1. (a-c) Schematic drawings of structures; Ferroelectric domains illustrating (d) up-graded film, (e) down-graded film and (f) BFO thin film; Schematic diagrams of crystal structures of BFO films (g) before and (h, i) after doping

2. Structures and cross section morphologies of thin films(a) XRD patterns and SEM images of (b) up-graded films and (f) down-graded films. Bismuth, iron and calcium mappings of (c-e) up-graded BFO films and (g-i) down-graded BFO films

3. Surface morphologies and domain structures of thin films(a-c) AFM images, (d-f) out-of-plane PFM images, (g-i) accordingly asymmetric phase and amplitude loops versus tip bias voltage of (a, d, g) up-graded films, (b, e, h) down-graded films and (c, f, i) BFO films

4. J-V characteristics of thin films(a) Up-graded BFO films and down-graded BFO films; (b) BCFO (x=0, 0.05, 0.10, 0.15, 0.20) films; Colorful figures are available on website

5. Ferroelectric properties of BFO compositionally graded films(a) Up-graded films; (b) Down-graded films; (c) BFO thin films; (d) Three kinds of films at the electric field intensity of 1500 kV/cm; (e) Remanent polarization, P_r and (f) coercive field, E_c changed with applied voltage; Colorful figures are available on website

6. Valence analysis of thin filmsNarrow scan spectra of thin films: (a) O1s, (b) Fe2p, and (c) Ca2p of Bi_1-xCa_xFeO₃ samples with x=0, 0.05, 0.10, 0.15 and 0.20; (d) Ratio of Fe³⁺/Fe²⁺ and (e) percentage of the oxygen vacancy concentration of BFCO with the increase of x

S1. AFM images, PFM images and asymmetric phase and amplitude loops versus tip bias voltage of BCFO (x=0.05, 0.10, 0.15, 0.20) films

S2. Ferroelectric properties of Bi_1-xCa_xFeO₃ films(a) x=0; (b) x=0.05; (c) x=0.10; (d) x=0.15; (e) x=0.20; (f) The remanent polarization P_r, the coercive field E_c measured under the electric field intensity of 1800 kV·cm^-1

S3. Narrow scan spectra of Bi4f of Bi_1-xCa_xFeO₃ samples with x=0, 0.05, 0.10, 0.15 and 0.20

S3. Mechanical displacement of (a) up-graded BFO films and (b) down-graded BFO films as a function of time under an applied sinusoidal voltage