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

Ordered Electronic Reconstruction of the (112¯011ar2011ar{2}0) ZnO Single Crystal

Abstract Three‐dimensional (3D) charge‐written periodic peak and valley nanoarray surfaces are fabricated on a (112¯0$11ar{2}0$) ZnO single crystal grown via chemical vapor transport. Because the grown ZnO crystals exhibit uniform n‐type conduction, 3D periodic nanoarray patterns are formed via oxygen annealing. These periodically decorated structures show that the peak arrays are conducting at the nanoampere level, whereas the valley arrays are less conductive. Energy dispersive spectroscopy indicates that the valley arrays are deficient in zinc by ≈4–6 at%, and that the peak arrays are deficient in oxygen, respectively. Kelvin probe force microscopy reveals the presence of periodic wiggles featuring variations of ≈70–140‐meV between the peak and valley arrays. A significant decrease in the Fermi level of the valley region is observed (≈190 meV), which corresponds to a high zinc vacancy doping density of 2 × 1018 cm−3. This result indicates the periodic generation of an extremely large electric field (≈11 000 V cm−1) in the vicinity of the peak–valley arrays. Computational analysis corroborates the experimentally observed generation of VZn and the preferential formation of surface protrusions on ZnO (112¯0$11ar{2}0$) rather than on (0001), based on surface effects, along with the generation of peak and valley features