fig2

Figure 2. PFM amplitude images of (A) the initial state, (B) V_a = -7 V at position #1, and (C) V_a = -7 V at positions #2 and #3 following V_a = +7 V at position #1. The white scale bar represents 200 nm; (D) Voltage-dependent PFM amplitude curves measured at positions #1 (red squares), #2 (blue circles), and #3 (green triangles) under negative [V_a(-), closed symbols] and positive [V_a(+), open symbols] bias. The conventional amplitude response near the tested c-domain (dotted black line) is shown for reference. Black arrows indicate the direction of the voltage sweeps. PFM: Piezoelectric force microscopy.