Charge storage in Co nanoclusters embedded in SiO2 by scanning force microscopy

Scanning force microscopy was used to study localized charge deposition and subsequent transport in Co nanoclusters embedded in ${\mathrm{SiO}}_{\mathrm{2}}$ deposited on an n-type Si substrate. Co nanoclusters were charged by applying a bias voltage pulse between tip and sample, and electrostatic force microscopy was used to image charged areas, to determine quantitatively the amount of stored charge, and to characterize the discharging process. Charge was deposited controllably and reproducibly within areas ∼20–50 nm in radius, and an exponential decay in the peak charge density was observed. Longer decay times were measured for positive than for negative charge; this is interpreted as a consequence of the Coulomb-blockade energy associated with single-electron charging of the Co nanoclusters.