Dipolar polarization and piezoelectricity of a hexagonal boron nitride sheet decorated with hydrogen and fluorine

In contrast to graphene, a hexagonal boron nitride (h-BN) monolayer is piezoelectric because it is non-centrosymmetric. However, h-BN shows neither in-plane nor out-of-plane dipole moments due to its three-fold symmetry on the plane and the fact that it is completely flat. Here, we show that the controlled adsorption of hydrogen and/or fluorine atoms on both sides of a pristine h-BN sheet induces flatness distortion in a chair form and an out-of plane dipole moment. In contrast, a boat form has no out-of-plane dipole moment due to the alternating boron and nitrogen positions normal to the plane. Consequently, the chair form of surface-modified h-BN shows both in-plane and out-of-plane piezoelectric responses; while pristine h-BN and the boat form of decorated h-BN have only in-plane piezoelectric responses. These in-plane and out-of-plane piezoelectric responses of the modified h-BN are comparable to those in known three-dimensional piezoelectric materials. Such an engineered piezoelectric two-dimensional boron nitride monolayer can be a candidate material for various nano-electromechanical applications.