Artificial design for new ferroelectrics using nanosheet-architectonics concept

Bandgap Tunable Oxynitride LaNb2 O7-x Nx Nanosheets

Bandgap tunable lanthanum niobium oxynitride [LaNb₂ O_7-x N_x ]^(1+x)- nanosheet is prepared by the delamination of a Ruddlesden-Popper phase perovskite oxynitride via ion-exchange and two-step intercalation processes. The lanthanum niobium oxynitride nanosheets have a homogeneous thickness of 1.6 nm and exhibit a variety of chromatic colors depending on the nitridation temperature of the parent-layered oxynitride. The bandgap energy of the nanosheets is determined by ultraviolet photoemission spectroscopy, Mott-Schottky, and photoelectrochemical measurements and is found to be tunable in the range of 2.03-2.63 eV. Furthermore, the oxide/oxynitride superlattice structures are fabricated by face-to-face stacking of 2D crystals using oxynitride [LaNb₂ O_7-x N_x ]^(1+x)- and oxide [Ca₂ Nb₃ O₁₀ ]^- nanosheets as building blocks. Moreover, the superlattices-like restacked oxynitride/oxide nanosheets hybrid exhibits unique proton conductivity and dielectric properties strongly influenced by the oxynitride nanosheets and enhanced photocatalytic activity under visible light irradiation.

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 (Ca₂Na_m-3Nb_mO_3m+1^-; 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.

Nanoarchitectonics in dielectric/ferroelectric layered perovskites: from bulk 3D systems to 2D nanosheets

We present an overview of recent investigations on the dielectric/ferroelectric properties of Dion-Jacobson-type perovskites, including bulk 3D layered systems and their exfoliated 2D nanosheets. In contrast to the Ruddlesden-Popper and Aurivillius phases, the Dion-Jacobson phases in bulk 3D systems have not been important targets for constructing dielectric/ferroelectric materials. However, recent investigations on Dion-Jacobson phases have provided new impetus to dielectric/ferroelectric materials. Dion-Jacobson perovskites can also facilitate delamination into 2D nanosheets. Layer-by-layer engineering of 2D perovskite nanosheets has a great potential for the rational design of new high-k dielectric/ferroelectric materials and nanodevices.