fig4

Figure 4. PFM amplitude images of (A) the initial state, (B) V_a = -4V followed by + 4V at position #6, and (C) V_a = +7 V at positions #7 and #8. The white scale bar represents 200 nm; (D) Voltage-dependent PFM amplitude curves measured at positions #6 (red squares), #7 (blue circles), and #8 (green triangles) under bipolar (V_a, closed symbols) and positive [V_a(+), open symbols] bias. The conventional amplitude response near the tested c-domain (dotted black lines) is shown for reference. Black arrows indicate the direction of the voltage sweeps. PFM: Piezoelectric force microscopy.