Mechanisms of Temperature-Dependent Hysteresis in Freestanding BaTiO3/MoS2 Heterostructures.

Freestanding ferroelectric oxides integrated with 2D semiconductors offer a platform for reconfigurable electronic functionalities beyond conventional dielectric gating. However, once released from epitaxial constraint and transferred directly onto metallic gates, the BaTiO3 (BTO) membranes under study preferentially adopt an in-plane polarization configuration, suppressing out-of-plane ferroelectric coupling at room temperature. Here, we demonstrate that single-layer MoS2 field-effect transistors gated by 25 nm freestanding BTO exhibit excellent electrostatic control at room temperature, with subthreshold swings down to 85 mV dec- 1 and high on/off ratios, yet negligible hysteresis, highlighting the effectiveness of depolarized BTO as an ultrahigh-κ dielectric. Upon cooling, robust counter-clockwise ferroelectric hysteresis emerges, with memory windows of ∼0.4 V, stable over 500 cycles and retention times exceeding 105 s. Temperature-dependent x-ray diffraction reveals structural signatures consistent with the bulk phase transitions sequence of BTO and confirms a predominant in-plane lattice orientation near room temperature. The combined electrical and structural analysis indicates that phase-dependent polarization anisotropy, together with domain-wall dynamics and interfacial screening processes, stabilizes an out-of-plane polarization component at low temperature in the rhombohedral and orthorhombic phases.