Layer control of Sr_1.8Bi_0.2Na_n-3Nb_nO_3n+1 (n = 3–5) perovskite nanosheets: dielectric to ferroelectric transition of film deposited by Langmuir Blodgett method

Solution-based processable high-k 2-dimensional (2D) ferroelectrics have attracted significant interest for use in next-generation nanoelectronics. Although few studies on potential 2D ferroelectric nanosheets in local areas have been conducted, reports on the thin-film characteristics applicable to the device are insufficient. In this study, we successfully synthesize high-k 2D Sr_1.8Bi_0.2Na_n-3Nb_nO_3n+1 (octahedral units, n = 3–5) nanosheets by the engineering of the n of NbO₆ octahedral layers with A-site modification, and realized ferroelectric characteristics in ultrathin films (below 10 nm). The nanosheets are synthesized by a solution-based cation exchange process and deposited using the Langmuir-Blodgett (LB) method. As increasing the NbO₆ octahedral layer, the thickness of the nanosheets increased and the band gaps are tuned to 3.80 eV (n = 3), 3.76 eV (n = 4), and 3.70 eV (n = 5). In addition, the dielectric permittivity of the 5-layer stacked nanofilm increase to 26 (n = 3), 33 (n = 4), and 62 (n = 5). In particular, the increased perovskite layer exhibits large distortions due to the size mismatch of Sr/Bi/Na ions at the A-site and promotes local ferroelectric instability due to its spontaneous polarization along the c-axis caused by an odd n number. We investigate the stable ferroelectricity in Pt/ 5-layer Sr_1.8Bi_0.2Na₂Nb₅O₁₆ / Nb:STO capacitor by polarization-electric field (P-E) hysteresis; the coercive electric field (E_c) was 338 kV cm⁻¹ and the remnant polarization (P_r) 2.36 μC cm⁻². The ferroelectric properties of ultrathin 2D materials could drive interesting innovations in next-generation electronics.