Modified cantilevers to probe unambiguously out-of-plane piezoresponse

We demonstrate and investigate the coupling of contributions from both in-plane (IP) polarization and out-of-plane (OP) components in BiFeO3 (BFO) thin films polarization probed by piezoresponse force microscopy (PFM). Such coupling leads to image artefacts which prevent the correct determination of OP polarization vector directions and the corresponding piezoelectric coefficient d33. Using material strength theory with a 1D modelling of the cantilever oscillation amplitude under electrostatic and elastic forces as function of the tip length, we have evidenced the impact of IP piezoresponse to the OP signal for tip length longer than 4 μm. The IP polarization vector induces a significant longitudinal bending of the cantilever, due to the small spring constant of long tips, which provokes a normal deviation superimposed to the OP piezoresponse. These artefacts can be reduced by increasing the longitudinal spring constant of the cantilever by shortening the tip length. Standard cantilevers with 15 μm-long tips were modified to reach the desired tip length, using focused ion beam techniques and tested using PFM on the same BFO thin film. Tip length shortening has strongly reduced IP artefacts as expected, while the impact of non-local electrostatic forces, becoming predominant for tips shorter than 1 μm, have led to a non-negligible deflection offset. For shorter tips, strong electric field from cantilever beam, can induce polarization switching as observed for 0.5 μm-long tip. Tip length ranging from 1 to 4 μm allowed minimizing both artefacts to probe unambiguously OP piezoresponse and quantify the d33 piezoelectric coefficient.