  Electromechanical response of solids underpins image formation mechanism of
several scanning probe microscopy techniques including the piezoresponse force
microscopy (PFM) and electrochemical strain microscopy (ESM). While the theory
of linear piezoelectric and ionic responses are well developed, the
contributions of quadratic effects including electrostriction and capacitive
tip-surface forces to measured signal remain poorly understood. Here we analyze
the electrostrictive and capacitive contributions to the PFM and ESM signals
and discuss the implications of the dielectric tip-surface gap on these
interactions.
