  Local domain structures of ferroelectrics have been studied extensively using
various modes of scanning probes at the nanoscale, including piezoresponse
force microscopy (PFM) and Kelvin probe force microscopy (KPFM), though none of
these techniques measure the polarization directly, and the fast formation
kinetics of domains and screening charges cannot be captured by these
quasi-static measurements. In this study, we used charge gradient microscopy
(CGM) to image ferroelectric domains of lithium niobate based on current
measured during fast scanning, and applied principal component analysis (PCA)
to enhance the signal-to-noise ratio of noisy raw data. We found that the CGM
signal increases linearly with the scan speed while decreases with the
temperature under power-law, consistent with proposed imaging mechanisms of
scraping and refilling of surface charges within domains, and polarization
change across domain wall. We then, based on CGM mappings, estimated the
spontaneous polarization and the density of surface charges with order of
magnitude agreement with literature data. The study demonstrates that PCA is a
powerful method in imaging analysis of scanning probe microscopy (SPM), with
which quantitative analysis of noisy raw data becomes possible.
