nitride(hBN) layers with a small twist angle between them occur in two dominant stacking orders, known as parallel and antiparallel, where the crystal alignments are at 0 and 180 degrees, respectively. Only the parallel stacking order produces triangular domains with alternating out-of-plane polarizations, since pairs of boron and nitrogen atoms belonging to opposite layers are aligned. According to recent reports this stacking order hosts ferroelectricity, out-of-plane piezoelectricity, and flexoelectricity. As the out-of-plane polarization is formed by interlayer dipole, the Van der Waals bonds are relatively weak and the piezoelectricity coefficient is expected to be big, hence a significant mechanical deformation happens under strong vertical electrical field. We studied small angle twisted hBN samples using several modes of atomic force microscopy(AFM) in the presence of a variable back gate voltage. AFM topography reveals triangular moiré patterns with very large height variations which we attribute to a strong piezoelectric effect. We further find that the topographical contrast can be completely reversed by switching the gate voltage polarity.