Thickness-dependent structural and electromechanical properties of (Na_0.85K_0.15)_0.5Bi_0.5TiO₃ multilayer thin film-based heterostructures

(Na_0.85K_0.15)_0.5Bi_0.5TiO₃ (NKBT) multilayer thin films with different thicknesses of 100–700 nm, corresponding to 2–14 layers with each layer of ~50 nm thickness, are synthesized on Pt(111)/Ti/SiO₂/Si substrates to form Pt/NKBT/Pt/Ti/SiO₂/Si heterostructures using different spin-coating and annealing conditions in a modified aqueous sol-gel process. The multilayer thin films spin-coated by two steps (step 1/2) at 600/4000 rpm for 6/30 s and annealed at 700 °C for 5 min with a heating rate of 30 °C/s show a dense, uniform, and continuous morphology as well as a pure perovskite structure with a rhombohedral–tetragonal phase transition at ~140 °C and no preferential orientation in the heterostructures. Their structural and electromechanical properties exhibit consistent improvement trends with increasing thickness from 100 to 550 nm (i.e., 2–11 layers). The 550 nm-thick, 11-layer films demonstrate the best ferroelectric, dielectric, piezoelectric, and electric performance in terms of the highest remnant polarization, saturation polarization, dielectric constant, and effective piezoelectric constant of 18.3 μC/cm², 53.6 μC/cm², 463, and 64 pm/V, as well as the lowest coercive field, dielectric loss tangent, and leakage current density of 116 kV/cm, 0.057, and 27 μA/cm², respectively. The observed thickness-dependent improvement is explained by an interfacial passive layer effect where the motion of both 180° and non-180° domain walls is enhanced in the thicker multilayer thin films by weakening the influence of domain pinning in the interfacial passive layers between the multilayer thin films and the substrates.