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A force microscope has been used in a new application to deposit and image localized surface charge on insulators. The lateral resolution for imaging surpasses that of currently available techniques. By applying voltage pulses to an etched nickel microscope tip, micron-sized regions of approximately 2×10−16 C were created on polymethylmethacrylate and single-crystal sapphire surfaces. After depositing the charge, high-contrast images of the charged region were obtained as contours of constant force gradient. The contrast was observed to decay over approximately 1 h, providing evidence for surface charge mobility. The minimum detectable surface charge was estimated to be on the order of 100 electrons.
Stern et al. (Mon,) studied this question.