NONRESONANT DETECTION OF ELECTRIC FORCE GRADIENTS BY DYNAMIC FORCE MICROSCOPY

A mechanoelectric cross-modulation scheme is proposed to enable direct and nonresonant detection of electric force gradients by dynamic force microscopy. The technique employs an ultrasonic vibration of the sample against a conductive tip, to which an alternating voltage is applied at a frequency near that of mechanical modulation, chosen well above the resonance frequency of the cantilever. The heterodyne mixing between the mechanically and the electrically induced oscillating charge densities on the tip gives rise to low frequency cross-modulation forces, proportional to the electric force gradients. The nonresonant detection of the first- and the second-order electric force gradients has successfully been performed, resulting in a dramatic enhancement of lateral resolution in surface potential imaging. © 1994 American Institute of Physics.