  A simple implementation of Kelvin probe force microscopy is reported that
enables recording topographic images in the absence of any component of the
electrostatic force. Our approach is based on a close loop z-spectroscopy
operated in data cube mode. Curves of the tip-sample distance as a function of
time are recorded onto a 2D grid. A dedicated circuit holds the KPFM
compensation bias and subsequently cut off the modulation voltage during
well-defined time-windows within the spectroscopic acquisition. Topographic
images are recalculated from the matrix of spectroscopic curves. This approach
is applied to the case of transition metal dichalcogenides (TMD) monolayers
grown by chemical vapour deposition on silicon oxide substrates. In addition,
we check to what extent a proper stacking height estimation can also be
performed by recording series of images for decreasing values of the bias
modulation amplitude. The outputs of both approaches are shown to be fully
consistent. The results exemplify how in the operating conditions of
non-contact AFM under ultra-high vacuum, the stacking height values can
dramatically be overestimated due to variations in the tip-surface capacitive
gradient, even though the KPFM controller nullifies the potential difference.
We show that the number of atomic layers of a TMD can be safely assessed, only
if the KPFM measurement is performed with a modulated bias amplitude reduced at
its strict minimum or, even better, without any modulated bias. Last, the
spectroscopic data reveal that defects at the TMD/oxide interface can have a
counterintuitive impact on the electrostatic landscape, resulting in an
apparent decrease of the measured stacking height by conventional nc-AFM/KPFM
compared to non-defective sample areas. Hence, electrostatic free z-imaging
proves to be a promising tool to assess the existence of defects in atomically
thin TMD layers grown on oxides
