Facet Engineering by Sculpting Artificial Edges on 2D Black Phosphorus for Localized and Selective Piezoelectric Response

The ability to achieve orientation selectivity in piezoelectric response and thereby identifying its active piezoelectric facets with superior response is imperative for the breakthrough in piezoelectricity; still, it remains a challenge due to the presence of the mixed facets in contemporary semicrystalline piezoelectric materials. Accurate determination of in-plane and out-of-plane piezoelectric coefficients in these materials is practically impossible due to the involvement of components of other piezoelectric tensors arising from the randomly oriented molecules. This report delves into the exploration of the piezoelectric and ferroelectric response at piezoelectric facets such as basal plane and edges of two-dimensional (2D) black phosphorus (BP) flake and thereby measures the in-plane and out-of-plane piezoelectric coefficients in their pure form. In this report, the localized piezoelectric facets (artificial edges) are meticulously sculpted using a low-power focused laser, resulting in precise piezoelectric response selectivity at facets of the BP flake. The experimental findings, supported by theoretical calculations, demonstrate that the in-plane piezoelectric response is prominent at the edges, while the out-of-plane response is significant at the basal plane of the flakes. Moreover, thinner flakes exhibit lower coercive voltage (V-C) and voltage at the spontaneous polarization (V-S), indicating a stronger ferroelectric response as compared to thicker flakes. The experimental results offer an innovative path for deciphering the active piezoelectric coefficient along different crystal facets of 2D materials, offering exciting prospects for further research beyond the confinement of contemporary piezoelectric materials.