Charge-Ferroelectric Transition in Ultrathin Na0.5 Bi4.5 Ti4 O15 Flakes Probed via a Dual-Gated Full van der Waals Transistor.

Ferroelectric field-effect transistors (FeFETs) have recently attracted enormous attention owing to their applications in nonvolatile memories and low-power logic electronics. However, the current mainstream thin-film-based ferroelectrics lack good compatibility with the emergent 2D van der Waals (vdW) heterostructures. In this work, the synthesis of thin ferroelectric Na0.5 Bi4.5 Ti4 O15 (NBIT) flakes by a molten-salt method is reported. With a dry-transferred NBIT flake serving as the top-gate dielectric, dual-gate molybdenum disulfide (MoS2 ) FeFETs are fabricated in a full vdW stacking structure. Barrier-free graphene contacts allow the investigation of intrinsic carrier transport of MoS2 governed by lattice scattering. Thanks to the high dielectric constant of ≈94 in NBIT, a metal to insulator transition with a high electron concentration of 3.0 × 1013 cm-2 is achieved in MoS2 under top-gate modulation. The electron field-effect mobility as high as 182 cm2 V-1 s-1 at 88 K is obtained. The as-fabricated MoS2 FeFET exhibits clockwise hysteresis transfer curves that originate from charge trapping/release with either top-gate or back-gate modulation. Interestingly, hysteresis behavior can be controlled from clockwise to counterclockwise using dual-gate. A multifunctional device utilizing this unique property of NBIT, which is switchable electrostatically between short-term memory and nonvolatile ferroelectric memory, is envisaged.