Rational Assembly of Two-Dimensional Perovskite Nanosheets as Building Blocks for New Ferroelectrics

Artificial materials in the form of superlattices have been studied actively in quest of new engineering methods or design rules for the development of desired functionalities, in particular high-k ferroelectricity, ferromagnetism, and high mobility electron gas. This work presents a controlled assembly strategy for fabricating atomically precise interfaces of two-dimensional (2D) homologous perovskite nanosheets (Ca₂NaNbO₃^-; m = 3-6) to construct artificial superlattices. The distinctive thickness of each 2D homologous perovskite nanosheets attributed to the presence of different number of NbO₆ octahedra provides an exquisite control to engineer interfacial properties for tailored design of superior high-k properties and emergence of ferroelectricity. The higher dielectric constant (ε_r = 427) and development of ferroelectricity for (Ca₂Nb₃O₁₀^-/Ca₂Na₂Nb₅O₁₆^-)₆ superlattice indicate that superlattice films with both odd number of NbO₆ octahedra possess extended polarization due to the potential effect of heterointerface and ferroelectric instabilities. Furthermore, the increased discontinuities/offsets in Ca₂Nb₃O₁₀^- and Ca₂Na₃Nb₆O₁₉^- nanosheets band alignment results in superior insulating properties (∼1 × 10^-11 A cm^-2 at 1 V) for (Ca₂Nb₃O₁₀^-/Ca₂Na₃Nb₆O₁₉^-)₆ superlattice. These findings exhibit new research opportunities for the development of novel artificial high-k dielectric/ferroelectric via precise control of interfaces at the atomic level and can be extended to the large family of 2D perovskite compounds.