Layer control of Sr1.8Bi0.2Nan-3NbnO3n+1 (n = 3-5) perovskite nanosheets: dielectric to ferroelectric transition of film deposited by Langmuir Blodgett method

Abstract

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 Sr1.8Bi0.2Nan-3NbnO3n+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 Sr1.8Bi0.2Na2Nb5O16 / Nb:STO capacitor by polarization-electric field (P-E) hysteresis; the coercive electric field (Ec) was 338？kV？cm？1 and the remnant polarization (Pr) 2.36 μC cm？2. The ferroelectric properties of ultrathin 2D materials could drive interesting innovations in next-generation electronics.