Texture-Induced Self-Polarization of Flexible BiFeO₃ Thin Films

Conventional poling methods, such as contact poling and corona poling, are frequently hampered by susceptibility to dielectric breakdown and film failure. Achieving self-poling without external poling procedure required in lead-free ferroelectric thin films is critical for advancing flexible piezoelectric devices. Here, we introduce a LaNiO₃ seed layer on flexible mica substrates to induce (001)-textured BiFeO₃ films for self-poling, while systematically evaluating the impact of Mn doping on film properties. The resulting self-poled BiFeO₃ films achieve a piezoelectric coefficient d_{33, d}≈50 pC/N and demonstrate exceptional bending stability, retaining over 90% of their initial d_{33, d} after more than 5000 bending cycles. This work presents a compelling new pathway for the development of high-performance lead-free flexible ferroelectric devices.

Mica substrates were prepared via blade-cleaving, followed by sequential 3-minute ultrasonic cleaning in anhydrous ethanol, acetone, and anhydrous ethanol to remove contaminants. LaNiO₃ solution was synthesized by sol–gel processing: stoichiometric lanthanum nitrate and nickel acetate (La: Ni = 1:1 mol ratio, 0.2 mol/L concentration) were dissolved in glacial acetic acid with stirring at 80℃ until complete dissolution, then stirred for 2 h at 25℃. The solution was aged for 48 h at 25℃.Subsequently, LaNiO₃ solution was spin-coated onto mica substrates at 5000 rpm, dried on a hotplate at 150℃, and thermally processed in a rapid thermal annealing furnace under air atmosphere. This involved a pyrolysis step at 400℃ for 5 min, followed by crystallization at 700℃ for 1 min. Layer-by-layer spin-coating with intermediate annealing achieved the target LaNiO₃ electrode thickness. Mn-doped BiFeO₃ solutions (0, 1%, 3%, 5% Mn-doping) were prepared by dissolving bismuth nitrate (10% excess), iron nitrate, and manganese acetate in ethylene glycol monomethyl ether (0.4 mol/L concentration), adding citric acid (1:1 molar ratio to metal ions) as a chelating agent, magnetically stirring for 4 h at 25℃, and aging for 48 h. These sols were spin-coated onto LaNiO₃/mica substrates, followed by 10 iterative cycles of thermal treatment: drying at 200℃ (5 min), pyrolysis at 400℃ (5 min), and crystallization at 550℃ (5 min) per layer to attain the final film thickness.

The LaNiO₃ seed layer promotes a dominant (001) texture in BiFe_1–xMn_xO₃ (0%, 1%, 3%, 5% Mn) thin films. However, increasing Mn dopant concentration induces compressive stress relaxation, significantly degrading the film texture from 92.3% to 56.7%. Undoped and 0.01% Mn-doped films exhibit pronounced out-of-plane self-polarization, yielding piezoelectric coefficients d_{33, d} = 48.6 pC/N and 47.4 pC/N. Internal bias in PFM voltage-phase/amplitude curves and unipolar conduction in I-V characteristics unequivocally confirm a built-in electric field. This field weakens at higher Mn concentrations (x = 0.03 and 0.05), concurrently diminishing self-polarization—A causal relationship demonstrating that self-polarization originates from this internal field. Although increased Mn doping reduces self-polarization, it, however, significantly improves electrical performance: leakage current decreases by 78% at x = 0.05 compared to undoped films. Therefore, optimal Mn doping (x = 0.01) synergistically preserves self-polarization while enhancing electrical performance. As flexible films, BiFe_1–xMn_xO₃ demonstrates exceptional bending stability and mechanical fatigue resistance. P–E hysteresis loops remain invariant under compressive/tensile stresses compared to undeformed states. After 5000 bending cycles at tensile strain (r =10 mm), ferroelectric properties and piezoelectric coefficient d_{33, d} degrade by＜10%, confirming outstanding operational durability.

This work demonstrates that LaNiO₃ seed layers deposited on mica substrates induce a dominant (001) texture in BiFeO₃-based thin films. Undoped and 0.01% Mn-doped compositions exhibit significant self-polarization driven by a built-in electric field, while increased Mn doping reduces texture degree from 92.3% to 56.7% and weakens self-polarization. The latter is evidenced by the decrease of piezoelectric coefficient, d_{33, d}, from 48.6 pC/N to 14.6 pC/N. Crucially, despite diminishing self-polarization, Mn doping substantially enhances electrical properties (78% leakage reduction at x=0.05). All films endow exceptional mechanical resilience: after 5,000 bending cycles (r = 10 mm), d_{33, d} degrades＜10%, with P–E hysteresis loops remaining unaffected. By synergistically integrating texture engineering and doping optimization, we achieve self-poling with a d_{33, d}≈50 pC/N, establishing a novel technological pathway for lead-free flexible ferroelectric devices.