Imaging ferroelectrics: reinterpreting charge gradient microscopy as potential gradient microscopy

Charge gradient microscopy (CGM) is a scanning probe imaging mode, particularly well-suited for the characterization of ferroelectrics. The implementation of the technique is straightforward; it involves monitoring currents that spontaneously develop between a passive conducting atomic force microscopy tip and Earth, as the tip is scanned across the specimen surface. However, details on the fundamental origin of contrast and what images mean, in terms of associated ferroelectric microstructures, are not yet fully understood. Here, by comparing information from CGM and Kelvin probe force microscopy, obtained from the same sets of ferroelectric domains (in both lithium niobate and barium titanate), it is shown that CGM reasonably reflects the spatial derivative of the measured surface potential. This is conceptually different from measuring local gradients in the surface bound-charge density or in any associated screening charges: after all, clear CGM signals are seen, even when polarization is entirely in-plane (where the bound charge density is uniformly zero, but gradients in surface potential are still fully expected). It is therefore suggested that CGM in ferroelectrics may be more accurately called potential gradient microscopy.