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In 2014, Charge Gradient Microscopy (CGM) was first reported as a newscanning probe imaging mode, particularly well-suited for the characterisationof ferroelectrics. The implementation of the technique is straightforward; itinvolves monitoring currents that spontaneously develop between a passiveconducting atomic force microscopy tip and Earth, as the tip is scanned acrossthe specimen surface. However, details on the fundamental origin of contrastand what images mean, in terms of associated ferroelectric microstructures, arenot yet fully understood. Here, by comparing information from CGM and KelvinProbe Force Microscopy (KPFM), obtained from the same sets of ferroelectricdomains (in both lithium niobate and barium titanate), we show that CGMreasonably reflects the spatial derivative of the measured surface potential.This is conceptually different from measuring local gradients in the surfacebound-charge density or in any associated screening charges: after all, we seeclear CGM signals, even when polarisation is entirely in-plane. We thereforesuggest that CGM in ferroelectrics might be more accurately called PotentialGradient Microscopy (PGM). Intriguingly, in all cases examined, the measuredsurface potential (determined both through KPFM and by integrating the CGMsignal) is of the opposite sign to that intuitively expected for a completelyclean ferroelectric in vacuum. This is commonly observed and presumed due to acharge accumulation on the ferroelectric surface which is not easily removed.

Imaging Ferroelectrics: Charge Gradient Microscopy (CGM) versus Potential Gradient Microscopy (PGM)