  In 2014, Charge Gradient Microscopy (CGM) was first reported as a new
scanning probe imaging mode, particularly well-suited for the characterisation
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 (KPFM), obtained from the same sets of ferroelectric
domains (in both lithium niobate and barium titanate), we show 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, we see
clear CGM signals, even when polarisation is entirely in-plane. We therefore
suggest that CGM in ferroelectrics might be more accurately called Potential
Gradient Microscopy (PGM). Intriguingly, in all cases examined, the measured
surface potential (determined both through KPFM and by integrating the CGM
signal) is of the opposite sign to that intuitively expected for a completely
clean ferroelectric in vacuum. This is commonly observed and presumed due to a
charge accumulation on the ferroelectric surface which is not easily removed.
